Pectinid Coquinas and Their Palaeoenvironmental Implications Ð Examples from the Early Miocene of Northeastern Egypt

Palaeogeography, Palaeoclimatology, Palaeoecology 172 (2001) 171±191 www.elsevier.com/locate/palaeo Pectinid coquinas and their palaeoenvironmental implications Ð examples from the early Miocene of northeastern Egypt Oleg Mandica, Werner E. Pillerb,* aInstitute for Palaeontology, University of Vienna, Geozentrum, Althanstrasse 14, A-1090 Vienna, Austria bInstitute for Geology and Palaeontology, Karl-Franzens University of Graz, Heinrichstrasse 26, A-8010 Graz, Austria Received 16 February 2000; accepted for publication 26 March 2001 Abstract The Early Miocene (Burdigalian) section at Gebel Gharra (SW Suez, Egypt) represents a mixed carbonate/siliciclastic regime and is composed of a series of ®ning- and deepening-upward sequences. Mass occurrences of scutellid echinoids and pectinid bivalves are remarkable features within this section. Five pectinid horizons, occurring in two superpositioned ®ning-upward sequences, are qualitatively and quantitatively studied in detail. The taxonomic composition of respective pectinid accumulations clearly support a deepening-upward interpretation with decreasing pectinid diversity, decreasing convexity, shell thickness, and sculpturing. The taphonomic parameters of the pectinid beds, in combination with the study of co-occurring biota and frequency and density of shell encrustations and bioerosion, clearly point to different mechanisms for shell accumulations, namely as `composite concentrations' and `reworked event-concentrations'. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Pectinidae; Coquinas; Taphonomy; Deepening-upward sequences; Burdigalian; Egypt 1. Introduction 1991) and the entire group is characterised by a distinct radiation during the Neogene (Demarcq, Considering the fossil record, pectinids are one of 1990). the most important bivalve groups due to their predo- Early Miocene shallow marine sections in the East- minantly calcitic shell mineralogy (e.g. Taylor et al., ern desert of Egypt (NW Suez) were recently studied 1969; Carter et al., 1998). These calcitic shells have a lithologically, micropalaeontologically and biostrati- high preservation potential and even in shallow water graphically by Abdelghany and Piller (1999). Within carbonate sediments where aragonitic mollusc shells this study, rich fossil occurrences particularly of echi- are usually dissolved, they are well-preserved. In noids and bivalves were discovered. On the base of addition to this high preservation potential, pectinids this information the best section Ð at Gebel Gharra occur frequently and are widely distributed in shallow (Fig. 1) Ð was selected and a detailed sedimento- marine environments. Moreover, distinct morphologi- logical and palaeontological study was conducted in cal groups developed different life strategies (Waller, order to provide a serious base for environmental reconstruction. The most striking fossils from a taxo- nomical, taphonomical, and palaeoecological per- * Corresponding author. Tel.: 143-316-380-5582; fax: 143-316- 380-9871. spective are pectinid bivalves and echinoids both E-mail address: [email protected] (W.E. Piller). occurring not only scattered in the sediment but also 0031-0182/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0031-0182(01)00268-1 172 O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191 Fig. 1. Location and geological setting of the studied section. as mass occurrences in several coquinas (Piller et al., information on the depositional history of the 1998). Since the echinoids are currently under study sequence. (compare Nebelsick and Kroh, 1998) we focus in this paper on the pectinid bivalves. The relatively high abundance of pectinids in the 2. General setting studied section provides a good base for palaeoecological investigations of these bivalves. The occurrence Neogene rocks are well exposed in the Cairo±Suez of coquinas offers the opportunities to reconstruct the district and form the low hills of that area (Abdel- environmental conditions which led to the origin of ghany and Piller, 1999; Sadek, 1959). Sedimentation these Fossillagerstatten. around Suez is bound to two shallow tectonic basins The occurrence of a rich and diverse pectinid fauna (Gineifa-Gharra-basin in the north; Hagul-Sukhna- in the Suez district was already mentioned by Fraas basin in the south), which are ®lled with shallow (1867) and Fuchs (1877). A ®rst taxonomic descrip- marine, mixed carbonate-siliciclastic sediments. The tion of pectinids was carried out by Fuchs (1883), Neogene sequences overlay either shallow marine followed by Blanckenhorn (1901, 1903), and partially Eocene carbonates or non-marine Oligocene silici- by Deperet and Roman (1902±1928), and Roger clastics (Hermina et al., 1989). The Oligocene age is (1939). Additional taxonomic studies were performed not well dated but inferred from the regional geologi- by Said and Yallouse (1956) and Cherif (1974). More cal situation (Said, 1990). Due to rapid lateral facies recently, an attempt for biostratigraphic use of this changes lithostratigraphic classi®cation is widely rich fauna was made by Hamza (1992). unclear and controversial (Abdelghany and Piller, Within the studied section we want to: (1) docu- 1999). The Gebel Gharra is located approximately ment the diversity of pectinids; (2) detect pectinid 30 km WNW of Suez and the measured section is communities in 'normal' sediments; (3) study the on its east-facing side (base of measured section: pectinid coquinas qualitatively and quantitatively; 30810.60N/32818.40E; Fig. 1). Its study earlier in detail (4) reconstruct the environmental conditions which by Souaya (1961, 1963) and by Cherif (1974) gave rise to the coquinas on the base of sedimento- remained, however, biostratigraphically unclear. The logical and palaeontological data, and (5) add general recent dating by Abdelghany and Piller (1999) using O. Mandic, W.E. Piller / Palaeogeography, Palaeoclimatology, Palaeoecology 172 02001) 171±191 173 planktic foraminifera, calcareous nannoplankton and and sorting by shape were studied in quadrants normal the larger foraminiferal genus Miogypsina clearly to bedding plane of de®ned areas within the pectinid allowed a dating into the Burdigalian (M2±M4 after coquinas (Tables 1±5 and 7). Where possible, several Berggren et al., 1995; NN4 after Martini, 1971; upper quadrants were studied within one coquina to get an part of SB 25 after Cahuzac and Poignant, 1997). estimate of variance (Tables 1±5 and 7). In coquinas Since Miogypsina is found up to the top of the section, with preferred horizontal orientation of valves convex a Middle Miocene age can de®nitely be excluded. side up/down ratios of valves were also obtained. 3. Methods 4. Results The palaeoenvironmental analysis has been carried 4.1. General description of the section out considering sedimentological and microfacial data, as well as taxonomic and taphonomic parameters The section at Gebel Gharra is approximately and functional morphology. 140 m in thickness (Fig. 2). The footwall is made up Taxonomy enables the de®nition of assemblages, of red siliciclastic sediments with abundant chert quantitative data give information on diversity allow- pebbles, which are interpreted as non-marine Oligo- ing calculation of several diversity indexes (e.g. cene deposits (e.g. Said, 1990). A direct contact Legendre and Legendre, 1998) (Plates 1 and 2). between these sediments and those under study in Since different indexes focus on different parameters the measured section was not found. The base of the we applied 2 indexes (Tables 1±5): Simpson index section consists of an alternation of cross-bedded ranges from 0 to 1, whereby higher values character- sands and silts, partly bioturbated and with rare ise higher diversities. Rare taxa have a low in¯uence plant remains, which are interpreted to represent a on its value. Therefore, it re¯ects mainly the distribu- ¯uvial±¯uviomarine depositional system. tion of the frequent species in a given sample. The Unequivocal marine sedimentation starts above an Shannon index is similar to the Simpson index, erosional and strongly bioturbated (Ophiomorpha) however, its value is strongly in¯uenced also by the level with (calcareous) sandstones with mud pebbles rare taxa of a sample. It characterises the diversity (beds 3,4) and three coquinas of densely packed irre- through number of occurring taxa and, additionally, gular echinoids (Parascutella) (Nebelsick and Kroh, by relative dominance of these taxa. This information 1998). Above the coquinas follows a poorly exposed is the base for the palaeoecological reconstruction sequence of bioturbated sands and marls (c. 15 m; using an actuopalaeontological approach. beds 5, 6) exhibiting a general ®ning-upward trend. Aspects of functional morphology were covered by The next package (beds 7±9) again starts with a analysing the following parameters: average shell bioturbated base, which is overlain by Planostegina size, convexity, shell thickness, and degree of orna- rudstones with mud pebbles. Above follow several mentation. For all parameters a rank scale was applied bioclastic sandstone beds with three distinct pectinid ranging from 1 to 3 (Table 6). Shell size was calcu- coquinas (for details see Section 4.2), which grade lated as length 1 height/2. A resulting value between into a more than 5 m thick unfossiliferous marl. 0 and 40 mm equals 1 on the rank scale, 41±60 mm This is overlain by another sediment cycle (beds equals

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