| Field Trip Description: The Wilberns Formation, deposited in Central Texas during the upper Middle to Late Cambrian, first consists of the Welge Sandstone, a transgressive sandstone, on top of which the Morgan Creek Limestone, a uniform fossiliferous, and glauconitic calcarenite, accumulated. Then, the Point Peak Member, characterized by an increase of silt deposition, was deposited in a shallow water, wave-agitated unit (ripples) and included some of the most spectacular stromatolitic bioherms observed in North America. The San Saba Member, the final Late Cambrian unit in the Wilberns Formation, contains shallow marine sediment and a series of stromatolite bioherms. During the two days-long post GSA 2008 Meeting field trip, the Point Peak and San Saba stromatolites and thrombolites will be visited and studied. During the first day (October 10, 2008), a six hours-long float by canoes and kayaks down the Llano River in the area of Mason will give unique access to a wide varieties of bioherms and biostromes, most likely microbial in origin instead of algal as previous described.
The second day will consist of visiting different out crops of the stromatolites and thrombolites along the San Saba and James rivers in public land and private ranches. Interpretation of the Wilber stromatolite and thrombolite depositional setting will be built on the recent studies of modern stromatolites in the Bahamas and Western Australia. |