Linking carbonate cyclicity in platforms to depositional and diagenetic overprints: an example from the Lower Eocene Drunka Formation, west of Assiut-Minia stretch, Western Desert, Egypt

摘要

Early diagenetic modifications in carbonate sequences can be traced to fluctuations in relative sea level and, hence, sequence stratigraphy. These variations in carbonate cyclicity can have an impact on the spatial and temporal distribution of diagenetic alterations. This study aims to investigate the depositional and diagenetic attributes of early Eocene Drunka carbonate sequences in the Western Desert of Egypt as a function of changes in cyclicity and relative sea-level variations. Drunka Formation is a thick carbonate platform consisting of two members; the lower member (34 m thick) consists of four depositional shallowing-upward or emergence carbonate cycles that range in thickness from 4 to 10.75 m. Each cycle commences with a thin lime mudstone and/or algal skeletal wackestone and capped with thick oolitic-peloidal grainstone. These cycles represent nongradual cycles that signify no regular balance between subsidence and sedimentation rates. They indicate high-frequency sea-level fluctuations and/or short-time sea-level oscillations, accompanied with high production of carbonates. The upper member (99.75 m thick) comprises of emergence carbonate cycles that begin with a thick lime mudstone, followed by algal wackestone/packstone and capped by a thin bed of nummulitic algal packstone or peloidal grainstone (0.5 to 5 m thick). These cycles represent gradual cycles that denote regular relative vertical increase in sea level concurrent with increase in sedimentation rate. They resemble low-frequency sea-level fluctuations with high rate of subsidence outpacing the increase in sea-level rise. The drowning of carbonate platform is evidenced by increase in thickness of emergent carbonate cycles upward especially in the upper member, the increase of thickness in the lower parts of cycles (lime mudstone and wackestone) at the expense of cycle cap (packstone and grainstone), and the decrease of skeletal particles. The carbonate components of the grainstone lithofacies (topmost part of cycles) are characterized by luminescence characteristics and depleted oxygen isotopic values (-7.57 to -9.07 aEuro degrees Vienna Pee Dee Belemnite (VPDB); av. = -8.32 aEuro degrees VPDB) and carbon isotopic values (-4.90 to -6.45 aEuro degrees VPDB; av. = -5.675 aEuro degrees VPDB). These values reflect formation/stabilization under meteoric phreatic conditions. The coated grains, which show aggrading neomorphism, display oxygen isotopic values that range between -6.48 and -9.48 aEuro degrees VPDB (av. = -7.98 aEuro degrees VPDB). The close and relatively depleted delta 18 O values of the base of cycles (lime mudstone, wackestone, and packstone) have probably suffered neomorphic stabilization in freshwater-dominated environment on the spatial and temporal distribution of diagenetic alterations.