THE TILTING OF TEXAS

The Edwards Group of central Texas consists of shallow marine/tidal flat carbonate /evaporitic facies that indicate much of the region was very near sea level at 105 to 100 Ma. Near downtown Austin, these strata are near 150 m elevation. Just west of the Balcones escarpment, these strata are near 240 m, an elevation a few tens of meters higher than the shoreline at 100 Ma according to the Exxon or Hallam global sea level reconstructions. The equivalent Cretaceous strata near Junction are at ~550 m elevation, near San Angelo at ~ 700 m, and near Midland at ~800 m. This indicates much of west Texas has risen since 100 Ma creating a regional tilt of about 0.5° west of a hinge near the Balcones escarpment. The regionally uniform uplift of west Texas and crustal tilting caused the major rivers across the state to have similar size and near-parallel, southeastward paths that, combined with longshore drift, evenly load the shelf.

The 100± Ma shoreline facies of the Edwards Group that has risen 500 m or so in west Texas predates possible subsidence related to the 80 to 40 Ma flattening of the subducting Farallon plate. The Edwards strata were deposited when the Sevier thrust belt was causing crustal thickening. However, the eastern limit to the thin-skinned crustal thickening and possible load-induced subsidence is in western Arizona-Utah. Laramide thick-skinned deformation propagated into west Texas in the very latest Cretaceous to Eocene. It is proposed that the tilting of much of Texas is a manifestation of lower crustal flowage towards the east driven by Laramide crustal thickening in the west. As thickening by lower crustal flow would have been isostatically compensated, the magnitude of tilting indicates the crust was inflated by about 3 km near Midland and petered out to near zero roughly parallel the trend of the Balcones Escarpment. The tilting has caused an enormous quantity of sediment to become transported across Texas in the rivers flowing towards the Gulf of Mexico. The sediment has buried the Edwards Group southeast of Austin and asymmetrical loading has mobilized the thick Jurassic salt to spread seaward, creating the spectacular thin-skinned normal fault system of southeast Texas.