ABSTRACT

The Pheasant-Francitas area, in the central part of the Texas Gulf Coast is typical of a portion of the Frio producing trend in which deposition was controlled largely by contemporaneous movement along faults. Widespread structural movement began near the end of Vicksburg time, resulting in regional down-to-the-coast normal faults in relatively shallow water near the ancient shore line. Both the alignment and displacement of these faults seems to have been affected by deep-seated structures which predate the faulting. These faults were active during deposition of the lower and middle Frio sediments, so that thick sections of mud and sand were deposited on the downthrown sides, while comparatively thin sections were being deposited on the upthrown sides of the faults. During early Frio time the fault movement formed miniature depositional basins on the downthrown blocks, centering in the areas of maximum displacement. These minature basins are characterized by dip and thickening of the middle and lower Frio sediments to the northwest into the controlling fault. Movement along the faults had diminished by late Frio time and its influence on deposition became minor. The upper Frio and overlying beds dip normally southeastward toward the Gulf of Mexico. The crests of anticlinal closures shift southeastward with increasing depth due to the rather abrupt thinning of the middle and lower Frio sections in that direction.