Shoal environment as a biodiversity hotspot: A case from the Barremian-Albian strata of Gabal Lagama (North Sinai, Egypt)

摘要

Recent studies demonstrated that shoal habitats include exceptionally diverse marine communities. Therefore, understanding the dynamics of these shoals under natural climatic changes is essential for biodiversity conservation. The Barremian-Albian succession of Gabal Lagama (North Sinai, Egypt) represents high-energy carbonates deposited on a complex oolite shoal. Behind these shoals, restricted lagoons and inter/supratidal mudflats environments of lesser extent were formed. Both rock and fossil samples were subjected to quantitative analyses. Nonmetric Multidimensional Scaling (NMDS) based on quantitative data from the microfacies suggested that the percentage of ooids, echinoderms, and benthic foraminifers are the major contributors to the variation in the facies association. Based on macrofaunal samples, three main benthic communities were identified. These communities display a clear connection to hydrodynamics and substrate type (grain size and consistency level). Indeed, high-energy shoal environment is characterized by a polyspecific association, which is dominated by epifaunal suspension-feeders (Amphidonte association). In contrast, two associations of low-energy environment is dominated by few species (i.e. paucispecific) and therefore, they reflect stressed environments; the lagoonal (Nucula association) and the middle ramp (Trigonia association). The facies architecture arranged the Barremian-Albian succession into five Type 2 sequences, which document 3rd order sea-level oscillation. The Barremian delta-dominated setting changed into a tide-dominated homoclinal ramp in the early Aptian and finally to a wave-dominated ramp by the late Aptian, which continued to the early Albian. The functional attributes of the benthic communities (epifauna/infauna and deposit/suspension feeders) have cyclic pattern correlated to the third order sea-level fluctuations.