GEOLOGIC NOTE AUTHORS

Emilio Ramos  Geodynamics and Basin

Stratigraphy and sedimentology Analysis Research Group, Universitat de
Barcelona, Martı´Franquès s/n, 08028 Barcelona,
Spain; emilio.ramos@ub.edu
of the Middle Ordovician Emilio obtained his Ph.D. in geology from
the Universitat de Barcelona in 1988. Since
Hawaz Formation then, he has been a lecturer in basin analysis
and petroleum geology. He has been involved
(Murzuq Basin, Libya) in several research projects on sedimentology
and basin analysis in Spain, northern Africa,
Emilio Ramos, Mariano Marzo, Jordi M. de Gibert, Antarctica and South America. His present-day
research interests include three-dimensional
Khaeri S. Tawengi, Abdalla A. Khoja, and
modeling of sedimentary bodies and reservoirs.
Néstor D. Bolatti
Mariano Marzo  Geodynamics and
Basin Analysis Research Group, Universitat
de Barcelona, Martı´ Franquès s/n, 08028
ABSTRACT Barcelona, Spain; mariano.marzo@ub.edu
The Middle Ordovician Hawaz Formation is a 200-m (660-ft)-thick A professor of stratigraphy at the Universitat
succession made up of fine-grained quartzarenites displaying a vari- de Barcelona, M. Marzo’s research interest
able degree of bioturbation. It records the deposition in a large-scale, focuses on the application of clastic sedimen-
low-gradient estuary, which was partially controlled by tectonic tology, sequence stratigraphy, reservoir mod-
extension. The upper boundary of the formation is marked by two eling, and basin analysis to the exploration
and production of hydrocarbons. He has been
erosion surfaces (unconformities U1 and U2), related to the Late
involved in several research projects funded
Ordovician glaciation. The U1 and U2 erosion surfaces generated a
by oil companies in southern Europe, the
pronounced paleotopography that controlled the deposition of the
North Sea, South America, and northern Africa.
Upper Ordovician sequences.
Tectonism influenced the paleogeography, although faults were Jordi M. de Gibert  Departament d’Estra-
unimportant from the point of view of sedimentary thickness. tigrafia, Paleontologia i Geociències Marines,
Tectonic subsidence was moderate, and accumulation rates were Universitat de Barcelona, Martı´ Franquès s/n,
low. Physiography favored tidal power, especially during transgres- 08028 Barcelona, Spain; jmdegibert@ub.edu
sive episodes, when the coastal embayment was flooded. Jordi M. de Gibert received his Ph.D. from
We defined 11 lithofacies, forming 6 facies associations. These the Universitat de Barcelona in 1996. After a
associations are subtidal sandstones; storm-reworked, shoreface sand- period at the University of Utah, he returned
stones; shoreface-to-beach sandstones; channel-sandstone bodies; to Barcelona in 1999, where he currently holds
a position as a tenure-track lecturer. His in-
nearshore to inner-platform sandstones; and K-bentonites. Trace-
terests and areas of expertise include trace
fossil assemblages match Skolithos and Cruziana archetypal ichno-
fossils, their paleobiological significance, and
facies. On the basis of the dominant facies associations and ichno- their implications for understanding ancient
facies, we divided the formation into three informal units, from base depositional environments.
to top: HW.1, HW.2 and HW.3.
Periodically, volcanic ash was supplied to the basin from distal Khaeri S. Tawengi  REPSOL Exploration in
eruptive centers and was preserved as thin beds of K-bentonite Murzuq S.A., Dat el Imad Tower Complex,
interstratified with the shoreface sandstones, but not with the tidal- Tower 3, Floor 8, Tripoli, Libya;
ktawengi@ryremsa.com
dominated sandstones.
K. Tawengi received a B.Sc. degree in geology
from Alfateh University, Libya, in 1984, and
an M.Sc. degree in sedimentology and stra-
Copyright #2006. The American Association of Petroleum Geologists. All rights reserved. tigraphy from Durham University, England, in
Manuscript received April 22, 2005; provisional acceptance July 21, 2005; revised manuscript received 1996. He worked as an explorationist with Agip
February 21, 2006; final acceptance March 9, 2006.
DOI:10.1306/03090605075

AAPG Bulletin, v. 90, no. 9 (September 2006), pp. 1309 – 1336 1309
 Oil Company in Libya from 1985 to 2000 and We divided the Hawaz Formation into five third-order de-
since then has worked as a senior exploration positional sequences. Lowstand deposits were not identified. The
member with REPSOL Exploration in Murzuq lower boundaries of transgressive systems tracts are tidal ravinement
S.A. His main fields of interest are sedimentol- surfaces or sequence boundaries, whereas the upper boundaries are
ogy, stratigraphy, and subsurface geology.
flooding surfaces. The transgressive systems tracts are constituted by
Abdalla A. Khoja  National Oil Corpora- early transgressive tidal deposits separated by a wave ravinement
tion, Tripoli, Libya surface from the late transgressive storm-dominated deposits. High-
A. Khoja graduated in 1972 from the University stand systems tracts consist of bioturbated shoreface-to-beach sand-
of Libya. He received a diploma in petroleum stones, which record seaward, shoreline progradation.
geosciences from Oxford Polytechnic (1991)
and an M.Sc. degree from Oxford Brookes Uni-
versity (1993). He joined the National Oil INTRODUCTION
Corporation of Libya in 1972 and is presently
the regional studies superintendent in the For a long time, petroleum exploration and oil production in Libya
National Oil Exploration Department. focused mainly on the northerly Ghadamis, Sirt, and Cyrenaica
Néstor D. Bolatti  REPSOL-YPF, Praia basins, whereas the southerly remote Murzuq and Al Kufrah basins
de Botafogo, 300, 7 andar. Rio de Janeiro, (Figure 1) remained little known. However, in recent decades, a
Brazil; ndbolattik@repsolypf.com major exploration effort has been devoted to the Murzuq Basin,
where several finds have brought oil in place up to 5200 MMBO
Néstor obtained his degree in geology from
Córdoba University (Argentina) in 1982 and his and recoverable reserves up to 1600 MMBO (Hallet, 2002).
postgraduate in petroleum geology in the The present knowledge of surface and subsurface geology of the
University of Cuyo, Argentina. He joined YPF Murzuq Basin was gathered by Sola and Worsley (2000). As in
in 1984 and worked in their exploratory de- previous works (i.e., Boote et al., 1998), these authors consider the
partment in Mendoza, Plaza Huincul, and Lower Silurian hot shales of the Tanezzuft Formation (Figure 2) as
Neuquen until the year 2000. Subsequently, the source rocks, whereas the Upper Ordovician sandstones of the
he became Libya team leader for REPSOL- Mamuniyat Formation are regarded as the main reservoir, leading to
YPF in Madrid. He is currently director of ex- the assumption that the Tanezzuft–Mamuniyat formations repre-
ploration and production in Brazil. sent the only petroleum system in the Murzuq Basin. However,
recent finds have also indicated that the sandstones of the Middle
Ordovician Hawaz Formation also act as a reservoir, particularly in
ACKNOWLEDGEMENTS
the central regions of the basin, where the Mamuniyat Formation is
This work is based on a regional field study missing and the Tanezzuft shales rest directly on the Hawaz For-
supported by a collaborative grant between mation (Hallet, 2002).
REPSOL Exploration in Murzuq S.A. (REMSA), This article characterizes the stratigraphy and sedimentology of
Tripoli, and the University of Barcelona (UB).
the oil-bearing Hawaz sandstones based on a detailed field study of
We thank REMSA and the NOC (National
Oil Corporation, Libya) for permission to pub- excellent exposures in the Gargaf uplift at the northern limit of the
lish the results. The work was partly funded Murzuq Basin.
by the Spanish Government Project CGL 05816-
C02-02-3650 and the Geodynamics and Basin
Analysis Research Group (2001 SGR 00074). GEOLOGICAL SETTING
It benefited from collaboration with D. Barsó
and D. Garcı́a (UB), R. Arnez, J. Arregui, N. During the Ordovician, the African craton was part of the Gond-
McDougall, and J. Vilá (REPSOL-YPF), H. C. wana supercontinent. Paleogeographic reconstructions of Gond-
Fernández (REMSA), and J. M. Samsó. We also wana (cf. Matte, 2001; Cocks and Torsvik, 2002; Kuhn and Barnes,
acknowledge valuable reviews by T. M. Olson, 2005) show that northern Africa made the western boundary of
M. L. Sweet, D. L. Wooddrop, and an anonymous Gondwana, which constituted a passive margin rimmed by a wide,
referee and comments from AAPG Editor E. A.
shallow-marine platform, located south of the Iapetus Ocean, where
Mancini, which helped improve the original
sedimentation occurred in terrigenous, transitional to shallow-marine
manuscript.
depositional environments. This margin experienced an extensional
regime during the Early and Middle Ordovician, leading to the sepa-
ration of the Avalonia and Baltica terranes (Badalini et al., 2002). In

1310 Geologic Note

 Figure 1. Geological map
of Libya showing the location
of the Murzuq Basin and the
main Paleozoic outcrops. The
inset shows the studied area
in the western Gargaf high.

north Africa, the thickness distribution of the Ordovi- most of Gondwana (Ghienne, 2003; Monod et al.,
cian deposits ranges from 2500 m (8200 ft) in the Anti- 2003; Deynoux and Ghienne, 2004; Young et al., 2004).
Atlas region (Morocco) to almost 0 m in some of the Related to the Late Ordovician glacial period, an im-
north Saharian intracratonic basins. These important portant erosive phase is recognized (Ghienne et al.,
thickness variations could be related to changes in subsi- 2003). The resulting unconformity may also underlie
dence linked to the extensional regime mentioned above. thickness variations affecting the Lower and Middle
According to Boote et al. (1998) and Klitzsch (2000), Ordovician successions.
sedimentation in the eastern Sahara zone during the early
Paleozoic was controlled by the development, during The Murzuq Basin
the Cambrian and the Early – Middle Ordovician, of a
north-south to north-northwest–south-southeast-oriented Davidson et al. (2000) describe the Murzuq Basin as a
system of horsts and grabens, forming wide elongate troughs postdepositional basin, an erosional remnant of a larger
and basinal areas. Structurally depressed areas were covered Paleozoic sedimentary basin that originally extended
by marine sequences, whereas the intervening uplifted over much of north Africa. The present-day boundaries
horsts were subject to low sedimentation or erosion. of the basin (Figure 1) are defined by erosion resulting
Another crucial element in the sedimentary evo- from multiphase tectonic uplifts. They comprise the
lution of north Africa was the occurrence during the Tihemboka high to the west, the Tibesti high to the
Late Ordovician of a widespread glaciation that covered southeast, and the Gargaf and Atshan highs to the north

Ramos et al. 1311

Figure 2. Stratigraphic chart summariz-
ing the Paleozoic depositional history
and major tectonic events recorded in the
Murzuq Basin infill. U1 and U2 = minor
unconformities described in the text.

and northwest. These uplifts were generated by various According to Davidson et al. (2000), seismic evi-
tectonic events ranging from the early Paleozoic to the dence shows only one Ordovician phase of extensional
Tertiary, but the main periods of uplift occurred during faulting. Nevertheless, the faults are widely spaced and
the middle Cretaceous to early Tertiary Alpine phase bear relatively small displacement, commonly less than
(Davidson et al., 2000; Badalini et al., 2002). 100 m (330 ft). Other phases of extensional movement
There is little evidence that these present-day basin- may have occurred, but if so, they were never of major
bounding uplifts were active during the early Paleozoic, significance, and their effects are now obscured by sub-
when the Ordovician formations were deposited. Tec- sequent compressional and/or transpressional faulting
tonics probably had a more significant effect in the north- reactivation.
east part of the present-day basin, resulting in a progres-
sive thinning of the Silurian Tanezzuft Formation toward
the northeast (Davidson et al., 2000). Several genera- The Murzuq Sedimentary Infill
tions of structuring, mainly compressional and trans-
pressional in nature, are recognized within the Murzuq The maximum sedimentary thickness in the present-
Basin, but the cumulative structural deformation is day Murzuq Basin is about 4000 m (13,100 ft). Da-
relatively minor. Fault arrangement shows considerable vidson et al. (2000) speculate that, despite successive
variation, although the north-south trend is dominant. erosive episodes during several phases of uplift and

1312 Geologic Note

erosion throughout the history of the basin, the maxi- partial stratigraphic sections (Figure 3) have been
mum sedimentary thickness probably never exceeded studied. Correlation between these partial stratigraphic
5000 m (16,400 ft). The age, lithostratigraphic sub- logs allows us to propose the composite type section
division, bounding unconformities, and major tectonic shown in Figure 3. The composite thickness of the
events controlling the Paleozoic sedimentary infill of Hawaz Formation in the Gargaf area reaches about 200 m
the Murzuq Basin are shown in Figure 2, which has (660 ft). However, it is important to highlight that its
been synthesized from the data by Aziz (2000), upper boundary is an erosive surface, and therefore, the
Davidson et al. (2000), Echikh and Sola (2000), and estimated value is a minimum. Toward the south, in the
Craik et al. (2001). The age of the Murzuq Basin infill Murzuq subsurface, thickness ranges from 0 to 280 m
ranges from Cambrian to Cretaceous. It has been clas- (0 to 918 ft) (Aziz, 2000), whereas in Dor el Gussa, in
sically divided into four major sedimentary sequences: the southeast margin, Mamgain (1980) attributed a
(1) Cambrian – Ordovician, (2) Silurian, (3) Devonian thickness of only 50 m (164 ft).
to Carboniferous, and (4) Mesozoic. The Hawaz Formation is mostly composed of well-
The basement is composed of high-grade meta- stratified sandstones, minor intercalations of silty shales,
morphic rocks associated with plutonic rocks, as well as and several K-bentonite beds. Trace fossils are frequent
low-grade metamorphic to unmetamorphic rocks of Pre- and, locally, abundant enough to overprint most primary
cambrian age (Mourizidie Formation). Both assemblages sedimentary structures. The sandstones are well-sorted,
are cut by a lower Paleozoic unconformity (Pan-African very fine- to fine-grained quartzarenites, although occa-
unconformity) overlain by a Cambrian–Ordovician clas- sionally, they are medium grained with a bimodal grain-
tic sequence known as the Gargaf Group, after Burollet size distribution. Grains are rounded to well rounded,
(1960). From the base to the top, the Gargaf Group with roundness increasing with grain size. The major
comprises five formations (Figure 2): Hasawnah, Ash detrital constituent is quartz (mono- and polycrystalline).
Shabiyat, Hawaz, Melaz Shuqran, and Mamuniyat. All Feldspar grains are uncommon, as are heavy-mineral
boundaries between formations are unconformities, ex- grains, whose major constituent is mica. Other heavy
cept for the one between the Ash Shabiyat and Hawaz minerals observed include well-rounded zircons. The
formations. The Ash Shabiyat and Hawaz formations most widely distributed cement is a syntaxial silica over-
mostly consist of quartzarenites; the Hasawnah and growth, but kaolinite, iron oxides, and carbonate cements
Mamuniyat formations consist of conglomeratic to micro- have also been locally identified. Porosity is mainly inter-
conglomeratic quartzarenites; and the Melaz Shuqran granular, but some samples show microfracture pores.
Formation is dominantly muddy. Interbedded within the sandstones, several clay-
The Ordovician succession is truncated by the rich beds, interpreted as K-bentonite, have been rec-
Taconian unconformity, which is transgressively over- ognized (Ramos et al., 2003). These bentonite beds are
lain by the Silurian Tanezzuft Formation (Figure 2). formed by a sericitic matrix with angular quartz, albite,
and potassium-feldspar porphyroclasts. Other accessory
The Hawaz Formation minerals are biotite, leucoxene and idiomorphic zircon,
and tourmaline crystals. The dominant clay mineral is
The Hawaz Formation was first defined by Massa and kaolinite.
Collomb (1960) as the entire sandy Ordovician suc- The paleocurrents show two main trends. Smaller
cession outcropping in the Gargaf area, being marked scale structures (ripples and small sigmoidal cross-bedded
by two unconformities developed atop the Hasawnah sets) show a wide dispersion. In contrast, larger scale
Formation and below the Melaz Shuqran and/or the structures, which characterize the upper and lower parts
Mamuniyat formations. Later, Collomb (1962) sub- of the formation, display a paleocurrent trend toward
divided the Hawaz Formation into three members, and the northeast-northwest and, locally, bidirectionality
Havlicek and Massa (1973) introduced the term Ash (Figure 3).
Shabiyat Formation for the lower and part of the mid- No agreement exists regarding the age of the Hawaz
dle members described by Collomb (1962). In this way, Formation. The only available chronostratigraphic data
the original Hawaz Formation of Massa and Collomb on the formation in the Murzuq Basin are provided by
(1960) was subdivided into two new conformable for- acritarchs and chitinozoans (Aziz, 2000; N. H. Miles,
mations, named Ash Shabiyat and Hawaz (Figure 2). 2001, personal communication). According to these
The Hawaz sandstones crop out extensively in the sources, the top of the Hawaz Formation is no younger
western part of the Gargaf high (Figure 1), where three than Llandeilian, as defined by the presence of abundant

Ramos et al. 1313

Figure 3. Correlations among the studied sections, lithostratigraphic subdivision, and proposed composite type section for the
Hawaz Formation in the western Gargaf. The Hawaz Formation type section includes the degree of bioturbation (ichnofabric index)
and the measured gamma-ray log. See the location of sections in the geological map (Figure 12).

1314 Geologic Note

Figure 4. Field view looking northwest and sketch showing the relationships between U1 and U2 at the top of the Hawaz Formation.
MN and MS = Upper Ordovician Mamuniyat and Melaz Shuqran formations, respectively. HW.3 = top of the Hawaz Formation. See
the location of this point in the geological map (Figure 12).

Acanthodiacrodium sp., whereas the base is no older than The Hawaz sandstones are cut by two erosion
Llanvirnian, as defined by the presence of Villosacapsula surfaces (U1 and U2 in Figure 2), recognizable both in
irrorata and Villosacapsula setosapellicula. Thus, a outcrop (Figure 4) and subsurface. These surfaces are
Llanvirnian – Llandeilian age, following the traditional attributable to a glacial period lasting from the Cara-
Ordovician British series, is assigned by these sources to docian to the late Ashgillian.
the Hawaz Formation. These series correlate with the
Darriwilian and the base of the fifth stage of the global
Ordovician subdivision (Finney, 2005). However, fol- FACIES ANALYSIS
lowing the acritarch biohorizons proposal of Vecoli and
Le Hérissé (2004) for the northern margin of Gond- Eleven lithofacies have been defined in terms of li-
wana (which includes some Ordovician Libyan basins), thology, sedimentary structures, and degree of bio-
the first-appearance biohorizon of V. setosapellicula cor- turbation. These facies ( Table 1) have been grouped
responds to the boundary between the Middle and Late into four sets: nonbioturbated sandstones (facies S1 to
Ordovician. Accordingly, a Middle Ordovician age, prob- S7; see Figure 5), bioturbated sandstones (facies Sb1
ably including the base of the Upper Ordovician, is pro- and Sb2; see Figure 6 ), heterolithics (facies H), and K-
posed for the Hawaz Formation. Furthermore, in the bentonites (facies Kb; see Figure 6D). Table 1 contains
neighboring Al Kufrah Basin (Figure 1), Seilacher et al. a brief description and sedimentary interpretation for
(2002) considered the Hawaz Formation as Arenigian each individual lithofacies.
(a British series that can be correlated to the second and
third stages, Lower–Middle Ordovician, of the global Ichnofacies
Ordovician subdivision, Finney, 2005) based on the
occurrence of certain ichnospecies of the trace fossil Trace fossils are abundant and diverse in the Hawaz
Cruziana. Taking the most conservative biostratigraph- sandstones and can aid in characterizing lithofacies and
ic data by N. H. Miles (2001, personal communica- interpreting their depositional setting. The recognized
tion), the Hawaz sandstones therefore cover a time trace-fossil assemblages can be assigned to two arche-
span of 9 – 12.5 m.y. typal ichnofacies: Skolithos and Cruziana.
Bioturbation is a prominent feature of the Hawaz The most common Skolithos ichnofacies is re-
sandstones. Ichnofabric indices (ii after Droser and Bott- corded as a pipe-rock ichnofabric (ii 4 –5) of Skolithos
jer, 1986) and ichnofacies distribution have been consid- linearis (Figure 7B) and Skolithos isp. (Figure 7C) and
ered for the stratigraphic characterization of the stud- characterizes facies Sb1 and Sb2 (Figure 6A–C). The
ied sections (Figure 3). Assemblages corresponding to Skolithos ichnofacies is also found in some thin-bedded
Skolithos and Cruziana ichnofacies (Seilacher, 1964; sandstones intercalated in the heterolithic facies (H).
Pemberton et al., 2001) have been recognized. However, it is sparser here and typically constitutes a

Ramos et al. 1315

1316

Table 1. Lithofacies Description and Interpretation

Lithofacies Facies Description Bioturbation Interpretation

Geologic Note

S1 (Figure 5A) Large-scale, sigmoidal cross-bedded sandstones Absent to sparse; occasionally Large subaqueous meso- to macroforms up to
– Very fine- to fine-grained sandstones 0.8 – 1.5 m with horizontal trace fossils 2 m (6.6 ft) height
(2.6 – 4.9 ft) thick, planar to trough cross-bedded (Cruziana ichnofacies) Unsteady, intermittent tidal regime
sets; individual sets with sigmoidal foreset packages Elongated ebb-tidal sand ridges in a subtidal
– 10-m (33-ft)-thick and 10 – 100-m (33 – 330-ft)-long environment
cosets
– Presence of reactivation surfaces with mud drapes
– Dominance of unidirectional paleocurrents toward
the north-northwest

S2 Medium-scale, sigmoidal cross-bedded sandstones Absent to sparse; occasionally Medium-scale dunes or megaripples
– Very fine- to fine-grained sandstones 20 – 80 cm with horizontal trace fossils Unsteady, intermittent tidal regime
(8 – 31 in.) thick, sigmoidal cross-bedded sets (Cruziana ichnofacies) Elongated ebb-tidal sand ridges under subtidal
– Common occurrence of mud clasts currents
– Dominance of unidirectional paleocurrents toward
the north-northwest

S3 (Figure 5B) Trough cross-bedded sandstones Unbioturbated Migration of three-dimensional dunes or

– Fine-grained sandstones megaripples induced by nearshore currents
– 20 – 50-cm (8 – 19-in.)-thick sets forming
cosets of trough cross-bedded

S4 (Figure 5C) Parallel-laminated sandstones Absent to sparse; occasional Sand deposition by nearshore currents under
– Very fine- to medium-grained sandstones horizontal trace fossils high-energy, upper flow regime
30 – 50-cm (12 – 19-in.)-thick beds
– Parallel lamination and occasionally parting
lineation

S5 (Figure 5D) Low-angle, swaley to hummocky cross-stratified Unbioturbated; occasionally with Result of storm events in a nearshore
sandstones sparse Skolithos. environment
– Fine-grained sandstones; thin to medium-bedded
– Low angle and/or swaley to hummocky
cross-stratification
– Hummocky sets 20 – 50 cm (8 – 19 in.) thick,
with wavelength 1 – 2 m (3.3 – 6.6 ft)
 S6 (Figure 5E) Ripple cross-laminated sandstones Scarce horizontal trace fossils Deposition from relatively low-velocity currents
– Very fine- to fine-grained sandstone; beds up to (Cruziana ichnofacies) and/or moderate wave action in a nearshore
50 cm (19 in.) thick environment
– Up to 3-cm (1.2-in.)-thick sets of current and/or Mud flakes are indicative of an unsteady
wave-ripple cross-lamination fluctuating flow regime
– Locally associated with millimeter-thick
mud flakes (flaser)

S7 (Figure 5F) Massive sandstones No signs of intensive bioturbation The massive appearance of this facies can
– Very fine- to medium-grained sandstones be interpreted as the result of early
– Beds 5 – 80 cm (2 – 31 in.) thick, massive, structureless, postdepositional processes involving
but locally with some vestiges of parallel lamination, dewatering and partial fluidization
cross-bedding, and convolute lamination
– Erosive bases floored with mud clasts

Sb1 (Figure 6A, B) Thinly bedded, parallel-laminated, bioturbated Highly bioturbated by Skolithos 6 – 8 mm Deposition in a marine environment
sandstones (0.23 – 0.31 in.) in diameter, and less characterized by low sedimentation rate
– Very fine- to fine-grained sandstones than 30 cm (12 in.) long Dominance of parallel lamination points to
– Thinly bedded; mainly parallel laminated, deposition above fair-weather-wave base
with occasional combined flow-ripple in a nearshore environment
cross-lamination

Sb2 (Figure 6C) Thick-bedded, massive, bioturbated sandstones Intensively bioturbated by Skolithos Deposition in a marine environment with low
– Very fine- to fine-grained sandstone up to 80 cm (31 in.) long sedimentation rate
– Massive beds 30 – 80 cm (12 – 31 in.) thick;
form pipe-rocks

H Heterolithic silty sandstones Uncommon vertical and horizontal trace Deposited in a nearshore, tidal- and
– Thin-bedded (1 – 20-cm [0.4 – 8-in.]-thick) alternating fossils of Skolithos and Cruziana ichnofacies storm-influenced environment below
very fine-grained sandstones with silty mudstones fair-weather-wave base
– Sandstones are ripple (current and wave)
cross-laminated with mud drapes (flaser structures)
– Silty mudstones contain rippled sand lenses
Ramos et al.

(linsen structures)

Kb (Figure 6D) K-bentonites Unbioturbated The result of diagenetic alteration of volcanic

– Whitish silty to clay-rich rocks ash beds deposited in a shallow-marine
– Massive to laminated; beds 10 – 20 cm environment; the volcanic ash accumulation
(4 – 8 in.) thick, but laterally extensive was originated by fallout processes following
multiple volcanic eruptions
1317
Figure 5. Field views of the nonbioturbated sandstone facies. (A) S1: large-scale, sigmoidal cross-bedded sandstones. (B) S3:
trough cross-bedded sandstones. (C) S4: parallel-laminated sandstones. (D) S5: low-angle, swaley to hummocky cross-stratified
sandstones. (E) S6: ripple cross-laminated sandstones. (F) S7: massive sandstones with erosive basal surface (dashed line).
Scales are 150 cm (59 in.) (Jacob’s staff, part A), 35 cm (13.7 in.) (geologist’s hammer, parts B – D, F), and 14 cm (5.5 in.) (pencil,
part E).

1318 Geologic Note

Figure 6. Field views of the bioturbated sandstone and K-bentonite facies. (A, B) Sb1: thinly bedded, parallel-laminated, bio-
turbated. sandstones. (C) Sb2: thick-bedded, massive, bioturbated sandstones. (D) Kb = K-bentonite. Scales are as in Figure 5.

less-bioturbated ichnofabric (ii 3– 4) of S. linearis, although the first two can locally occur in high densities.
which is typically smaller and more homogeneous in The higher diversity of the Cruziana ichnofacies and
size. This might record opportunistic, poststorm colo- the dominance of traces produced by detritus and de-
nization of sandy substrates. posit feeders are consistent with lower energy condi-
The Cruziana ichnofacies is typically associated tions and finer grained substrates. Similar Ordovician
with the heterolithic facies (H). The assemblage consists assemblages are characteristic of lower intertidal to
of locomotion, feeding, and resting trace fossils. De- shallow subtidal environments (Mángano et al., 1996).
spite the fact that bioturbation can be intense at the Nevertheless, the absence of associated structures in-
base or top of some sandstone beds, ichnofabric indices dicating subaerial exposure in the Hawaz Formation
are generally low (ii 2) because most traces are horizon- favors the interpretation that these assemblages were
tal. Ichnodiversity is much higher than in the Skolithos formed under subtidal conditions.
ichnofacies. Trilobite resting and crawling traces, Ruso- A Cruziana ichnofacies with a different ichnotax-
phycus isp. (Figure 7D) and Cruziana isp. (Figure 7E, F), onomic and ichnofabric signature is locally found in
respectively, are among the most abundant trace fos- the bioturbated sandstones at the base of the formation
sils. Other common ichnotaxa are Arthrophycus linearis (see Figure 3). It is characterized by a highly bioturbated
(Figure 7G) and Planolites isp., whereas less frequent ichnofabric (ii 4– 5) consisting of Thalassinoides isp.
trace fossils include Lockeia siliquaria, Daedalus multi- (Figure 7A) and Teichichnus rectus. Its association with
plex (Figure 7H), Aulichnites isp., and Bergaueria cf. sucta, pipe-rocks suggests a lower to middle shoreface setting.

Ramos et al. 1319

Figure 7. Characteristic trace fossils from the Hawaz Formation. (A) Thalassinoides isp.; (B) Skolithos linearis; (C) Skolithos isp.;
(D) Rusophycus isp.; (E, F) Cruziana isp.; (G) Arthrophycus linearis; and (H) Daedalus multiplex. Scales are as in Figure 5.

1320 Geologic Note

Table 2. Facies Association*

Facies
Association Main Lithofacies Subordinate Lithofacies Interpretation

T S1, S2, S6 S4, S7, H Large sand ridges, shoals, and intershoal depressions locally dissected
by subaqueous channels in a sandy subtidal environment
S S4, S5, S3, S6 S7, H Deposition in a storm-reworked shoreface locally incised by tidal inlets
B Sb1, S6 H Deposition in a prograding lower shoreface to beach; locally incised
by tidal inlets
Ch S7, S3 S2, S4, S6 Channel-fill deposits (subtidal channels or tidal inlets)
P Sb2 Terrigenous marine inner platform
Kb Kb Volcanic ash

*See vertical arrangement in Figure 3.

Facies Associations defined, thickening-upward sequences (H, S6, S2, S1)

are locally observable. Paleocurrents show a northwest
The lithofacies described in Table 1 have been grouped to north-northwest trend, although locally, a bidirec-
into six assemblages called facies association T (subtidal tionality trend is observed.
sandstones), S (storm-reworked, shoreface sandstones),
B (shoreface-to-beach sandstones), Ch (channel-sandstone
bodies), P (nearshore-inner-platform sandstones), and Interpretation
Kb (K-bentonites). They are synthesized in Table 2. Facies association T contains most of the diagnostic pri-
The vertical distribution and sequential arrangement of mary structures that characterize deposition in offshore
each facies association are shown in Figure 3. No sig- tidal sand ridges, such as tidal cross-bedding, tidal bun-
nificant lateral facies changes have been observed across dles, mud drapes, and reactivation surfaces (Dalrymple,
the studied area. 1992; Johnson and Baldwin, 1996). We interpret this
( Table 2) as having been deposited in an extensive,
sandy, subtidal environment linked to a broad estuary
Facies Association T mouth or tidal-dominated delta front. The subtidal de-
Facies association T is primarily characterized by fa- posits would have included large sand waves (a few me-
cies S1, S2, and S6, with S4, S7, and H as subordi- ters high, tens to hundreds of meters wide, and a hun-
nate elements ( Table 2). It forms packages ranging in dred to thousands of meters long), shoals floored with
thickness between 0.6 and 15 m (1.96 and 49 ft), with smaller mesoforms (dunes) and microforms (ripples),
average values of about 2–4 m (6.6–13 ft). It occurs and intershoal depressions occasionally dissected by a
as continuous, erosively based, sheetlike bodies that shifting network of rapidly aggrading, shallow and broad,
can be traced for kilometers. Despite their lateral con- subaqueous channels (see the section on facies associa-
tinuity, these sheets possess a complex multilateral and tion Ch). Normally, the tidal sand waves display high-
multistorey internal architecture resulting from the lat- angle cross-bedding, with dips of 20–30j, pronounced
eral and vertical stacking of cosets of large- to medium- asymmetry, and a dominant direction of the foresets
scale, sigmoidal, cross-bedded sandstones with reac- (Figure 5A). They have few erosional breaks and scarce
tivation surfaces and mud drapes (facies S1 and S2). mud content. According to Allen (1980), these sand
Cosets of parallel-laminated (facies S4) and ripple waves characterized tidal conditions in which the sub-
cross-laminated sandstones (facies S6) are less com- ordinate currents (floods in our case) remained below
mon, whereas intercalations of massive sandstones the threshold of the sediment movement.
(facies S7) and remnants of thin-bedded heterolithic According to Allen (1984), there is a relationship
intervals (facies H) are uncommon. The degree of between sand-wave height and water depth. Applying
bioturbation in this facies association is low and ex- his equation, one can postulate that the observed tidal
clusively formed by trace fossils of the Cruziana ich- sand waves of the Hawaz Formation, which reach up
nofacies in the heterolithic facies. The vertical ar- to 2 m (6.6 ft) in height, were deposited at depths of
rangement of the above-mentioned facies tends to about 14 m (45 ft). However, other authors (i.e., Lanck-
be random, although 2–3-m (6.6–10-ft)-thick, poorly neus and de Moor, 1995) studying current bed forms in

Ramos et al. 1321

shallow, tidal sand ridges have not found any relation Interpretation
between these two parameters. Instead, they found a Following Walker and Plint (1992) and Johnson and
wide range of sand-wave heights (0.6–5.5 m [1.96–18 ft] Baldwin (1996), the features described here for facies as-
high) at all water depths in the interval 7–18 m (23–59 ft) sociation S suggest sandy deposition in storm-dominated
deep. Consequently, water-depth estimations for facies shallow-marine settings ( Table 2) mainly because of the
association T must be considered with caution. Never- presence of hummocky and swaley cross-stratification
theless, we can tentatively suggest a water depth of a (Figure 5D). Storm reworking of previously deposited
few decameters (10– 30 m; 33 –100 ft). subtidal sand sheets and contemporaneous shoreline
The occasional heterolithic intervals (facies H) sands could be related to a general shoreline retreat.
represent deposition in relatively lower energy, distal, The biggest convex-upward bodies most likely repre-
or protected areas. From proximal to distal, this facies sent shoreface storm sand bars (up to 1.5 m [4.9 ft] high
association would ideally show a progressive substitu- and tens of meters to 100 m [330 ft] wide) flanked by
tion of facies S1 by facies S2, as well as increasing per- hummocky and ripple sculptured sand sheets. The lat-
centages of S6 and H. ter microforms and the heterolithic facies probably de-
In general, the facies association T is character- veloped preferentially in relatively deeper, lower shore-
ized by a low degree of bioturbation attributable ex- face areas, where K-bentonites seem to be preferentially
clusively to the Cruziana ichnofacies, although the preserved as well. Occasionally, the storm-dominated,
ichnofabric index increases with the increase in litho- shoreface ensemble was incised by tidal inlets (see the
facies S6 and H. section on facies association Ch).
Bioturbation intensity in facies association S is very
Facies Association S low, but some thin sandstone beds that intercalated
Facies association S is mostly formed from facies S4, within the heterolithic facies (H) may display moder-
S5, S3, and S6, with minor contributions of facies S7 ate bioturbation (ii 3–4) attributable to small, homog-
and H ( Table 2). It forms packages ranging in thick- eneously sized, S. linearis, recording opportunistic,
ness between 0.5 and 20 m (1.6 and 66 ft), with average poststorm colonization of sandy substrates.
values of about 2 –5 m (6.6 –16 ft). It is characterized
by commonly stacked convex-upward bodies, trace- Facies Association B
able laterally for tens of meters and showing distinct Facies association B mainly consists of facies Sb1 and
compensational geometries. The topmost part of each S6, with facies H as a subordinate component (see
individual body (up to 1.5 m [4.9 ft] thick) comprises Table 2). It forms tabular units ranging in thickness be-
cosets of parallel-laminated and low-angle, swaley to tween 0.75 and 12 m (2.4 and 39 ft), with average values
hummocky cross-stratified sandstones (facies S4 and of about 1–4 m (3.3–13 ft). It is characterized by pack-
S5), which overlie and merge laterally into cosets of rip- ages of thinly bedded, parallel-laminated, intensively
ple cross-laminated sandstones (facies S6), sometimes bioturbated sandstones (facies Sb1), which, in some
interbedded with thin heterolithic beds (facies H). cases, overlie packages of ripple cross-laminated sand-
Each body shows a slightly coarsening- and thickening- stones (facies S6) delineating poorly defined thickening-
upward trend (H-S6-S4/S5), although the contact be- upward parasequences (S6-Sb1) 1 – 3 m (3.3 –10 ft)
tween S6 and the higher energy facies (S4 and S5) is thick. It is locally interbedded with K-bentonite beds.
rather abrupt and commonly erosive. The degree of bio- This facies association is characterized by a high de-
turbation for this facies association is very low, and it gree of bioturbation of the Skolithos ichnofacies (pipe-
is attributable to scarce trace fossils belonging to the rocks). The abundance of Skolithos tubes results in a
Cruziana and Skolithos ichnofacies. When laterally ex- high degree of textural heterogeneity. The sedimen-
tensive outcrops are examined, the compensational ge- tary bodies of this facies association are laterally con-
ometries of successive bodies can result in facies alter- tinuous over kilometric distances, forming topographic
nations that are random, abrupt, slightly coarsening markers.
upward, or multiple coarsening and fining upward.
Paleocurrents, mostly measured in ripples, show a wide Interpretation
dispersion. Eastward and westward currents are dom- Features such as those of facies association B are in-
inant, although a northward component (northeast- terpreted as more or less complete lower shoreface
northwest) is always present. Locally, facies association (S6) to beach (Sb1) parasequences ( Walker and Plint,
S is interbedded with K-bentonite beds (see below). 1992; Johnson and Baldwin, 1996) deposited in an

1322 Geologic Note

overall prograding context ( Table 2). These deposits acterized by a high degree of bioturbation (ii 4 – 5),
are occasionally cut by tidal inlet bodies of the facies typical of large Skolithos-forming pipe-rocks. In this case,
association Ch. K-bentonites seem to be preferentially the high degree of bioturbation and the vertical persis-
preserved in lower shoreface subenvironments. tence of the ichnofabric indicate the continuing exis-
The Sb1 lithofacies display very abundant Skolithos tence of stable paleoenvironmental conditions. This
burrows, resulting in an extremely high degree of bio- ichnofabric probably represents upper to middle
turbation. These Skolithos pipe-rocks are common in shoreface settings. We propose, without making any
the lower Paleozoic (particularly in the Cambrian) in a precise claims, a sandy nearshore environment for the
variety of shallow-marine and nearshore environments deposition of facies association P. A possible inner-
(Droser, 1991). This assemblage is typically found in platform interpretation could also be put forward on
shifting substrates deposited in moderate- to high- the basis of the extremely tabular morphology of their
energy settings, affected by periodic erosion and/or sedi- beds ( Table 2).
mentation. These conditions favored the establishment
of a low-diversity community of suspension feeders in- Facies Association Kb
habiting vertical burrows. Facies association Kb consists of a single facies (Kb)
and generally appears as thin (10 –20-cm [4 – 8-in.]-
Facies Association Ch thick), laterally extensive, white to reddish colored,
Facies association Ch is formed by facies S3 and S7 massive to laminated, clay-rich beds (Figure 6D). This
with subordinate occurrences of S2, S4, and S6. It is facies association is characteristically unbioturbated,
characterized by concave-upward, channelized litho- despite the fact that these thin beds of K-bentonite
somes, up to 8 m (26 ft) thick and 100 m (330 ft) facies are commonly interbedded within sandstones of
wide, encased in the three previously described facies facies associations S and B that display some degree of
associations. These bodies show a multilateral and multi- bioturbation. No K-bentonite beds have been found
storey infill. Massive (S7) and trough cross-bedded interbedded with facies associations T, Ch, or P.
sandstones (S3) are the dominant sedimentary facies,
although some vestiges of cross-bedding with mud Interpretation
drapes (facies S2), parallel lamination (S4), and rippled Facies association Kb is interpreted ( Table 2) as the
intervals are occasionally observable. Commonly, this result of the alteration of volcanic ash beds caused by
facies association appears unbioturbated. fallout processes (e.g., Fisher and Schmincke, 1984) fol-
lowing multiple eruptions and deposited in a shallow-
Interpretation marine environment.
Because of its stratigraphic relationships with other fa-
cies associations and its channelized nature, facies asso-
ciation Ch is interpreted to be the result of channel-fill
deposits ( Table 2). These channels may have been sub- HAWAZ SUBDIVISION
tidal channels (when associated with facies association T )
or tidal inlets (when incised in facies associations S or B). Considering lithofacies, bioturbation, facies associations,
the geometry of the sedimentary bodies, and gamma-
Facies Association P ray response, three mappable, informal lithostratigraphic
The facies association P consists of a single facies (Sb2) units have been differentiated (see Figure 3). From base
and is exclusively represented by the thick-bedded, mas- to top, these are the lower, middle, and upper units
sive, bioturbated sandstones ( pipe-rocks) of facies Sb2 (HW.1, HW.2 and HW.3, respectively). The thickness
(Figure 6C). The most characteristic feature of this fa- distribution of facies associations and of the intervals
cies association is its intensive Skolithos bioturbation. with a given ichnofabric index (ichnograms in the sense
Mud sedimentation is not recorded. Facies association P of Bottjer and Droser, 1991) in the different Hawaz
occurs as a sedimentary package up to 25 m (82 ft) thick. subunits is shown in Figure 8. The lower unit charac-
terizes the deposition in nearshore to inner-platform
Interpretation settings with very intense bioturbation. The middle unit
The lack of preserved primary sedimentary structures is characterized by the deposition in shoreface storm-
in facies association P renders difficult a precise envi- reworked to beach settings and displays the maximum
ronmental interpretation. This facies association is char- values of volcanic ash contribution. Because of that, it

Ramos et al. 1323

 thinly bedded, tabular, and cross-laminated sandstone
beds. As the succession becomes finer grained upward,
ichnofabric index decreases, and Cruziana (Figure 7E, F)
and Planolites become the most abundant ichnofossils.
Gamma-ray values for this unit have been mea-
sured in the field along section 1 (Figure 3). The log
records relatively low values, especially toward the lower
part, becoming progressively higher upward. Low val-
ues are associated with fine- to medium-grained, well-
sorted quartzarenites. The progressive upward increase
in the radiation values records the slightly fining-upward
trend. Radiation values are mainly related to thorium.
The contribution of uranium is not significant, and that
of potassium is nil.

Middle Unit (HW.2: Volcanoclastic Unit)

This unit mostly consists of strata of facies associations S

and B, with secondary levels of T and Ch (Figure 8).
Facies association P is not present. The most charac-
teristic feature of this unit is that it is home to the
maximum content of facies association Kb. This fa-
cies composition results in the middle unit being
formed mostly by tabular and laterally continuous,
thinly stratified siltstone to fine-grained sandstone
packages, intercalating thin K-bentonite beds and oc-
casionally slightly coarser grained lenticular sand bod-
Figure 8. Percentage distribution of the facies associations and ies (Figure 10).
ichnofabric indices in the three Hawaz subunits. See text for The tabular thin-bedded sandstones include a
the description of the facies associations. Class ‘‘Und.’’ refers to
large variety of internal sedimentary structures, such
unexposed or undetermined sedimentary record.
as swaley to hummocky cross-bedding, planar lami-
nation, large-scale cross-bedding, current and wave-
ripple cross-lamination, and flaser and linsen struc-
can be referred to as a volcanoclastic unit. The upper tures. In general, these laminated sandstones show
unit is characterized by the dominance of subtidal sand- low ichnofabric indices (ii 1 – 2) because trace fossils
stones (facies association T) displaying the lowest de- are mostly horizontal and restricted to bedding planes.
gree of bioturbation. Rusophycus and Cruziana are the dominant ichno-
genera (Figure 7D – F), but Planolites, Arthrophycus
Lower Unit (HW.1: Nearshore to Inner-Platform Sandstones) (Figure 7G), and Lockeia are also present. Skolithos is
restricted to some thin sandstone beds, except in the
This unit is dominated by remarkably tabular strata, upper part of the unit where highly bioturbated pipe-
25 – 80 cm (10 –31 in.) thick (Figure 9), of medium- to rocks are found in thicker packages. The K-bentonites
fine-grained sandstones attributable to facies associa- appear as 10–20-cm (4–8-in.)-thick beds, of whitish
tion P (Figure 8). Bioturbation is very intense through massive or laminated mudstones (Figure 6D). When
most of the unit (ichnofabric index: ii 4 – 5), forming a iron content is high, bentonites may also take on a red-
pipe-rock constituted by closely spaced Skolithos bur- dish color. Typically, K-bentonites show little or no
rows (Figures 6C, 7C). Thalassinoides (Figure 7A) and bioturbation.
Teichichnus are also found, but only in the lowermost Erosively based, lenticular sand bodies, tens to sev-
part of the unit. eral hundreds of meters wide and 50 cm (19 in.) to 10 m
The lower unit shows a thinning-upward trend, with (33 ft) thick, have been occasionally observed encased
pipe-rock sandstones alternating toward the top with in the above-mentioned lithofacies (Figure 10B). The

1324 Geologic Note

Figure 9. Field view of the lower unit (HW.1 or nearshore to inner-platform sandstones). The picture shows the lower part of
section 1 (Figure 3). Notice its tabular, well-stratified appearance. Beds dominantly consist of facies Sb2. The outcrop is about 40 m
(131 ft) high.

Figure 10. (A) Field view of the tabular, thinly stratified, middle unit (HW.2 or volcanoclastic unit). The outcrop, about 170 m (557 ft)
wide, mainly consists of facies Sb1. (B) Field view of a lenticular body of facies S7 encased in thinly stratified facies Sb1 belonging to the
middle unit.

Ramos et al. 1325

larger bodies occasionally contain mud clasts above The massive, structureless lenticular bodies dis-
their erosive bases. No preferential vertical or lateral play clear concave-upward, channelized geometries.
distribution of these lenticular bodies has been detected They are filled with medium-grained sandstones, with
in the middle unit. On the contrary, the slightly bio- frequent mud clasts toward the erosive base. The sig-
turbated tabular beds are most frequent in the lower- moidal cross-bedded bodies occasionally include wave
most half of the unit, whereas the highly bioturbated ripples and plane-laminated sandstone beds. Locally,
tabular beds dominate in the upper half. paleocurrents show bidirectionality.
The gamma-ray log of the middle unit (Figure 3) Bioturbation is very low in the upper unit. The
reflects the heterolithic nature of the succession. The thick sandstone bodies exhibit nil to low bioturbation
lower values correspond to highly bioturbated and (only isolated S. linearis and uncommon Cruziana oc-
well-sorted quartzarenites. The scarcely bioturbated, cur at the base of the thinner sandstone beds). However,
ripple-laminated, or cross-bedded sandstones contain- some heterolithic intervals exhibit a higher abundance
ing mud chips display intermediate values, whereas of trace fossils. Rippled, centimeter-thick sandstone
the K-bentonite beds are responsible for the highest beds are topped by trace fossils (ii 2), which include
radiation peaks. Gamma-ray spectrometry clearly in- Aulichnites and Planolites. At least two massive sand-
dicates that the high radioactivity of the K-bentonite stone beds display intense Daedalus multiplex bio-
beds is caused by a significant contribution of thorium, turbation (ii 5) (Figure 7H).
uranium, and potassium, whereas in the clean quartz- Gamma ray of the upper unit is characterized by
arenites, the radiation is almost exclusively linked to relatively homogeneous, low values except for some
thorium. channelized bodies containing abundant clay chips
and the thick, matrix-rich beds related to K-bentonites
Upper Unit (HW.3: Subtidal Sandstones) (Figure 3).

The upper unit consists primarily of subtidal sand-

stones (facies association T), with minor amounts of MAPPING AND PALEORELIEF ON TOP OF THE
all the other facies associations except for associa- HAWAZ FORMATION
tion P, which is not represented (Figure 8). It consti-
tutes a thick, sandstone-dominated package. Most of The three lithostratigraphic units of the Hawaz Forma-
the unit is formed by laterally amalgamated and verti- tion and its lower and upper boundaries have been mapped.
cally stacked, lenticular, fine- to medium-grained sand- The resulting map (Figure 12) illustrates the nature of
stone bodies (Figure 11) showing either a massive inter- its relationships with the other Ordovician units (Figure 2)
nal structure or well-developed, sigmoidal cross-bedding. and, particularly, the characteristics of the paleotopog-
It includes scarce K-bentonite beds and/or graywackes raphy developed above the Hawaz sandstones.
made of a mixture of reworked quartzarenites and The Hawaz Formation crops out forming a mono-
bentonites. clinal block, gently dipping (2–4j) to the west (Figure 13).

Figure 11. Field view and sketch of the laterally amalgamated and vertically stacked lenticular sand bodies of the upper unit
(HW.3 or subtidal sandstones), dominated by facies associations T and Ch. The picture shows the lower third of section 3
(Figure 3).

1326 Geologic Note

Figure 12. Geological map of the outcropping Hawaz Formation in the western Gargaf. The map shows the location of the studied
sections 1 –3 represented in Figure 3 and the location of Figures 4 and 13C. See cross sections AA and BB in Figure 13.

Ramos et al. 1327

Figure 13. (A, B) Geological cross sections AA and BB, showing the geometry of the Upper Ordovician paleorelief. (C) Field
view facing northeast of U2 forming an inlier on the Hawaz sandstones. See legend and location in Figure 12.

Its lower boundary is a conformity with the underly- the upper and younger U2 constitutes the upper bound-
ing Lower Ordovician Ash Shabiyat Formation. This ary of the Hawaz sandstones (Figures 4, 12). Mapping
boundary crops out in extensive areas to the east of the demonstrates that U2 locally truncates the older U1,
mapped area. The upper boundary of the Hawaz For- and that the magnitude of the erosional hiatus linked
mation is formed by two erosive surfaces, unconfor- to U2 (where this is the sum of U1 and U2) increases to
mities 1 and 2 (U1 and U2, respectively, in Figures 2, 4), the east. The erosion produced by U2 would have
the former being older. U1 underlies the Melaz Shuqran included eastward the total thickness of the Melaz
Formation, whereas U2 constitutes the basal boundary Shuqran plus the Hawaz formations.
of the Mamuniyat Formation. The characteristics of the paleotopography gener-
According to the ages proposed by N. H. Miles ated along the erosive surface U2 can be recognized in
(2001, personal communication), the hiatus linked to the geological map. In the central zone of the mapped
U1 represents a time span of about 21 m.y. covering, at area, the Upper Ordovician Mamuniyat Formation fills a
least, the whole Caradocian, whereas the hiatus related paleovalley cutting down the Hawaz Formation. The
to U2 covers the Cautleyan and Rawtheyan (middle paleovalley is elongated in a south-north to south-
Ashgillian), representing a 1.1 m.y. time span. In the southeast –north-northwest direction, 5 – 13 km (3.1 –
western outcrops of the Hawaz sandstones (Figure 12), 8 mi) wide and, based on the geological cross sections
the hiatus is overlain by the Melaz Shuqran Formation, (Figure 13), appears to cut down about 100 m (330 ft)
and the lower and older U1 constitutes the upper bound- into the Hawaz sandstones. The direction of this paleo-
ary of the Hawaz Formation. Toward the east, the valley is in agreement with the measured directions of
Mamuniyat Formation directly rests upon the Hawaz glacial grooves on top of the Hawaz sandstones (170–
sandstones (or even the Ash Shabiyat Formation), and 350j) and data by Ghienne et al. (2003) and Deynoux

1328 Geologic Note

and Ghienne (2004). In the northeastern corner of the nant shore processes. These authors concluded that
mapped area (Figure 12), the remnants of another 5-km during a marine transgression related to a sea level rise,
(3.1-mi)-wide, southeast-northwest–directed paleoval- coasts tend to be embayed by the flooding of previously
ley can be observed. In this case, it is excavated into the incised valleys, and the generation of estuaries occurs.
lower unit (HW.1) and partially into the Ash Shabiyat However, during high sea level and/or marine regres-
Formation. In addition to these incised paleovalleys, sion related to a sea level fall, coasts tend to be more
some paleohighs can also be recognized. In the south- linear or lobate by progradation of strand plains and/or
east corner of the geological map, two inliers of the delta systems.
Hawaz Formation, bounded by U2 and overlapped by the In the tide-dominated estuarine facies model pro-
Mamuniyat strata, can be observed (Figures 12, 13B). In posed by Dalrymple (1992) (Figure 14A), tidal sand
this case, these inliers constitute paleohighs of the ridges record deposition in the outer estuary during ma-
Hawaz Formation formed by units HW.1, HW.2, and a rine transgression, where tidal-marine processes control
part of HW.3. This suggests that the vertical incision deposition. Storm-dominated, shallow-marine, and up-
into the Hawaz Formation related to U2 can be esti- per shoreface-to-beach sandstones, which are deposited
mated to be about 100 m (330 ft), whereas the areal in shallower, wave- or storm-dominated settings, record
extent of these paleohighs is about 4  6 and 1.5  progressive estuary infill and coastal progradation pro-
2.5 km (2.5  3.7 and 0.9  1.5 mi). Based on seismic duced during high sea level and subsequent sea level fall
data, similar paleovalleys and paleohighs have been (Figure 14B, C).
described in the subsurface of the Murzuq Basin by Thus, the proposed sedimentary model (Figure 14)
Aziz (2000) and Khoja et al. (2000). for the deposition of the Hawaz sandstones in the Gargaf
area is linked to three main evolutionary geomorphic
scenarios reflecting differences in wave and tidal power,
as well as transgressive versus regressive conditions.
SEDIMENTARY MODEL, EVOLUTION, AND These scenarios were probably located in a large-scale,
SEQUENCE STRATIGRAPHY north-northwest– to south-southeast–oriented, estua-
rine setting developed along a vast depressed area cen-
The facies associations described above characterize tered around the so-called ‘‘Murzuq-Djado trough’’ (cf.
sandy deposition in shallow-marine settings that appear figure 1 of Klitzsch, 2000). This formed an embayment
vertically arranged: offshore tidal ridges (facies associa- whose shoaling and convergence produced local tidal
tion T), storm-reworked shoreface (facies association S), amplification, increasing the tidal range and current ve-
upper shoreface to beach (facies association B), and locities, thus favoring the action of tidal processes dur-
nearshore to inner platform (facies association P). The ing deposition. This sandy sedimentary environment
sedimentary record also includes channel sand bodies (fa- was punctuated by distal, fine-grained volcanic contri-
cies association Ch) and shoreface sandstones interbed- butions in the form of ash falls.
ded with thin K-bentonite beds (facies association Kb). Contrary to Vos (1981), who indicated the pres-
Models for terrigenous deposition in shallow- ence of upper delta-plain to alluvial-plain facies in the
marine settings are widely discussed in Dalrymple Hawaz sandstones, in our opinion, the shallowest re-
(1992), Dalrymple et al. (1992), Walker and Plint corded facies are the upper shoreface sandstones be-
(1992), and Johnson and Baldwin (1996), among others. cause neither supratidal nor onshore deposits revealing
According to Dalrymple (1992), offshore tidal sand subaerial exposure have been recognized.
ridges can occur in estuaries, deltas, or open shallow-
marine settings. The tide-dominated estuarine facies
model proposed by Dalrymple et al. (1992) and its Sequence Stratigraphy
evolution through time proposed by Boyd et al. (1992)
provide us with the best available equivalent for the Shallow-marine systems are potentially sensitive set-
depositional model of the Hawaz sandstones. The evo- tings for recording sea level changes, particularly tidal
lutionary model proposed by the latter authors has been systems. In the proposed evolutionary coastal model
tested on a large number of cases on the present-day (Boyd et al., 1992), cycles of relative change of sea level
Australian coast (Harris et al., 2002). are recorded as cycles of estuarine basin expansion
The coastal classification proposed by Boyd et al. during sea level rises and estuarine filling by coastal
(1992) is based on geomorphologic features and domi- progradation during high sea level. Falling sea levels

Ramos et al. 1329

 and consequent forced regression commonly do not
produce sediment accumulation in shallow-marine set-
tings (Baum and Vail, 1988) and are only attested to by
the incision of fluvial valleys.
During transgressive episodes, tidal processes tend
to increase because of the increase in tidal power un-
dergone as valleys are progressively flooded and the
coast becomes embayed. For this reason, transgressive
deposits are commonly bounded by an erosive basal
surface (tidal ravinement surface). The sedimentary
infill of estuaries is complex because of the interaction
of marine (tidal, storm, and waves) and fluvial processes.
Nevertheless, Cattaneo and Steel (2003) consider that
deposits associated to transgressions in tide-dominated
estuaries can be divided into two transgressive tracts:
(1) an early transgressive tract, deposited where tidal
processes dominate over storm or wave processes,
bounded by a basal tidal ravinement surface and a top
wave ravinement surface; and (2) a late transgressive
tract, deposited where storm and wave processes domi-
nate and bounded by a wave ravinement surface below
and the maximum flooding surface (MFS) above.
Considering the proposed depositional model
(Figure 14) and the above-exposed division of trans-
gressive systems tracts and key surfaces (Figure 15),
the following sequence-stratigraphic scenario has been
adopted:

1. Lowstand systems tracts are not represented in the

Gargaf outcrops of the Hawaz Formation.
2. The subtidal sandstones (facies association T) are re-
lated to an early transgressive systems tract (TST-1,
Figure 14A). The basal boundaries of TST-1s are typ-
ically tidal ravinement surfaces, whereas the upper
boundaries are wave ravinement surfaces (Figure 15).
3. Sequence boundaries (SB in Figure 15) are mostly
represented by the tidal ravinement surface at the

Figure 14. Evolutionary sedimentary model for the deposition

of the Hawaz sandstones. (A) Early transgressive system tract
(TST-1); (B) late transgressive system tract (TST-2); (C) high- Figure 15. Sequence-stratigraphic model displaying systems
stand system tract (HST). tracts and key surfaces. See text for further explanations.

1330 Geologic Note

 base of the tide-dominated, early transgressive systems by an MFS from the HST, formed by facies associa-
tract, but in the absence of the early transgressive tion B. The TST/HST minimum thickness ratio of this
tide-dominated deposits, the SB could be represented sequence is about 3.6.
by a wave ravinement surface. These sequence bound- Sequence 2 is about 44 m (144 ft) thick. It is fully
aries might be proposed to delimit broad (kilometer- developed and comprises an early TST, dominated by
scale) channelized estuarine bodies previously in- facies association T, overlain by a late TST matching
cised during the relative sea level falls. facies association S. The HST is formed by several
4. The storm-reworked, shoreface sandstones (facies as- shallowing-upward parasequences of facies associa-
sociation S) represent the late transgressive systems tion B, locally cut by channels of facies association Ch.
tract (TST-2, Figure 14B). TST-2s are bounded by Interbedded K-bentonites are found both in the late
a wave ravinement surface at the base (which, lo- TST and in the HST. The sequence boundary is a tidal
cally, in the absence of the TST-1, could coincide ravinement surface. The surface separating the early
with the SB) and the flooding surface (FS) at the top TST from the late TST has not been observed, whereas
(Figure 15). the surface separating the TST from the overlying HST
5. The intensively bioturbated, shoreface-to-beach is a flooding surface. The TST/HST thickness ratio of
sandstones (facies association B) are linked to pro- sequence 2 is approximately 2.0.
grading systems, which constitute the highstand sys- Sequence 3 is about 21 m (68 ft) thick. It is in-
tems tract (HST, Figure 14C). Highstand systems complete, possibly because of the erosive superposition
tracts are separated from the late transgressive sys- of sequence 4. The sequence boundary is a tidal, locally
tems tracts by the MFS at the base (Figure 15) and channelized, ravinement surface. The TST is represented
bounded by the following SB at the top. by stacked deepening-upward parasequences consist-
6. The channel sand bodies (facies association Ch), ing of facies associations T and S. A major K-bentonite
which have been interpreted as subtidal channels or interval is present. The HST is not preserved.
tidal inlets, can be related to either the transgressive Sequence 4 is 49 m (160 ft) thick. It is character-
or highstand systems tracts. ized by a well-developed early TST dominated by facies
7. The nearshore to inner-platform sandstones (facies association T, with minor occurrences of facies asso-
association P) could tentatively be ascribed to a late ciations Ch and S. The sequence boundary is a tidal
transgressive systems tract. ravinement surface. The late TST is formed by several
8. In terms of ichnofacies, early transgressive systems deepening-upward parasequences constituted by facies
tracts are characterized by assemblages of the Cruz- associations S and T. The surface separating the early
iana ichnofacies, whereas highstand systems tracts TST from the late TST is a wave ravinement surface.
correspond to pipe-rocks of the Skolithos ichnofa- The HST, poorly exposed, is dominated by facies asso-
cies. The only exceptions are the nearshore to inner- ciation B, although some minor intervals of facies asso-
platform sandstones (facies association P). ciations T and S can be recognized. K-bentonites are
also present in the HST. The surface separating the TST
Using this approach, five sequences (sequences 1–5) from the overlying HST is a flooding surface. The TST/
have been differentiated (Figure 16). Considering that HST thickness ratio is about 1.7.
the time span covered by the whole Hawaz Formation Sequence 5 has not been fully characterized. Only
is about 9 m.y., an average duration of about 2 m.y. is the basal part of the TST, represented by a 15-m (49-ft)-
estimated for each of the five differentiated sequences. thick package of facies association T, has been studied in
Thus, they can be regarded as third-order sequences. some detail. The precise nature of the sequence bound-
Higher frequency cycles (with a periodicity below 1 m.y.) ary is also unknown, although most likely, it is a tidal
are clearly recorded in sequences 3 and 4 (see sequential ravinement surface.
trend in Figure 16). These five third-order sequences
constitute a part of the basal (NA 1) second-order se-
quence defined by Carr (2002) for the Paleozoic of
north Africa. DISCUSSION
Sequence 1 is about 61 m (200 ft) thick, but it has
not been fully characterized because its lower bound- The sedimentary record of the Hawaz Formation in
ary has not been identified. The TST is exclusively the Gargaf area allows us to reconstruct the evolution
represented by facies association P, and it is separated of a coastal zone of the Gondwana continental margin,

Ramos et al. 1331

Figure 16. Sequence stratigraphy interpretation proposed for the Hawaz Formation. See legends in Figures 3 and 15. FS = flooding
surface; M/S ratio = mudstone/sand ratio. tRs = tidal ravinement surface; wRs = wave ravinement surface.

1332 Geologic Note

which was characterized by shallow-marine, siliciclas- mately kept apace with rates of sea level changes and
tic sedimentation. The factors controlling the evolu- subsidence.
tion in coastal and shallow-marine settings are cyclic Despite the sedimentary record being incomplete,
changes in relative sea level and rates of subsidence as indicated by the erosive surface marking the upper
and sediment supply (Dalrymple, 1992; Dalrymple boundary of the Hawaz Formation, it records the de-
et al., 1992; Walker and Plint, 1992; Johnson and Bald- position of a 200-m (660-ft)-thick sandstone succession
win, 1996). during a time span between 9 and 12.5 m.y. Considering
During sea level rises, valleys were flooded, pro- a volume loss by compaction averaging 21.6% for the
ducing estuaries in embayed coasts, whereas during whole of the formation (Barsó et al., 2005), sedimen-
stages of high sea levels, the shoreline migrated seaward, tation rate can be estimated as less than 26.9 m/m.y.
producing the progradation of strand plains, beaches, (88.2 ft/m.y.). No abundant literature exists concern-
or deltas in a lobate to linear coast (Boyd et al., 1992; ing accumulation rates in these environments, but
Harris et al., 2002). The embayed morphology of coast- Dalrymple et al. (1992) indicate that sedimentation
al areas was enhanced by tectonism, which controlled would have been high, and that estuaries commonly fill
the size and subsidence of the basin, causing a large- in a short time. Einsele (2000) gives values for sedi-
scale depressed area elongated in a direction approxi- mentation rates ranging from 10 to 100 m/m.y. (33 to
mately north-south (Klitzsch, 2000). Such a large-scale 330 ft/m.y.) for a more general siliciclastic shelf set-
embayment displaying a low gradient increases tidal ting. In any case, it appears that the long-period average
power. Under the same tidal range, the water volume sedimentation rate of 26.9 m/m.y. (88.2 ft/m.y.) cal-
transferred from the open ocean to the estuarine basin culated for the whole of the Hawaz Formation must be
and vice versa during each tide cycle, whereas tidal cur- considered as a moderate to low rate when compared
rent speeds are larger in a low steeped large basin than in with the expected values for these depositional settings.
a steeper and/or small-scale one (Dalrymple et al., If we bear in mind that a moderate to low sedimen-
1992). This favors the development of tide instead of tation rate was in equilibrium with the total subsidence,
wave-dominated estuaries. Unfortunately, the outcrops we can infer that the total subsidence rate was also low,
of the Gargaf area were located at an outer estuarine including tectonic subsidence rates. Using the above
position, and we have no data about the central and inner data, we have calculated the tectonic subsidence by the
parts of the basin. Because of this, a precise paleogeo- backstripping method (Watts, 1981), resulting in a tec-
graphical reconstruction is not possible. tonic subsidence rate less than 19 m/m.y. (62 ft/m.y.) as
Commonly, estuarine coastal progradation during an average for the whole of the 9 m.y. This is a rate larger
a high sea level stage and subsequent estuary infill oc- than expected for a passive continental margin, but low-
curs rapidly. Woodroffe et al. (1989) estimated that er than common values for a synrift sequence (Einsele,
the progradation of the modern coastline of the inner 2000). It can be interpreted as the result of a continen-
South Alligator River estuary has migrated seaward tal margin subjected to limited tectonic extension.
more than 20 km (12 mi) since the end of the Holocene These data, obtained from the outcrops of the
transgression, resulting in an average progradation of Gargaf zone, are in agreement with Davidson et al.
shoreline of about 2 km/k.y. (1.2 mi/k.y.). Another (2000), who, based on seismic data, suggested that the
modern example was studied by Curray et al. (1969), Ordovician tectonic extension was small and produced
which assessed a coastal progradation of 15 km (9.3 mi) few, widely separated faults with small net slips.
during the last 3600 yr at some locations of the Pacific Little is known about sediment supply. The sedi-
Coast of Mexico, resulting in a coastal progradation of mentation rate and the time needed to infill an estuary
about 4 km/k.y. (2.5 mi/k.y.). The outer position with are functions not only of sediment supply and total
respect to the estuary of the outcrops studied herein subsidence rates, but also of the geometry and size of
makes it possible that, despite the rapid progradation the basin, which controls accommodation. In the case
of the shoreline, this zone of the estuary was never studied here, the large-scale Murzuq-Djado trough
filled with sediments. (Klitzsch, 2000) genetically controlled the estuary, and
The sandstones of the Hawaz Formation accumu- thus, high accommodation can be interpreted. As well
lated in marine environments shallower than 20–30 m as the detrital sediment input supplied to the estuary
(66–100 ft) along the whole of the succession. Facies by the fluvial system, the basin was periodically punc-
analysis indicated that no major deepening of the basin tuated by volcanic ash inputs supplied by relatively dis-
occurred, indicating that sedimentation rate approxi- tal, explosive volcanic eruptions. Deposits of volcanic

Ramos et al. 1333

ash have been preserved as thin K-bentonite beds in- Khoja et al. (2000) have documented this situation in
terstratified within the shallow sandy deposits (facies the subsurface of the Murzuq Basin, south of the Gargaf
associations S and B). This volcanic sediment supply area.
may have contributed significantly to the total sedi-
ment supplied to the basin and may have favored and
enhanced the partial infill and shallowing of the basin. CONCLUSIONS
On the contrary, it is also possible that the preservation
of volcanic ash deposits was controlled by the environ- In the Gargaf high, the Hawaz Formation is a 200-m
mental energy of the basin. In tide-dominated estuar- (660-ft)-thick succession made up of fine-grained
ies, the outer zone reaches maximum energy, whereas quartzarenites displaying diverse degrees of bioturba-
the central and coastal zones have relatively low energy tion and containing thin K-bentonite beds.
(Dalrymple et al., 1992). Therefore, the outer, tidal- The sandstones form six facies associations called
dominated zone, affected by strong, ebb-dominated cur- T, S, B, Ch, P, and Kb and have been interpreted as
rents, provided a low preservation potential for volcanic deposited in subtidal (T), storm-reworked shoreface (S),
ashes, which, if deposited, probably were bypassed off- shoreface to beach (B), channels (Ch), and nearshore
shore, whereas the preservation potential of these de- to inner-platform (P) settings. The association Kb is
posits increased in the low-energy central and internal interpreted as deposits of volcanic ash.
estuarine zones. As a whole, the Hawaz Formation records sedi-
As opposed to the clean quartzarenites, the mentation in a shallow-marine environment. The ver-
K-bentonite beds have, characteristically, high gamma- tical arrangement of the facies associations is controlled
ray values. Because they commonly display large, basin- by relative sea level variations. During sea level rises,
scale distribution, they are potentially useful as a tool for valleys were flooded, and estuarine sedimentation oc-
subsurface correlation. Gamma-ray spectrometry peaks curred. The transgressive deposits form two systems
related to the K-bentonite beds can be characterized not tracts: an early TST dominated by subtidal deposits, fol-
only by their high values but also by their high uranium lowed by a late TST dominated by shallow-water, storm-
and thorium content compared to potassium. reworked deposits. During high sea levels, shorelines
The sequence-stratigraphic partitioning proposed tend to prograde, and shoreface-to-beach sandstones
in this article points to third (and higher)-order relative were accumulated as HST deposits. Sediments record-
sea level fluctuations, leading to successive transgressive- ing the lowstand systems tract have been not recog-
regressive episodes across the basin. Although these nized. Based on this, the Hawaz Formation is divided
changes could have been eustatic in origin, their use- into five third-order sequences (sequences 1–5). Never-
fulness as a basin-scale correlation marker is masked by theless, higher frequency cycles are also locally recorded.
the homogeneous, sand-rich nature of the basin infill Although these sequences are related to sea level
and the prominent gamma-ray response of the inter- fluctuations leading to successive transgressive-regressive
bedded K-bentonite beds. episodes across the basin, they are not particularly use-
During the Late Ordovician, the Hawaz Formation ful as basin-scale correlation tools because of the ho-
was affected by intensive erosion processes related to mogeneous, sand-rich nature of the Hawaz Forma-
the Late Ordovician glaciation, which covered most of tion. More certain correlations can be made using the
the Gondwana region (Ghienne, 2003; Monod et al., prominent gamma-ray response of the interbedded
2003; Young et al., 2004). This produced the incision K-bentonite beds, which could constitute useful key
of a network of glacial valleys and a landscape over- beds for subsurface correlation. They are easily iden-
printed by glacial morphology. This glacial-related paleo- tifiable not only by their gamma-ray peak, but also by
relief, which has been made clear by geological map- the nature of the radiation, with high values in U and
ping (Figures 12, 13), has fundamental consequences Th and a low contribution from K.
because the Hawaz Formation may have locally acted The presence of the K-bentonite beds interbedded
as a reservoir for the source Silurian Tanezzuf Forma- within the Hawaz sandstones points to synsedimen-
tion (Figure 2) or the main reservoir Upper Ordovician tary calc-alkaline volcanic activity possibly related to
Mamuniyat Formation directly resting above or beside the extensional regime experienced by the northern
the Hawaz Formation by the erosive absence of the margin of Gondwana during the Ordovician.
dominantly muddy intervening Upper Ordovician Me- The dominance of the different facies associa-
laz Shuqran Formation (see Figure 2). Aziz (2000) and tions in the vertical succession allows us to divide the

1334 Geologic Note

Hawaz Formation into three mappable lithostrati- Carr, I. D., 2002, Second-order sequence stratigraphy of the Palae-
ozoic of north Africa: Journal of Petroleum Geology, v. 25,
graphic subunits (HW.1 to HW.3). The lower subunit
p. 259 – 280.
(HW.1) is dominated by deposition in nearshore to Cattaneo, A., and R. J. Steel, 2003, Transgressive deposits: A re-
inner-platform settings; the middle subunit (HW.2) view of their variability: Earth-Science Reviews, v. 62, p. 187 –
contains the greatest volcanic ash contribution to the 228.
Cocks, L. R. M., and T. M. Torsvik, 2002, Earth geography from
basin; and the upper subunit (HW.3) is characterized 500 to 400 million years ago: A faunal and palaeomagnetic
by the dominance of subtidal deposits. Consequently, review: Journal of the Geological Society (London), v. 159,
each subunit displays different sedimentary architec- p. 631 – 644.
Collomb, G. R., 1962, Etude geologique du Jebel Fezzan et de sa
tural features and distribution of their heterogeneities. bordure Paleozoique: Notes et Mémoires Compagnie Française
The upper boundary of the Hawaz Formation is du Pétrole, v. 1, p. 735.
an erosion surface related to the Upper Ordovician Craik, D., S. Quesada, R. Lemaire, A. Odriozola, and N. D. Bolatti,
2001, Sistema petrolero Tanezzuf-Mamuniyat. Cuenca de
glaciation. The mapping of the three Hawaz subunits
Murzuq, Libia: Boletı́n de Informaciones Petroleras, v. 68,
illustrates the nature of this erosion surface and the p. 97 – 108.
scale and features of the resulting paleorelief. Curray, J. R., F. J. Emmel, and P. J. S. Crampton, 1969, Holocene
The paleogeographic reconstruction shows the Gar- history of a strand plain, lagoonal coast, Nayarit, Mexico, in
A. A. Castanares and F. B. Phleger, eds., Coastal lagoons — A
gaf zone as the outer zone of a large estuary occupying a symposium: Mexico, Universidad Nacional Autónoma, p. 63 –
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south–striking trough controlled by tectonism. Howev- Dalrymple, R. W., 1992, Tidal depositional systems, in R. G. Walker
and N. P. James, eds., Facies models. Response to sea level
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218.
Dalrymple, R. W., B. A. Zaitlin, and R. Boyd, 1992, Estuarine
facies models: Conceptual basis and stratigraphic implications:
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1336 Geologic Note