Paleoenvironmental setting and facies analysis of the Middle Miocene carbonate platform in north Western Desert, Egypt

Abstract The Marmarica Formation in the north Western Desert (NWD) of Egypt is predominantly composed of Middle Miocene carbonate rocks. A detailed microfacies and paleoenvironmental analysis was conducted on six representative surface sections extending from the Siwa to Matruh areas: Dakrur, North Siwa (NS), Siwa-Matruh 1 (SM1), Siwa-Matruh 2 (SM2), Abu Khrait, and Agiba. Field observations and lithological data indicate that the Marmarica Formation can be subdivided into three informal members: lower, middle, and upper. A distinct facies shift is observed from interbedded shale and carbonate in the Siwa area to predominantly carbonate facies towards the north in the Siwa-Matruh and Matruh areas. Microfacies analysis of the studied sections identified twelve distinct microfacies types, which are grouped into three main facies associations, corresponding to three paleoenvironmental settings: restricted lagoonal platform interior, open marine shelf (platform top), and high-energy platform shoal. Based on the vertical and lateral distribution of these facies, a homoclinal ramp depositional model is proposed, revealing a clear deepening trend from south to north. In the southern Siwa area, the depositional system evolves vertically from a protected, restricted lagoon to open shelf and shoal settings. Towards the north, the environment becomes progressively deeper, dominated by fluctuations between open marine shelf and high-energy shoal environments. Sequence stratigraphic analysis identified three third-order transgressive-regressive (T-R) depositional sequences (SQ1, SQ2, SQ3), each comprising transgressive system tracts (TST) and highstand system tracts (HST) and are separated by two sequence boundaries (SB1 and SB2). These sequences correspond to global eustatic cycles—such as Haq et al. 1987 and Hardenbol et al. 1998- and correlate with equivalent Miocene carbonate systems in the Tethyan realm highlighting the interplay of eustatic sea-level changes and regional tectonics on carbonate platform evolution during the Langhian–Serravallian stages.