The Pheasant-Francitas area of southwestern Matagorda County and southeastern Jackson County, in the central part of the Texas Gulf Coast, is typical of that part of the Frio trend in which deposition of the lower and middle Frio strata was controlled largely by faults along which movement was contemporaneous with sedimentation. Widespread structural movement beginning near the end of Vicksburg time resulted in regional down-to-the-coast faults in relatively shallow water near the ancient shore line. Both the alignment and displacement of these faults seem 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 miniature basins are characterized by dip and thickening of the middle and lower Frio sediments toward the northwest into the controlling fault. Movement along the faults had diminished by late Frio time and its influence

End_Page 2514------------------------------

on deposition became minor. The upper Frio and overlying beds dip normally southeastward toward the Gulf of Mexico. The crests of structural closures within the miniature basins shift with depth due to the rather abrupt changes in the thickness of the middle and lower Frio sections.

End_of_Article - Last_Page 2515------------