The Geology of Texas - Vol. 1

WALNUT FORMATION

Nomenclature. -- The Walnut clays were named by Hill (772, p. 512; 780, p. 86) to indicate the yellow clays, flaggy limestone, and shell masses of Exogyra texana and Gryphaea marcoui, lying above the Paluxy sands near Comanche Peak. The type locality is at Walnut Springs, Bosque County.

Stratigraphic position and contacts.—In north-central Texas the basal part of the type Walnut is invaded by sand, up to the level of the prominent Gryphaea marcoui shell aggregate, and the basal portion is thus indistinguishable from Paluxy sand. It is therefore probable that, north of Fort Worth, some Walnut sand may be included in the Antlers formation; at least the upper Antlers contains Exogyra texana and Ostrea crenulimargo, which are known from the Fredericksburg group. The Walnut-Comanche Peak contact is apparently conformable. In the southern part of the Edwards Plateau and in northern Trans-Pecos Texas, the Walnut is either reduced locally to an insignificant "break" (receding ledge) less than 1 foot thick, or else is not positively identifiable. In the southern Pecos Valley, the underlying formation is Glen Rose limestone, but near Fort Stockton it is the basement sand. These contacts appear to be concordant, but whether they are conformable has not been discovered.

Facies. -- The formation is in the neritic facies over most of its extent, and consists of clays, limestone seams and shell aggregates. From Fort Worth to the Red River valley its basal part is sand; this is possibly true at places in Trans-Pecos Texas, as at Kent. 

Areal outcrop, local sections.--The section near the type locality represents only an extreme of the Walnut formation in Texas. To the north the base rapidly becomes sandy, and the only persistent feature is the aggregate of Gryphaea marcoui shells; to the south the oyster shell beds first became unconsolidated (in Williamson County), and then vanish, leaving only an insignificant and indistinctive marl, containing scattered oyster shells and few or no diagnostic markers, and, finally, in the southern part of the Edwards Plateau, only a thin "break" or receding ledge.  

Near Goodland, Oklahoma the probable equivalent of the Walnut is 3 to 6 feet of persistent, hard, thin, coquina-like limestone, with interbedded thin layers of dark marly shale (1530, p. 135). On Red River, north of Gainesville, the Walnut is represented by 4 feet of marly clay containing Exogyra texana and Gryphaea marcoui (189, p. 15). Near Marysville, the Walnut consists of a small thickness of marl and marly limestone with the usual fossils. At Preston, and in Little Mineral Creek, Grayson County, the Walnut is represented by a few clay layers containing Exogyra texana (803, p. 208). The difficulties in mapping and identifying the Walnut in this district have been most clearly realized and expressed by Winton (1791, pp. 16-18) in treating the Denton County area. The top of the formation is a widely persistent, consolidated, and mappable Gryphaea marcoui shell aggregate, 16–18 feet thick. Under this cap are sands, sandstones, and a few clay seams, which continue down to a coarse red sandstone (Paluxy), which lithologically is distinguishable from the finer-grained and lighter-colored Walnut sands. On the Clear Fork of the Trinity west of Fort Worth, the shell aggregate of the Walnut is 5 to 8 feet thick, and is underlain by about 100 feet of Walnut sands. At Decatur (803, p. 203), the Walnut is about 27 feet thick, with only a thin shell agglomerate at the top. Wells in Tarrant and Johnson counties show about 100 feet of Walnut, with some water sands. Logs of Dallas County wells indicate 50 feet or more of the formation. In Johnson County, both outcrop and wells indicate a thickness of about 100 feet, of which the upper 25 feet is shell aggregate and the lower part mostly soft light-colored sands, with a few thin ledges of grayish sandstone, which form a water reservoir of small volume (1790, p. 20). At Comanche Peak, the formation consists of 30 feet of shale, marly limestone, and sands (803, p. 209) . Various wells in eastern Hill County show 145–215 feet of Walnut; in western Hill County, 120–135 feet.