Late Mississippian gastropods of the Chainman Shale, west-central Utah

The Chainman Shale of Mississippian (Osagean to late Chesterian) age, well exposed in the Confusion Range of western Utah, has yielded a profusion of fossils during investigations conducted by the U.S. Geological Survey in the past 30 years. Conspicuous among these fossils are gastropods, which range in age from latest Meramecian to late Chesterian. In west-central Utah, not far from the State boundary, the Chainman outcrop belt stretches from Granite Mountain south to the northern part of the Needle Range, a distance of69 miles (110 km). The Chainman thickens from north to south; the section at Granite Mountain is 1,315 feet (401 m) thick and that at Jensen Wash in the Burbank Hills, 2,203 feet (671 m).

The rocks of the Chainman Shale record a general though irregular shallowing of the area from moderate depths of 330 feet (100 m) or so to quite shallow depths of perhaps locally little more than 3-6 feet (1-2 m). Most of the gastropods occur with ammonoids in a facies of shale or shale containing phosphatic limestone concretions, In this lutaceous facies, Glabrocingulum is predominant and Lunulazona and Retispira are common; these genera are represented by a succession of species. A thick limestone unit is present in some areas in the upper part of the formation, particularly in the vicinity of Skunk Spring, where it is 318 feet (97 m) thick. This limestone unit represents a calcareous shoal facies having an entirely different gastropod fauna, characterized by Catazona and species of Naticopsis.

The Chainman Shale could be easily zoned by gastropods, but we are not proposing such azonation. A framework of ammonoid and foraminiferal zones already is available, and we prefer to regard the gastropod assemblages as part of this framework. The assemblages are confined to the major ammonoid and foraminiferal zones, and only three of the gastropod species seem to range across major zonal boundaries. These species are Bellerophon (Bellerophon vespertinus Gordon and Yochelson and Straparollus (Euomphalus intermedius Gordon and Yochelson, both of which are present in Mamet Foraminifer Zones 17 and 18, and Bellazona polita n. sp., which locally seems to range from Mamet Foraminifer Zone 16s into the basal part of Zone 17.

Eight assemblages, seven of them in ascending stratigraphic order, are recognized within the gastropod fauna of the Chainman Shale; the eighth assemblage is a facies equivalent of the sixth highest. The seven mud-dwelling assemblages are characterized mainly by species of Glabrocingulum and Lunulazona, which together account for 80 percent of the gastropod specimens in our Chainman collections. The eighth assemblage, that in the shallow-water carbonate facies, is the one characterized by Catazona and species of Naticopsis.

The lowermost gastropod assemblage, that of Lunulazona nodomarginata (McChesney), includes 10 species and is restricted to the northern end of the study area, where it occurs in the upper part of the Goniatites americanus Ammonoid Zone, in beds equivalent to the lower part of Mamet's Foraminifer Zone 16i. We regard the entire G. americanus Zone as late Meramecian in age. All the zones higher in the Chainman are Chesterian in age. The second assemblage is that of Lunulazona costata Sadlick and Neilsen, which includes six gastropod species; it occurs in the Goniatites granos us Ammonoid Zone, equivalent to Mamet's Foraminifer Zone 16s.

Three gastropod assemblages are recognized within the Paracravenoceras barnettense Ammonoid Zone, equivalent to Mamet's Foraminifer Zone 17. The earliest, that of Lunulazona sadlicki, includes five species; the intermediate, that of Glabrocingulum hosei n. sp., four species; and the highest, that of Glabrocingulum confusionense n, sp., two species (the second being G. hosei).

Two laterally equivalent facies-controlled assemblages are present within the Cravenoceras hesperium Ammonoid Zone, most of which is equivalent to Mamet's Foraminifer Zone 18