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Biostratinomy and facies analysis of the upper Cretaceous oyster storm shell beds of the Duwi formation, Qusseir District, Red Sea Region, Egypt
Journal of African Earth Sciences 39 (2004) 421–428 www.elsevier.com/locate/jafrearsci
Biostratinomy and facies analysis of the upper Cretaceous oyster storm shell beds of the Duwi formation, Qusseir District, Red Sea Region, Egypt Abdalla M. El-Ayyat *, Ahmed S. Kassab Faculty of Science, Department of Geology, Assiut University, Assiut, Egypt
Abstract The Maastrichtian oyster Lopha villei (Coquand) occurs abundantly in the upper part of the Duwi Formation in the Red Sea region, Egypt. It forms thin undulating sharp-based shell beds that comprise both reclining and encrusting morphotypes of Lopha villei. Bed-by-bed biostratinomic and facies analyses of these shell beds confirm their shallow marine origin and formation by storm events. A tempestite model, explaining the mode of formation and the idealized sequence of events of such storm shell beds, has been inferred. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Biostratinomy; Facies analysis; Oyster storm beds; Duwi formation; Qusseir District; Red Sea; Egypt
1. Introduction Storm events have significant effects on recent shelf sedimentation and sediment transport, as well as in building stratigraphic sequences in shallow marine basins (Aigner, 1982, 1985; Einsele and Seilacher, 1982; Raif, 1982; Kassab, 1995). Dynamic stratigraphy (Matthews, 1984) and biostratinomy are important tools to reconstruct the processes that controlled deposition in shallow marine basins, based on the integration of stratigraphic, sedimentologic and paleoecologic data. Such tools focus on the analysis of the causes and processes of stratification, and the relationships between organisms and their environment after death and before and during burial, respectively, to interpret the stratigraphic record in terms of a process-oriented stratigraphy (analytical stratigraphy).
*
Corresponding author.
0899-5362/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2004.07.024
In Egypt, Upper Cretaceous shallow marine sequences are widely distributed and well exposed at several localities. One of the most interesting sections is the phosphate bearing succession of the upper part of the Duwi Formation, exposed in the Qusseir District, Red Sea coast, Egypt. The descriptive stratigraphy and fauna of the Duwi Formation of the Red Sea region and Eastern Desert have been discussed in several publications (e.g. Youssef, 1954, 1957; El Naggar, 1966; Abdel Razik, 1970; Hamama and Kassab, 1990; Kassab and Mohamed, 1996). However, publications on biostratinomy and storm events of the Egyptian Upper Cretaceous shallow marine sequences, in general, are very scarce (e.g. Kassab and Zakhera, 1994; Kassab, 1995; Keheila and Kassab, 2001). The aim of the present paper is to analyze biostratinomically the Upper Cretaceous oyster shell beds of the Duwi Formation exposed in the Qusseir region, to document their storm origin and to demonstrate examples of the tempestite and storm events.
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2. Material and methods The present study is mainly confined to the oyster shell beds of the upper part of the Duwi Formation exposed at Hamadat and Atshan. The measured sections are located about 16 km from Qusseir, facing Gebel Duwi, and then about 5 km and 9 km along Wadi Karim, to the south of the Quift-Qusseir asphaltic road (Fig. 1). At both localities, sediments and associated fossils have been surveyed and studied in detail in exposed faces and on bedding planes, and from petrological thin sections prepared for microfacies analysis. Macrofossil assemblages have been collected bed-by-bed, and the cord quadrate method (Ager, 1963) used to study the bedding planes. The preservational state of the oysters and their side view orientation (convex-or concavedownward, vertical or oblique) were described and counted. Directions of beak-commissure planes of the oysters have been measured and histograms and rose diagrams constructed for the data.
composed of phosphorites and phosphatic beds intercalated with oyster limestones, claystone, marls, and calcareous siltstones. Lithological characteristics and inferred sea level changes of the concerned beds are given in Fig. 2. These beds are mainly composed of calcareous phosphatic fine sandstone–siltstone, dolomitized lime-mudstone, phosphatic bioclastic wackestone, bioclastic oyster floatstone with lime mud-to wackestone matrix, and phosphatic oyster rudstone microfacies associations.
3. Stratigraphic framework Ghorab (1956) suggested the formal name ‘‘Duwi Formation’’ for the phosphate-bearing succession of the Gebel Duwi, Qusseir District, overlying the marginal marine shales and siltstones of the Qusseir Formation (Ghorab, 1956) and underlying the deep marine shales and marls of the Dakhla Formation (Said, 1961). The age and stratigraphic position of the Duwi Formation, in Egypt, has been discussed in detail by Hamama and Kassab (1990), and Kassab and Mohamed (1996) confirming its Campanian–Maastrichtian age. In its type section, the Duwi Formation (65 m thick) consists of alternating beds of claystone, sandstone, siltstone, marlstone and oyster limestone enclosing a number of phosphate and phosphatic interbeds of variable thickness (Youssef, 1957). In the present study, the uppermost 20 m of the formation were measured and analyzed in detail. The examined succession is mainly
Fig. 2. Lithology and sea-level changes of the upper part of the Duwi Formation. 1, Silicified band; 2, Bioturbation; 3, Marlstone; 4, Phosphatic concretions; 5, Silicified concretions; 6, Mudstone; 7, Oyster beds; 8, Rock fragments; 9, Fining upward microsequences; 10, Irregular surface; 11, Calcareous Siltstone; 12, Phosphorite.
Fig. 1. Location map of the measured sections, upper part of the Duwi Formation, Red Sea Region, Egypt.
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The phosphorite and phosphatic beds are normally associated with the oyster banks, hardgrounds, erosion surfaces, and sea level changes. Phosphorites either overlie the oyster banks or are separated from them by fine-grained siliciclastic beds (Fig. 2). At certain levels, oyster banks enclose a few phosphorite bodies of ellipsoidal shapes. Petrographically, the phosphorites vary from wackestones with a small content of phosphatic grains to friable phosphatic arenites. The phosphatic components are granular, comprising pellets, rock fragments, bones, teeth, and skeletal debris. Macrofossils collected from the studied interval include the bivalves Lopha villei (Coquand), Abruptalopha abrupta (DÕOrbigny), Arca multidentata (Newton), Arca gauldrina (DÕOrbigny), Veniella drui (Munier-Chalmas), Nuculana producta (Nilsson), and Glossus chargehensis (Mayer-Eymar) and the sphenodiscid ammonites Sphenodiscus lobatus (Tuomey) and Indioceras paluchistanense Arkell, Kummel and Wright, as well as a few baculitid ammonite, nautiloid, gastropod, and echinoderm species. These faunal assemblages indicate an Early Maastrichtian age for the studied interval.
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Detailed facies analysis has revealed that: pinching out, bifurcation, interlayering of phosphorite and oyster packages, hard-grounds, sharp-based shell beds, bioturbation, fining-upward microsequences, graded bedding, erosive surfaces, silicification, intraformational conglomeratic bands, dolomitization, flaser and lenticular bedding, and hummocky cross-stratification are the most characteristic features of the measured succession (Figs. 2–4). Such features document the accumulation of the phosphate bearing succession by storm-induced events, tempestites (Ahmed, 1995; Ahmed and Kurzweil, 2002). Cyclicity in the sequence is also clear from the presence of alternating shell-rich and shell-poor beds. The sharp undulating base of the shell beds points to erosion prior to sedimentation of overlying layers. Tidal influences and high-energy conditions prevailed throughout deposition of the succession. However, at certain horizons, low-energy conditions also prevailed, as indicated by the presence of the interbedded sediments dominated by deposit-feeder traces and body fossils of low diversity. Such features point to the prevalence of
Fig. 3. Field views of some common features of the measured succession, upper part of the Duwi Formation, showing: hummocky cross stratification (A); graded bedding and undulating sharp-based shell beds (B); hard-ground (C).
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Fig. 4. Field views of some common features of the measured succession, upper part of the Duwi Formation, showing inter-bedding of phosphorite and oyster beds with erosive undulating base and basal shell lags (A, C); cyclicity and fining upward microsequences (A,B,C); sharp-based shell beds (B,C,D); silicification in the phosphorite beds (dark bands) interbedded with oyster limestone beds (light horizons), bioturbation (lower part), and irregular lower surfaces with discontinuity (C); pinching out and bifurcation (D).
synsedimentary tectonic episodes and events of biologic activities during the Early Maastrichtian time. Aspects of the benthic fauna and ichnofossil assemblages together with the general sedimentological framework point to deposition of the succession in a shallow marine inner shelf setting, below fair-weather wave base and under variable high to low energy conditions, produced during highly fluctuating sea-levels resulting in several transgressive/regressive cycles (Fig. 2). These cycles are probably due to a combined effect of changes in sea level and synsedimentary tectonism as an echo of the Syrian Arc System and the global Laramide Orogeny (Kassab, 1995; Keheila and Kassab, 2001). Large-scale or even small-scale synsedimentary deformations in the phosphorite-bearing successions of the study area and other Egyptian regions were recorded by certain authors (e.g. Hendriks and Luger, 1987; Ahmed and Kurzweil, 2002).
shell thickening, or mud accretion as adults (Seilacher, 1984). The main morphological features of this oyster can be summarized as follows (Fig. 5): the shell is triangular in outline, fan shaped, subequivalves to nearly equivalves, inequilateral, having a zigzag commissure and highly crenulated shell margins. Left valves are highly convex with smaller less convex right valves. The beak is orthogyral, and unpointed with a narrow umbonal area. The posterior margin is concave and the dorsal, anterior and ventral margins are broadly convex. The attachment area is generally small and rounded in most shells,
4. Mode of life of the oyster Lopha villei As a general rule, living and fossil oysters have a planktic larval stage, which subsequently becomes attached to a firm substrate by its convex left valve after the metamorphosis stage (Stenzel, 1971). The true softbottom dwellers use shell substrates only in their early growth stages and become stabilized by size increase,
Fig. 5. Morphological features of the oyster Lopha villei (Coquand).
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some shells show xenomorphism. The two valves are sculptured by plicate radial ribs, with wide interspaces crossed by weak growth bands. The high convexity of the shell in Lopha villei as well as the small rounded attachment area, document adaptation to life on a soft substratum. The oyster populations in this study are composed only of mature shells. The absence of juveniles and very small individuals is probably due either to low juvenile mortality, biogenic destruction or winnowing by currents. The zigzag folding of the commissure was probably an adaptation to reduce the danger of sediment intake, and guarantees that in either left or right valve position, one side of the folded commissure is well above the sediment level (Seilacher, 1984). It seems likely that Lopha villei inhabited shallow marine inner shelf environments and lived with the larger left valve convex downward. It belongs to the fanshaped heavyweight recliners of Seilacher (1984), which secondarily adapted to life on a soft substratum. Adhesion to the sediment surface may be further increased by the development of strong plicate sculpture. This oyster may have remained as an encruster throughout its life growing into hard substrates (such as pebbles, hardgrounds, shells, etc.) as evidenced by the xenomorphic sculpture on some shells (Fig. 5F, G). This means that we are dealing with reclining and encrusting associations of Lopha in the studied shell beds. Such observation points to the wide tolerance of Lopha villei to fluctuations in substratum conditions.
5. Biostratinomy and storm origin The studied shell beds are mainly composed of calcitic shells of the oyster Lopha villei occurring either scattered throughout the sediments, or concentrated in beds to form shell banks. In terms of the preservational state, the skeletal accumulations consist either of complete articulated shells, disarticulated or broken shells, or a mixture of both. Complete shells are generally dominant. Fig. 6 illustrates the great variations of cross-sectional shell orientations in two measured shell bank concentrations. It shows that shells having a convexdown orientation constitute the largest fraction (45%)
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Fig. 7. Oyster orientation rose diagrams of the analyzed storm shell beds at Arshan (A) and Hamadat (B) localities.
in the Atshan locality, whereas oblique to vertically oriented shells are dominant in the Hamadat area contributing the largest proportion of the total shell content (50%). It is interesting to note that the percentage of convex-up oriented shells is the same at the two localities (30%). Measurements of the beak-commissure (dorso-ventral axis) orientation direction of the shells from their plan-view, plotted as rose diagrams in Fig. 7, indicate a polymodal distribution with no preferred orientation. Such a random pattern is also documented by the crosssectional orientation of the valves (Fig. 6). Most of the studied shell beds exhibit a sharp, rarely flat, but more commonly undulating erosive base. Occasionally, these erosive bases are highly bioturbated by Thalassinoides. Shell fragments are piped downwards into the underlying sediments. Generally, the bioclastic debris present is sand-sized or finer. Some shell beds are matrix-supported (floatstones and wackestones), but most of them are coarsely bioclast-supported (packstones and rudstones). The later are usually densely packed, often as a result of over-compaction (as evidenced by distorted and/or fractured shells, the fragments of which are still in place). Stacking, nesting and frequently imbrication of shells are commonly observed. Composite shell beds (amalgamated) in which successive beds cut erosively into underlying units are common, in one case, wave ripples are developed on the top of a shell bed. Commonly, the shell beds are vertically graded. Gradation is often discontinuous with a lower coarser shell interval with large shell fragments separated from an upper interval of medium to finely laminated shell debris. At certain levels, moderate to strong bioturbation modifies the biofabric. The composition of the matrix in the shell beds ranges from bioclastic-rich lime-mudstones to wackestones. Bioturbation increases markedly towards the
Fig. 6. Oyster orientation histograms of the analyzed storm shell beds at Arshan (A) and Hamadat (B) localities.
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top of the shell beds as the energy of deposition declines to allow colonization by benthonic faunal assemblages. The random orientation of the shells in the beds points to a rapid dumping by storm flows. The assemblage of sedimentary features present in these successions strongly supports a storm-based origin for the shell beds examined in this succession (e.g. Aigner, 1982, 1985; Einsele and Seilacher, 1982; Kreisa and Bambach, 1982; Kassab, 1995).
6. Proximal-distal trends In general, the effects of storm activity on sediment deposition have been demonstrated to decrease with increasing water depth (Aigner and Reineck, 1982), and like turbidites, to show a clear facies change laterally in a proximal to distal direction (Aigner, 1982). The shell beds in this succession have been classified into five bed types based on these proximal-distal trends (Fig. 8: 1–5): Type (1) consists of composite (amalgamated) skeletal (shell) accumulations reflecting several episodes of storm generated erosion and redeposition. Such beds are most likely to be of nearshore shallow water origin (Fu¨rsich and Oschman, 1986). Type (2) is made up of a single thick, massive bed also with an erosive base showing extensive bioturbation. Type (3) commences with an erosive base followed by fining-upward gradation. The upper finer part of this unit is bioturbated or cut by the erosive, undulating surface of the next shell beds or phosphorite units.
Fig. 8. The studied storm shell beds arranged to the degree of proximality (the terminology after Aigner, 1982; Aigner and Reineck, 1982).
Type (4) consists of a couplet of oyster shell accumulations, at the base, showing different states of preservation (articulated to disarticulated; complete to broken shells), followed by faintly laminated bioclastic debris which grades upward into skeletal calcarenitic marly limestone. Type (5) represents the distal end member of the storm events and is generally thin and dominated by mud-sedimentation. The unit has a sharp erosive base followed by a layer of fragmental shell material, which grades upward into ripple-laminated skeletal lime silt/mud. At certain levels, bioturbation and dolomitization have drastically affected this interval. The bed types identified above represent a proximal to distal sequence characterized by a gradual lateral decrease in bed thickness, in grain size, in amalgamation phenomena, faunal mixing and energy levels. These variations are interpreted as resulting from a general decrease in storm influence.
7. Tempestite model Fig. 9 shows a model of the idealized sequence of events, which led to the formation of the Lopha storm shell beds. It can be summarized as follows: – In a near-shore position and below fair-weather wave base, background sedimentation took place (covering a pre-existing hard-ground which formed by a sealevel low stand) producing a sea floor of soft calcare-
Fig. 9. Conceptual model illustrating the idealized sequence of events during the accumulation of the analyzed storm shell beds of the upper part of the Duwi Formation.
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ous sediments, which was eventually colonized by and populated with epibenthic oysters and infaunal bivalves. – After formation and before lithification, the unconsolidated substrate was exhumed, partly eroded, reworked and the fine sediments present winnowed away during a storm event, while oyster and other bivalve shells were concentrated in pavements and exposed on the sea floor to form a hard substrate. Later on, the resultant hard-ground was encrusted by oysters and other bivalves and bored by lithophagid bivalves and serpulids. Borings were filled later with fine skeletal sand/silt material. – Repeated erosion and exhumation by subsequent episodic storm events led to further erosion, winnowing, disarticulation, fragmentation, in-situ reworking and finally redeposition of the oyster shells in extensive banks. – Periods of lower storm intensity led, in some instances, to deposition of beds of finer-grained, graded shell debris. As the storms subsided, fine grained clastic or carbonate particles settled from suspension completing the fining-upward tempestite succession cycle.
8. Discussion and conclusions The bivalve oyster Lopha villei (Coquand) forms the main components of the shell beds which characterize the Maastrichtian succession of the upper part of the Duwi Formation exposed at Hamadat and Atshan localities, Red Sea region. The present study provides evidence for an ancient storm deposition system during the Maastrichtian in Egypt, and underlines the important role that such local episodic storm events have played in the formation of such shell bed successions. A tempestite model, explaining the mode of formation and the idealized sequence of events of such storm shell beds, has been inferred. A combination of storm generated wave and wind effects of varying intensity on the basin floor is probably the principal physical process responsible for the formation of these storm shell beds. Facies and faunal analyses of the examined shell beds point to deposition in a shallow marine inner shelf environment, ranging between a littoral and a shallow infrasublittoral setting under low to high energy conditions. The transgressive/regressive episodes recognized within the studied sequence are probably due to a combined effect of changes in sea level and synsedimentary tectonism. Morphological features and biostratinomical analysis refer the oyster Lopha villei to the epibenthic suspension feeders of the fan-shaped recliners, which secondarily adapted to life on soft substrates. This oyster shows a considerable variation range in morphological features reflecting a wide tolerance to fluctuations in substratum
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conditions. It appears most likely that these oysters initially colonized relatively hard ground surfaces on the sea floor. The textures and biofabrics of the lithified oyster beds suggest that periodically considerable amounts of mud were winnowed away and that powerful physical processes into layers and banks concentrated the shells. This can be explained by occurrence of a pre-existing hard ground which formed by a sea-level lowstand and which were blanketed in mud during a late-period when sea-level was higher. Periodically this soft oyster-rich sediment was swept clean of mud by storms, cutting down into the underlying hard ground and concentrating the oyster shells. This process was repeated until another sea-level lowstand when energy levels were so high as to preclude sedimentation and so a further hard ground formed.
Acknowledgments We are most indebted to Dr. Mark Woods and Dr. Graham Lott for fruitful comments, critical revision and valuable corrections that improved the manuscript.
References Abdel Razik, T.M., 1970. Stratigraphy of the sedimentary cover of Anz-Atshan, south Duwi district. Geological Survey and Mineral Resources Department, 153–179. Ager, D.V., 1963. Principles of palaeoecology. Mc-Graw Hill Book Comp. Inc., New York, p. 347. Ahmed, E.A., 1995. Microbial mediation and geochemical aspects of the Quseir-Safaga phosphorites (U. Cretaceous), Red Sea, Egypt. Bulletin Faculty Sciences, Assiut University 17, 157–169. Ahmed, E.A., Kurzweil, J., 2002. Sedimentological, mineralogical and geochemical characteristics of Upper Cretaceous Egyptian phosphorites with special reference to the microbial role in phosphogenesis. In: Wagreich, M. (Ed.), Aspects of Cretaceous ¨ sterr. Akad. Wiss., Stratigraphy and Palaeobiogeography, 15. O Schriftenr. Erdwiss. Komm., pp. 11–34. Aigner, T., 1982. Event-stratification in Nummulite accumulations and in shell beds from the Eocene of Egypt. In: Einsele, G., Seilacher, A. (Eds.), Cyclic and Event Stratification. Springer Verlag, pp. 248–262. Aigner, T., 1985. Storm Depositional Systems: Dynamic stratigraphy in modern and ancient shallow marine sequences. Lecture notes in Earth Sciences 3, 1–174. Aigner, T., Reineck, H.E., 1982. Proximality trends in modern storm sands from the Helgoland Bight (North Sea) and their implications for basin analysis. Senckenbergiana Marit. 14, 183–215. Einsele, G., Seilacher, A. (Eds.), 1982. Cyclic and event stratification. Springer Verlag, p. 536. El Naggar, Z.R., 1966. Stratigraphy and planktonic foraminifera of the Upper Cretaceous-Lower Tertiary succession in the Esna-Idfu region, Nile Valley, Egypt, U.A.R. Bulletin British Museum National History Series Geology 2, 1–291. Fu¨rsich, F.T., Oschman, W., 1986. Storm shell beds of Nanogyra virgula in the Upper Jurassic of France. N. Jb. Geol. and Palaeont., Abh. 172, 141–161.
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Ghorab, M.A., 1956. A summary of proposed rock stratigraphic classification for the Upper Cretaceous rocks in Egypt. Read before the Geological Society of Egypt, 19th June, Abstract, Cairo. Hamama, H., Kassab, A.S., 1990. Upper Cretaceous ammonites of Duwi Formation in Gabal Abu Had and Wadi Hamama, Eastern Desert, Egypt. Journal African Earth Sciences 10, 453– 464. Hendriks, F., Luger, P., 1987. The Rakhiyat formation of gebel qreya area: evidence of middle campanian to early maastrichtian synsedimentary tectonism. Brliner Geowiss. Abh. A 75.3, 83–96. Kassab, A.S., 1995. Biostratinomy of the upper cenomanian oyster storm shell beds from the Northeastern Desert of Egypt. Bollettino della Societa Paleontologica Italiana 34, 139–145. Kassab, A.S., Mohamed, A.S., 1996. Upper Cretaceous macrofossils from the Duwi Formation of the Nile Valley, southern Egypt. N. Jb. Geol. Palaeont. Abh. 200, 259–284. Kassab, A.S., Zakhera, M.S., 1994. Sexual dimorphism in the Late Cretaceous oyster Exogyra overwegi Von Buch of the Egyptian Eastern Desert. N. Jb. Geol. Palaeont., Abh. 193, 383–399. Keheila, E.A., Kassab, A.S., 2001. Stratigraphy and sedimentationtectonics of the Campanian-Thanetian succession in north Wadi Qena and southern Galala, Eastern Desert, Egypt: Evidence for major and regional erosive unconformities. Bulletin Faculty Sciences, Assiut University 30, 73–109.
Kreisa, R.D., Bambach, R.K., 1982. The role of storm processes in generating shell beds in Paleozoic shelf environments. In: Einsele, G., Seilacher, A. (Eds.), Cyclic and Event Stratification. Springer Verlag, pp. 200–207. Matthews, R.K., 1984. Dynamic stratigraphy. Prentise-Hall, New Jersey, p. 489. Raif, W., 1982. Muschelkalk/Keuper bone-beds(Middle Triassic, South West Germany): storm condensation in a regressive cycle. In: Einsele, G., Seilacher, A. (Eds.), Cyclic and event stratification. Springer Verlag, pp. 299–325. Said, R., 1961. Tectonic framework of Egypt and its influence on distribution of foraminifera. American Association Petroleum Geology Bulletin 45, 192–218. Seilacher, A., 1984. Constructional morphology of bivalves: evolutionary pathways in primary versus secondary soft bottom dwellers. Palaeontology 27, 207–237. Stenzel, H.B., 1971. Oysters. In: Moore, R.C., Teichert, C. (Eds.): Treatise on Invertebrate Paleontology, Part N, Mollusca 6, Bivalvia 3, N953-N1224, Geological Society America, Boulder and University Kansas, Lawrence. Youssef, M.I., 1954. Stratigraphy of the Gebel Oweina section, near Esna, Upper Egypt. Bull. Inst. Desert Egypt, Cairo 4, 83–93. Youssef, M.I., 1957. Upper Cretaceous rocks in Kosseir area, Bull. Inst. Desert Egypt 7, 35–54.
Biostratinomy and facies analysis of the upper Cretaceous oyster storm shell beds of the Duwi formation, Qusseir District, Red Sea Region, Egypt Abdalla M. El-Ayyat *, Ahmed S. Kassab Faculty of Science, Department of Geology, Assiut University, Assiut, Egypt
Abstract The Maastrichtian oyster Lopha villei (Coquand) occurs abundantly in the upper part of the Duwi Formation in the Red Sea region, Egypt. It forms thin undulating sharp-based shell beds that comprise both reclining and encrusting morphotypes of Lopha villei. Bed-by-bed biostratinomic and facies analyses of these shell beds confirm their shallow marine origin and formation by storm events. A tempestite model, explaining the mode of formation and the idealized sequence of events of such storm shell beds, has been inferred. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Biostratinomy; Facies analysis; Oyster storm beds; Duwi formation; Qusseir District; Red Sea; Egypt
1. Introduction Storm events have significant effects on recent shelf sedimentation and sediment transport, as well as in building stratigraphic sequences in shallow marine basins (Aigner, 1982, 1985; Einsele and Seilacher, 1982; Raif, 1982; Kassab, 1995). Dynamic stratigraphy (Matthews, 1984) and biostratinomy are important tools to reconstruct the processes that controlled deposition in shallow marine basins, based on the integration of stratigraphic, sedimentologic and paleoecologic data. Such tools focus on the analysis of the causes and processes of stratification, and the relationships between organisms and their environment after death and before and during burial, respectively, to interpret the stratigraphic record in terms of a process-oriented stratigraphy (analytical stratigraphy).
*
Corresponding author.
0899-5362/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2004.07.024
In Egypt, Upper Cretaceous shallow marine sequences are widely distributed and well exposed at several localities. One of the most interesting sections is the phosphate bearing succession of the upper part of the Duwi Formation, exposed in the Qusseir District, Red Sea coast, Egypt. The descriptive stratigraphy and fauna of the Duwi Formation of the Red Sea region and Eastern Desert have been discussed in several publications (e.g. Youssef, 1954, 1957; El Naggar, 1966; Abdel Razik, 1970; Hamama and Kassab, 1990; Kassab and Mohamed, 1996). However, publications on biostratinomy and storm events of the Egyptian Upper Cretaceous shallow marine sequences, in general, are very scarce (e.g. Kassab and Zakhera, 1994; Kassab, 1995; Keheila and Kassab, 2001). The aim of the present paper is to analyze biostratinomically the Upper Cretaceous oyster shell beds of the Duwi Formation exposed in the Qusseir region, to document their storm origin and to demonstrate examples of the tempestite and storm events.
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2. Material and methods The present study is mainly confined to the oyster shell beds of the upper part of the Duwi Formation exposed at Hamadat and Atshan. The measured sections are located about 16 km from Qusseir, facing Gebel Duwi, and then about 5 km and 9 km along Wadi Karim, to the south of the Quift-Qusseir asphaltic road (Fig. 1). At both localities, sediments and associated fossils have been surveyed and studied in detail in exposed faces and on bedding planes, and from petrological thin sections prepared for microfacies analysis. Macrofossil assemblages have been collected bed-by-bed, and the cord quadrate method (Ager, 1963) used to study the bedding planes. The preservational state of the oysters and their side view orientation (convex-or concavedownward, vertical or oblique) were described and counted. Directions of beak-commissure planes of the oysters have been measured and histograms and rose diagrams constructed for the data.
composed of phosphorites and phosphatic beds intercalated with oyster limestones, claystone, marls, and calcareous siltstones. Lithological characteristics and inferred sea level changes of the concerned beds are given in Fig. 2. These beds are mainly composed of calcareous phosphatic fine sandstone–siltstone, dolomitized lime-mudstone, phosphatic bioclastic wackestone, bioclastic oyster floatstone with lime mud-to wackestone matrix, and phosphatic oyster rudstone microfacies associations.
3. Stratigraphic framework Ghorab (1956) suggested the formal name ‘‘Duwi Formation’’ for the phosphate-bearing succession of the Gebel Duwi, Qusseir District, overlying the marginal marine shales and siltstones of the Qusseir Formation (Ghorab, 1956) and underlying the deep marine shales and marls of the Dakhla Formation (Said, 1961). The age and stratigraphic position of the Duwi Formation, in Egypt, has been discussed in detail by Hamama and Kassab (1990), and Kassab and Mohamed (1996) confirming its Campanian–Maastrichtian age. In its type section, the Duwi Formation (65 m thick) consists of alternating beds of claystone, sandstone, siltstone, marlstone and oyster limestone enclosing a number of phosphate and phosphatic interbeds of variable thickness (Youssef, 1957). In the present study, the uppermost 20 m of the formation were measured and analyzed in detail. The examined succession is mainly
Fig. 2. Lithology and sea-level changes of the upper part of the Duwi Formation. 1, Silicified band; 2, Bioturbation; 3, Marlstone; 4, Phosphatic concretions; 5, Silicified concretions; 6, Mudstone; 7, Oyster beds; 8, Rock fragments; 9, Fining upward microsequences; 10, Irregular surface; 11, Calcareous Siltstone; 12, Phosphorite.
Fig. 1. Location map of the measured sections, upper part of the Duwi Formation, Red Sea Region, Egypt.
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The phosphorite and phosphatic beds are normally associated with the oyster banks, hardgrounds, erosion surfaces, and sea level changes. Phosphorites either overlie the oyster banks or are separated from them by fine-grained siliciclastic beds (Fig. 2). At certain levels, oyster banks enclose a few phosphorite bodies of ellipsoidal shapes. Petrographically, the phosphorites vary from wackestones with a small content of phosphatic grains to friable phosphatic arenites. The phosphatic components are granular, comprising pellets, rock fragments, bones, teeth, and skeletal debris. Macrofossils collected from the studied interval include the bivalves Lopha villei (Coquand), Abruptalopha abrupta (DÕOrbigny), Arca multidentata (Newton), Arca gauldrina (DÕOrbigny), Veniella drui (Munier-Chalmas), Nuculana producta (Nilsson), and Glossus chargehensis (Mayer-Eymar) and the sphenodiscid ammonites Sphenodiscus lobatus (Tuomey) and Indioceras paluchistanense Arkell, Kummel and Wright, as well as a few baculitid ammonite, nautiloid, gastropod, and echinoderm species. These faunal assemblages indicate an Early Maastrichtian age for the studied interval.
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Detailed facies analysis has revealed that: pinching out, bifurcation, interlayering of phosphorite and oyster packages, hard-grounds, sharp-based shell beds, bioturbation, fining-upward microsequences, graded bedding, erosive surfaces, silicification, intraformational conglomeratic bands, dolomitization, flaser and lenticular bedding, and hummocky cross-stratification are the most characteristic features of the measured succession (Figs. 2–4). Such features document the accumulation of the phosphate bearing succession by storm-induced events, tempestites (Ahmed, 1995; Ahmed and Kurzweil, 2002). Cyclicity in the sequence is also clear from the presence of alternating shell-rich and shell-poor beds. The sharp undulating base of the shell beds points to erosion prior to sedimentation of overlying layers. Tidal influences and high-energy conditions prevailed throughout deposition of the succession. However, at certain horizons, low-energy conditions also prevailed, as indicated by the presence of the interbedded sediments dominated by deposit-feeder traces and body fossils of low diversity. Such features point to the prevalence of
Fig. 3. Field views of some common features of the measured succession, upper part of the Duwi Formation, showing: hummocky cross stratification (A); graded bedding and undulating sharp-based shell beds (B); hard-ground (C).
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Fig. 4. Field views of some common features of the measured succession, upper part of the Duwi Formation, showing inter-bedding of phosphorite and oyster beds with erosive undulating base and basal shell lags (A, C); cyclicity and fining upward microsequences (A,B,C); sharp-based shell beds (B,C,D); silicification in the phosphorite beds (dark bands) interbedded with oyster limestone beds (light horizons), bioturbation (lower part), and irregular lower surfaces with discontinuity (C); pinching out and bifurcation (D).
synsedimentary tectonic episodes and events of biologic activities during the Early Maastrichtian time. Aspects of the benthic fauna and ichnofossil assemblages together with the general sedimentological framework point to deposition of the succession in a shallow marine inner shelf setting, below fair-weather wave base and under variable high to low energy conditions, produced during highly fluctuating sea-levels resulting in several transgressive/regressive cycles (Fig. 2). These cycles are probably due to a combined effect of changes in sea level and synsedimentary tectonism as an echo of the Syrian Arc System and the global Laramide Orogeny (Kassab, 1995; Keheila and Kassab, 2001). Large-scale or even small-scale synsedimentary deformations in the phosphorite-bearing successions of the study area and other Egyptian regions were recorded by certain authors (e.g. Hendriks and Luger, 1987; Ahmed and Kurzweil, 2002).
shell thickening, or mud accretion as adults (Seilacher, 1984). The main morphological features of this oyster can be summarized as follows (Fig. 5): the shell is triangular in outline, fan shaped, subequivalves to nearly equivalves, inequilateral, having a zigzag commissure and highly crenulated shell margins. Left valves are highly convex with smaller less convex right valves. The beak is orthogyral, and unpointed with a narrow umbonal area. The posterior margin is concave and the dorsal, anterior and ventral margins are broadly convex. The attachment area is generally small and rounded in most shells,
4. Mode of life of the oyster Lopha villei As a general rule, living and fossil oysters have a planktic larval stage, which subsequently becomes attached to a firm substrate by its convex left valve after the metamorphosis stage (Stenzel, 1971). The true softbottom dwellers use shell substrates only in their early growth stages and become stabilized by size increase,
Fig. 5. Morphological features of the oyster Lopha villei (Coquand).
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some shells show xenomorphism. The two valves are sculptured by plicate radial ribs, with wide interspaces crossed by weak growth bands. The high convexity of the shell in Lopha villei as well as the small rounded attachment area, document adaptation to life on a soft substratum. The oyster populations in this study are composed only of mature shells. The absence of juveniles and very small individuals is probably due either to low juvenile mortality, biogenic destruction or winnowing by currents. The zigzag folding of the commissure was probably an adaptation to reduce the danger of sediment intake, and guarantees that in either left or right valve position, one side of the folded commissure is well above the sediment level (Seilacher, 1984). It seems likely that Lopha villei inhabited shallow marine inner shelf environments and lived with the larger left valve convex downward. It belongs to the fanshaped heavyweight recliners of Seilacher (1984), which secondarily adapted to life on a soft substratum. Adhesion to the sediment surface may be further increased by the development of strong plicate sculpture. This oyster may have remained as an encruster throughout its life growing into hard substrates (such as pebbles, hardgrounds, shells, etc.) as evidenced by the xenomorphic sculpture on some shells (Fig. 5F, G). This means that we are dealing with reclining and encrusting associations of Lopha in the studied shell beds. Such observation points to the wide tolerance of Lopha villei to fluctuations in substratum conditions.
5. Biostratinomy and storm origin The studied shell beds are mainly composed of calcitic shells of the oyster Lopha villei occurring either scattered throughout the sediments, or concentrated in beds to form shell banks. In terms of the preservational state, the skeletal accumulations consist either of complete articulated shells, disarticulated or broken shells, or a mixture of both. Complete shells are generally dominant. Fig. 6 illustrates the great variations of cross-sectional shell orientations in two measured shell bank concentrations. It shows that shells having a convexdown orientation constitute the largest fraction (45%)
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Fig. 7. Oyster orientation rose diagrams of the analyzed storm shell beds at Arshan (A) and Hamadat (B) localities.
in the Atshan locality, whereas oblique to vertically oriented shells are dominant in the Hamadat area contributing the largest proportion of the total shell content (50%). It is interesting to note that the percentage of convex-up oriented shells is the same at the two localities (30%). Measurements of the beak-commissure (dorso-ventral axis) orientation direction of the shells from their plan-view, plotted as rose diagrams in Fig. 7, indicate a polymodal distribution with no preferred orientation. Such a random pattern is also documented by the crosssectional orientation of the valves (Fig. 6). Most of the studied shell beds exhibit a sharp, rarely flat, but more commonly undulating erosive base. Occasionally, these erosive bases are highly bioturbated by Thalassinoides. Shell fragments are piped downwards into the underlying sediments. Generally, the bioclastic debris present is sand-sized or finer. Some shell beds are matrix-supported (floatstones and wackestones), but most of them are coarsely bioclast-supported (packstones and rudstones). The later are usually densely packed, often as a result of over-compaction (as evidenced by distorted and/or fractured shells, the fragments of which are still in place). Stacking, nesting and frequently imbrication of shells are commonly observed. Composite shell beds (amalgamated) in which successive beds cut erosively into underlying units are common, in one case, wave ripples are developed on the top of a shell bed. Commonly, the shell beds are vertically graded. Gradation is often discontinuous with a lower coarser shell interval with large shell fragments separated from an upper interval of medium to finely laminated shell debris. At certain levels, moderate to strong bioturbation modifies the biofabric. The composition of the matrix in the shell beds ranges from bioclastic-rich lime-mudstones to wackestones. Bioturbation increases markedly towards the
Fig. 6. Oyster orientation histograms of the analyzed storm shell beds at Arshan (A) and Hamadat (B) localities.
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top of the shell beds as the energy of deposition declines to allow colonization by benthonic faunal assemblages. The random orientation of the shells in the beds points to a rapid dumping by storm flows. The assemblage of sedimentary features present in these successions strongly supports a storm-based origin for the shell beds examined in this succession (e.g. Aigner, 1982, 1985; Einsele and Seilacher, 1982; Kreisa and Bambach, 1982; Kassab, 1995).
6. Proximal-distal trends In general, the effects of storm activity on sediment deposition have been demonstrated to decrease with increasing water depth (Aigner and Reineck, 1982), and like turbidites, to show a clear facies change laterally in a proximal to distal direction (Aigner, 1982). The shell beds in this succession have been classified into five bed types based on these proximal-distal trends (Fig. 8: 1–5): Type (1) consists of composite (amalgamated) skeletal (shell) accumulations reflecting several episodes of storm generated erosion and redeposition. Such beds are most likely to be of nearshore shallow water origin (Fu¨rsich and Oschman, 1986). Type (2) is made up of a single thick, massive bed also with an erosive base showing extensive bioturbation. Type (3) commences with an erosive base followed by fining-upward gradation. The upper finer part of this unit is bioturbated or cut by the erosive, undulating surface of the next shell beds or phosphorite units.
Fig. 8. The studied storm shell beds arranged to the degree of proximality (the terminology after Aigner, 1982; Aigner and Reineck, 1982).
Type (4) consists of a couplet of oyster shell accumulations, at the base, showing different states of preservation (articulated to disarticulated; complete to broken shells), followed by faintly laminated bioclastic debris which grades upward into skeletal calcarenitic marly limestone. Type (5) represents the distal end member of the storm events and is generally thin and dominated by mud-sedimentation. The unit has a sharp erosive base followed by a layer of fragmental shell material, which grades upward into ripple-laminated skeletal lime silt/mud. At certain levels, bioturbation and dolomitization have drastically affected this interval. The bed types identified above represent a proximal to distal sequence characterized by a gradual lateral decrease in bed thickness, in grain size, in amalgamation phenomena, faunal mixing and energy levels. These variations are interpreted as resulting from a general decrease in storm influence.
7. Tempestite model Fig. 9 shows a model of the idealized sequence of events, which led to the formation of the Lopha storm shell beds. It can be summarized as follows: – In a near-shore position and below fair-weather wave base, background sedimentation took place (covering a pre-existing hard-ground which formed by a sealevel low stand) producing a sea floor of soft calcare-
Fig. 9. Conceptual model illustrating the idealized sequence of events during the accumulation of the analyzed storm shell beds of the upper part of the Duwi Formation.
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ous sediments, which was eventually colonized by and populated with epibenthic oysters and infaunal bivalves. – After formation and before lithification, the unconsolidated substrate was exhumed, partly eroded, reworked and the fine sediments present winnowed away during a storm event, while oyster and other bivalve shells were concentrated in pavements and exposed on the sea floor to form a hard substrate. Later on, the resultant hard-ground was encrusted by oysters and other bivalves and bored by lithophagid bivalves and serpulids. Borings were filled later with fine skeletal sand/silt material. – Repeated erosion and exhumation by subsequent episodic storm events led to further erosion, winnowing, disarticulation, fragmentation, in-situ reworking and finally redeposition of the oyster shells in extensive banks. – Periods of lower storm intensity led, in some instances, to deposition of beds of finer-grained, graded shell debris. As the storms subsided, fine grained clastic or carbonate particles settled from suspension completing the fining-upward tempestite succession cycle.
8. Discussion and conclusions The bivalve oyster Lopha villei (Coquand) forms the main components of the shell beds which characterize the Maastrichtian succession of the upper part of the Duwi Formation exposed at Hamadat and Atshan localities, Red Sea region. The present study provides evidence for an ancient storm deposition system during the Maastrichtian in Egypt, and underlines the important role that such local episodic storm events have played in the formation of such shell bed successions. A tempestite model, explaining the mode of formation and the idealized sequence of events of such storm shell beds, has been inferred. A combination of storm generated wave and wind effects of varying intensity on the basin floor is probably the principal physical process responsible for the formation of these storm shell beds. Facies and faunal analyses of the examined shell beds point to deposition in a shallow marine inner shelf environment, ranging between a littoral and a shallow infrasublittoral setting under low to high energy conditions. The transgressive/regressive episodes recognized within the studied sequence are probably due to a combined effect of changes in sea level and synsedimentary tectonism. Morphological features and biostratinomical analysis refer the oyster Lopha villei to the epibenthic suspension feeders of the fan-shaped recliners, which secondarily adapted to life on soft substrates. This oyster shows a considerable variation range in morphological features reflecting a wide tolerance to fluctuations in substratum
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conditions. It appears most likely that these oysters initially colonized relatively hard ground surfaces on the sea floor. The textures and biofabrics of the lithified oyster beds suggest that periodically considerable amounts of mud were winnowed away and that powerful physical processes into layers and banks concentrated the shells. This can be explained by occurrence of a pre-existing hard ground which formed by a sea-level lowstand and which were blanketed in mud during a late-period when sea-level was higher. Periodically this soft oyster-rich sediment was swept clean of mud by storms, cutting down into the underlying hard ground and concentrating the oyster shells. This process was repeated until another sea-level lowstand when energy levels were so high as to preclude sedimentation and so a further hard ground formed.
Acknowledgments We are most indebted to Dr. Mark Woods and Dr. Graham Lott for fruitful comments, critical revision and valuable corrections that improved the manuscript.
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