SEDIMENTOLOGY AND GEOCHEMISTRY OF THE LOWER TRIASSIC (SPATHIAN) SILVER LAKE FORMATION

The Silver Lake Formation is a sequence of Lower Triassic (Spathian) metamorphosed sedimentary rocks located in the Soda Mountains northwest of Baker, California. The Silver Lake Formation is comprised mainly of limestone and cherty limestone with thinner beds of siltstone and channelized conglomerates. The unit is laterally equivalent to, but lithologically distinct from the shallow water facies of the Virgin Limestone (Moenkopi Formation) and deeper water facies of the Union Wash Formation (Walker and Wardlaw, 1989). Walker and Wardlaw (1989) provide a general description of the unit and a late Early Triassic age (Spathian) based on conodonts, and propose that the Silver Lake Formation was deposited in a shallow marine setting, however, little else is known about the unit. The Silver Lake Formation was analyzed at the Spectre Spur, CA locality as part of a graduate class project (GEOL 510T - Carbonate Sedimentology) in order to better determine the depositional setting and geochemistry of the unit. Each student was assigned a specific aspect of the unit – sedimentology, sedimentary petrography, paleontology, carbon isotopes, trace elements, organic matter content, and reported their results at the end of the 15 – week semester. Results indicate that carbonates found within the Silver Lake Formation consist of interbedded peloidal limestone and wackestone, with cherty limestone becoming more common higher in the sequence. Fossils include ostracodes, microgastropods, thin – shelled bivalves, sponge spicules and rare scaphopods. Macrofossils are typically micritized, and the entire unit has undergone a substantial degree of recrystallization as the result of metamorphism. The carbonate units are occasionally cross-cut by channelized conglomerates that include clasts of limestone, siltstone, chert, gneiss and volcanic rocks. Carbon isotopes confirm a Spathian to lowermost Middle Triassic age (Aegean), while trace elements indicate deposition under well-aerated conditions. Overall, the faunal composition and extensive micritization indicate a semi-restricted shallow lagoon depositional setting, while conglomerate – filled channels developed during brief periods of exposure that allowed the carbonate platform to be periodically incised without the development of extensive paleokarst features.