GEOLOGIC MAPPING OF MIDDLE JURASSIC MICROBIAL BUILDUPS IN THE INDIAN PASS QUADRANGLE (1:24,000-SCALE), CODY, WYOMING

Geologic mapping of the Indian Pass Quadrangle (1:24,000-scale), near Cody, Wyoming, facilitates examining the controls on Middle Jurassic microbial reef development. The Middle Jurassic is represented by the Gypsum Spring and “lower” Sundance Formations. Mapping of the Indian Pass Quadrangle indicates that the Middle Jurassic section is restricted to the limbs of a series of modern-day anticlines and synclines that trend approximately N-S across the map area. The thickness of the Middle Jurassic section, especially the Gypsum Spring Formation, is irregular and is apparently affected by local paleohighs (of Triassic Chugwater Formation) that existed during the Middle Jurassic.

During the summer 2002, student and faculty researchers from Wichita State University discovered well-exposed outcrops of thrombolite buildups in Middle Jurassic strata (Bajocian- to Bathonian-age) in the Indian Pass Quadrangle. These are some of the earliest known worldwide occurrence of Jurassic thrombolite reefs. During the following 2003 field season, the distribution of these reefs was mapped as part of a USGS EDMAP mapping project of the Indian Pass Quadrangle. The buildups occur in a limited stratigraphic interval within a green-gray mudstone and limestone section that occurs above the basal gypsum and red shale lithofacies of the Gypsum Spring Formation. Numerous thrombolite reefs are exposed on this surface and were mapped across the quadrangle. Specific mapping goals included determining the location, dimensions, thickness and spacing between the reefs. Initial examinations indicate that these microbial buildups developed in shallow water, inner ramp environments. Since many Jurassic thrombolite reefs discussed in the literature have been interpreted to have developed in “deep” water settings, a thorough mapping of these well-exposed shallow-water reefs from Wyoming provides important contributions to extend the understanding of the distribution of thrombolite reefs into shallow water environments.