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Subsurface Lower Cretaceous Stratigraphy, Central Texas
Lower Cretaceous strata in central Texas are divisible into genetically related rock bodies. The Lower Trinity Group contains a lower terrigenous near shore deposit (Hosston formation) which interfingers with an overlying shallow water, offshore limestone-dolomite sequence (Sligo limestone); this Group may contain a barrier reef section downdip.
The Middle Trinity Group, a sequence of grey to black shale and limestone, does not intercalate with either the underlying or overlying rock bodies. Updip, near the Texas craton hinge line, it is separated from the overlying and underlying rocks by disconformities.
Downdip the Stuart City reef, a coral rudistid limestone, exists apparently continuously between the top of the Middle Trinity Group and part of the Georgetown formation of the Washita Division. Back reef deposits are separable into Upper Trinity Group, Fredericksburg Group and part of the Washita Division.
The Upper Trinity Group consists of the basal Hensel sand (mostly updip outside the area of study) which intercalates going downdip with the usually dolomitic Glen Rose limestone. The middle part of the lower Glen Rose contains a reefal limestone section which is restricted to the area near and over the San Marcos arch and near the Stuart City reef. The upper Glen Rose is a sequence of dominantly evenly-bedded, dolomitic, foraminiferal limestone.
The Fredericksburg Group includes the East Texas basin lagoon deposits of the Walnut and Comanche Peak formations and the rudistid limestone deposits of the Lower Edwards. The lower Edwards intercalates with part of the lower units of the Walnut over the San Marcos arch and with successively higher units toward the East Texas basin; lower Edwards and Comanche Peak produce a similar pattern.
The basal unit of the Washita Division is a widespread black shale (Kiamichi) and shaley limestone middle (Kiamichi) Edwards which is absent by onlap around the Belton high (proposed new term). The dolomitic, rudist limestone of the upper Edwards interfingers with the lower beds in the Georgetown limestone north of the San Marcos arch. The remainder of the Georgetown and the overlying Del Rio clay are laterally continuous throughout the area of study except in Southern Gonzales County where the Del Rio may be absent. The Buda interval pinches out near the edge of the Texas craton and thickens going southeast and downdip. All stratigraphic units studied thicken into depositional troughs in southeastern Gonzales, northwestern Karnes and central Fayette counties; thinning occurs on the southeast flank of the trough; correlation of units from the trough margin to the Stuart City reef area is difficult, if not impossible, with presently available data.
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