TRINITY GROUP/FORMATION

Age: Early Cretaceous Period, Comanchian Series
Distribution: Gulf Coastal Plain in southwest Arkansas. Parts of Little River, Sevier, Howard, Hempstead, Pike, Clark, and Nevada counties; Texas, Louisiana, Oklahoma
Geology: The Trinity Group is comprised of sand, gravel, clay, limestone, and evaporite deposits. Gypsum is mined commercially from this unit. Prominent members of the Trinity Group include the Pike Gravel Member, the Dierks Limestone Member, and the DeQueen Limestone Member. The Pike Gravel, the basal member of the Trinity Group, is a bedded, 0- to 100-foot thick, pale-yellow to medium-orange, predominantly gravel-bearing deposit. The Dierks Limestone Member is a 0- to 70-foot thick, interbedded, greenish, calcareous clay and gray, fossiliferous limestone present in the lower part of the Trinity. The DeQueen Limestone Member, which varies from 0 to 100 feet in thickness, is present in the middle part of the Trinity Group sequence and is composed of interbedded green and gray calcareous clay, limestone, gypsum and celestite. The limestones are thin-bedded and sandy, but crystalline and fossiliferous intervals are present. The DeQueen Limestone Member is also noted for a dinosaur track-way site discovered in a mine near Nashville in Howard County. The upper part of the Trinity Group is mostly fine-grained, cross-bedded sand, usually weathered reddish. Marginal marine fossils are noted from the Trinity Group and carbonized logs are found between the Dierks and DeQueen Limestone Members. The base of the Trinity Group rests unconformably on a surface of upturned and eroded Paleozoic rocks. The Trinity Group ranges up to 1,000 feet in thickness ,although it is usually much thinner.
Original Reference: R. T. Hill, 1888, Science, v. 11, p. 21
Type locality: Named for exposures on the Trinity River of Texas


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