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Carbonate lithofacies of the lower member of the El Hefhuf Formation (Turonian, Coniasian-Santonian) in the Bahariya Oases, Western Desert of Egypt comprise dolostone, burrowed dolostone, cherty dolostone, calcareous dedolostone, dolomitic quartzose lime-mudstone, and caliche. The dolostone and cherty dolostone formed in intertidal to supratidal environments, while dedolostone and caliche were formed during subaerial exposure. The dolomitic quartzose lime-mudstone was deposited in a restricted subtidal environment. Stable isotope analyses of the dolostone lithofacies are consistent with dolomitization by normal marine to evaporatively enriched dolomitizing fluids with only slight mixing of meteoric water. Therefore, dolomitization occurred in the intertidal and supratidal environments immediately following deposition. The dolostone and cherty dolostone lithofacies display pronounced cyclicity at the sub-meter scale, with individual cycles consisting of one (monolithic), two (diad) or at most three (triad) lithofacies. Most of all cycles show evidence of subaerial exposure at the top, such as brecciations, and many cycles are capped by calcareous palaeosol layers (caliche). In the absence of evidence for cycle periods at Milankovitch-scale frequencies, we reject orbital forcing as the cause of the cyclicity and suggest instead autocyclic depositional processes in the peritidal environment as the primary control on cyclicity, possibly modified by eustatic fluctuations at periods longer than the cycles recorded here. The significance of this study is to suggest formation of dolomite cycles due to high-frequency in sea level with intermittent subaerial exposure. [ABSTRACT FROM AUTHOR]

Abstract: The Upper Triassic (Adamanian LVF) Ojo Huelos Member of the San Pedro Arroyo Formation (Chinle Group) is a distinctive, carbonate-rich unit that occurs in the lower Chinle section of central New Mexico. The member consists mainly of micritic lime mudstones, ostracodal wackestones to grainstones, peloidal grainstones and distinctive pisolitic rudstones, interbedded with fine-grained siliciclastic mudstones. Most limestones exhibit some evidence of pedogenic brecciation and root penetration, and porous fabrics similar to those of modern limestone tufas occur locally. The interbedded mudstones are typically lenticular and commonly display a blocky ped fabric in which subequant peds are separated by sparry calcite veins. Fossils from the Ojo Huelos Member are freshwater (darwinulid) ostracodes, various freshwater fishes and aquatic/amphibious tetrapods–metoposaurs and phytosaurs. We interpret the carbonate facies as the deposits of carbonate lakes, ponds and wetlands that were partly spring-fed, whereas the interbedded and surrounding mudstones were alluvial in origin. The groundwater and overland hydrology of the region was likely controlled by the relative proximity to an upland recharge area in the Mogollon Highlands to the south, but sedimentary fabrics record strong overprinting by desiccation and pedogenic reworking. Consequently, we interpret the Ojo Huelos Member as recording a climate that varied from subhumid to semi-arid, which caused episodic falls in the hydrologic base-level. This resulted in landscape degradation, exemplified by significant pedogenic and erosional reworking of the carbonate sediments and fluvial incision. [Copyright &y& Elsevier]