TESTING IF THIN CARBONATE UNITS ARE MARINE OR NON-MARINE: APPLYING THE HARDIE APPROACH TO THE TRIASSIC ALCOVA LIMESTONE OF WYOMING

The Triassic Alcova Limestone in the Chugwater Group is a thin (<6 m) and persistent (>30,000 km²) limestone unit in Wyoming, USA. Though most workers have interpreted the Alcova as a major marine transgression, controversy over marine or non-marine environments of deposition and source waters persists

To attempt to address this controversy we have measured detailed sections through the Alcova Limestone in at four locations in Wyoming. In outcrop the Alcova Limestone is a carbonate mudstone ranging in thickness from 77 cm near Thermopolis to 360 cm at Alcova. The dominate sedimentary structures are upward doming and wavy algal laminations, ripple marks, starved ripple cross-bedding, mud cracks, discontinuous disturbed laminations, lenticular bedding, and mud partings. Other than the algal laminations the only fossils or trace fossils we have found are ambiguous shell casts and molds and vertebrate tracks on the uppermost surface of the unit. Thin section petrography shows a mostly recrystallized carbonate with peloidal ghosts and angular quartz silt and sand. Bracketing units are thinly bedded or laminated claystone, siltstone, sandstone, and rare thin (<30 cm) carbonate mudstones. The most common sedimentary structures in these units are wavy laminations, wave ripples, small blocky peds, soil slickensides, and a flat pebble conglomerate directly overlying the Alcova.

To address this problem, we use the approach described in Hardie’s classic paper “Evaporites: Marine or non-marine” (1984). We have modified Hardie’s five criteria for making distinctions between marine and non-marine evaporites to apply to this ambiguous carbonate. The sedimentary structures, fossils, facies associations within the unit, and associated clastic rocks are consistent with deposition in a non-marine environments with non-marine saline source waters. The Alcova Limestone likely represents a period of higher groundwater resulting in a large lake or saline pan in a continental landscape previously dominated by smaller lakes and saline pans and surrounding mudflats with abundant soil formation. A lacustrine interpretation for the Alcova Limestone is more consistent with the surrounding units and better fits the evidence.