We've Moved!
Visit SDSU’s new digital collections website at https://digitalcollections.sdsu.edu
Description
Member C of the Mountain Springs Formation at Sheep Mountain and the Spring Mountains and lateral equivalents at the Nopah Range located in the southern Great Basin provide a record of sedimentation of a westward thickening wedge of carbonate strata deposited in cratonal, cratonic margin and miogeoclinal paleoenvironments under supratidal, intertidal and subtidal settings in Early to Medial Devonian time, Deposition occurred on a very shallow, wide, stable platform and was initiated by Early Devonian transgression which submerged the platform. Drowning of the platform resulted in the deposition of moderately fossiliferous and burrowed units at the base of the study sections. Sandy barrier shoals at the Nopah Range define the shallow western edge of the platform. Subjacent strata west of the study area were deposited in shelf, slope and basinal paleoenvironments and are considerably thicker. Most of member C is unfossiliferous, fine-grained, and medium- to thin-bedded, deposited in shallowing-upward, restricted peritidal flat paleoenvironments during a long period of offlap. Evidence of former evaporites and collapse breccias occur in the latest stages of offlap. The rocks are pervasively dolomitized to the southeast but limestones occur in the upper part of the unit to the northwest. Although dolomitization continued in paleoenvironments shoreward, the limestones represent the cessation of dolomitization in offshore areas. Geochemical information gathered using X-ray diffraction, atomic absorption, scanning electron microscope and the electron microprobe, and petrographic observations shed light on diagenetic conditions and events which led to pervasive dolomitization of member C. The relationship between paleogeography, depositional facies and diagenetic environments appears reasonable for member C dolomites. The most abundant CaCO3 precursors were aragonite and high magnesium calcite which were rapidly replaced in the supratidal environments. Subordinate amounts of primary dolomite were precipitated under supratidal-evaporative conditions. Diagenesis in the vadose/meteoric zones occurred early, and remained a relatively closed system in landward areas. Rocks which record evidence of former evaporites contain late stage dolomite or calcite filling growth structures interbedded with early dolomite. Late stage sparry dolomite has undergone multiple dissolutions and recrystallizations in conditions locally open. Diagenetic systems were more open in northwesterly or subtidal rocks associated with freshwater/marine phreatic zones. The bulk of dolomite was formed during "transitional" dolomitization of supratidal, intertidal and subtidally-deposited carbonates under mixed seawater/freshwater conditions. Pressure solution facilitated by moderately high temperatures during burial resulted in the reorganization of the dolomite to its present pervasive fabric without significantly altering or homogenizing prior formational geochemical signatures.
