Slope Assemblage - Shale, graywacke, siltstone, chert, conglomerate, and limestone

Carbonaceous shale, black chert and argillite, graywacke, chert-pebble conglomerate, and detrital limestone are the primary lithologies described from all of the rocks assigned to this unit, representing a mixed slope and basinal facies. On other maps these rocks have been included in a variety of units including the foreland basin and Devonian siliceous and transitional rocks. Mapping and new biostratigraphic data gathered in the last 30 years have shown that many of these rocks mapped only as Devonian also contain Early Mississippian fossils, thus making it difficult to distinguish them from known lithologically similar Lower Mississippian rocks. Although this unit is everywhere structurally bounded by faults, a stratigraphic link to older Slope assemblage rocks is possible. These rocks are imbricated with units DCs, IPMcl, OCc, Ocq, DSt, Dc, and MDcl. Whether there is a definable continuous Early Mississippian through Devonian sequence within this unit is unknown, but is suggested in the Carlin-Piñon Range (Smith and Ketner, 1978). The Slaven Chert first described in the Shoshone Range (Gilluly and Gates, 1965) is black chert with carbonaceous shale beds 4–10 feet thick, limy brown-weathering sandstone as much as four ft thick with coarse fragments of chert, shale, greenstone, limestone, graywacke, feldspathic siltstone, and brown-weathering limestone 2–20 ft thick, and contains Late Devonian radiolarians (Boundy-Sanders, Sandberg, and others, 1999). The Mississippian Waterpipe Canyon Formation is a similar formation with basal medium-grained graywacke with interlayered black, carbonaceous shale; chert-pebble conglomerate; and bedded chert grading upward into sandstone layers with black, well-rounded quartz and a black, pyritic, phosphate- and barite-bearing, argillaceous matrix interlayered with black, platy, quartz siltstone and fine-grained graywacke interbeds. It contains Early Mississippian radiolarians (Peters, Armstrong, and others, 2003). In the HD Range in northeastern Elko County, an undated, light-gray weathering, brittle, black shale, structurally underlies the other thrust sheets and was referred to as the Chainman Shale by Riva (1970), but is included here in unit MDst. In the Windermere Hills a fissile black argillite with sporadic interbeds of quartz-chert arenite is poorly exposed with variable dips suggesting a complex structure (Oversby, 1972). In the Cockalorum Wash quadrangle along the Eureka-Nye County boundary, a pale yellow-brown, organic-detrital limestone contains quartz and chert grains locally interbedded with and succeeded upward by light-colored siliceous mudstone, claystone, and siltstone. The basal limestone contains mixed Mississippian and Devonian faunas; a thin chert from a higher zone has Osagean radiolarians (Hose, 1983). In the northern Adobe Range, this unit is recognized as dark siliceous rocks consisting of shale, argillite, and bedded chert. They are faulted and folded with sparse collections of Kinderhookian and Famennian radiolarians and conodonts (Ketner and Ross, 1990). The Webb Formation in the Carlin-Piñon Range is a gray siliceous mudstone with black to gray, tan-weathering, dense limestone in lenses near the top (Smith and Ketner, 1978). The argillite of Lee Canyon is a black siliceous argillite with a little black chert and very little conglomerate and sandstone near the top (Smith and Ketner, 1978). In the Sulphur Spring Range, the Bruffey sequence (Carlisle and Nelson, 1990) is a black chert pebble to boulder conglomerate and well-bedded gritty limestone, chert and limestone conglomerate, gray limy shale, and minor sandstone. Smith and Ketner (1978) describe the same rocks as gray limestone, sandy limestone, chert, and chert-pebble conglomerate. The Woodruff Formation from the same area is described by Carlisle and Nelson (1990) as a gray fissile shale, dolomitic siltstone, and black and brown bedded chert. Smith and Ketner (1978) describe the Woodruff as dark gray to black siliceous mudstone and chert, with lesser amounts of shale, siltstone, dolomitic siltstone, dolomite, and limestone. In the Shoshone Range, pale-red to pale-brown weathering, platy, silty dolomite interbedded with black chert in the basal 50 ft of rocks referred to as the Pilot Shale by both Gilluly and Gates (1965) and Wrucke (1974) is included here. In the southern Independence Range, this unit consists of fine-grained limestone, bedded chert, shale, conglomerate, and prominent ledges of limy sandstone with Famennian and Frasnian (Late Devonian) conodonts (Ketner, 1998). In Welches Canyon northwest of Carlin, this unit is gray to black limestone, fine grained, and thin to thick bedded with common sand- and silt-size clasts of quartz and chert grains. It also contains pebbles and cobbles of chert, and interlayered chert and siliceous shale as much as 50 feet thick (Evans, 1974). In the Snake Mountains, the unit is dark carbonaceous limestone apparently overlain by a light-gray, siliceous platy siltstone. Other outcrops that belong with unit MDst, but are not mapped separately on a regional scale from Slope or Basin assemblage units DCs and DOts include the Pinecone sequence in the Toquima Range (Coles and Snyder, 1985), and gold-bearing chert (Theodore, T., oral commun., 2006) mapped informally as the “Rodeo Creek Formation” (Peters, 1997b) in the Carlin area.
State Nevada
Name Slope Assemblage - Shale, graywacke, siltstone, chert, conglomerate, and limestone
Geologic age Lower Mississippian and Devonian
Lithologic constituents
NGMDB product
Counties Elko - Eureka - Lander - Nye