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Description
Member B of the Mountain Springs Formation represents shallow subtidal, intertidal, and supratidal carbonates deposited along the cratonic margin of the Cordilleran miogeosyncline in late Medial to Late Ordovician time. Six sections, in a roughly north-south line, across 101 km (63 miles) of southeastern California and southern Nevada were measured and sampled for petrographic and trace element analysis. Member B consists of nearly pure well-ordered dolomite, with only extremely minor amounts of sand, silt, clay, and calcite. According to depositional texture the rocks are mainly mudstone. Minor amounts of wackestone and packstone are attributed to a basal transgressive deposit where energy was slightly higher, or to storm deposits in a low energy environment. The most characteristic lithology of member B is a light and medium gray mottled mudstone, containing few bioclasts (mostly crinoid fragments), which was deposited in a very low energy, subtidal environment. Scarcity of bioclasts, mottling due to incomplete bioturbation of the sediment, and gray color due to unoxidized organic matter and reduced iron, indicate a restricted environment of higher salinity or low oxygen. Overlying intertidal and supratidal rocks suggest a shallow water environment. Intertidal rocks are medium gray mudstone with discontinuously laminated intervals often displaying dolomite spar-filled vugs attributed to gas voids created directly above an organic-stained area. Several sequences of medium gray, coarse crystalline, massive mudstone grading upwards to dark gray, fine crystalline, laminated wackestone are interpreted as ponds formed behind tidal channel levees. Supratidal deposits occur near the top of most sections. They are identified by a tan color, fine crystalline texture, cryptalgal laminae, and birdseye structures. Intraclasts are common, and are attributed to storm deposits, desiccation, and possibly intertidal channel deposits. Absence of evaporites or pseudomorphs suggests a humid climate. Trace element geochemical data obtained by atomic absorption analysis of bulk rock samples show slight concentration variability (with respect to lithology) possibly due to differences in depositional environment. Groups obtained by plotting one trace element versus another for each lithotype have considerable overlap which prevents correlation of depositional environment with Na, Sr, Mn, or Fe concentration. Pervasive dolomitization of subtidal, intertidal, as well as supratidal deposits and absence of evaporites provide evidence for secondary dolomitization of originally aragonitic and calcitic sediment. Petrographic analysis reveals a broad range of crystal sizes, possibly related to original permeability and texture and that dolomitization occurred before compaction and stylolite formation. This, along with absence of interbedded limestones, suggest dolomitization in the shallow subsurface by mixing of meteoric water with marine pore water. Trace element data provide strong evidence supporting dolomitization by meteoric water mixing with marine water. Recrystallizations and dolomitization occurred in contact with water containing less Sr, and more Mn and Fe, relative to Ca, than sea water, probably closer to meteoric water composition. Higher Na concentrations are not consistent with water salinity lower than sea water, however, and may indicate slight hypersalinity, or Na complexing with organic matter during diagenesis.
