654 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



one of very gentle seaward dips, except where locally disturbed by salt 

 plugs, high-angle faults, and gentle warpings. A southward-migrating 

 trough line has been postulated such that the maximum thicknesses of 

 the different stratigraphic units are not superposed. 



STRUCTURAL GEOLOGY 



Balcones and Luling-Mexia Fault Zones 



A complex assembly of faults follows approximately the border of the 

 Tertiary and Cretaceous formations of the Coastal Plain in Texas. See map, 

 Fig. 41.1. The zone of faults is located between the Edwards plateau of 

 comparatively flat Cretaceous strata, and the seaward dipping Tertiary 

 beds of the Coastal Plain. It crosses the San Marcos arch, which is a broad 

 southeastward-plunging nose of the Llano dome. 



The zone of faults has been divided in several ways. The Tectonic Map 

 of the United States shows the southwestern part to be called the Bal- 

 cones fault zone, and the northeastern part the Luling-Mexia. Weeks 

 (1945) believes the Luling and Mexia are distinct and describes the three 

 belts as follows: 



Balcones Fault Zone. 



Extending through the vicinities of the towns of Georgetown, Austin, San 

 Marcos, New Braunfels, and some distance north of the towns of San Antonio, 

 Hondo, and Uvalde is the Balcones fault zone with downthrown side principally 

 on the southeast. 



This zone of faults is located between the comparatively flat dip of the Ed- 

 wards Plateau and the more steeply dipping beds of the Coastal Plain, and 

 crosses the San Marcos arch, a broad nose which plunges southeastward from 

 the Llano uplift. In the vicinity of Austin, Travis County, the total throw across 

 the Balcones zone of faults approximates 900 feet; in northwestern Bexar 

 County, 1200 feet; in northeastern Uvalde County, 500 feet; and in southwest- 

 ern Uvalde County, 200 feet. In Kinney County the Balcones zone of faults 

 dies out. 



Fig. 41.4 Distribution, thickness, and structure contour maps of the Mississippi embayment and 

 delta regions, after Murray, 1947. The black areas are the areas of outcrop, the solid lines are 

 structure contours, and the dotted lines are isopachs. 



