Earth-Science Reviews 203 (2020) 103137

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Earth-Science Reviews
journal homepage: www.elsevier.com/locate/earscirev

Sequence stratigraphy in organic-rich marine mudstone successions using T

chemostratigraphic datasets
⁎
Maya T. LaGrange , Kurt O. Konhauser, Octavian Catuneanu, Brette S. Harris, Tiffany L. Playter,
Murray K. Gingras
Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, AB T6G 2E3, Canada

A R T I C LE I N FO A B S T R A C T

Keywords: Sequence stratigraphy is commonly used to understand basin history and the distribution of conventional re-
Sequence stratigraphy servoir facies. Establishing a sequence stratigraphic framework in organic-rich mudstone successions is chal-
Chemostratigraphy lenging because macroscale sedimentological and petrophysical variations can be subtle, while biostratigraphic
Organic-rich mudstone and seismic data may be unavailable or of limited use. For these reasons, it is becoming increasingly common for
Black shale
chemostratigraphic profiles to be integrated with other datasets to facilitate sequence stratigraphic interpreta-
Geochemical proxies
tion. This paper summarizes the whole-rock inorganic geochemical proxies relevant to sequence stratigraphic
analysis in fine-grained, organic-rich marine units and reviews studies that have incorporated chemostrati-
graphic trends for sequence stratigraphy. This synthesis demonstrates that chemostratigraphic datasets are
useful in identification of transgressive-regressive cycles, allowing for a preliminary summary of the chemos-
tratigraphic characteristics of the maximum flooding surface, maximum regressive surface, transgressive systems
tract, and regressive systems tract to be established based on existing work. A preliminary synthesis of the
chemostratigraphic characteristics of the highstand systems tract is also possible for highstand systems tracts
recognized using other criteria. However, a chemostratigraphic means of identifying the correlative conformity
and basal surface of forced regression in order to subdivide the regressive systems tract into the lowstand systems
tract, falling-stage systems tract, and highstand systems has not yet been demonstrated. Further work is also
required in order to establish the differences in the chemostratigraphic signature of surfaces and systems tract
depending on the depositional setting. Chemostratigraphic proxies are an emergent and promising tool for the
identification of cyclicity in organic-rich mudstone intervals, which will become increasingly useful as further
research is conducted on the topic.

1. Introduction Sequence stratigraphic units and bounding surfaces can be observed at

different scales (i.e., hierarchical levels), depending on the purpose of
Sequence stratigraphy was defined by Posamentier and Allen (1999) study and/or the resolution of the data available (Catuneanu, 2019b).
as “the analysis of cyclic sedimentation patterns that are present in The interpretations drawn from sequence stratigraphic analysis are
stratigraphic successions, as they develop in response to variations in used to gain insights into the evolution of sedimentary basins and
sediment supply and space available for sediment to accumulate” (p.1). predict facies distributions for hydrocarbon and mineral exploration
In sequence stratigraphy, the unit deposited during one of these com- (Embry, 2009; Catuneanu et al., 2011). Recently, the development of
plete cycles is classified as a sequence (Catuneanu et al., 2009), and shale resource plays has resulted in an increased focus on under-
sequences are subdivided into systems tracts that consist of coeval de- standing and predicting lateral and vertical lithological variations in
positional systems (Brown Jr. and Fisher, 1977). Systems tracts can be organic-rich mudstone successions (e.g., Egenhoff and Fishman, 2013;
further subdivided into smaller scale sequence stratigraphic cycles (i.e., Taylor and Macquaker, 2014; Wilson and Schieber, 2015; Birgenheier
sequences), allostratigraphic cycles (i.e., parasequences), or sedi- et al., 2017; Li and Schieber, 2020). As a result, establishing a sequence
mentological cycles (i.e., beds and bedsets) (Catuneanu, 2019a). Both stratigraphic framework is a valuable tool for the evaluation of these
sequences and systems tracts are bounded by sequence stratigraphic successions because it allows for the delineation and subsurface corre-
surfaces (Embry, 2002; Catuneanu, 2006; Catuneanu et al., 2009). lation of higher quality reservoir intervals (Slatt and Rodriguez, 2012;

⁎
Corresponding author.
E-mail address: maya1@ualberta.ca (M.T. LaGrange).

https://doi.org/10.1016/j.earscirev.2020.103137
Received 4 July 2019; Received in revised form 13 February 2020; Accepted 14 February 2020
Available online 19 February 2020
0012-8252/ © 2020 Elsevier B.V. All rights reserved.
M.T. LaGrange, et al. Earth-Science Reviews 203 (2020) 103137

Knapp et al., 2019).

However, making stratigraphic correlations in mudstone intervals
presents unique challenges. These successions are characterized by
more subtle macroscale sedimentological variations than those ob-
served in coarser-grained clastic and carbonate successions, making
lithological trends less feasible unless thin sections are available at a
high resolution (Ratcliffe et al., 2012a; Borcovsky et al., 2017). Fur-
thermore, the subtle compositional variability characteristic of fine-
grained successions can be difficult to infer from petrophysical datasets
(i.e., well logs), which are commonly used as a basis for stratigraphic
correlations (Pearce et al., 2005; Turner et al., 2016; Knapp et al.,
2019). In addition, as these mudstone intervals commonly represent
sedimentation in distal areas of restricted basins, biostratigraphic da-
tasets may be unavailable or of limited use (Ratcliffe et al., 2012a).
Moreover, high-resolution biostratigraphic datasets are typically un-
available for Paleozoic or older mudstone intervals (Slatt and
Rodriguez, 2012). The stratal geometries that allow researchers to
make interpretations using seismic profiles are often absent (Pearce
et al., 2005), seismic datasets may not exist, or the scale of the sequence
stratigraphic analysis may be sub-seismic (Catuneanu, 2019b).
Given these challenges, it is becoming increasingly common for
chemostratigraphic data to be integrated with other datasets in the Fig. 1. Sequence stratigraphic terminology used herein. Acronyms: NR–normal
study of mudstone successions (e.g., Pearce et al., 1999; Ratcliffe et al., regression; FR–forced regression; TR–transgression; RST–regressive systems
2010, 2012a, 2012b; Sano et al., 2013; Turner et al., 2015, 2016; El tract; LST–lowstand systems tract; FSST–falling-stage systems tract;
Attar and Pranter, 2016; Nyhuis et al., 2016; Pyle and Gal, 2016; HST–highstand systems tract; TST–transgressive systems tract; MRS–maximum
Playter et al., 2018; DeReuil and Birgenheier, 2019). This type of regressive surface; CC–correlative conformity; BSFR–basal surface of forced
stratigraphy uses inorganic geochemical properties of whole-rock regression; and MFS–maximum flooding surface. Modified from Catuneanu
(2006).
samples to characterize and correlate stratigraphic units (Pearce et al.,
1999). More specifically, certain elements and elemental ratios are used
as proxies for sediment provenance, paleoproductivity, paleoredox Hierarchical Cluster Analysis, or Q-mode Factor Analysis to determine
conditions in the water column and/or sediment pile, and other en- elemental associations (e.g., Pearce et al., 2005; Ver Straeten et al.,
vironmental conditions at the time of sediment deposition. Indeed, 2011; Ratcliffe et al., 2012a; Sano et al., 2013; Turner et al., 2016;
observed variations in these proxies with depth have been used to Playter et al., 2018). Table 2 summarizes the purpose of each elemental
identify chemostratigraphic units with distinct geochemical character- proxy discussed below. It is critical that elemental data are tied to
istics (Pearce et al., 1999). As a result, chemostratigraphic units are mineralogical-data calibration points to ensure sound and accurate
now frequently applied as a means of subdivision and correlation (e.g., depositional and diagenetic interpretations of elemental data as they
Pearce et al., 1999; Ratcliffe et al., 2010, 2012a; El Attar and Pranter, relate to mineralogical distribution.
2016; Pyle and Gal, 2016). Chemostratigraphic datasets have also been
used to increase the resolution of sequence stratigraphic interpretations
within mudstone intervals (e.g., Algeo et al., 2004; Ver Straeten et al., 2.1. Detrital sediment
2011; Hammes and Frébourg, 2012; Sano et al., 2013; Turner et al.,
2015, 2016). The abundance of elements that are predominantly associated with
In this work, we review the elements and elemental ratios that can detrital sediment can be used to make inferences about the abundance
be used as chemostratigraphic proxies for sequence stratigraphic in- of terrigenous (i.e., continental) sediment input into the basin (Chen
terpretation of organic-rich mudstone successions. Through this review, et al., 2013). These elements will vary depending on the interval but
we aim to provide a systematic framework for the chemostratigraphic commonly include Al, K, Ti, and Zr (e.g., Pearce et al., 2005; Ratcliffe
identification of sequence stratigraphic surfaces, systems tracts, and et al., 2012b; Sano et al., 2013; Nyhuis et al., 2016; Turner et al., 2016).
sequences in organic-rich mudstone intervals. This work follows the In addition to single element patterns, summing a combination of the
sequence stratigraphic terminology of Catuneanu (2019a) (Fig. 1). elements has also proven effective as a proxy for abundance of con-
Herein, ‘mudstone’ is used as a general term referring to all fine-grained tinental input (e.g., Al2O3 + TiO2 + Fe2O3 + K2O) as was used by
sedimentary rocks and is defined as a rock composed of greater than Ratcliffe et al. (2012b) to characterize chemostratigraphic units in the
50% silt and clay size grains following Lazar et al. (2015). Horn River Formation (British Columbia, Canada). Similarly, Sano et al.
(2013) used Al2O3 + K2O + TiO2 in conjunction with other proxies to
2. Chemostratigraphic proxies establish a chronostratigraphic framework in the Haynesville Forma-
tion (Texas and Louisiana). Chen et al. (2013) suggested that because Ti
The following section reviews many of the elemental proxies useful is present in heavier minerals (e.g., rutile, ilmenite) that settle out
to chemostratigraphy of mudstone intervals and how they are inter- sooner than Al bearing minerals, the molar Ti/Al ratio can be used as a
preted. Table 1 summarizes the main mineralogical controls on these proxy for changes in proximity to detrital sediment sources and varia-
elements, although there are other ways for these elements to become tion in relative sea level in intervals where sediment provenance re-
enriched in sediment (e.g., incorporation into nodules; adsorption onto mains constant. They observed similar trends in the reconstructed sea
mineral particles in the water column; adsorption onto, or accumula- level profile based on benthic δ18O profile and the Ti/Al profile of se-
tion into, organic matter; diagenetic and metamorphic reactions, and diment cores from the South China Sea and Japan Sea, with lower Ti/Al
interactions with secondary fluids). To confirm the primary controls on in sediment deposited during interglacial periods and higher Ti/Al in
each element, many studies compare elemental composition data to sediments from glacial periods suggesting that increases in Ti/Al reflect
mineralogical data obtained through x-ray diffraction analysis and increased continental sediment supply to the basin.
perform statistical analyses such as Principal Component Analysis,

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M.T. LaGrange, et al. Earth-Science Reviews 203 (2020) 103137

Table 1
Primary mineralogical controls on elements commonly used as chemostratigraphic proxies in organic-rich mudstone successions.
Element Typical mineralogical affinity Reference

Al Clay and feldspar. Ratcliffe et al. (2004), Brumsack (2006), Tribovillard et al. (2006),
Piper and Calvert (2009)
Cr Chromite, clay minerals, ferromagnesian minerals. Tribovillard et al. (2006)
K Clay and feldspar. Ratcliffe et al. (2004), Turner et al. (2016)
Nb Titanium oxides, silicates, or distinct mineral phase, clay minerals. Bonjour and Dabard (1991), Dinelli et al. (2007)
Rb Clay and feldspar. Ratcliffe et al. (2004), Dinelli et al. (2007)
Th Detrital. Associated with clay minerals, accessory minerals or adsorbed to mineral surfaces. Myers and Wignall (1987), Rowe et al. (2017)
Ti Titanium oxides, chlorite, illite/mica, biotite. Ratcliffe et al. (2004), Pearce et al. (2005), El Attar and Pranter
(2016)
U Detrital or authigenic. Accessory minerals or adsorbed to mineral surfaces. Can also be adsorbed to Myers and Wignall (1987), Wignall and Twitchett (1996),
organic matter or precipitated as uranium oxides. Tribovillard et al. (2006)
Y Heavy minerals (zircon, garnet, monzanite, apatite, hornblende). Dinelli et al. (2007)
Zr Zircon. Patchett et al. (1984), Ratcliffe et al. (2004), Mongelli et al.
(2006), El Attar and Pranter (2016)

Table 2
Elemental proxies useful for interpreting systems tracts, bounding surfaces, and sequences in marine organic-rich mudstone successions.
Parameter Potential proxies Details Reference

Detrital Sediment Elements predominantly associated with Vary depending on the interval but commonly include Al, K, Zr, and e.g., Pearce et al. (2005), Ratcliffe
detrital sediment Ti. et al. (2012b), Sano et al. (2013),
Nyhuis et al. (2016), Turner et al.
(2016)
Sums of detrital elements For example, Al2O3 + TiO2 + Fe2O3 + K2O used by Ratcliffe et al. e.g., Ratcliffe et al. (2012b), Sano
(2012b) or Al2O3 + K2O + TiO2 used by Sano et al. (2013). et al. (2013)
Ti/Al Higher Ti/Al as terrigenous input increases. Chen et al. (2013)
Grain Size Si/Al If silica is primarily detrital, can reflect changes in abundance of e.g., Ratcliffe et al. (2004), Dinelli
coarse sediment supply. et al. (2007), Ratcliffe et al. (2012c)
Zr/Al May reflect changes in abundance of coarse sediment supply. e.g., Ratcliffe et al. (2004), Dinelli
et al. (2007)
Zr/Nb May reflect changes in abundance of coarse sediment supply. e.g., Ratcliffe et al. (2006), Sano et al.
(2013)
Zr/Rb May reflect changes in abundance of coarse sediment supply. e.g., Dinelli et al. (2007)
Y/Al Reflects changes in abundance of coarse sediment supply. e.g., Dinelli et al. (2007)
Y/Rb May reflect changes in abundance of coarse sediment supply. e.g., Dinelli et al. (2007)
Paleoredox V Can become enriched in anoxic to euxinic environments. Wanty and Goldhaber (1992), Calvert
and Pedersen (1993), Tribovillard
et al. (2006)
Re Can become enriched in anoxic to euxinic environments. Colodner et al. (1993), Crusius et al.
(1996), Yamashita et al. (2007)
Cr Can become enriched in anoxic to euxinic environments. Calvert and Pedersen (1993), Algeo
and Maynard (2004), Tribovillard
et al. (2006)
U Can become enriched in anoxic to euxinic environments. Myers & Wignall (1987), Wignall and
Twitchett (1996), McManus et al.
(2005), Tribovillard et al. (2006)
Mo Can become enriched in euxinic environments. Erickson and Helz (2000)
Ni Can become enriched in euxinic environments Huerta-Diaz and Morse (1992),
Calvert and Pedersen (1993),
Tribovillard et al. (2006)
Th/U Th/U < 2 reflects anoxic conditions, Th/U 2–7 suggests oxic Wignall and Twitchett (1996)
environments, and Th/U > 7 is indicative of sediment deposited
under highly oxidizing conditions.
Basin Restriction Mo/TOC Mo/TOC > 35 × 10−4 in weakly restricted settings, Algeo & Lyons (2006)
~15–35 × 10−4 in environments characterized by moderate
restriction, and < 15 × 10−4 in strongly restricted settings.
Excess Silica Peaks in Si/Al Al typically reflects clay abundance in mudstones (Sageman and Davis et al. (1999), Sageman & Lyons
Lyons, 2004; Rowe et al., 2017). (2004), Turner et al. (2015, 2016),
Gambacorta et al. (2016), Zhang et al.
(2019)
Si-Al crossplot See Fig. 2. Tribovillard et al. (2006), Rowe et al.
(2012), El Attar and Pranter (2016)
Siexcess = Sisample(Si/Al)average shale Excess silica relative to average shale. Ross and Bustin (2009), Shen et al.
(2014), Arsairai et al. (2016)

Acronyms: TOC–total organic carbon.

2.2. Grain size clay size fraction (e.g., Ratcliffe et al., 2004; Dinelli et al., 2007;
Ratcliffe et al., 2012c). Similarly, changes in Zr/Nb and Zr/Al can in-
Chemostratigraphic proxies can also be used to infer grain size dicate changes in the abundance of coarse sediment supply if these
variations. For instance, if the Si present in an interval is predominantly ratios are primarily controlled by variations in the abundance of silt-
detrital, trends in Si/Al can reflect variations in the proportion of silt to size detrital zircon rather than variations in provenance (Ratcliffe et al.,

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M.T. LaGrange, et al. Earth-Science Reviews 203 (2020) 103137

2004, 2006; Dinelli et al., 2007; Sano et al., 2013). In this case, Ratcliffe euxinic conditions where Re(VII) is reduced to Re(IV) (Colodner et al.,
et al. (2004) suggested that increasing Zr/Al results from an increased 1993; Crusius et al., 1996; Yamashita et al., 2007) and precipitated as
proportion of silt-size detrital zircon compared to clay minerals, sulfides or adsorbed to organic matter (Kendall et al., 2010). In anoxic
whereas Ratcliffe et al. (2006) and Sano et al. (2013) observed that Zr/ settings, the conversion of Cr(VI) to Cr(III) leads to the latter being
Nb followed grain size trends with higher Zr/Nb in coarser grained incorporated into organic compounds or Fe(III) and Mn(IV) oxyhydr-
intervals. Additionally, in a study of fine-grained Pleistocene and Ho- oxides (Calvert and Pedersen, 1993; Algeo and Maynard, 2004).
locene sediments, Dinelli et al. (2007) showed that ratios of Zr/Rb, Zr/ Chromium has been used to interpret paleoredox conditions in some
Al, Y/Al, and Y/Rb correlated well to the coarse silt-size sediment studies (e.g., El Attar and Pranter, 2016), although the possibility of Cr
fraction. Bloemsma et al. (2012) assessed geochemical grain size loss from remineralization of organic matter in euxinic conditions
proxies for three offshore fine-grained quaternary sediment cores. Two (Algeo and Maynard, 2004) and the potential for Cr enrichment from an
of these cores were retrieved from different locations off the coast of increased proportion of Cr in detrital sediment means that this proxy
West Africa, whereas the third core was collected off the coast of Chile. should be used with caution (Tribovillard et al., 2006).
In the two cores from West Africa, Si exhibited the strongest correla- Uranium is delivered to the oceans in detrital accessory minerals
tions with mean grain size, whereas Ti showed the strongest correla- and in solution in the form of U(VI) (Myers and Wignall, 1987). Under
tions with mean grain size for core collected offshore of Chile. Based on oxic conditions, U(VI) binds with bicarbonate to form a soluble anion,
these results, Bloemsma et al. (2012) concluded that geochemical grain while reducing conditions lead to the conversion of U(VI) to U(IV) and
size proxies vary depending on the particular setting. The above studies can lead to sedimentary enrichment of U in the form of uraninite, UO2
suggest that geochemical grain size proxies should be determined for (Myers and Wignall, 1987; Klinkhammer and Palmer, 1991; Wignall
each interval on a case by case basis through comparison with grain size and Twitchett, 1996; McManus et al., 2005; Partin et al., 2013).
data. Molybdenum accumulates in sediment under euxinic conditions
(Helz et al., 1996; Tribovillard et al., 2006; Scott and Lyons, 2012;
2.3. Paleoredox Robbins et al., 2016). In seawater, Mo is present as molybdate
(MoO42−) and in the presence of H2S reacts to form particle-reactive
Variations in redox conditions changes the solubility of many ele- thiomolybdates (e.g., MoS42−) (Helz et al., 1996), which is enriched in
ments, with decreased oxygen levels at the sediment water interface sediment through complexation with organic matter or reduced com-
resulting in the enrichment of certain metals in sediment; in this paper pounds (Helz et al., 1996; Erickson and Helz, 2000). Similarly, euxinic
we focus on mudstone successions. When discussing paleoredox con- conditions result in enrichment of Ni through incorporation of NiS into
ditions, the terms oxic, suboxic, anoxic, and euxinic are used. The sulfides (Huerta-Diaz and Morse, 1992; Calvert and Pedersen, 1993).
distinction between oxic, suboxic, and anoxic has commonly been used The Th/U ratio can be used as an indicator of paleoredox conditions
to distinguish the amount of dissolved oxygen available, with different (Myers and Wignall, 1987; Wignall and Twitchett, 1996; Kimura and
authors using different concentrations (e.g., oxic, > 4.5 uM; suboxic, Watanabe, 2001; Sano et al., 2013). As previously discussed, U can
4.5 uM – 10 nM; anoxic, < 10 nM; Morrison et al., 1999; Revsbech become enriched in sediment under anoxic conditions (Myers and
et al., 2009; Tyson and Pearson, 1991). The terms are similarly used to Wignall, 1987; Wignall and Twitchett, 1996; Partin et al., 2013). In
refer to specific microbial metabolisms and the terminal electron ac- contrast, the abundance of Th in sediment is controlled by detrital input
ceptor (TEA) coupled to the oxidation of organic carbon (e.g., Froelich as it is insoluble in seawater, and consequently it does not become
et al., 1979; Canfield and Thamdrup, 2009). Thus, oxic refers to aerobic authigenically enriched (Myers and Wignall, 1987; Wignall and
respiration (where cells use O2), suboxic refers to using nitrate (NO3−), Twitchett, 1996). Owing to their differing mobility, Th/U < 2 is in-
while anoxic refers to using Mn(IV), Fe(III) or sulfate (SO42−) terpreted to reflect deposition under anoxic conditions, Th/U of 2–7
(Konhauser, 2007). Additionally, the term euxinic will be used herein to suggests oxic environments, and Th/U > 7 are indicative of sediment
describe anoxic conditions in which sulfate reduction results in the deposited under highly oxidizing conditions (Wignall and Twitchett,
presence of hydrogen sulfide in the water column. 1996).
Trace element enrichment is commonly compared to an average
shale with the enrichment factor for a given element (X) calculated as 2.4. Basin restriction
follows (Brumsack, 2006; Tribovillard et al., 2006):
In basins characterized by a degree of restriction from the global
EFX = (X /Al)sample /(X /Al)average shale (1)
oceans, Algeo & Lyons (2006) proposed that Mo/total organic carbon
An element is considered to be enriched if this ratio exceeds 1 and (TOC) is an indicator of the extent of basin isolation. In that study,
depleted if the ratio is below 1 (Brumsack, 2006; Tribovillard et al., Mo–TOC relationships of sediment were demonstrated to vary as a
2006). Alternatively, Brumsack (2006) argued that for intervals with function of deep-water dissolved Mo and deep-water renewal time in
low detrital input and, therefore low Al abundance, it is more suitable several restricted modern anoxic environments. Algeo & Lyons (2006)
to evaluate enrichment of an element (X) by considering its non-detrital observed sedimentary Mo/TOC values of > 35 × 10−4 in weakly re-
fraction, which can be calculated using the following formula: stricted settings, ~15–35 × 10−4 in environments characterized by
moderate restriction, and < 15 × 10−4 in strongly restricted settings.
Xnon − detrital = Xsample –Al sample (X /Al)average shale (2)
This trend was interpreted to reflect a growing drawdown of the aqu-
Elemental concentrations from Average Shale (Wedepohl, 1971) or eous Mo reservoir in increasingly restricted settings caused by Mo re-
Post-Archean Average Australian Shale (Taylor and McLennan, 1985) moval to sediment outpacing Mo re-supply. These relationships have
are commonly used as comparators (e.g., Ross and Bustin, 2009; Rowe been used to interpret the level of basin isolation in several studies of
et al., 2009; Zhou and Jiang, 2009; Gambacorta et al., 2016). organic-rich mudstone intervals including Harris et al. (2013), Turner
Several trace elements, including V, Re, Cr, and U, may become and Slatt (2016), and Hines et al. (2019).
enriched in anoxic to euxinic sediment. Vanadium is fixed in sediment
under mildly anoxic conditions if the insoluble hydroxide VO(OH)2 is 2.5. Biogenic sediment
formed once V(V) is reduced to V(IV) (Wanty and Goldhaber, 1992;
Calvert and Pedersen, 1993). Under euxinic conditions, sedimentary V Certain elements, elemental ratios, and cross-plots are used to es-
enrichment occurs through precipitation of V2O3 or V(OH)3 upon re- timate the proportion of biogenic silica (i.e., silica derived from or-
duction of V(IV) to V(III) (Wanty and Goldhaber, 1992; Calvert and ganisms such as diatoms, radiolarians, or siliceous sponges) relative to
Pedersen, 1993). Sedimentary enrichment of Re occurs under anoxic to other sources of silica. Understanding variations in the abundance of

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M.T. LaGrange, et al. Earth-Science Reviews 203 (2020) 103137

these studies (e.g., Dean and Arthur, 1998; Hart, 2015) suggesting that
Ca could potentially be used as a proxy for pelagic carbonate if this
relationship is first confirmed through comparison to XRD and sedi-
mentological datasets.

2.6. Additional influences on composition

A number of factors can influence elemental distributions other than

environmental conditions at the time of deposition (Pearce et al., 2005).
These include; provenance, bioturbation, post-depositional re-oxida-
tion, diagenesis, oil generation and expulsion, and chemical weathering
of outcrops. For example, although Th/U can be a paleoredox indicator
(Myers and Wignall, 1987; Wignall and Twitchett, 1996; Kimura and
Watanabe, 2001; Sano et al., 2013), it can also vary based on prove-
nance and sedimentary recycling (McLennan et al., 1993). Similarly,
trends in Zr/Nb can indicate grain size variations (e.g., Ratcliffe et al.,
2006; Sano et al., 2013) or changes in sedimentary provenance, with
Fig. 2. Example of a Si–Al cross-plot used to identify excess silica. Data is from
the Barnett Shale in the 1-Blakely core in Wise County, Texas. Reproduced from
higher Zr/Nb resulting from an increased proportion of zircon derived
Rowe et al. (2012) with permission. from a mature, recycled sedimentary provenance (Ratcliffe et al.,
2006).
Bioturbation also affects trace element distribution. Using elemental
biogenic silica is important because these trends can be used to inter-
maps obtained by synchrotron rapid scanning X-ray fluorescence,
pret variations in productivity (Schieber et al., 2000; Harris et al., 2018)
Harazim et al. (2015) observed partitioning of trace elements (e.g., Cr,
or variations in the degree of clastic dilution, with higher biogenic silica
V, and Zr) between burrows and burrow halos in Cretaceous continental
suggestive of condensed deposition (Bohacs, 1998; Gutierrez Parades
mudstone samples, which was interpreted to have been caused by
et al., 2017). Sources of Si in mudstone successions include detrital Si
mechanical sorting of grains by the trace maker. In marine sedimentary
from fluvial or aeolian sources, precipitation of silica during clay di-
rocks, elemental distributions have also been shown to be heavily im-
agenesis, and biogenic silica (Schieber, 1996; Taylor & Macquaker,
pacted by the presence of bioturbation (e.g., Over, 1990). This high-
2014). Hydrothermal input can additionally serve as a potential source
lights the importance of understanding the scale and distribution of
of silica in mudstone intervals (e.g., Adachi et al., 1986; Arsairai et al.,
bioturbation in the interval of interest and assessing geochemical het-
2016). Aluminium content typically reflects clay abundance in mud-
erogeneity caused by animal-sediment interactions where possible.
stone intervals (Sageman and Lyons, 2004; Rowe et al., 2017). Excess Si
Many major and trace elements may be affected by diagenesis, and
relative to Al is typically identified in the following three ways: (1)
some elements are more prone to diagenetic remobilization than others
peaks in Si/Al (e.g. Dean and Arthur, 1998; Davis et al., 1999; Sageman
(Pearce et al., 1999). For example, of elements often associated with
and Lyons, 2004; Turner et al., 2016); (2) Si–Al crossplots (Fig. 2;
detrital input (e.g., Al, Ti) are effectively immobile during diagenesis
Tribovillard et al., 2006; Rowe et al., 2012; El Attar and Pranter, 2016);
(e.g., Thomson et al., 1998; Brumsack, 2006; Tribovillard et al., 2006),
and (3) calculating excess silica relative to the average shale using eq. 3
although the observation of authigenic kaolinite in the Cretaceous
(e.g., Ross and Bustin, 2009; Shen et al., 2014; Arsairai et al., 2016).
Mancos Shale suggests that Si and Al can be mobile on at least a local
Where excess Si may be biogenic, aeolian, or hydrothermal in origin.
scale (Taylor & Macquaker, 2014). Silica can be released during diag-
Si excess = Sisample –Al sample (Si/Al)average shale (3) enesis through clay mineral transformations (e.g., smectite to illite or
illite to muscovite) or through dissolution of silicate minerals or bio-
Al–Fe–Mn ternary diagrams can be used to distinguish non-hydro- genic silica (Schieber, 1996; Taylor & Macquaker, 2014). The ob-
thermal silica from hydrothermal silica (see Adachi et al., 1986; servation of early diagenetic quartz suggests that the low permeability
Arsairai et al., 2016). Sageman and Lyons (2004) suggest that biogenic and low diffusion coefficients of mudstone intervals prevents significant
and aeolian silica can be distinguished by considering depositional diagenetic mobility of silica (Taylor & Macquaker, 2014 and references
environment, studying sedimentary textures, and by comparison with therein). Similarly, K can be incorporated into clays during illitization,
other elemental proxies. For example, the presence of biogenic silica is whereby a precursor smectite phase can be altered through pore-water
supported if the Zr/Al and Ti/Al profiles, which are interpreted to re- enrichment in K+. Two main mechanisms for smectite to illite trans-
flect the proportion of continental input, do not correspond to Si/Al formation have been inferred, including the dissolution of the smectite
peaks (Turner et al., 2015, 2016; Gambacorta et al., 2016). Similarly, lattice and growth of illite crystals due to the Ostwald ripening-like
Zhang et al. (2019) suggest that corresponding peaks in both Si/Al and effect (Nadeau et al., 1985) and a biological role in which sedimentary
Si/Ti indicate that excess silica is more likely biogenic than detrital. microbes concentrated K+ into their biomass which then becomes lib-
Petrographic analysis and scanning electron microscopy with asso- erated during heterotrophic degradation (Aubineau et al., 2019).
ciated imaging techniques (e.g., cathodoluminescence or charge con- Post-depositional diffusion of O2 into bottom sediments, which can
trast imaging) can also be used to differentiate biogenic silica and occur in situations such as glacial-interglacial transitions or deposition
detrital quartz (e.g., Schieber et al., 2000; Buckman et al., 2017). of turbidites in anoxic settings, leads to re-oxidation of reduced species
Planktonic organisms can also have calcareous rather than siliceous which can, in turn, result in remobilization of certain elements
skeletons (e.g., coccoliths, foraminifera). Several authors have proposed (Morford et al., 2001; Tribovillard et al., 2006). This has been observed
that lithological cycles observed in successions dominated by pelagic to potentially affect Cr, Mo, Ni, U, and V (e.g., Thomson et al., 1993,
carbonates, marlstones, and mudstones are related to variations in re- 1995; Rosenthal et al., 1995). Tribovillard et al. (2006) suggested that
lative sea level (e.g., Elder et al., 1994; Dean and Arthur, 1998; Hart, redox-sensitive trace metals associated with sulfides (e.g., Mo, Ni, Re)
2015). More specifically, Hart (2015) proposed that pelagic carbonate should be immobile if post-depositional re-oxidation does not occur
content increases upwards through transgression to the maximum because sulfides are typically stable during diagenesis in sediments
flooding surface and then decreases upwards during regression. CaCO3 where organic supply is sufficient to support sulfate reduction, and
or calcite profiles have been used in conjunction with other datasets hence sulfide generation.
(e.g., sedimentological, petrophysical) to identify cycles in some of The enrichment of Ni and V has been observed in oils derived from

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M.T. LaGrange, et al. Earth-Science Reviews 203 (2020) 103137

type II kerogens (Lewan and Maynard, 1982; Lewan, 1984). This may Shale of the Arkoma Basin, Oklahoma. The third-order maximum
result in lower than expected enrichment of certain trace metals in flooding surface identified in this interval is characterized by a max-
organic-rich mudstone units that have undergone oil generation and imum gamma ray response, relatively high Si/Al, local minimum con-
expulsion (Harris et al., 2013). Chemical weathering also affects com- centration of terrigenous elements (Al, K, Ti, Zr). This surface also se-
position (Nesbitt and Young, 1982). For example, in a study of two parates underlying strata with lower Mo and V abundance from
organic-rich mudstone intervals from the Guizhou province in China, overlying strata displaying higher Mo and V concentrations (Fig. 3).
Liu et al. (2016) observed the loss of major elements, such as Na, Ca, Higher-frequency surfaces that mark the shift from regression to
Mg, Fe, and trace elements, including V and Ni, in outcrop samples transgression were recognized chemostratigraphically and are marked
compared to core samples. Weathering of organic matter in mudstone by local minima in terrigenous elements (e.g., Al, K, Ti, Zr) and com-
intervals has been observed to cause a pronounced breakdown of Ni and monly coincide with local maxima in Si/Al and Ca. Turner et al. (2016)
V metallo-organic complexes (Grosjean et al., 2004). In a study of the refer to these as chemostratigraphic flooding surfaces, herein, these
effects of weathering on proxies used for paleoenvironmental inter- surfaces are interpreted as higher frequency maximum flooding sur-
pretation in mudstone successions, Marynowski et al. (2017) observed faces. Ratcliffe et al. (2012c) and Sano et al. (2013) identified max-
significant decreases in the concentrations of trace metals including, imum flooding surfaces in the Upper Jurassic Haynesville Shale, an
Mo, Ni, and U, which were attributed to degradation of organic matter organic-rich mudstone deposited on the slope of a carbonate platform
and oxidation of pyrite. The depletion of Re from weathering of or- (Frébourg et al., 2013) in a restricted basin setting (Hammes et al.,
ganic-rich mudstones has also been observed (e.g., Peucker-Ehrenbrink 2011). These studies placed maximum flooding surfaces in the Hay-
and Hannigan, 2000; Jaffe et al., 2002). nesville Shale at minimums in Zr/Nb, while chemostratigraphic data
Due to complexities inherent in element and mineral distributions, presented in Sano et al. (2013) also illustrates that maximum flooding
good practice confirms element affinities using cross plots, compar- surfaces in the Haynesville Shale are characterized by minimums in V
isons, and statistical analyses. When possible, integrating chemostrati- abundance (Fig. 4). Data presented in Ratcliffe et al. (2012c) shows that
graphic interpretations with other datasets is also recommended (e.g., maximum flooding surfaces in this interval typical fall at local lows in
Algeo et al., 2004; Ver Straeten et al., 2011; Hammes and Frébourg, terrigenous content. Harris et al. (2018) presented geochemical data for
2012). cores from the Duvernay Formation — an organic-rich mudstone de-
posited in inter-reef settings during the Late Devonian in Alberta
2.7. Normalization to aluminium and the average shale (Knapp et al., 2019) — that are included in a previously established
sequence stratigraphic framework based on sedimentological and pet-
In many studies, elemental concentrations have been normalized to rophysical attributes. Harris et al. (2018) use the formula for excess Si
Al with the aim of accounting for dilution by other components and (Eq. (3)) and interpret all excess Si to be biogenic in origin, potentially
facilitating comparison between different locations and intervals because of the presence of siliceous radiolaria in several of the facies
(Pearce et al., 1999; Van der Weijden, 2002; Algeo et al., 2004; Algeo observed by Knapp et al. (2017). Maximum flooding surfaces in cores
and Maynard, 2004; Harris et al., 2013). Moreover, elements are nor- from the West Shale Basin are often near lows in Al and typically cor-
malized to Al in order to compare with average shale values, which respond to highs in excess Si and Mo/Al. In the East Shale Basin,
then provides an estimate of enrichment of those elements (Brumsack, maximum flooding surfaces are typically near minima in Al, and se-
2006; Tribovillard et al., 2006). However, Van der Weijden (2002) parate intervals of lower Al with overlying intervals characterized by
demonstrated that normalization to Al can result in apparent correla- higher Al abundance. Patterns of excess Si enrichment and Mo/Al do
tions between unrelated variables or vice versa, especially if the coef- not match those observed in the West Shale Basin, which Harris et al.
ficient of variation of Al is large compared to the coefficient of variation (2018) attributed to higher carbonate dilution, lower productivity, and
of the elements being normalized. Furthermore, Algeo & Lyons (2006) higher dissolved oxygenation in the East compared to West Shale Basin.
argued that given trace elements and TOC are both affected by dilution, Results from these studies suggest that maximum flooding surfaces
trace metals should not be normalized to Al for comparison with trends are typically characterized by relatively low levels of terrigenous ele-
in TOC. Finally, understanding the relative proportion and abundance ments (e.g., Al, K, Ti, Zr), minima in grain size proxies (e.g. Zr/Nb), and
of different elements or minerals is useful for sequence stratigraphy maxima in Si/Al (Table 3). The enrichment of redox sensitive trace
(e.g., identifying condensed sections) and normalizing to Al conflates elements (e.g., Mo, V) at these surfaces appears to be variable. Seeing as
the dataset. Consequently, normalization to Al should only be per- maximum flooding surface records a time when continental sediment is
formed if the coefficient of variation of Al is similar to that of the ele- stored in landward settings, which is accompanied by the formation of
ment of interest and it is also recommended to consider both normal- condensed sections in the marine environment (Loutit et al., 1988), a
ized profiles and raw profiles. Given its lower solubility, some trace minimum abundance of detrital elements is expected (Turner et al.,
element studies of ancient shales have instead relied on metal/Ti ratios 2016). In shallow-marine systems, a maximum flooding surface also
as a proxy for authigenic enrichments (e.g., Reinhard et al., 2013; Fru records the change from fining upwards to coarsening upwards (Embry,
et al., 2016). 2010). Peaks in Si/Al or excess Si observed by Turner et al. (2015,
2016) and Harris et al. (2018), respectively, were interpreted to reflect
3. Observed chemostratigraphic expression of surfaces a higher proportion of biogenic silica. Both studies observed increases
in biogenic silica proxies at maximum flooding surfaces, suggesting that
The geochemical expressions of sequence stratigraphic surfaces higher biogenic silica content is associated with maximum flooding
observed in fine-grained organic-rich intervals are summarized in surfaces, which is also expected in condensed sections (e.g., Bohacs,
Table 3. 1998; Gutierrez Parades et al., 2017). This has been interpreted as the
result of increased primary productivity (e.g., Harris et al., 2018), de-
3.1. Maximum flooding surface creased clastic dilution, or a combination thereof (Turner et al., 2016).
Differences in redox sensitive trace metal enrichment associated
The maximum flooding surface marks the switch from transgression with maximum flooding surfaces from study to study likely occur be-
to regression and the time of the maximum landward position of the cause of differences in the paleohydrographic characteristics of the
shoreline (Posamentier and Allen, 1999). A few studies provide ex- setting. As water depth at a point on the open shelf increases, dissolved
amples of the chemostratigraphic expression of this surface. Turner oxygen at this location typically declines (Wilde et al., 1996). In
et al. (2016) produced a sequence stratigraphic framework for two modern oceans, consumption of oxygen by degrading organic matter
outcrops and three cores in the organic-rich Upper Devonian Woodford results in the decrease of dissolved oxygen from the surface to a

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Table 3
The observed chemostratigraphic expression of sequence stratigraphic surfaces in organic-rich mudstone units.
Sequence stratigraphic surface Observed chemostratigraphic signature Examples

Maximum flooding surface Low abundance of terrigenous proxies Ratcliffe et al. (2012c), Turner et al. (2015, 2016), Harris et al. (2018)
Minima in grain size proxies Ratcliffe et al. (2012c), Sano et al. (2013)
Increase in proxies for biogenic silica Turner et al. (2015, 2016), Harris et al. (2018)
Variable levels of redox proxies depending on paleohydrography Sano et al. (2013), Turner et al. (2015, 2016), Harris et al. (2018)
Maximum regressive surface Peak in terrigenous proxies Ratcliffe et al. (2012c), Harris et al. (2018)
Peak in grain size proxies Ratcliffe et al. (2012c), Sano et al. (2013)
Minima in proxies for biogenic silica Harris et al. (2018)
Variable levels of redox proxies depending on paleohydrography Sano et al. (2013), Harris et al. (2018)

Fig. 3. Chemostratigraphic profiles from the Woodford Formation in the Wyche Farm Quarry Core–1 in Pontotoc County, Oklahoma. Gamma ray log data and
chemostratigraphic profiles are presented with the interpreted transgressive-regressive cycles shown to the left of each dataset. The shoreline trajectory shown
presents the inferred local shoreline trajectory. The solid grey lines at the change from transgression to regression are herein interpreted as higher frequency
maximum flooding surfaces. Acronyms: GR–gamma ray; HST–highstand systems tract; MFS–maximum flooding surface; TST–transgressive systems tract. Reproduced
with permission from Turner et al. (2016).

minimum in the zone of lowest circulation (Wyrtki, 1962; Berry and In restricted basin settings, oxygen becomes depleted through decay
Wilde, 1978). Below this depth, dissolved oxygen increases as a result of organic matter and is not replenished because these basins lack cir-
of ventilation by cold, dense, oxygenated waters produced at high la- culation and ventilation (Berry and Wilde, 1978; Schönfeld et al.,
titudes during the formation of sea ice (Wyrtki, 1962; Berry and Wilde, 2015). If transgression results in increased connectivity to the open
1978). The depth of the oxygen minimum can intersect the seafloor ocean, oxygenation can increase from input of ventilated water and
from the shelf to the slope (Reichart et al., 1998; Helly and Levin, improved circulation (Savrda and Bottjer, 1989; Bohacs, 1998). Ad-
2004). At depths near the oxygen minimum, transgression could result ditionally, Hines et al. (2019) attributed increased bottom water oxy-
in a shift to more oxygenated conditions, whereas for positions well genation during times of transgression to lower productivity, with
above the oxygen minimum, transgression should lead to a decline in higher productivity during regression resulting from an increased
dissolved oxygen. Additionally, transgression can result in rising wave supply of terrigenous sediment and nutrients. For example, in a study of
base, causing decreased water column mixing and oxygenation, with the Woodford Shale, Turner and Slatt (2016) used Mo/TOC relation-
the opposite occurring during falling relative sea level (Harris et al., ships and a decreasing upwards trend in redox-sensitive trace metal
2013). This change in oxygen is reflected by the ichnofacies model enrichment to interpret renewed circulation and decreased basin iso-
(MacEachern et al., 2009a) with trace fossil assemblages transitioning lation resulting from transgression. Similarly, Sano et al. (2013) con-
to those made by organisms more tolerant of suboxic to anoxic condi- cluded that anoxic conditions during deposition of the Haynesville
tions as water depth increases and mixing from waves becomes less Shale, which occurred in a restricted intra-shelf basin, prevailed at the
common resulting in lower dissolved oxygen at the sediment-water onset of transgression. This conclusion has also been made using other
interface (MacEachern et al., 2009b; Dashtgard and MacEachern, datasets. For example, by comparing trace fossil assemblages to orga-
2016). nic‑carbon and carbonate content in the Upper Cretaceous Niobrara

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M.T. LaGrange, et al. Earth-Science Reviews 203 (2020) 103137

Fig. 4. Chemostratigraphic profiles for the Upper Jurassic strata of three wells in eastern Louisiana and western Texas and associated sequence stratigraphic
interpretation. Package 1 corresponds to the Smackover Formation, package 2 comprises the Gilmer Lime Formation, and package 3 and 4 are the Haynesville
Formation. The solid Zr/Nb profile represents a moving average with n = 2 and the associated squares are the raw data. Acronyms: T-R–transgressive-regressive;
EFV–enrichment factor of Vanadium. Reproduced from Sano et al. (2013). Copyright ©2013 by The American Association of Petroleum Geologists. Reprinted by
permission of the AAPG whose permission is required for further use.

Formation (Colorado), Savrda and Bottjer (1989) interpreted intervals 3.2. Maximum regressive surface
of increased oxygenation to reflect open marine circulation in the
Western Interior Seaway during transgression. Because of differences in The maximum regressive surface occurs at the change from re-
the effect of relative sea level on water column oxygenation depending gression to transgression and marks the maximum basinward shoreline
on the setting, the relative enrichment of redox-sensitive trace metals at position (Helland-Hansen and Martinsen, 1996). Chemostratigraphic
maximum flooding surfaces depends on whether a shift from trans- examples of maximum regressive surfaces in the Haynesville Shale can
gression to regression resulted in increased or decreased water column be found in Ratcliffe et al. (2012c) and Sano et al. (2013) where they
oxygenation. are identified by maxima in Zr/Nb. The terrigenous input profile (sum
of Al, K, Na, Ti) plotted by Ratcliffe et al. (2012c) typically displays
peaks at maximum regressive surfaces, while the V enrichment profile
in Sano et al. (2013) shows that maximum flooding surfaces most often

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M.T. LaGrange, et al. Earth-Science Reviews 203 (2020) 103137

mark the shift from increasing V enrichment to decreasing V enrich- Turner et al. (2015 and 2016) interpret a third-order transgressive
ment (Fig. 4). The chemostratigraphic data presented in Harris et al. systems tract in the organic-rich Woodford Shale. In this example, the
(2018) shows that the maximum regressive surface identified in the transgressive systems tract is characterized by an overall decline in
Duvernay Formation West Shale Basin by Knapp et al. (2019) is char- terrigenous elements (e.g., Ti, Zr, K, Al) and in redox sensitive elements
acterized by relatively high Al, minimum Mo/Al with values near 0, and (e.g., Mo, V) reflecting decreasing restriction, and often by increasing
low levels of biogenic Si. Si/Al that arises from an increasing proportion of biogenic silica
In marine shelf environments, the maximum regressive surface ty- (Fig. 3). However, in certain intervals, V is concentrated in phosphatic
pically records the shift from coarsening upwards to fining upwards nodules, resulting in V abundance peaks that are not associated with
(Catuneanu, 2002; Embry, 2010), which is reflected in these examples reduced circulation (Turner et al., 2016). Turner et al. (2016) also
by peaks in grain size proxies or detrital elements. Results from Harris identified fourth-order T-R cycles based on trends in Ti and Zr with
et al. (2018) suggest that maximum regressive surfaces are character- trends of decreasing concentration characterizing transgression.
ized by a low abundance of biogenic silica. As previously discussed, Other studies focus on the Upper Jurassic Haynesville Shale of Texas
changes in relative sea level can have variable effects on water column and Louisiana. Ratcliffe et al. (2012c) identified lower-frequency T-R
oxygenation depending on the setting. The shift from increasing to cycles in the Haynesville Formation with higher-frequency T-R cycles
decreasing V enrichment associated with maximum regressive surfaces superimposed on these trends. Higher-rank transgressive packages are
in the Haynesville Shale is likely the product of highest levels of basin characterized by a declining abundance of terrigenous content, which is
isolation at this time. Harris et al. (2018) interpreted that times of the sum of Al2O3, TiO2, Na2O, and K2O. Lower-rank transgressive in-
higher sea level during deposition of the Duvernay Formation were tervals are characterized by declining Zr/Nb and SiO2/Al2O3. Trends in
characterized by lower bottom water oxygenation because of reduced Zr/Nb and SiO2/Al2O3 follow one another, indicating that they are
water column mixing, high productivity, and inflow of oxygen depleted associated with the fraction of silt size sediment and that biogenic silica
bottom waters. This implies that minima in Mo/Al associated with the is proportionately less important in the Haynesville Formation
maximum regressive surface in the Duvernay Formation of the West (Ratcliffe et al., 2012c). This interpretation is supported by the clear
Shale Basin can be explained by highest levels of mixing, lowest pro- positive trend between Zr and Si observed in the Si–Zr cross plot for the
ductivity, and minimum inflow of oxygen depleted bottom waters. Haynesville Formation (Ratcliffe et al., 2012c). Sano et al. (2013)
identified T-R sequences in the same three localities as Ratcliffe et al.,
2012c (Fig. 4). In this case, V enrichment data is also presented, and
4. Observed Geochemical Expression of Systems Tracts
those authors interpreted that anoxia decreases throughout transgres-
sion.
The geochemical expressions of systems tracts observed in fine-
Hammes & Frebourg (2012) also studied the Haynesville Shale. In
grained organic-rich intervals are summarized in Table 4.
this dataset, the transgressive systems tract is expressed by lower Al
concentration than the overlying highstand systems tract. In contrast,
4.1. Transgressive systems tract Si/Al, Ti/Al, and Zr/Al are elevated compared to the overlying high-
stand systems tract. The authors contended that an increased abun-
The transgressive systems tract comprises strata deposited when the dance of Ti and Zr supports the interpretation that bottom waters were
rate of relative sea-level rise surpasses the rate of shoreline sediment euxinic because they are redox sensitive elements. With that said, Ti
supply (Posamentier and Allen, 1999). Examples of the chemostrati- and Zr are generally taken to reflect detrital input (Bhatia and Crook,
graphic expression of the transgressive systems tract include Turner 1986; Plank and Langmuir, 1998; Piper and Calvert, 2009), and have
et al. (2015, 2016), Sano et al. (2013), Ratcliffe et al. (2012c), Hammes not been shown by previous studies to be redox sensitive. The opposite
& Frebourg (2012), Ver Straeten et al. (2011), and Harris et al. (2018).

Table 4
The observed chemostratigraphic expression of systems tracts in fine-grained organic-rich intervals.
Systems tract Observed chemostratigraphic signature Examples

TST Decreasing Al Hammes & Frebourg (2012), Ver Straeten et al. (2011),Turner et al. (2015, 2016), Harris et al.
(2018)
Decreasing sums of terrigenous elements (e.g., Ratcliffe et al. (2012c)
Al + Ti + Na + K)
Variable trends in elements associated with heavy minerals (Ti
and Zr):
Decreases Ver Straeten et al. (2011), Turner et al. (2016)
Increases Hammes & Frebourg (2012), Ver Straeten et al. (2011)
Decreasing grain size proxies Ratcliffe et al. (2012c), Sano et al. (2013)
Elevated proxies for biogenic silica Turner et al. (2015, 2016), Harris et al. (2018)
Variable levels of redox proxies depending on paleohydrography Hammes & Frebourg (2012), Ver Straeten et al. (2011), Sano et al. (2013), Turner et al. (2015,
2016), Harris et al. (2018)
HST Increasing Al Hammes & Frebourg (2012), Turner et al. (2015, 2016), Harris et al. (2018)
Variable trends in elements associated with heavy minerals (Ti
and Zr):
Decreases Hammes & Frebourg (2012)
Increases Turner et al. (2015, 2016)
Variable levels of biogenic silica proxies Harris et al. (2018)
Variable levels of redox proxies depending on paleohydrography Turner et al. (2015, 2016), Harris et al. (2018)
RST Increasing Al Turner et al. (2015, 2016), Ver Straeten et al. (2011)
Variable trends in elements associated with heavy minerals (Ti
and Zr):
Decreases Ver Straeten et al. (2011)
Increases Turner et al. (2015, 2016), Ver Straeten et al. (2011)
Increasing grain size proxies Ratcliffe et al. (2012c), Sano et al. (2013)

Abbreviations: TST–transgressive systems tract; HST–highstand systems tract; RST–regressive systems tracts.

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M.T. LaGrange, et al. Earth-Science Reviews 203 (2020) 103137

trends in the Al profile compared to the Si/Al, Ti/Al, and Zr/Al suggest sections develop because of a decline in the abundance of terrigenous
decoupling between the sources of Al versus Si, Ti, and Zr, which could sediment input during transgression (Loutit et al., 1988; Galloway,
potentially occur if Si, Ti, and Zr are primarily associated with the 1989). The proportion of biogenic sediment increases in these intervals
wind-blown detrital fraction. Hammes & Frebourg (2012) did propose as dilution from detrital sediment declines (Arthur and Sageman, 2005;
that the detrital fraction in the transgressive strata is likely primarily Ver Straeten et al., 2011). High levels of biogenic silica in transgressive
aeolian. The Mo enrichment profile is also elevated compared to the systems tracts can also be interpreted as the product of increased bio-
overlying highstand systems tract. logical productivity (e.g., Harris et al., 2018). Ver Straeten et al. (2011)
Ver Straeten et al. (2011) identified third-order transgressive sys- interpreted that rising levels of Si/Al and Ti/Al in the transgressive
tems tracts in the Middle Devonian Oatka Creek and Skaneateles for- systems tract of the Skaneateles Formation likely reflects higher aeolian
mations deposited in the Appalachian Basin in western New York or volcanic input.
during the Middle Devonian. These formations are both included in the Trends in redox sensitive trace metal enrichment through the
Hamilton Group and the Oatka Creek Formation is also a member of the transgressive systems tract are variable. Certain studies show de-
Marcellus Subgroup (Arthur and Sageman, 2005). At this location, the creasing paleoredox proxies (e.g., Sano et al., 2013; Ver Straeten et al.,
Oatka Creek Formation dominantly comprises organic-rich shale with a 2011; Turner et al., 2016), whereas others show increases (e.g.,
few limestone interbeds and is interpreted to reflect deposition in pri- Hammes & Frebourg, 2012; Harris et al., 2018), likely because of dif-
marily anoxic conditions (Ver Straeten et al., 2011). In this interval, the ferences between the paleohydrographic conditions of each deposi-
transgressive systems tract is characterized by a small decrease in Al tional setting.
and Ti/Al, and stable levels of Si/Al in the organic-rich mudstone in-
tervals. This systems tract also shows markedly lower Mo abundance 4.2. Highstand systems tract
than the overlying regressive strata, suggesting a restricted depositional
setting with increased connectivity to the open ocean in the trans- Strata of the highstand systems tract are deposited when the rate of
gressive systems tract. The Appalachian Basin was a restricted epicra- relative sea-level rise decreases such that the rate of sediment supply
tonic basin (Rowe et al., 2008) and, therefore, increased oxygenation equals or exceeds the rate of accommodation generation at the shore-
accompanying the transgressive systems tract is expected. The over- line (Posamentier and Allen, 1999). Examples of the geochemical ex-
lying Skaneateles Formation is composed primarily of interbedded pression of highstand systems tract deposits were presented by Turner
mudstone and organic-rich mudstone and was deposited in a dom- et al. (2015, 2016) where they interpreted a third-order highstand
inantly suboxic environment (Ver Straeten et al., 2011). In this for- systems tract at the top of the Woodford Shale characterized by in-
mation, the transgressive systems tract is expressed chemostrati- creasing Ti, Zr, Al, and K, decreasing Si/Al, and relatively low Mo and V
graphically by overall low Mo, which is expected in a suboxic interval. (Fig. 3). Similarly, Hammes and Frebourg (2011) suggested that the
Here the transgressive systems tract also shows a significant decrease in organic-rich Upper Jurassic Bossier Formation (Texas and Louisiana)
Al while Ti/Al and Si/Al show increasing trends that are opposite to records a second-order highstand systems tract, characterized by in-
trends in Al. Ver Straeten et al. (2011) suggested that increasing Ti/Al creasing Al upwards, and decreasing Si/Al, Ti/Al, and Zr/Al. Highstand
and Si/Al can be attributed to elevated levels of silica from aeolian or systems tracts in the Duvernay Formation of the West Shale Basin show
volcanic sources. Data presented by Harris et al. (2018) shows that increasing Al. In this example, Mo/Al typically remains elevated
transgressive systems tracts in the Devonian Duvernay Formation of the moving from the maximum flooding surface until the mid-highstand
West Shale Basin in Alberta are characterized by decreasing Al2O3, systems tract, where it then begins to decline. The lowermost highstand
rising excess Si, and elevated Mo/Al. systems tract exhibits decreasing biogenic Si, but subsequent highstand
Together, the aforementioned studies suggest that the geochemical systems tract record continuing elevated levels of excess Si following
signature of a transgressive systems tract includes decreasing Al transgressive systems tracts (Harris et al., 2018). It is important to note
(Table 4). Other elements associated with detrital minerals (e.g., Ti, Zr) that the highstand systems tracts in all of the above examples were
may also decrease (e.g., transgressive systems tract in the Oatka Creek identified based on other datasets. At this point, criteria to identify the
Formation presented in Ver Straeten et al., 2011; Turner et al., 2016), highstand systems tract chemostratigraphically are poorly constrained
and composite profiles of terrigenous elements may show declining because no geochemical signature for the basal surface of forced re-
trends (e.g., the terrigenous input profile of Ratcliffe et al., 2012c). gression has been defined. Nonetheless, in some cases it is possible to
Although in some instances, certain detrital elements (e.g., Ti, Zr) show identify the highstand, falling-stage, and lowstand systems tracts in
increasing trends (e.g., transgressive systems tract in the Skaneateles fine-grained unconventional plays using other datasets such as seismic
Formation of Ver Straeten et al. (2011); Hammes & Frebourg, 2012). (e.g., Dominguez and Catuneanu, 2017).
During a transgression, sediment trapping in fluvial and/or coastal The highstand systems tracts described above are all characterized
environments while the shoreline moves landward results in a decrease by increasing Al content, likely reflecting progradation associated with
in the abundance of fluvial detrital sediment that reaches the shelf and relative sea level highstand. Titanium and Zr also increase in some in-
slope (Loutit et al., 1988; Mann and Stein, 1997; Catuneanu, 2006). stances and stable Ti/Al observed in one case likely reflects propor-
This is likely the cause for the observed decreases in the abundance of tional increases in both Ti and Al. However, Zr/Al and Ti/Al decline
Al and of other terrigenous elements where they occur. Turner et al. throughout the highstand systems tracts in other studies, potentially
(2016) observed stronger declines in elements associated with heavy due to falling aeolian or volcanic input. Decreasing Si/Al interpreted as
minerals (e.g., Ti and Zr) compared to elements associated with clay declining biogenic silica moving upwards through highstand systems
minerals (e.g., Al and K) and proposed that this is explained by ter- tracts is likely the product of higher dilution by detrital sediment or
restrial heavy minerals settling out sooner than clay minerals. Increases lower productivity. Conversely, increases in proxies for biogenic silica
in Ti/Al and Zr/Al observed in certain cases may be the product of (e.g., Si/Al, excess Si) through the highstand systems tract can be in-
increased aeolian or volcanic input as suggested by Ver Straeten et al. terpreted as continuing elevated levels of biological productivity (e.g.,
(2011). In marine shelf environments, transgressions are also typically Harris et al., 2018). The signature of redox proxies through the high-
characterized by fining-upwards grain size trends (Catuneanu et al., stand systems tract depends on paleohydrographic conditions. For ex-
2009; Embry, 2010), reflected by declining Zr/Nb and Si/Al in Hay- ample, low levels of Mo and V observed by Turner et al. (2016) during a
nesville Formation (e.g., Ratcliffe et al., 2012c; Sano et al., 2013). highstand systems tract in the Woodford Shale were interpreted as the
These studies all display elevated proxies for excess silica during product of greater oxygenation caused by a higher degree of con-
transgressive systems tracts. In certain cases, this was interpreted as nectivity between the Arkoma Basin and the Palaeotethys at this time.
biogenic silica (e.g., Turner et al., 2016; Harris et al., 2018). Condensed Harris et al. (2018) suggested that the Mo/Al patterns observed in

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M.T. LaGrange, et al. Earth-Science Reviews 203 (2020) 103137

highstand systems tracts of the Duvernay Formation in the West Shale Bohacs and Schwalbach, 1992; Macquaker and Taylor, 1996; Bohacs,
Basin are attributed to higher input of anoxic nutrient-rich seawater 1998; Schieber, 1998; Williams et al., 2001; Bohacs et al., 2005;
from lower basin restriction leading to higher productivity and more Macquaker et al., 2007). For these reasons, it is not yet possible to
reducing conditions during the upper transgressive systems tracts and define the chemostratigraphic expression of the correlative conformity
lower highstand systems tracts. and basal surface of forced regression. During a transgression, the
transgressive surface of erosion forms from erosion by waves or tides
4.3. Regressive systems tract (Swift, 1975; Catuneanu, 2002), and typically forms in coastal and
upper shoreface settings (Catuneanu, 2002). Similarly, the regressive
The regressive systems tract constitutes all strata deposited when surface of marine erosion is cut by waves during forced regression (Plint
the shoreline moves basinward (Embry, 2002; Embry and Johannessen, and Nummedal, 2000) and this erosion takes place on the inner shelf
1993). The regressive systems tract includes strata of the highstand, (Plint and Nummedal, 2000; Catuneanu, 2002). Until recently, fine-
falling-stage, and lowstand systems tracts (Catuneanu, 2002). Turner grained organic-rich successions were viewed as largely distal, deep-
et al. (2015 and 2016) interpreted fourth-order regressive systems water deposits, which would suggest that the transgressive surface of
tracts superimposed on third-order trends in the Woodford Shale based erosion and regressive surface or marine erosion are not relevant to
on Ti and Zr profiles, which displayed similar signatures to trends in K these units. However, many organic-rich mudstone successions are now
and Al. These regressive intervals are typically characterized by in- being re-interpreted to reflect deposition in more proximal environ-
creasing Ti, Zr, K, and Al (Fig. 3). Lower rank regressive systems tracts ments than originally thought (e.g., Schieber, 1994; Smith et al., 2019).
are interpreted in the Haynesville Shale by Ratcliffe et al. (2012c) and This may lead to increased recognition of these surfaces and allow for
Sano et al. (2013). These regressive trends are characterized by in- future delineation of their chemostratigraphic characteristics.
creasing Zr/Nb and Si/Al in Ratcliffe et al. (2012c) and increasing Zr/ Flooding surfaces and parasequences are not discussed in this work.
Nb and V in Sano et al. (2013); Fig. 4). Ratcliffe et al. (2012c) interprets A flooding surface (i.e., parasequence boundary) records an abrupt
that Zr/Nb and Si/Al record grain size variations in the Haynesville water deepening (Van Wagoner et al., 1988), which may or may not
Formation. Ver Straeten et al. (2011) presented geochemical profiles record a change in stratal stacking pattern (Catuneanu, 2019a); as such,
along with a previously established sequence stratigraphic interpreta- it is a surface of allostratigraphy rather than sequence stratigraphy
tion for the Devonian Oatka Creek and Skaneateles formations. They (Catuneanu, 2019a). Certain studies of mudstone intervals still make
use the term ‘early highstand systems tract’ for the highstand systems use of flooding surfaces and parasequences rather than lower rank se-
tract and the term ‘late highstand systems tract’ rather than ‘falling- quence stratigraphic surfaces and sequences (e.g., Bohacs et al., 2014;
stage systems tract’ for the interval deposited during relative sea level Birgenheier et al., 2017; Borcovsky et al., 2017; Turner et al., 2015,
fall, following the Van Wagoner et al. (1988) model. In their inter- 2016), even though the parasequence boundaries no longer comply
pretations, these are grouped together as the highstand systems tract with the original definition of a flooding surface. Therefore, following
rather than separated into the early highstand systems tract and late Catuneanu (2019a), this paper recommends that the use of scale-in-
highstand systems tract. Ver Straeten et al. (2011) suggested that dependent terminology (i.e., the recognition of higher-frequency se-
lowstand systems tracts are not present above these highstand systems quence stratigraphic surfaces and systems tracts) provides a superior
tracts but are instead overlain by transgressive systems tracts. These alternative for stratigraphic mapping and correlation than the deli-
highstand systems tracts are herein considered as regressive systems neation of parasequences (see Catuneanu, 2019a, for a full discussion).
tracts as they may include both normal and forced regressive deposits. The chemostratigraphic expression of lowstand systems tracts and
The third-order regressive systems tract in the Oatka Creek Formation is falling-stage systems tracts have not yet been established, likely for
characterized by increasing Al, decreasing Si/Al, and Ti/Al, and a similar reasons as discussed for the basal surface of forced regression
marked increase in Mo relative to the underlying transgressive systems and correlative conformity. Additionally, although all systems tracts
tract. The regressive systems tract in the overlying Skaneateles For- may be present, higher frequency cycles identified using chemostrati-
mation displays rising Al and Si/Al, with relatively stable Ti/Al. Mo- graphic profiles in these units are commonly limited to the identifica-
lybdenum remains quite low through the entire interval. tion of transgression and regression rather than parsing out normal
These studies suggest that regressive systems tracts are typically from forced regressions (e.g., Ratcliffe et al., 2012c; Sano et al., 2013;
characterized by an increasing abundance of detrital elements and grain Turner et al., 2015, 2016). As a result, only two systems tracts are
size proxies reflecting ongoing progradation. An exception to this is commonly recognized: the transgressive systems tract and regressive
present in the regressive systems tract of the Oatka Creek Formation, systems tract. This is likely the case because in shelfal settings the re-
where Si/Al and Ti/Al decline, which Ver Straeten et al. (2011) inter- gressive systems tract records ongoing progradation, making the basal
preted to have been caused by falling levels of aeolian input. surface of forced regression and correlative conformity cryptic in stu-
dies using log, core, or outcrop data (Catuneanu, 2006). This, therefore,
5. Discussion leads to difficulty in distinguishing the highstand systems tract, low-
stand systems tract, and falling-stage systems tract in these settings. It is
To our knowledge, there are no existing examples of the chemos- also possible that certain systems tracts are not developed in particular
tratigraphic expression of certain surfaces. These include the subaerial depositional settings.
unconformity, correlative conformity, transgressive surface of erosion, One disadvantage of using an undifferentiated regressive systems
regressive surface of marine erosion, and basal surface of forced re- tract is that it amalgamates different genetic types of regressive strata,
gression. The subaerial unconformity is the unconformable portion of which lowers the resolution of the sequence stratigraphic study.
the surface that marks the end of forced regression and is typically Considering normal and forced regressive deposits together may also be
developed in shelf or platform settings (Hunt and Tucker, 1992). This disadvantageous if they have different characteristics related to their
surface is defined by non-marine strata on top (Catuneanu, 2006; unconventional reservoir potential, for example variations in TOC have
Shanmugam, 1988) and is not relevant to the marine mudstone suc- been recorded between forced vs. normal regressive systems tracts in
cessions discussed in this work. As the concept of integrating chemos- unconventional plays (Dominguez et al., 2016; Dominguez and
tratigraphic datasets for sequence stratigraphy is fairly recent, there are Catuneanu, 2017). These are limitations of using chemostratigraphic
relatively few studies that have compared elemental proxies to pre- datasets to identify sequence stratigraphic cycles in fine-grained or-
viously established sequence stratigraphic frameworks or used che- ganic-rich units.
mostratigraphy to help identify surfaces. The recognition of sequence The geochemical characteristics of sequence stratigraphic surfaces
stratigraphic cyclicity within mudstone units is also relatively new (e.g., and systems tracts in organic-rich mudstone intervals are expected to

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M.T. LaGrange, et al. Earth-Science Reviews 203 (2020) 103137

vary depending on the depositional setting. For example, based on 2017), and for certain elements, poor accuracy relative to accepted
comparison with coarser grained intervals, we expect that there are values or concentrations obtained by other laboratory-based analyses
differences between the chemostratigraphic expression of certain sur- (e.g., Rowe et al., 2012; Hines et al., 2019). The use of helium flow or
faces in more proximal settings influenced by progradation compared to high-performance detectors is now allowing for measurement of lighter
more distal settings beyond the influence of progradation where sedi- elements (e.g., Al and Si), which were traditionally poorly detected by
ment-gravity deposition and pelagic sedimentation are dominant. pXRF, although lower accuracy has been reported for Al compared to
Examples of environments influenced by progradation are the con- elements such as Si, Ti, and in some cases, Zr (Lemiere, 2018). None-
tinental shelf or a ramp setting under the influence of shoreline pro- theless, in a study specific to mudstone intervals, Rowe et al. (2012)
gradation. The slope or abyssal plain of a shelf-slope system or a ramp observed high accuracy for Al and Si. Of the other elements discussed
setting where sedimentation is dominated by sediment-gravity flows herein, Rowe et al. (2012) observed less reliable Cr pXRF measurements
and pelagic fallout are examples of settings that are beyond the influ- when concentrations were below 70 ppm, and unreliable U and Th
ence of shoreline progradation. Herein, environments experiencing results when pXRF data was compared to data collected using wave-
progradation will be referred to as shallow-water settings, whereas length dispersive x-ray fluorescence. Uranium can be difficult to mea-
settings beyond the influence of progradation where sedimentation is sure with XRF because of inter-element interference and low abundance
dominated by sediment-gravity flows and pelagic settling will be re- in many samples (Rowe et al., 2017), and along with Zr, Cr, and Ni, U is
ferred to as deep-water settings. During relative sea-level fall, the deep- one of the elements for which Hines et al. (2019) observed poor cor-
water environment experiences maximum supply of detrital sediment, relations between pXRF data and other laboratory-based measurements
and the detrital sediment supplied to these settings is coarser than the (fusion XRF and ICP-MS).
sediment that accumulates during relative sea-level rise (Posamentier An additional drawback related to the integration of chemostrati-
and Kolla, 2003). Unlike for prograding environments, deposition in graphic datasets for sequence stratigraphic interpretation is the re-
deep-water is less predictable in terms of the locus of accumulation of quirement for fairly continuous measurements from core or outcrop
depositional elements. However, general trends can still be established intervals in order to identify surfaces (Pearce et al., 2010). For example,
at the scale of composite profiles that integrate regional data (e.g., fig. studies that identified sequence stratigraphic surfaces and systems
36 in Catuneanu, 2019b). The main contrasts between the shallow- and tracts (e.g., Ver Straeten et al., 2011; Sano et al., 2013; Turner et al.,
deep-water settings are recorded by the grain-size changes associated 2016) sampled cores and outcrops at intervals ranging from 5 to
with the correlative conformity and the maximum regressive surface. In 120 cm. These high-resolution sampling rates are not possible when
shallow-water environments, the correlative conformity is more diffi- relying on drill cuttings rather than core and outcrop samples because
cult to distinguish because it occurs within a prograding and coarsening drill cuttings are typically taken at coarser intervals (Pearce et al.,
upwards interval (Catuneanu, 2006), whereas the maximum regressive 1999). Given the limitations associated with the use of chemostrati-
surface marks the change from coarsening to fining upwards grain size graphic datasets, chemostratigraphy is more effective when used in
trends (Catuneanu, 2002; Embry, 2010). In coarser grained deep-water conjunction with other datasets (e.g., sedimentological and petrophy-
intervals dominated by sediment-gravity flows, the composite vertical sical) to form robust sequence stratigraphic frameworks in organic-rich
profile coarsens upwards to the correlative conformity during relative mudstone intervals (e.g., Ver Straeten et al., 2011; Hammes and
sea-level fall, and then fines upwards through the lowstand systems Frébourg, 2012).
tract to the maximum flooding surface (Catuneanu, 2006, 2019a). In
this case, the maximum regressive surface is present within a fining 6. Conclusions
upwards interval and is more difficult to distinguish (Catuneanu,
2019b). These trends relate to the efficiency of transfer of fluvial se- This review of documented elemental stratigraphic changes in
diment across the shelf to the deep-water setting, which is highest marine mudstone successions and their interpretation allows for a
during forced regression (see full discussion in Catuneanu, 2019a, preliminary delineation of the chemostratigraphic expression of certain
2019b). We expect that this would affect the chemostratigraphic sig- sequence stratigraphic surfaces and systems tracts, namely the max-
natures of the correlative conformity and maximum regressive surface imum flooding surface, maximum regressive surface, transgressive
in organic-rich mudstone units depending on the setting, with the systems tract, and regressive systems tract. Chemostratigraphic char-
maximum regressive surface recording maxima in terrigenous proxies acteristics of the highstand systems tract are presented, although this
and grain size proxies in shallow-water settings, but the correlative systems tract has been identified using criteria other than chemos-
conformity recording these maxima in deep-water settings, although tratigraphic in all examples presented in this work, and presently
this has not yet been confirmed. The examples discussed herein seem to cannot be distinguished from the regressive systems tract based on
correspond to the expected expression of the maximum regressive chemostratigraphic proxies alone. Available sequence stratigraphic in-
surface in settings influenced by progradation. Further work is required terpretations of geochemical proxy datasets presented herein demon-
to provide examples of the chemostratigraphic expression of these strate that chemostratigraphic datasets are quite useful in interpreting
surfaces and systems tracts in shallow- versus deep-water settings so transgressive-regressive cycles (regressive systems tracts and trans-
that the chemostratigraphic expression can be confirmed. gressive systems tracts separated by maximum regressive surfaces and
In chemostratigraphic studies of mudstone successions, geochemical maximum flooding surfaces, respectively). Also apparent is the need for
composition data is typically collected using either inductively coupled integration with other datasets to further subdivide the regressive sys-
plasma (ICP) and inductively coupled plasma mass spectrometry (ICP- tems tract and confirm proxy signatures to produce robust sequence
MS) or inductively coupled plasma optical emission spectrometry (ICP- stratigraphic frameworks. To date, the utility of incorporating che-
OES) (e.g., Harris et al., 2013, 2018; Sano et al., 2013; Playter et al., mostratigraphic datasets for the study of sequence stratigraphic cycli-
2018), or by portable energy-dispersive X-ray fluorescence (pXRF) (e.g., city in organic-rich mudstone successions has been demonstrated by a
Hammes and Frébourg, 2012; Sano et al., 2013; Turner et al., 2015, handful of studies. Nonetheless, the field remains quite new with lim-
2016; El Attar and Pranter, 2016; Hines et al., 2019). Advantages of ited published examples of the geochemical expression of surfaces and
pXRF compared to ICP methods include time and cost efficiency as well systems tracts. Furthermore, because of the lack of consensus and on-
as the non-destructive nature of the analysis, potentially allowing for going shift in our understanding of mudstone depositional systems, it is
the collection of higher-resolution datasets (Rowe et al., 2012; Rowe not yet possible to confirm many of the expected differences in the
et al., 2017; Lemiere, 2018; Zhang et al., 2019). There are, however, chemostratigraphic expression of surfaces and systems tracts depending
some drawbacks associated with pXRF, including a more limited suite on the depositional setting. The following are suggestions aimed at
of elements that can be obtained compared to ICP analyses (Rowe et al., advancing the field. First, the publication of studies comparing

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M.T. LaGrange, et al. Earth-Science Reviews 203 (2020) 103137

geochemical proxies to previously established sequence stratigraphic Schwalbach, J.R., Wegner, M.B., Simo, J.T., 2005. Production, destruction, and di-
frameworks (based on other datasets) for fine-grained organic-rich in- lution—the many paths to source-rock development. In: Harris, N.B. (Ed.), The
Deposition of Organic-Carbon-Rich Sediments: Models, Mechanisms, and
tervals will enable further geochemical characterization of surfaces and Consequences: Society of Economic Paleontologists and Mineralogists Special
systems tracts and allow chemostratigraphy to be better integrated with Publication. 82. pp. 61–101.
other datasets. Incorporating chemostratigraphic datasets while devel- Bohacs, K.M., Lazar, O.R., Demko, T.M., 2014. Parasequence types in shelfal mudstone
strata—quantitative observations of lithofacies and stacking patterns, and conceptual
oping sequence stratigraphic interpretations in organic-rich intervals link to modern depositional regimes. Geology 42 (2), 131–134.
where these frameworks do not yet exist will serve a similar purpose. As Bonjour, J.L., Dabard, M.P., 1991. Ti/Nb ratios of clastic terrigenous sediments used as an
our understanding of depositional systems for these units increases, indicator of provenance. Chem. Geol. 91 (3), 257–267.
Borcovsky, D., Egenhoff, S., Fishman, N., Maletz, J., Boehlke, A., Lowers, H., 2017.
characterizing the differences in chemostratigraphic expression of sur- Sedimentology, facies architecture, and sequence stratigraphy of a Mississippian
faces and systems tracts associated with variations in depositional set- black mudstone succession—the upper member of the Bakken Formation, North
ting will also further the utility of chemostratigraphic data to sequence Dakota, United States. Am. Assoc. Pet. Geol. Bull. 101 (10), 1625–1673.
Brown Jr., L.F., Fisher, W.L., 1977. Seismic stratigraphic interpretation of depositional
stratigraphic studies of mudstone successions.
systems: examples from Brazilian rift and pull apart basins. In: Payton, C.E. (Ed.),
Seismic Stratigraphy––Applications to Hydrocarbon Exploration: American
Declaration of competing interest Association of Petroleum Geologists Memoir. 26. pp. 213–248.
Brumsack, H.J., 2006. The trace metal content of recent organic carbon-rich sediments:
implications for Cretaceous black shale formation. Palaeogeogr. Palaeoclimatol.
None. Palaeoecol. 232, 344–361.
Buckman, J., Mahoney, C., März, C., Wagner, T., Blanco, V., 2017. Identifying biogenic
Acknowledgments silica: mudrock micro-fabric explored through charge contrast imaging. Am. Mineral.
102 (4), 833–844.
Calvert, S.E., Pedersen, T.F., 1993. Geochemistry of recent oxic and anoxic marine se-
The authors would like to thank Kathryn Fiess, Viktor Terlaky, Dave diments: implications for the geological record. Mar. Geol. 113 (1–2), 67–88.
Herbers, Carolyn Furlong, and Sara Biddle for their advice and gui- Canfield, D.E., Thamdrup, B., 2009. Towards a consistent classification scheme for geo-
chemical environments, or, why we wish the term ‘suboxic’would go away.
dance. Maya LaGrange, Brette Harris, and Murray Gingras acknowledge Geobiology 7 (4), 385–392.
the Northwest Territories Geological Survey for generously providing Catuneanu, O., 2002. Sequence stratigraphy of clastic systems: concepts, merits, and
funding of this work. Kurt Konhauser, Octavian Catuneanu, and Murray pitfalls. J. Afr. Earth Sci. 35, 1–43.
Catuneanu, O., 2006. Principles of Sequence Stratigraphy. Elsevier, Amsterdam.
Gingras thank the Natural Sciences and Engineering Research Council Catuneanu, O., 2019a. Model-independent sequence stratigraphy. Earth Sci. Rev. 188,
of Canada (NSERC) for their financial support. We would also like to 312–388.
thank the editor Chris Fielding and reviewers Bruce Hart, Bryan Turner, Catuneanu, O., 2019b. Scale in sequence stratigraphy. Mar. Pet. Geol. 106, 128–159.
Catuneanu, O., Abreu, V., Bhattacharya, J.P., Blum, M.D., Dalrymple, R.W., Eriksson,
and Lauren Birgenheier for providing feedback that greatly improved
P.G., Fielding, C.R., Fisher, W.L., Galloway, W.E., Gibling, M.R., Giles, K.A.,
the quality of the manuscript. Holbrook, J.M., Jordan, R., Kendall, C.G.S., Macurda, B., Martinsen, O.J., Miall, A.D.,
Neal, J.E., Nummedal, D., Pomar, L., Posamentier, H.W., Pratt, B.R., Sarg, J.F.,
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