PRE-CRETACEOUS EROSION SURFACE IN TEXAS

During the early Mesozoic, an erosion surface had developed across Precambrian basement and Paleozoic strata in central, north, and west Texas. Then came widespread deposition of Cretaceous strata, followed by the modern cycle of subaerial erosion. We have applied ArcGIS^® technology to geologic and topographic maps, to record more than 15 thousand x-y-z (easting-northing-elevation) data points that are situated on the pre-Cretaceous erosion surface. Sources of data are (i) the surface where intersected in outcrop, (ii) well logs where the surface is buried, and (iii) places where Cretaceous strata were stripped off but underlying rocks are not eroded (relict surface).

Matlab^® software generates topographic contour maps from these 3-D data, and calculates surfaces projected mathematically through them. To a first approximation, both the modern and pre-Cretaceous landscapes of Texas are described as a ramp sloping gently downward to the east. Immediately before the Cretaceous flooding event, the surface was a vast, almost featureless plain in the northern and western region. Stratigraphic relationships indicate that the Llano Uplift (central Texas) had already become re-elevated topographically. Its surface was dissected by east-trending valleys associated with more than 100 meters of local relief. Remarkably, the courses of the modern Colorado, Llano, and Pedernales Rivers approximately follow these same paleovalleys. Modern erosion is exhuming a relict landscape created more than 100 million years ago.