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Parcell, William C.1, Monica K. Williams1 
(1) Wichita State University, Wichita, KS

ABSTRACT: Development of Middle Jurassic Shallow-Water Microbial Buildups in Association with Mixed Carbonate and Evaporite Deposition, Bighorn Basin, Wyoming

Thrombolite patch reefs are recognized in association with mixed carbonate and evaporite deposits in the Middle Jurassic of the Bighorn Basin, Wyoming. The Middle Jurassic in the Bighorn Basin is comprised of the mixed carbonate-evaporite Piper and Gypsum Spring Formations (Bajocian to Bathonian) and the carbonate-dominated Rierdon and “lower” Sundance Formations (Bathonian to Callovian). Laminated microbial horizons have been observed throughout the Piper and Gypsum Spring Formations by previous investigators. The thrombolites occur in a unit of interbedded carbonate mudstone and fossiliferous/oolitic packstone to grainstone, which is equivalent to the middle Firemoon Limestone Member of the Piper Formation in Montana. The observed buildups are approximately 3 meters across and 1 meter thick. Each buildup is composed of circular ‘heads’ of layered microbialite with an internal clotted texture (i.e. thrombolite). Individual heads are approximately 0.5 meters in diameter and 0.25 meters thick. Environmental interpretations of these microbial buildups suggest that they developed in shallow water (< 1 meter water depth). As many Jurassic thrombolite reefs discussed in the literature have been interpreted to have developed in deep-water settings, examination of these well-exposed Wyoming reefs provides important contributions to extend the distribution of thrombolite reefs into shallow-water environments.


AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.