Environmental change and stratigraphy of the Upper Triassic sediment succession in Ras Al-Khaimah, UAE

Abstract

In the United Arab Emirates, the Late Triassic events including the Carnian Pluvial Episode are relatively poorly studied compared to the carbon isotope excursion and extinction event at the Triassic-Jurassic boundary. This study presents an integrated approach using geochemical and sedimentological data to investigate the depositional and environmental changes through the Late Triassic into the earliest Jurassic. Upper Triassic sediments exposed in Wadi Milaha consists of the marine Milaha and Ghalilah formations. The upper part of the Milaha Formation comprises limestone (predominantly mudstones and wackestones), with subordinate sandstone, marl and shale deposited in a shallow marine environment with some evidence of high-energy shoal deposition represented by ooidal and bioclastic grainstones and packstones. Clastic input varies cyclically and correlates with higher-order sea-level fluctuations. Faunal content includes bivalves, green algae, echinoderms, and benthic foraminifers, and suggests deposition in a shallow semi-restricted to open marine environment. Elemental proxies including Fe and Mn enrichment factors show widespread oxygen deficiency during the Late Norian on this equatorial shelf of Panthalassa. The Late Norian-Hettangian Ghalilah Formation is further broken into the Asfal and Sumra members. The first of these members is dominated by floatstones and rudstones with a higher content of coarse siliciclastics, indicating deposition in regressive conditions. The Sumra Member shows a decrease in coarse siliciclastics and an increase in mudstones, wackstones and packstones indicating a transgressive sea level cycle following the sequence boundary at the top of Asfal. The XRF elemental data also indicate fluctuations in clastic input throughout the Asfal and Sumra members indicative of increased weathering fluxes likely associated with a change to more humid conditions through the Late Triassic. A loss of fauna as well as ooidal grainstones are present at the top of the Sumra Member and continue into the Sakhra Member of the Ghalilah Formation indicating the well-documented extinction at the Triassic–Jurassic boundary. These new data from the equatorial margin of Panthalassa highlight significant environmental and climatic shifts through the Late Norian to Hettangian.