Balcones, Luling, and Mexia Fault Zones in Texas

Abstract

The Balcones fault zone crosses the San Marcos arch and is located between the comparatively flat dip of the Edwards Plateau and the more steeply dipping beds of the Coastal Plain. It is composed principally of faults with downthrown side on the southeast. The throw on faults of this zone approximates 1,200 feet in northwestern Bexar County. The Luling fault zone lies coastward from the Balcones fault zone and is composed principally of faults with downthrown side on the northwest. In Caldwell County along San Marcos River a total throw of more than 1,500 feet is indicated by faults of the Luling type. The Mexia fault zone lies eastward and southeastward from the Luling zone and is characterized by faults with downthrown side on the southeast and by faults with downthrown side on the northwest. Both commonly occur with a graben of varying width between them. This zone of faults extends from north and northeast of Mexia, Limestone County, southwest through parts of Milam Burleson, Lee, Bastrop, Fayette, Gonzales, and probably through parts of De Witt, Karnes, Goliad, Bee, Live Oak, and Duval counties. Major movement along these faults probably occurred in late Catahoula or early Oakville (Miocene) time, thus affecting sedimentation of this period. Minor faults are present in Pliocene and basal Pleistocene formations. Some Recent movement is recorded.