Geologic Wonders of West Texas: a preview of Don’s new book
23 August, 2004
Speaker: Don McGookey
Don gave a 2½ hour overview of his new book, soon to be printed. His visual tour included the Ouchita-Marathon uplift, Llano uplift, Permian basin, Llano Estacado, Big Bend, and Trans-Pecos region. The Paleozoic basins of west Texas share many genetic similarities with the Paleozoic basins of the southern Rockies. The ancestral Rockies-Colorado trough is part of this system. Significant wrench fault couplets have been recognized.
Much of this region shares a broad marine transgression in the Paleozoic, retreat of seas and erosion in the Triassic-Jurassic, inundation again by the Western Interior Seaway extending from the Gulf of Mexico to the Arctic in the Cretaceous, and non-marine deposition in the Cenozoic.
The dividing line between west Texas and the Gulf Coastal Plain is a 1300 mile Paleozoic buried mountain chain—the Ouchita-Marathon uplift—extending from Mississippi through Arkansas, SE Oklahoma, Texas, into Mexico. Its tectonic style is similar to the Alps foldbelt, where a microcontinent-continent collision has produced an interior metamorphic zone associated with 1000’s feet of flysch. While the frontal zone, with vergence to the northwest, is exposed around Marathon, Tx., the interior Ouchita zone is well hidden; only one small outcrop of metamorphic rocks has been found in Mexico. One of the more interesting and puzzling units is the Devonian novaculite, essentially pure silica, perhaps formed of radiolarian ooze.
The Llano uplift in the Texas hill country is an exposure of 1 billion year old Town Mountain granite, about the same age as the Pikes Peak granite. It is quarried as high quality building stone. Older metamorphic rocks are also present. The mostly granitic interior of the uplift is a topographic low, as the surrounding Paleozoic-Cretaceous rocks are more resistant to erosion. This feature was uplifted in the early Pennsylvanian, and onlapped by thick Cretaceous carbonates. Enchanted Rock in the granitic interior is a classic exfoliation dome.
The Permian basin—consisting of the Delaware basin, Central Platform, and Midland basin—contains up to 25,000 feet sediment. This is a prolific oil and gas basin, due to many stratigraphic and structural trap types, multiple source rocks, and efficient Permian evaporitic top seals. The Central Basin Platform captures a large fetch area with respect to hydrocarbon migration. About 25 % of US production has come from this basin. One well in the Yates field tested 256,000 BOPD. The early Cambrian to Mississippian fill contains good subcrop traps. Sea level fluxuations in late Mississippian, due to southern hemisphere glaciation, produced many stratigraphic traps. The prolific Permian carbonate reefs were developed near the equator. Growth was periodically shut off from sediment infilling, generating additional turbidite targets in the deep basin. The Permian San Andres-Greyburg, because of its position below the major end-of-Permian top seal, contains over half of the basin production. Carlsbad Caverns’ genesis is interesting because the dissolution of carbonate was from the bottom up, involving the formation of sulphuric acid. This involved a delicate balance between the presence of organics (from migrating hydrocarbons), gypsum, fresh water, and connate water.
An astrobleme, 19 miles south of Fort Stockton, TX., has been recognized. It is 8 miles in diameter, and contains the typical indicators of bolides such as shocked quartz, fracturing, and a central rebound crater. There is a deep gas field under the astrobleme, where the top seal has not been breached.
The Llano Estacado of the high plains (Nebraska, Oklahoma, Kansas, Texas) contains the exceptional aquifer of the Miocene Ogallala formation. This apron of sediment, derived from the uplifting Rockies, is tilted to the east. Springs are present.
Trans-Pecos igneous activity started about 47 ma as low relief shield volcanoes. There are 6 volcanic centers in the Davis Mountains (37-35 ma) where volcanic flows have substantially changing composition. The Solatario is a huge laccolith. The Chisos Mountains is a late Eocene-early Oligocene volcanic center. The geology of this region is quite varied, setting up fantastic scenery.