ABSTRACT: Estimating Amplitudes of Fifth-Order Sea Level Fluctuations from Peritidal Through Basinal Carbonate Deposits, Lower Mississippian, Wyoming-Montana

Maya Elrick, J. Fred Read

Three types of 1-10-m upward-shallowing cycles are observed in the Lower Mississippian Lodgepole and lower Madison formations of Wyoming and Montana. Typical peritidal cycles have pellet grainstone bases overlain by algal laminites, which are rarely capped by paleosol/regolith horizons. Shallow ramp cycles have burrowed, pellet-skeletal wackestone bases overlain by cross-bedded ooid/crinoid grainstone caps. Deep ramp cycles are characterized by sub-wave base limestone/argillite, storm-deposited limestone, overlain by hummocky stratified grainstone caps. Average cycle periods range from 17-155 k.y. Thin, rhythmically bedded limestone/argillite deposits of basinal facies do not contain shallowing-upward cycles, but do contain 2-4 k.y. limestone/argillite rhythms. These sub- ave base deposits are associated with Waulsortian-type mud mounds which have >50 m synoptic relief. This relief provides minimum water depth estimates for the deposits, and implies storm-wave base was less than 50 m.

Two-dimensional computer modeling of cyclic platform through noncyclic basinal deposits allows for bracketing of fifth-order sea level fluctuation amplitudes, thought responsible for cycle formation. Computer models using fifth-order amplitudes less than 20 m do not produce cycles on the deep ramp (assuming a 25-30 m storm-wave base). Amplitudes >30 m produce water depths on the inner ramp that are too deep, and disconformities extend too far into the basin. The absence of meter-scale cycles in the basin suggests water depths were too great to record the effects of sea level oscillations occurring on the platform, or climatic fluctuations, associated with glacio-eustatic sea level oscillations, were not sufficient to affect hemipelagic depositional patterns in the tropical basin en ironment.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990