Contributions to Geology 22.1

Laramide compressional tectonics, southeastern Wyoming

D. L. Blackstone, Jr.

Pages
1-38

Keywords
Laramide compressional tectonics, southeastern Wyoming

Abstract
The Hanna-Carbon-Laramie basin complex of southeastern Wyoming is outlined by Rocky Mountain foreland type uplifts in which the Precambrian basement is exposed in the core of the uplifts and is locally overthrust onto the basin margins. The Laramie basin is the shallowest (-3700 ft; - 11277 m) at the Precambrian-sedimentary interface; and the Hanna basin is the deepest (-28,000 ft; - 8533 m). The deeper parts of the basins contain continental deposits of Late Cretaceous to Eocene age with the thickest section in the Hanna basin. Several examples of plunging folds in which the exposed basement core is flexed rather tightly with no disruption of the overlying sedimentary cover are discussed—Elk Mountain, Bald Mountain and Sheephead Mountain are typical.
Basement rocks are highly fractured and are flexed into folds as large as the basins themselves and adjacent uplifts, as well as at scales of a few hundred feed amplitude. The response of the basement to the Laramide compressional field is governed in part by major northeast trending shear zones of Precambrian age, as well as by systematic fracture patterns of indeterminate age.
Major faults on the basin margins are low angle, dip beneath the uplifts, and have slip of as much as 12,000 feet (3657 m). The thrust plates override the adjacent synclines in the footwall, a situation incompatible with an interpretation involving only vertical stress field. All folds in the sedimentary column along the basin margins where data from oil and gas exploration or production wells are adequate show reverse faulting.
The thesis developed on the basis of both surface and subsurface data is that the observed deformation can only be explained adequately by a stress field which was horizontally directed during the Laramide orogenic episode.

An Irvingtonian fauna fromthe oldest Quaternary alluvium in eastern Pumpkin Creek Valley, Morrill and Banner counties, Nebraska.

R. George Corner and Robert F. Diffendal, Jr.

Pages
39-43

Keywords
An Irvingtonian fauna fromthe oldest Quaternary alluvium in eastern Pumpkin Creek Valley, Morrill and Banner counties, Nebraska.

Abstract
Vertebrate fossils of Mammuthus meridionalis (Nesti) and Equus sp. cf. E. scotti Gidley occur at two sites in the capping alluvium of the highest strath terrace along the south side of Pumpkin Creek Valley, Nebraska. The presence of these two species supports the conclusion that the age of the capping sediments is probably early Irvingtonian (early Pleistocene).

Sedimentology and petrology of freshwater lacustrine carbonate: mid-Tertiary Camp Davis Formation, northwestern Wyoming

Rhonda L. Davis and Bruce H. Wilkson

Pages
45-55

Keywords
Sedimentology and petrology of freshwater lacustrine carbonate: mid-Tertiary Camp Davis Formation, northwestern Wyoming

Abstract
The lower part of the late Tertiary Camp Davis Formation of northwestern Wyoming records a transition from terrigenous clastic to freshwater carbonate deposition resulting from changes in basin drainage and sediment supply during movement along the Hoback normal fault. Conglomerate deposited in proximal to medial alluvial fan settings was subjected to varying amounts of weathering and erosion as it was transgressed by the lake shoreline. Overlying nearshore limestone facies include lithified carbonate mud, sand, and gravel. They record a spectrum of energy levels within depositional environments in the lake system. Synsedimentary lacustrine calcite cementation in lake margin settings is documented by the presence of reworked cemented algal crusts as large allochems in overlaying units. Lake-margin carbonate is overlain in turn by a sequence of fine-grained terrigenous pyroclastic sandstone and siltstone. These record basin-center deposition which occurred as transgression of the Camp Davis lake continued over basal conglomerate and nearshore lacustrine carbonate.
Facies associations along the 8.3 km outcrop, which parallels the northeastern lake paleoshoreline, demonstrate that the lateral and vertical distribution of various limestone rock types were controlled by the configuration of the lake shore. Micritic facies accumulated along low-energy interfan reentrants and grainstone facies accumulated along high-energy headlands developed along more proximal alluvial fan gravels.

A new species of centetodon (Mammalia, Insectivora, Geolabididae) from southwestern Montana and its biogeographical implications

Jason A. Lillegraven and Alan R. Tabrum

Pages
57-73

Keywords
A new species of centetodon (Mammalia, Insectivora, Geolabididae) from southwestern Montana and its biogeographical implications

Abstract
Mammalian fossils recently collected from several localities in the Jefferson and Three Forks basins of southwestern Montana allow definition of a new Species of geolabidid insectivore, Centetodon kuenzii. Centetodon kuenzii is readily distinguishable morphologically and mensurally from its closest know relative, C. chadronensis, under which name a smaller sample was originally included. The new species occurs in rocks ranging in age from late Eocene (late Uintan) to middle Oligocene (early Orellan), yet dental samples show no observable evolutionary change through this approximately ten million year sequence. Centetodon kuenzii appears endemic to the paleobasins of southwestern Montana, while C. chadronensis as presently defined ranged at least from central Wyoming southward to the Big Bend area of Texas. Geological considerations suggest that individuals of C. kuenzii lived in basin-margin paleoenvironmental settings while C. chadronensis was adapted to life on more open plains. Late Eocene topographic barriers between southwestern Montana and central Wyoming are proposed as a cause of geographical isolation of stocks ancestral to C. kuenzii and C. chadronensis. Dental specializations observed in C. kuenzii preclude likelihood of ancestry to other know species of Centetodon.