Abstract

The Siluro Devonian rocks of the subsurface of West Texas and southeastern New Mexico consist of four lithologic units: the Fusselman Formation, the “Upper Silurian” unit, the “Devonian” unit and the Woodford Formation. The Fusselman Formation consists of limestone and dolomite. The “Upper Silurian” unit contains shale, limestone, and dolomite. The “Devonian” unit consists of chert and siliceous limestone. The Woodford Formation is composed of dark shale. The Caballos Formation, which crops out in southwest Texas, is composed of chert and novaculite. It is thought to be correlative with all four of the subsurface units.

During the early Paleozoic a shallow depositional basin called the Tobosa basin developed in western Texas and southeastern New Mexico. The basin was bounded on the north and east by low-lying land masses and opened to the south into the subsiding Ouachita-Marathon geosyncline. Throughout Silurian and Early and Middle Devonian times the basin became deeper, but more restricted areally. During Silurian time broad, thick carbonate shelves developed around the northern, eastern and western margins of the basin. A sediment-starved condition developed in the deeper parts of the basin where a thin sequence of micrites and green shales was deposited in Late Silurian time. By Early Devonian time most of the shelf areas were exposed and only the deeper portions of the basin remained submerged. During Early and Middle Devonian times deep-water cherts and siliceous limestones were deposited.

The Caballos Novaculite indicates that the water depth in the Ouachita-Marathon geosyncline to the south also reached its maximum during Silurian and Devonian times. In late Middle or early Late Devonian time most of the remaining area of the Tobosa Basin was exposed by mild uplift. In Late Devonian time the area was again invaded by the sea, and the dark Woodford Shale was deposited, overlapping all of the previous Devonian and Silurian deposits.