ABSTRACT

A large mid-Eocene channel near Yoakum, Texas, is evident from a study of contour maps and cross sections assembled from the information provided by the electrical logs of more than 50 wells. The channel or canyon, clearly erosional, is filled largely with silty shale, which contrasts sharply on the well logs with the sandy character of the typical Wilcox strata through which it was gouged.

This canyon can be traced for more than 50 miles, from its mouth evidently near the southeast line of Lavaca County up-did in a north-northwesterly direction to the outcrop of the Wilcox in Bastrop County. At its maximum known development near the town of Yoakum it has a width of 10 miles and a depth of 3,000 feet.

A completely satisfactory explanation of the origin is difficult. The following factors are indicated:

A. The presence of a major stream.

B. The great differential in thickness of the Wilcox not known to exist, attaining as much as 8,000 feet at the outer edge of the former continental shelf, which placed an unstable mass of sediments adjacent to deep water and a subsiding sea floor.

C. Slumps and slides at the mouth of the large stream, triggered in part by the fault action known to have been prevalent during Wilcox time, stoped inland guided by the stream channel and provided a source of turbidity currents powerful enough to cut a gentle gradient to the sea bottom.

D. An abrupt transgression of the sea led to rapid filling of the gorge and deposition of a thin blanket of shale over a large area outside the channel. This was followed by regression during which the thick and extensive sands of the Carrizo were deposited. These clean massive sands of the uppermost Wilcox entirely obliterated any evidence of the great channel below.