SEISMIC IMAGING OF THE CATAHOULA ASH NEAR RIO GRANDE CITY, TX

The Oligocene Catahoula Formation of the Texas Gulf Coast consists of a vertical succession of volcaniclastic sediments interbedded with fine grained sandstones and shales. It occurs in a band sub-parallel to the modern coastline, from Louisiana to the border with Mexico. The focus of this study is within the South Texas portion of the Catahoula, adjacent to the Rio Grande floodplain. Near Rio Grande City, Texas, an anomalously thick (estimated thickness of 20m) deposit of Catahoula-age ash (27.5 Ma) representing a single event, outcrops in an area approximately 60x20 miles (95x30 km). This unit overlays the Frio formation, and unconformably underlies the mid-Miocene Goliad Formation conglomerate. This tuff is massive in appearance indicating a low-energy depositional environment.

This study used the seismic refraction imaging method to determine if the base of tuff was acoustically visible. Potential original bedding characterized by acoustic impedance boundaries were also sought. The recording system was composed of 12 geophones, a twelve pound sledgehammer, and aluminum strike plate. The total line distance of the seismic profile was 530 ft (160 m) with geophone interval of 10 ft (3 m). The acquisition rolled forward with 50% overlap of 24-station profiles across the line. Preliminary results show the presence of several refractor signals. These are analyzed for velocity and depth information along with potential sideswipe signal from a nearby outcrop.