Milankovitch cyclicity, stratigraphy, and sedimentology of the Ellenburger Group in Colorado Bend State Park, San Saba, Texas

The Ellenburger Group represents inner shelf deposits and was deposited during the Lower Ordovician as part of a large eperic platform that covered most of North America. The Ellenburger Group outcrops in central Texas and Colorado Bend State Park contains all three formations, the Honeycut, Gorman, and Tanyard. The Ellenburger is a large carbonate oil play in the west Texas Permian basin and has been well studied, however; in central Texas in the Llano Basin, Colorado Bend State Park specifically, little research has been conducted. The Llano Uplift has been uplifted several times and initial crustal thickening began during the Precambrian with onset of the Grenville orogeny. The Llano uplift may have been an important structural high during the Lower Ordovician and possibly influenced sedimentation in the central Texas region of the shelf. Interest for this study came from a brecciated zone with rip-up clasts found in Colorado Bend State Park which was interpreted to represent supratidal storm clasts. This horizon may reflect a time when the Ellenburger in central Texas was completely exposed and possibly not exposed in far west Texas. The purpose of this study is to compare west and central Texas, while describing the stratigraphy, developing sea level curves and depositional and diagenetic models for central Texas, which can be compared to that of west Texas.;Thirty six locations were chosen for stratigraphic survey from Digital Elevation maps prepared in ArcGis. Three hundred samples were collected from the park and were cut into billets for lab analysis. Sixty eight billets were sent off for thin sectioning. A composite section was developed from field and lab analyses of the Gorman Formation, as it was the best exposed unit. Six facies were identified: 1) algal dome boundstone; 2) minorly bioturbated boundstone; 3) moderately bioturbated boundstone; 4) moderately bioturbated mudstone and wackestone; 5) highly bioturbated mudstone/wackestone and packstone; and 6) grainstone. These facies were interpreted as tidal flat subenvironments and two depositional models were developed for the Gorman. Sea level curves were constructed from the composite section and cyclicity was identified as high frequency Milankovitch cycles. Colorado Bend State Park has been interpreted to record slightly deeper deposition and the Llano Uplift likely had little influence on the central Texas shelf.