Donald P. McGookey
The word “unique” comes to mind when looking at the features of the Llano Uplift of central Texas. This broad dome is one of a family of uplifts and basins formed across southwestern North American during Late Paleozoic time. Erosion of this dome by the Colorado River and its tributaries has provided a unique opportunity to view Late Proterozoic rocks in the United States. The next closest outcrop of Precambrian rocks of the same age (Grenvillian) is over a thousand miles northeast in Ontario and Quebec. About 60 percent of the Precambrian outcrops are regionally metamorphosed gneisses and schists dated at 1.12 b.y. The metamorphic rocks originally were a very thick section of sedimentary rocks with intervals of extrusive and intrusive rocks near the top of the sequence. The original deposits are typical of those found in an island arc, like the present-day Japanese islands. The balance of the Precambrian outcrops are large exposures of an approximately 1 billion year old granite batholith that intruded the older rocks.
The overlying Paleozoic rocks include a Late Cambrian section dominated by thick sandstones and an Early Ordovician section of carbonates. Late Ordovician, Silurian and early Devonian rocks were deposited over the uplift area, but were removed by erosion and are now recorded only by pebbles deposited in karst topography sink holes at the top of the Early Ordovician carbonates. An early Pennsylvanian (Late Atokan) uplift of the dome was accompanied by the development of numerous radial faults. An easily identified angular unconformity between Early and Middle Pennsylvanian rocks is present on the northeast, north and west flanks of the uplift. The radial faults do not displace rocks younger than Early Pennsylvanian rocks. Lower Cretaceous carbonate rocks were deposited over the uplift. They have been removed by erosion and are now present on the west, south and east flanks.