STRATIGRAPHY AND PETROGRAPHY OF HETTANGIAN TO SINEMURIAN LACUSTRINE CARBONATES OF THE GLEN CANYON GROUP, WASHINGTON COUNTY (SW UTAH)

Lacustrine deposits within the Whitmore Point Member and Kayenta Formation (Glen Canyon Group), with 9 to 25 m of total thickness, record environmental conditions in SW Utah during the Early Jurassic. These deposits are dominantly siliciclastic. Their mudstones are variegated, typical of dysoxia. Around 20 widespread and vertically distributed carbonate beds 2 to 60 cm thick occur within the Group. One bed extends for at least 18 km. The Whitmore Point's lacustrine units show a low carbonate/siliciclastic ratio (1/33) when compared to that of the Kayenta (1/6). Correlation of sections suggests that the lacustrine systems were eventually replaced by fluvial deposits. Some carbonate beds return up-section as isolated pond deposits in fluvial overbank environments. Most carbonates are micritic calcite or dolomite; some are laminated. Coarser-grained units (oolite, ostracod coquina, and rudstone) also occur. Fossils include ostracods, stromatolites (suggesting shallow water), and ganoid fish scales associated with Fe concretions. Carbonate petrography reveals a complex early diagenesis. Pedogenic features occur in most units, suggesting frequent lake desiccation. They include pseudomicrokarst, alveolar-septal structures, desiccation cracks, nodulization, bioturbation, marmorization, and brecciation. Other features are intra-sediment sulfate growth, bedded chert, and cauliflower chert. Opaline liesegang and dewatering fractures in chert suggest early silicification. Saline-alkaline water in some lakes is indicated in chert by the optical positive elongation of chalcedony and by the reticulate structure pointing to a magadiite precursor, which, along with the eogenetic sulfates (in other units) and the overall complex diagenesis, suggest a semi-arid climate. Carbonate lamination is not fully understood at this point. A redox front above or near the sediment-water interface could promote dysoxia and anoxia that preserved laminae. A setting of balanced-filled lakes is inferred, due to the occasional saline-alkaline waters, fauna rich in ostracods, and absence of evaporite beds. The lakes were ephemeral and shallow, given their frequent desiccation, lack of deltas, and restricted lake shore units. These lake deposits suggest regional semi-arid conditions in the Early Jurassic.