***A drive-by earthcache as parking is sparse.***
Oak Cliff, not just an iconic Dallas neighborhood, yet an actual geological cliff formation, unique to Dallas county.
The northern line of the cliff, once covered by oak trees, runs along I-30 and the Trinity River, south and west of downtown. Given area development, the geological features are easy to miss.
Part of the Gulf Coastal Plain's Black Prairie region and slanting east in the same direction as the Austin formation's dip, the cliff is located on the gentle back slope of the White Rock cuesta. Area topography is of low rolling hills, except a narrow portion in the north and northeast where the Trinity River cuts the resistant lower Austin, leaving high bluffs flanking the river. The bedrock consists of chalk and limestone layers belonging to the Austin formation, of Upper Cretaceous age.
Exposures occur in creek beds, banks and road-cuts. However, the Austin is mostly concealed by alluvium of Quaternary age. In Dallas County this formation is approximately 600 feet thick.
The Austin formation is divisible into these four parts:
1) Chalk with inter-bedded 1-to-2-inch layers of limestone and calcareous shale.
2) Limestone and calcareous shale with an inter-bedded 1-to-2-inch layers of chalk.
3) Chalk with inter-bedded 1-to-3-foot layers of limestone and calcareous shale.
4) Pebbly mix of rock and chalk.
Along Cedar Creek, near the Dallas Zoo, the faults are more abundant than adjoining areas. Here, faulting reverses the dip of the strata so they are inclined westward. This reversal is local and of minor character - a poorly defined anticline flexure in the chalky Cedar Creek bed.
The suggested cause of the faulting may be due to differential compaction of the underlying incompetent Eagle Ford shale. The small-scale faulting and warping is perhaps the result of the competent Austin adjusting to the differential settling of the underlying shale.
LOGGING REQUIREMENT: Submit your answer to the following question.
At posted coordinates on S. Marsalis Ave., is a rare exposed section of the Austin formation making up the oak cliff. Based on your observation of the exposed rock layers, which of the four Austin sub-divisions discussed above is present?