ABSTRACT

A subsurface study of Buda through Wilcox strata in Bastrop County reveals that most of the stratigraphic units thin toward the San Marcos arch and the outcrop. A "thin-down" channel exists in the lower part of the Austin Group, probably because of relatively stable anoxic conditions in the channel area. The Travis submarine volcanic activity occurred along fracture and fissure systems in the Austin sea; beach calcarenites formed in various locations around the volcanic mounds. Four mini-shelf packages in the Bergstrom Formation (Taylor Group) progressively downlap and pinch out onto the top of the Sprinkle Formation. A toplap type, regional angular unconformity exists at the top of the Taylor Group. The Navarro Group is composed of a number of coarsening-up, subparallel, divergent, and irregularly bedded cyclic shale packages which sometimes contain poorly sorted, thin sands. The Tertiary-Cretaceous boundary is probably conformable. The Midway Group marine shale grades upward into the marginal marine sediments of the lower Wilcox.

The major episodes of displacement of the Luling faults and the Smithville fault occurred sometime after the Early Eocene. The Paige fault area was unstable from Early Cretaceous to at least Early Eocene time.

The term "cryptoformity" is recommended for stratigraphic boundaries which are the apparently conformable equivalents of major unconformities, especially important Division- or Sequence-bracketing surfaces. Condensed zones and surfaces to toplapping, onlapping, and downlapping may also be indicated by a cryptoform symbol and be described as cryptoformable.