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William Aubrey Cobban

Publications and source records attributed to William Aubrey Cobban.

At least 19 recordsLinked to original sources
USGS1984

Molluscan record from a Mid-Cretaceous borehole in Weston County, Wyoming

A core borehole in the Osage oilfield on the west flank of the Black Hills uplift in eastern Wyoming penetrated, in decending order, most of the Carlile Shale, all of the Greenhorn Formation, and the upper part of the underlying Belle Fourche Shale. Molluscan fossils are abundant in parts of the core and indicate an age span of early Coniacian to the middle Cenomanian. Most of the fossils are bivalves and ammonites; gastropods are scarce. Fossils in the cores indicate the following zones: Lower Coniacian Cremnoceramus? waltersdorfensis Upper Turonian Scaphites coroensis S. nigricollensis S. whiifieldi S. warreni Middle Turonian Collignoniceras woollgari Lower Turonian Mytiloides mytiloides Mytiloides aff. M. duplicostatus Upper Cenomanian Sciponoceras gracile Dunveganoceras albertense D. pondi Middle Cenomanian Acanthoceras amphibolum

Professional Paper↗
USGS1983

Stratigraphy and paleontology of Mid-Cretaceous rocks in Minnesota and contiguous areas

PART A: Molluscan fossils are locally abundant at outcrops of Upper Cretaceous rocks in eastern North and South Dakota, northeastern Nebraska, northwestern Iowa, and western and northern Minnesota. Other Cretaceous mollusks have been found in the glacial deposits in Minnesota, Iowa, and Illinois. The oldest well dated marine mollusks are of earliest late Cenomanian age and occur in northwestern Iowa. Mollusks of marginal marine and nonmarine environments in northwestern Iowa and south-central Minnesota are probably of slightly younger late Cenomanian age. The youngest mollusks treated in this report are bivalves of Santonian age found in the Niobrara Formation in eastern North and South Dakota. The collections indicate the presence or former presence of the following ammonite zones in the northeastern part of the Western Interior seaway: Santonian: Scaphites depressus-Clioscaphites choteauensis Coniacian: Scaphites uentrocosus; Scaphites preuentricosus Turonian: Scaphites coruensis; Scaphites whitfieldi; Prionocyclus hyatti; Subprionocyclus percarinatus; Collignoniceras woollgari; Watinoceras coloradoense Cenomanian: Dunueganoceras albertense; Dunueganoceras pondi PART B: Sedimentary rocks of early Late Cretaceous age occur in the eastern parts of North Dakota, South Dakota, and Nebraska, and in Minnesota and western Iowa. They are generally included in, from oldest to youngest, the Dakota Formation, Graneros Shale, Greenhorn Formation, Carlile Shale, and Niobrara Formation. However, in eastern North Dakota, they are also assigned to, in ascending order, the Belle Fourche Shale, Greenhorn Formation, Carlile Shale, and Niobrara Formation. The Graneros Shale and laterally equivalent strata in the Belle Fourche Shale grade eastward into the Coleraine Formation of northeastern Minnesota and probably into the Windrow Formation of southeastern Minnesota. Cretaceous beds locally overlie rocks of Precambrian, Paleozoic, and Jurassic ages, and they are generally overlain by glacial drift and alluvium of Quaternary age. In Minnesota and adjoining areas, formations of mid-Cretaceous age commonly overlap the dissected surface of Precambrian rocks. The thickness of the lower Upper Cretaceous sequence ranges from about 223 m in eastern North Dakota and about 200 m in northeastern Nebraska to a featheredge in Minnesota and Iowa. These lower Upper Cretaceous formations are composed mainly of shale, siltstone, sandstone, and limestone units of marine and nonmarine origin and were deposited near the eastern shore of a transgressing and regressing epicontinental sea in Cenomanian Turonian Coniacian. and Santonian time. During the late Cenoanian, the strandline was in Minnesota and was oriented generally northnortheast. Interpretations of the depositional environments of the strata and of fossils from outcrops and from clasts in glacial drift indicate that the Cretaceous seaway extended from this region northeastward across Canada to Greenland in the Turonian Coniacian, and Santonian. The Cretaceous formations are deformed into broad, shallow synclines in eastern North and South Dakota and in northeastern Nebraska. A west-trending anticline, the Sioux uplift, separates the synclines in southeastern South Dakota. Sparse evidence of minor faulting in the formations exists in northeastern South Dakota, along the strike of a major southwest-trending zone of tectonism in the Precambrian basement rocks. The structural relief in the region, on the top of the Greenhorn Formation, is at least 250 m between eastern North Dakota and northeastern Minnesota, and at least 200 m between northeastern Nebraska and northwestern Iowa. A comparison of the Cretaceous stratigraphy of the Minnesota region and of areas in Wyoming indicates that some marine transgressions and regressions were synchronous in the two regions. The transgression during Greenhorn time, the regression during early Carlile time, and the transgression dur

Iowa, Minnesota, Nebraska, North Dakota, South Dak↗
USGS1973

Stratigraphy and geologic history of the Montana group and equivalent rocks, Montana, Wyoming, and North and South Dakota

During Late Cretaceous time a broad north-trending epicontinental sea covered much of the western interior of North America and extended from the Gulf of Mexico to the Arctic Ocean. The sea was bounded on the west by a narrow, unstable, and constantly rising cordillera which extended from Central America to Alaska and which separated the sea from Pacific oceanic waters. The east margin of the sea was bounded by the low-lying stable platform of the central part of the United States. Rocks of the type Montana Group in Montana and equivalent rocks in adjacent States, which consist of eastward-pointing wedges of shallow-water marine and nonmarine strata that enclose westward-pointing wedges of fine-grained marine strata, were deposited in and marginal to this sea. These rocks range in age from middle Santonian to early Maestrichtian and represent a time span of about 14 million years. Twenty-nine distinctive ammonite zones, each with a time span of about half a million years, characterize the marine strata. Persistent beds of bentonite in the transgressive part of the Claggett and Bearpaw Shales of Montana and equivalent rocks elsewhere represent periods of explosive volcanism and perhaps concurrent subsidence along the west shore in the vicinity of the Elkhorn Mountains and the Deer Creek volcanic fields in Montana. Seaward retreat of st randlines, marked by deposition of the Telegraph Creek, Eagle, Judith River, and Fox Hills Formations in Montana and the Mesaverde Formation in Wyoming, may be attributed to uplift in near-coastal areas and to an increase in volcaniclastic rocks delivered to the sea. Rates of transgression and regression determined for the Montana Group in central Montana reveal that the strandline movement was more rapid during times of transgression. The regression of the Telegraph Creek and Eagle strandlines averaged about 50 miles per million years compared with a rate of about 95 miles per million years for the advance of the strand-line during Claggett time. The Judith River regression averaged about 60 miles per million years compared with movement of the strandline during the Bearpaw advance of about 70 miles per million years. The final retreat of marine waters from Montana, marked by the Fox Hills regression, was about 35 miles per million years at first, but near the end of the regression it accelerated to a rate of about 500 miles per million years. Rates of sedimentation range from less than 50 feet per million years in the eastern parts of North and South Dakota to at least 2,500 feet in western Wyoming. The low rates in the Dakotas correspond well with modern rates in the open ocean, and the rates in western Wyoming approach the rate of present coastal sedimentation.

Montana, North Dakota, South Dakota, Wyoming↗