Phanerzoic Development of the Llano Uplift

Thomas E. Ewing
Bexar Geological Surveys, San Antonio, Texas

The outcrops of Precambrian and Paleozoic rocks we see today in the Llano Uplift were made possible by six broad arches, which intersected at today’s uplift.

• The Concho Arch thinned the Lower Paleozoic strata along a broad NW-SE axis from the Panhandle to Llano; previously the area had received relatively thick Cambrian and Lower Ordovician sediments.
• In the Pennsylvanian, the Llano was at the south end of the Bend Arch, which formed at the western margin of the Fort Worth foredeep basin of early Pennsylvanian age and the eastern margin of the late Pennsylvanian to Permian West Texas Basin. Normal and transtensional faulting of early Pennsylvanian age (the Llano fault zone) was related to foredeep development (at least in part).
• The Llano was also at the eastern end of the Late Pennsylvanian to Permian Ozona Arch, which was a forebulge arch associated with the Val Verde Basin to the south.
• In the early Mesozoic, the Llano Arch uplifted and eroded Paleozoic sediments along a NE-trending rift shoulder related to Gulf opening; this arch exposed the junction of previous arches.
• After Albian transgression, the San Marcos Arch formed a SE-trending forebulge in mid- and Late Cretaceous time to the rapidly subsiding Maverick Basin and northeast Mexico. The southeast end of the arch received sediment (with numerous unconformities), but the northwest end was a land area during much of the Late Cretaceous.

Finally, the area was uplifted in the Neogene as part of the overall uplift of the Cordillera. Differential subsidence and slippage created the Balcones fault zone along the old Ouachita lines of weakness.