Pectinid coquinas and their palaeoenvironmental implications — examples from the early Miocene of northeastern Egypt

The Early Miocene (Burdigalian) section at Gebel Gharra (SW Suez, Egypt) represents a mixed carbonate/siliciclastic regime and is composed of a series of fining- 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 fining-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’.