Geologic History of the Rio Grande Region

The Lower Rio Grande border region or "Rio Grande Embayment" (South Texas-Coahuila-Nuevo Leon-Tamauli-pas) lies at the junction of the Gulf of Mexico and Cordilleran tectonic provinces. To study the area and explore for mineral resources in the region, we must understand the complex history of the two contrasting tectonic regimes.

Pre-Middle Triassic: The 1100-Ma Llano Craton, the southern extension of Laurentia, was rimmed to the east and south by the east- to northeast-trending Late Paleozoic Ouachita thrust belt, caused by northwestward closure of an ocean or marginal basin. To the southwest, inferred Ouachita hinterland crust was covered with Permian volcanics and intruded by Permian and Triassic igneous rocks-an inferred southeast-trending "Cordilleran" Permo-Triassic volcanic and plutonic arc. South of the Parras-Galeana linear, deformed Paleo2oic and Precambrian rocks are related to North America, but were likely formed far northwest of their present location.

Opening of Tethys (Late Triassic-Late Jurassic) created a complex mosaic of crustal blocks, with "tiles" of high and low extension. At least four long northwest-trending discontinuities bound the mosaic tiles (Frio River Line/FRL; La Babia-Zapata Zone; San MarcosrSan Carlos Zone; Parras-Galeana Zone); they are inferred transcuirent fault zones with hundreds of km of left-lateral shear. Thick Callovian evaporites were deposited over highly extended crustal blocks in the Louann (Gulf Coast Basin) and Minas Viejas (Monterrey Trough) salt basins.

Border Rift Zone faulting (Late Jurassic(?)-Early Cretaceous) extended from Coahuila northwest through the Chi-huahua to the Bisbee Basin of southeastern Arizona. The less-extended tiles in Coahuila were reactivated as block uplifts, shedding coarse debris into the surrounding troughs. To the northeast, regional subsidence of the Gulf Coast Basin led to the creation of the peripheral graben system, and to the rise of salt pillows and diapirs.

Regional southwest focused subsidence (A1bian-Paleogene) created a major sedimentary basin, now highly eroded. Subsidence caused regional shelf-margin retreat in Aptian-Albian time. All later units (including significant Eagle Ford source rocks) thicken markedly southwest of the FRL hingeline. In latest Cretaceous time, clastic sediments filled the basin from the west, forming coal-bearing deltaic and shoreline complexes, and mobilizing salt structures northwest of Monter-rey. Over five km of sediments were deposited above the presently-exposed rocks of the Monterrey area, possibly including Oligocene volcanics.

Northeast-directed "Laramide" compression (Late Cretaceous-Eocene) created a wide variety of folds and thrust faults. Folding diminished to the northeast, just crossing the Rio Grande. Basement blocks became structural buttresses with broad folds. The northern Minas Viejas salt basin was deformed by high-relief, salt-cored folds flanked by high-relief, basement-involved thrusts. The Monterrey salient of the Sierra Madre Oriental (tight, very high relief, Jurassic-cored folds) filled the southern part of the evaporite trough. The deep "Rosita trough" in South Texas, caused either by rafting of Creta-ceous basinal carbonates or by evacuation of a salt wall, was filled with growth-faulted Lower Eocene deltaic sediments.

Large-scale, post-27 Ma uplift and exhumation was coupled with detachment faulting along the Gulf margin. The La Popa-Monterrey area was eroded to depths of 5-7 km during late Oligocene and early Miocene time. This erosion extended across the Rio Grande, accounting for higher coal ranks there. Oligocene-Miocene alkaline igneous rocks were intruded, with coeval volcanics preserved in the Tras-Pecos Texas region (NW) and in the Sierra de Tamaulipas (SE). The Edwards Plateau was also uplifted in Miocene time, bounded by Balcones extensional faults on the SE. An Early Oligocene shale-based detachment, expanding the Vicksburg Formation deltas, was followed by complex Late Oligocene to Miocene growth faulting in South Texas and adjacent Tamaulipas.

Implications for resource exploration in the Lower Rio Grande area: 4.U The Triassic to Jurassic history of transcur-rent faults and rift basins must have created complex structures, now masked by the post-Oxfordian stratigraphy. 42 Because of the southwest-focused Cretaceous subsidence, hydrocarbons may have migrated northeastward from the Mexi can "oil kitchens" into basin-edge traps in South Texas. AW Increased maximum burial depths towards the Rio Grande in South Texas suggest that the oil generation window of the Austin/Eagle Ford section becomes shallower in that direction, and may reach the surface in northeastern Mexico "Small crustal blocks similar to those of northeastern Mexico may be found beneath South Texas. Such blocks could be exploration targets for Smackover reservoirs, and could affect subsequent sedimentation. S Salt activity south of the Frio River Line is different than the standard Northern Gulf history. The absence of a peripheral graben system is consistent with southwest-directed subsidence. Exploration opportunities in and around the salt structures of South Texas differ from those to the northeast.