The Webb Formation is named for exposures near Webb Creek in Sec. 19, T. 31 N., R. 53 E., and in the SE 1/4 Sec. 13, T. 31 N., R. 52 E., in the Carlin-Pinon Range (Smith and Ketner, 1975).

The Webb Formation is Early Mississippian (Kinderhookian) in age based upon an abundant conodont fauna. It is an age equivalent of the Joana Limestone, Tripon Pass Limestone and the lowest portion of the Chainman Formation which locally overlies the Webb (Smith and Ketner, 1975). In the Carlin-Pinon Range, the base of the Webb is an erosional unconformity where it lies on the Devils Gate, Vinini or Woodruff Formations (Smith and Ketner, 1975). The upper contact with the Chainman Formation is both gradational and abrupt.

The Webb Formation is composed of variably silicified, thinly-bedded and laminated, gray, brown, or pink weathering claystone and mudstone (Smith and Ketner, 1975). Near the top of the formation, the mudstone is black and finer-grained and more argillitic than below. In thin section the mudstone consists of angular grains of quartz and chert, and in hand sample it is commonly very siliceous. Within the mudstone, medium to coarse-grained, brown sands are locally interbedded in thin layers, as is black to gray, dense, thin-bedded limestone in lenses from 1 to 30 feet in thickness and up to 1.5 miles long (Smith and Ketner, 1975). Beds of gray to black chert 1 to 3 inches thick are present locally in the Webb, as are barite-bearing claystone nodules that are concentrated near the top of the formation.

In the Carlin-Pinon area, the Webb Formation is very difficult to distinguish from the Woodruff Formation, and in fact may be miss-mapped locally. Smith and Ketner (1975) suggest that the Woodruff contains more chert, has dolomitic siltstone that is absent in the Webb, and contains no coarse-grained sandstones similar to the Webb.

The Webb is also present to the south in the Fish Creek Range, where it has been mapped in the Cockalorum Wash Quadrangle by Hose (1983). It is a pale yellow brown, organic detrital limestone with quartz and chert grains, interbedded with and succeeded upward by, light-colored siliceous mudstone, siltstone and grit, and thin chert beds which are concentrated in the upper portion of the formation. The unit contains both Kinderhook and Osage foraminifera and algae, and is considered allochthonous (Hose, 1983). In the Dobbin Summit area of the Monitor Range, Wise (1977) correlated about 450 feet of allochthonous, thin-bedded, calcareous argillite, calcareous shale, and minor impure thin to medium-bedded sandstone and siltstone with the Webb Formation.

The Webb varies from 20 to 800 feet thick in various partial sections exposed the Pinon Range with 735 to 800 feet being a typical maximum thickness (Smith and Ketner, 1975). In the Monitor Range about 450 feet of possible Webb are exposed (Wise, 1977).

The Webb is mapped within the Pinon and Fish Creek Ranges, and may be present in the Monitor Range.

The Webb Formation represents relatively rapid marine deposition of fine-grained marine clastics. The lack of fossils in these laminated sediments suggests hostile bottom conditions. The Webb was probably deposited below an oxygen minimum zone. Deposition of the para-autochthonous to allochthonous Webb overlapped the initial deposits of the Chainman Formation (Smith and Ketner, 1975). The exact nature of this "overlap" has been the subject of hot debate. It now appears that the Webb is allochthonous and structurally overlies the Chainman Formation (Johnson and Pendergast, 1981).