From the Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.
Bureau Seminar, March 13, 2009
Laser-Aided 3D Reconstruction of Carbonate Systems:
Lower Ordovician Paleokarst Example,
Franklin Mountains, Texas
Jerome A. Bellian
Bureau of Economic Geology
The Lower Ordovician El Paso Group is a >400-m-thick carbonate succession superbly exposed in the Franklin Mountains, El Paso, Texas. The Franklin Mountain outcrops also offer one of the most complete and relatively undeformed Lower Ordovician exposures in North America. The El Paso Group was deposited on a carbonate megabank, referred to as the "Great American Carbonate Bank," which covered half of Laurentia. Near the end of the Early Ordovician the Sauk global sea-level lowstand led to extensive subaerial exposure of this megabank. Evidence for this exposure has been identified across North America, including the Central Kansas Uplift, Texas, the Arbuckle Mountains, Newfoundland, and throughout the Appalachians as the Knox unconformity. In the Franklin Mountains evidence of the exposure indicates erosion or nondeposition of the uppermost Floian through Darwillian Global Stages (uppermost Lower Ordovician and entire Middle Ordovician). Surficial erosion (grikes, karren, and epikarst), as well as large vertically extensive and laterally extensive breccia bodies have been identified in the Franklin Mountains suggesting a meteoric paleocave system active during this sea-level lowstand. Breccia bodies representing the position of collapsed and infilled paleocaves have been mapped along the eastern face of the Franklin Mountains and within canyons on the western dipslope. These breccias range in size from a few square meters to over 120,000 m2 in outcrop area. Estimating the dimensions of these paleocaves and associated breccias requires precise definition of stratigraphic and spatial position to be compared against analogous modern karst systems.
Mapping these breccia shapes in outcrop requires a precise sequence stratigraphic model with accurate spatial positioning. In carbonate rocks significant subaerial exposure surfaces, even in excellent outcrops, can be easily overlooked. In order to map 20 km2 of carbonate stratigraphy at the sub-meter scale and accurately delineate geobody dimensions, a high-resolution laser survey was acquired of the southern Franklin Mountains. From this laser survey a submeter digital outcrop model with centimeter accuracy was produced for the entire study area. Hyperspectral image analysis was conducted to identify mineralogy changes over the north half of the study area which correlated dolomite-calcite changes related to breccias. 3D paleocave geometry was modeled by combining conventional field mapping and digital outcrop-model interpretation so that breccias could be identified to populate the model 3D grid. Results of this project offer spatial statistics on breccia dimensions for karst water and hydrocarbon systems within a sequence stratigraphic framework and set the stage for future research in fracture and geochemical analysis to better constrain the temporal evolution of these systems.