Sedimentology, sequence stratigraphy, palynology and diagenetic evaluation of the Triassic Jilh Formation. New insights from Saudi Arabia

Abstract

The Jilh Formation (late Anisian to Carnian) was deposited on the southwestern margin of the Neotethys Ocean recording the flooding of a broad shallow marine platform, and is characterized by restricted clastic-prone mudrocks and sabkha anhydrite with interbedded shallow marine dolomitized carbonates. Within the Lower Jilh Formation, deposits encompass tidal flat to low-energy inner ramp thrombolytic microbial deposits with local anhydrite nodules and fenestrae structures interbedded with ft-scale intraclast-rich floatstones with desiccation surfaces. Within the interbedded mudrocks, land-derived miospores (including common Lunatisporites spp. and other taeniate bisaccate pollen) and clastic material are common. During the Carnian, the higher sea level resulted in a change to tidal flat bioturbated and microbially induced mudstones, along with inner ramp subtidal peloidal sand banks and skeletal wackestones with benthic foraminifera within the Upper Jilh Formation. Terrestrially derived miospores are still common (assemblage including common Camerosporites secatus, Conipollenites arabicus, Duplicisporites spp., Samaropollenites speciosus), but an increasing marine influence is also noted during the Carnian from the presence of marine palynomorphs such as acritarchs and prasinophytes (including acanthomorphic acritarchs, Veryhachium spp. and Dictyotidium spp.). The backstepping of the platform is marked by the progressive flooding of the tidal flats and the deposition of open marine facies culminating in a regional 2nd order maximum flooding surface. Petrographic and geochemical analyses show the development of fabric-replacive planar-s to planar-e dolomite fabrics associated with negative δ18O values and positive δ13C values, suggesting precipitation of the dolomite on top of 3rd and 2nd order sequences from possible transgressive reflux syn or post deposition of the Jilh Formation. Zoned dolomite rhombs lining macropores might be related to burial diagenesis and pore-filling anhydrite and calcite are locally common. The local presence of native sulfur replacing primary anhydrite nodules, along with the minor calcitization of the anhydrite also suggest the onset of thermochemical sulfate reduction during the burial history at possibly high temperature.