Structural evolution of a detached delta system—the Vicksburg of south Texas

Abstract

The Oligocene Vicksburg formation of South Texas was deposited as a coarse clastic delta system on the fringe of the uplifted Sierra Madre mountains in Mexico. Earliest Vicksburg sediments were initially deposited in a simple delta lobe, which detached due to gravity from underlying Eocene formations and crept seaward as a block, creating a depositional center. The upper Early Vicksburg contains expanded sets of deltaic sediments which infilled a depression formed by the initial movement. Downlap surfaces indicate that the expansion was oriented northeast-southwest. Middle Vicksburg uplift in the Sierra Madre mountains to the south caused erosion and a change in strike direction to northeast-southwest and a significant change in structural style. West to easterly oriented ridges formed in conjunction with significant north-south, en-echelon, counter-regional growth faults. The ridges later accumulated hydrocarbons in deformed sandstones. Counter-regional faults formed as a deeper detachment within the underlying Eocene during Late Vicksburg time. This resulted in formation of two different structural provinces. The Slick Ranch Field province illustrates a complex extensional zone dominated by horst and graben structures. In contrast, the McAllen Ranch Field province is simpler, without Late Vicksburg extension. One of the keys to successful exploitation of the Vicksburg is unraveling extensive, low angle, discontinuous fault sets that dominate the majority of rotated fault blocks. These faults formed during growth and rotation of individual slump structures. Faulting occurred when curvature on listric fault planes was reduced and the slump rotated to a horizontal position. When rotation was complete, all the curvature from the base of the slump was transposed to the upper surface. Subsequent low angle faults dissected the crest of the slumps. The study area, in Starr and Hidalgo counties, South Texas (Figure 1), borders the Rio Grande River and Mexico. More than six trillion ft3 of gas and …