P.L. Inderwiesen, 1983. "Contemporaneous Fault System, Texas Gulf Coast", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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The seismic line depicting the contemporaneous fault system is located in Brooks County, Texas (index, Figure 1). The orientation of the line is in an east to west direction nearly perpendicular to the trend of the Sam Fordyce - Vanderbilt fault zone. The western-most part of the seismic line is approximately 1.6 km (1 mi) from the fault zone.
The Sam Fordyce - Vanderbilt fault zone, known also as the Vicksburg fault zone, is thought to be the location of the Jackson (upper Eocene Series) shelf-slope break (Hardin and Hardin, 1961) across which the Vicksburg and Lower Frio (Oligocene Series) formations thicken to the east. The thickening of the formations occurs on the downthrown sides of regional contemporaneous faults. Movement along such faults is contemporaneous with the accumulation of sediment. The faults form on the landward sides of major depocenters (Bruce, 1973).
The trend of the Sam Fordyce - Vanderbilt fault zone is in a northeast to southwest direction except in Starr County, Texas, where the system turns southeast (Figure 1). Murray (1961) describes the fault system as being made up of many individual contemporaneous faults, striking northeast to southwest, which are partly en echelon and have normal faulting down-to-the-basin. The fault displacements average thousands of feet. Also, in areas where more than one fault exists, the updip fault(s) are usually younger suggesting that faulting progresses away from the depocenter.
Loucks (1978) demonstrated that Vicksburg sands are most prevalent in Hidalgo County of the Lower Texas Gulf Coast (Figure 1), and diminish toward the Upper Texas Gulf Coast. Also, Boyd and Dyer (1966) noted that the Vicksburg and Frio formations thicken the greatest in the Rio Grande alley depocenter in Brooks, Hidalgo, and Kenedy counties, Texas. Both papers concluded that the ancestral Rio Grande River was the major source of clastic sediments.
Regional stratigraphy of the Middle and Upper Tertiary is shown in Figure 2. The Jackson formation is a massive, low-density/high-pressured shale on the east side of the Sam Fordyce -Vanderbilt Fault Zone. The Vicksburg and Frio formations are similar sandstone/shale sequences and therefore require the use of index foraminifera for identification (Loucks, 1978). The overall regional dip is toward the Gulf Coast Geosyncline with sandstones grading to shales in the downdip or easterly direction.