| dc.description.abstract | The Upper Cretaceous Buda Formation in central Texas is overlain by Eagle Ford group (or False Buda unit locally), and overlying the Del Rio Formation (Grayson Formation). The Buda Formation outcrop exposes from west Texas to the Dallas-Forth Worth area, and extends continuously into subsurface in central Texas. This study focuses on subsurface stratigraphy of the Buda Formation based on core analysis and outcrop descriptions to determine the regional distribution of depositional environment of the Buda Formation in central Texas.
Three facies (from deep to shallow) are interpreted from core to outcrops: foraminfera mudstone, skeletal wackestone/packstone, and massive mudstone. Outcrops are mainly composed of skeletal wackestone with massive mudstone, whilst cores are dominated by globigerinid mudstone and skeletal wackestone/packstone. Both outcrops and cores are extremely burrow mottled. The abundance of bioturbation and benthic forams across the depositional profile suggests deposition occurred along a well-oxygenated, low relief ramp.
2D strike and dip cross sections constrained by cores indicate that the thickness of the Buda Formation varies significantly from 20 to 100 feet through central Texas. Considerable thickening of the Buda Formation within the trough between Edwards and Sligo paleo shelf margins occurs in south Wilson, north Karnes and Gonzales counties of central Texas. The dramatic variations of the Buda Formation thickness along the dip direction are consistent with filling differential accommodation space across the paleotopographic profile of the pre-existing Lower Cretaceous reef trend. Depositional pattern shown on cross-section indicates possible transgression sequence followed by Eagle Ford transgression.
Typical reversed “J” pattern shows the relationship of δ^13CvPDB and δ^18OvPDB from core samples indicating the Buda Formation definitely being altered under meteoric conditions during diagenetic process. Cathodoluminescence results indicate two diagenetic events occurred in Buda Formation from early marine cementation to burial diagenesis under marine-meteoric mixing water conditions based on two luminesces color stages from dull to bright orange.
The False Buda unit (locally overlying the Buda Formation in central Texas) is quite similar to Maness shale (equally to lower Eagle Ford group in south Texas) composed of low Ca but high Si, Al, K based on geochemical analysis. Large negative excursion event is shown around the Lower Eagle Ford-False Buda contact, which possibly indicates that subaerial exposure or marine erosion occurred during sedimentation. | en |
