Geology of the Preston Hollow Quadrangle (Dallas, Collin, and Denton Counties, Texas)

Bedrock in the Preston Hollow Quadrangle consists of the upper Eagle Ford shale and the middle marl and lower chalk members of the Austin formation. The area is situated upon a gentle homocline with the beds dipping 50 to 60 feet per mile to the southeast. The Eagle Ford is principally shale with interbedded thin layers of shaly limestone and sandy shale. The Austin formation is readily divisible into a lower chalk, which is a rather hard unit, and an overlying softer marly unit. There are, however, many hard chalky beds in the marl. Many thin layers of an argillaceous material which weathers rapidly make the bedding in both units of the Austin very pronounced. What are interpreted as scour-and-fill structures indicate rather strong submarine currents during Austin time. Minor normal faults are numerous in the harder units. Beginning possibly as early as Tertiary time subaerial cycles of erosion and deposition formed three clearly defined terraces. The uppermost caps the highest hills and may be older than the Pleistocene. The lowest two are probably Pleistocene and were definitely formed by the Trinity River