GULF COASTAL PLAIN 



655 



Luling Fault Zone. 



The Luling fault zone lies coastward from the Balcones zone and is composed 

 principally of faults with downthrown side on the northwest. Examples of this 

 zone are: (1) the Staples, Larremore (along which the Larremore oil field is 

 located), and Lytton Springs line of faults in Guadalupe, Caldwell, and Bastrop 

 counties; (2) the Luling fault in Guadalupe and Caldwell counties along which 

 the Luling oil field is located, and which extends northeastward cross Caldwell 

 County and into Bastrop County; (3) the Darst Creek-Salt Flat line of faults 

 along which fields of these names are located in Guadalupe and Caldwell 

 counties; and (4) the Somerset and Alta Vista faults in Atascosa and Bexar 

 counties. All of these faults have considerable length. The average throw ap- 

 proximates 450 feet. 



In Caldwell County along San Marcos River, a total throw of more than 

 1,500 feet is indicated on faults of the Luling zone. The faults of this zone have 

 the greatest throw and are most numerous from Travis and Bastrop counties 

 southwest through Bexar County, thus crossing the San Marcos arch. 



Mexia Fault Zone. 



Farther down the dip than the fault zones described above is the Mexia 

 zone of faults characterized by faults with downthrown side on the south- 

 east and by faults with downthrown side on the northwest. Both faults 

 commonly occur together with a graben of varying width between them. In 

 the Mexia area, the name Tehuacana has been given to faults on the northwest 

 side of the graben. The Mexia zone of faults extends from the vicinity of Mexia, 

 Limestone County, northeastward and eastward around Tyler basin. Faults in 

 southwestern Arkansas probably are a part of this zone. From Mexia southwest- 

 ward this zone of faults extends far into South Texas and offsets down the dip 

 at various points. At Mexia, Midway beds at the surface are cut by the faults; in 

 Lee County, Mount Selman; in Bastrop and Fayette counties, Cook Mountain 

 and Yegua; and in Gonzales County, Yegua and Jackson. 



The zone of faults extending southwest through parts of DeWitt, Karnes, 

 Goliad, Bee, Live Oak, and Duval counties may be part of this zone. Repre- 

 sentative faults are those along which oil and gas have accumulated in northern 

 Bee County in the vicinity of Pettus. Considering all of these faults as belong- 

 ing to the Mexia zone, in Texas alone the length of the zone is over 500 miles. 



In the region of Mexia many of the faults along which oil and gas production 

 is obtained from the Woodbine are en echelon, with the south end of the fault 

 at the north passing west of the north end of the fault at the south. This causes 

 closure in this direction. There is a structural high in the region of Mexia, and 

 south of this high the strike of the beds and the strike of the faults tend to con- 

 verge at the south end of each fault structure and tend to divirge at the north 

 end. This lack of effective north closure, plus absence of Woodbine sand, 

 probably is the reason for barren structures on the south toward the Falls 

 County regional low. 



Minor movements may have occurred in Cretaceous time, but the main 

 displacements came in late Oligocene (late Catahoula) or Miocene (early 

 Oakville) time (Weeks, 1945). The sediments of the Catahoula and Oak- 

 ville reflect the movements. Because certain Pliocene beds are displaced 

 less than older beds, it follows that some movement in places has 

 occurred in post-Pliocene time. 



The structure of the Coastal Plain from the Bend arch of central Texas 

 eastward across the fault zones to tire Sabine uplift is shown in the cross 

 section of Fig. 41.5. 



Flexure Fault Zones 



Paralleling the coast of Texas and shoreward of the Miocene boundary 

 (Fig. 41.1) are three flexure and fault zones. These are called flexure 

 zones or flexure fault zones, and they are shown in the lower cross 

 section of Fig. 41.2. The Gulf side is down 500 to 1500 feet, but the 

 unusual aspect is the reverse (?) drag aspect of the beds on the down- 

 thrown side. This has been interpreted as sagging or slumping of the 

 beds incident to the tendency of fissure opening as down-tilting of the 

 block toward the Gulf occurs. The faults die out upward in the Miocene 

 and Pliocene sediments and hence are about mid-Tertiary in age. Need- 

 less to say the flexure fault zones are the sites of very productive belts 

 of oil fields. 



Tyler or East Texas Basin 



East of the Balcones and Mexia fault zones is the Tyler basin, so 

 called on the Tectonic Map of the United States, but often named the 

 East Texas basin. See cross section of Fig. 41.5. It is the result of gentle 

 dips eastward off the Bend arch of central Texas and westward off the 

 Sabine uplift. It consists of a thick Tertiary and Upper Cretaceous se- 

 quence of beds. The Lower Cretaceous succession has not been pc ni- 

 trated in the deeper parts of the basin, nor has the mother salt that 

 has spawned a score of salt domes within the basin. 



Sabine and Monroe Uplifts 



A large gentle dome in easternmost Texas and northwestern Louisiana 

 is reflected in the surficial Tertiary strata, and is known as the Sabine 



