Geology topics

F. Peterson

Publications and source records attributed to F. Peterson.

5 recordsLinked to original sources

USGS2004

Jurassic "savannah" - Plant taphonomy and climate of the Morrison Formation (Upper Jurassic, Western USA)

The Morrison Formation contains six plant taphofacies: Wood, whole-leaf, leaf-mat, root, common carbonaceous debris, and rare carbonaceous debris. None of these taphofacies is common in the Morrison; particularly striking is the paucity of wood, even in more reducing environments. The flora of the Morrison Formation is distinct in the Kimmeridgian and Tithonian parts of the section. The plant taphofacies are consistent with a predominantly herbaceous vegetation. Evidence from the plant taphofacies, floras, and sedimentology of the Morrison is consistent with a warm, seasonal, semi-arid climate throughout, changing from dry semi-arid to humid semi-arid from the Kimmeridgian to the Tithonian parts of the formation. ?? 2004 Published by Elsevier B.V.

Sedimentary Geology↗

USGS2004

Reconstruction of the Upper Jurassic Morrison Formation extinct ecosystem - A synthesis

A synthesis of recent and previous studies of the Morrison Formation and related beds, in the context of a conceptual climatic/hydrologic framework, permits reconstruction of the Late Jurassic dinosaurian ecosystem throughout the Western Interior of the United States and Canada. Climate models and geologic evidence indicate that a dry climate persisted in the Western Interior during the Late Jurassic. Early and Middle Kimmeridgian eolian deposits and Late Kimmeridgian alkaline, saline wetland/lacustrine deposits demonstrate that dryness persisted throughout the Kimmeridgian. Tithonian-age coal reflects lower evaporation rates associated with a slight cooling trend, but not a significant climate change. With a subtropical high over the Paleo-Pacific Ocean and atmospheric circulation generally toward the east, moisture carried by prevailing winds "rained out" progressively eastward, leaving the continental interior-and the Morrison depositional basin-dry. Within the basin, high evaporation rates associated with the southerly paleolatitude and greenhouse effects added to the dryness. Consequently, the two main sources of water-groundwater and surface water-originated outside the basin, through recharge of regional aquifers and streams that originated in the western uplands. Precipitation that fell west of the basin recharged aquifers that underlay the basin and discharged in wetlands and lakes in the distal, low-lying part of the basin. Precipitation west of the basin also fed intermittent and scarce perennial streams that flowed eastward. The streams were probably "losing" streams in their upstream reaches, and contributed to a locally raised water table. Elsewhere in the basin, where the floodplain intersected the water table, small lakes dotted the landscape. Seasonal storms, perhaps in part from the Paleo-Gulf of Mexico, brought some precipitation directly to the basin, although it was also subjected to "rain out" en route. Thus, meteoric input to the basin was appreciably less than groundwater and surface water contributions. The terrestrial Morrison ecosystem, which can be likened to a savannah, expanded with the northward retreat of the Late Jurassic Western Interior Seaway. The ecosystem was a complex mosaic, the components of which shifted through time. Riparian environments probably were the most diverse parts of the ecosystem, where a multi-storeyed canopy supported a diverse fauna, from insects to dinosaurs. Equable conditions also existed in wetlands, lakes, and elsewhere on the floodplain when seasonal rainfall brought an herbaceous groundcover to life. Eolian environments and alkaline, saline wetlands were inhospitable to life.Large herbivorous dinosaurs were adapted to this semi-arid landscape. Their size was an adaptive asset based on considerations of food requirements associated with a low metabolism and was also an advantage for migration during drought. Some of the large sauropods were adapted to browsing the higher vegetation associated with riparian environments; others to grazing the herbaceous groundcover on the floodplain and charophytes in the wetlands. The extensive distal wetlands may, in fact, have been refugia for some of these herbivores during the dry season and droughts. Extended periods of drought account for some of the dinosaur death assemblages; yet, the ecosystem could also sustain the most unusual life forms that ever roamed the Earth. ?? 2004 Elsevier B.V. All rights reserved.

Sedimentary Geology↗

USGS1988

Synthesis of late Paleozoic and Mesozoic eolian deposits of the Western Interior of the United States

Late Paleozoic and Mesozoic eolian deposits include rock units that were deposited in ergs (eolian sand seas), erg margins and dune fields. They form an important part of Middle Pennsylvanian through Upper Jurassic sedimentary rocks across the Western Interior of the United States. These sedimentary rock units comprise approximately three dozen major eolian-bearing sequences and several smaller ones. Isopach and facies maps and accompanying cross sections indicate that most eolian units display varied geometry and complex facies relations to adjacent non-eolian rocks. Paleozoic erg deposits are widespread from Montana to Arizona and include Pennsylvanian formations (Weber, Tensleep, Casper and Quadrant Sandstones) chiefly in the Northern and Central Rocky Mountains with some deposits (Hermosa and Supai Groups) on the Colorado Plateau. Lower Permian (Wolfcampian) erg deposits (Weber, Tensleep, Casper, Minnelusa, Ingleside, Cedar Mesa, Elephant Canyon, Queantoweap and Esplanade Formations) are more widespread and thicken into the central Colorado Plateau. Middle Permian (Leonardian I) erg deposits (De Chelly and Schnebly Hill Formations) are distributed across the southern Colorado Plateau on the north edge of the Holbrook basin. Leonardian II erg deposits (Coconino and Glorieta Sandstones) are slightly more widespread on the southern Colorado Plateau. Leonardian III erg deposits formed adjacent to the Toroweap-Kaibab sea in Utah and Arizona (Coconino and White Rim Sandstones) and in north-central Colorado (Lyons Sandstone). Recognized Triassic eolian deposits include major erg deposits in the Jelm Formation of central Colorado-Wyoming and smaller eolian deposits in the Rock Point Member of the Wingate Sandstone and upper Dolores Formation, both of the Four Corners region. None of these have as yet received a modern or thorough study. Jurassic deposits of eolian origin extend from the Black Hills to the southern Cordilleran arc terrain. Lower Jurassic intervals include the Jurassic part of the Wingate Sandstone and the Navajo-Aztec-Nugget complex and coeval deposits in the arc terrain to the south and west of the Colorado Plateau. Major Middle Jurassic deposits include the Page Sandstone on the Colorado Plateau and the widespread Entrada Sandstone, Sundance Formation, and coeval deposits. Less extensive eolian deposits occur in the Carmel Formation, Temple Cap Sandstone, Romana Sandstone and Moab Tongue of the Entrada Sandstone, mostly on the central and western Colorado Plateau. Upper Jurassic eolian deposits include the Bluff Sandstone Member and Recapture Member of the Morrison Formation and Junction Creek Sandstone, all of the Four Corners region, and smaller eolian deposits in the Morrison Formation of central Wyoming and apparently coeval Unkpapa Sandstone of the Black Hills. Late Paleozoic and Mesozoic eolian deposits responded to changing climatic, tectonic and eustatic controls that are documented elsewhere in this volume. All of the eolian deposits are intricately interbedded with non-eolian deposits, including units of fluvial, lacustrine and shallow-marine origin, clearly dispelling the myth that eolian sandstones are simple sheet-like bodies. Rather, these units form some of the most complex bodies in the stratigraphic record.

Western Interior↗

USGS1988

Pennsylvanian to Jurassic eolian transportation systems in the western United States

The direction of sediment transport in eolian sandstones of Pennsylvanian to Jurassic age was interpreted from crossbedding resultants (vector means) obtained from studies of eolian rocks in the western U.S., supplemented by data from the few eolian units of eastern North America. These were compiled from the published or unpublished (theses) literature, from unpublished field data contributed by colleagues, or from measurements made for this study. In addition, new paleogeographic maps were compiled to evaluate the influence of geographic features on the atmospheric circulation patterns that are inferred from the crossbedding studies. Regionally, the crossbedding indicates northeasterly, northerly, or northwesterly winds (present coordinates) from Pennsylvanian through most of Middle Jurassic time. A rather abrupt change in wind directions occurred in late Middle Jurassic time (late part of the Callovian Age) when westerly wind patterns developed. By the Late Jurassic the winds shifted to southwesterly. Calculations of the consistency factor (vector mean strength) made from region-wide analyses of the resultants indicate fairly unidirectional winds from the Pennsylvanian through the Early Jurassic. Middle Jurassic circulation was more varied, judging from crossbedding studies in the lower part of the Entrada Sandstone. Crossbedding in Upper Jurassic eolian rocks of Wyoming and South Dakota yielded a random pattern but Upper Jurassic rocks farther south on the Colorado Plateau and adjoining areas show a return to a fairly unidirectional pattern. Comparing the resultants with their reconstructed paleogeographic setting shows surprisingly little influence of major geographic features on overall circulation patterns. However, the greatest amount of local variation occurred at or near highly indented shorelines where the temperature contrast between land and water produces local wind currents that may vary appreciably from regional circulation patterns. Although they do not cause noticeable horizontal deflections in wind patterns, small and low topographic highs appear to be able to promote the development of a dune field if a source of sand is available and if streams do not enter the growing dune field.

western United States↗

USGS1988

Wind directions predicted from global circulation models and wind directions determined from eolian sandstones of the western United States - A comparison

Wind directions for Middle Pennsylvanian through Jurassic time are predicted from global circulation models for the western United States. These predictions are compared with paleowind directions interpreted from eolian sandstones of Middle Pennsylvanian through Jurassic age. Predicted regional wind directions correspond with at least three-quarters of the paleowind data from the sandstones; the rest of the data may indicate problems with correlation, local effects of paleogeography on winds, and lack of resolution of the circulation models. The data and predictions suggest the following paleoclimatic developments through the time interval studied: predominance of winter subtropical high-pressure circulation in the Late Pennsylvanian; predominance of summer subtropical high-pressure circulation in the Permian; predominance of summer monsoonal circulation in the Triassic and earliest Jurassic; and, during the remainder of the Jurassic, influence of both summer subtropical and summer monsoonal circulation, with the boundary between the two systems over the western United States. This sequence of climatic changes is largely owing to paleogeographic changes, which influenced the buildup and breakdown of the monsoonal circulation, and possibly owing partly to a decrease in the global temperature gradient, which might have lessened the influence of the subtropical high-pressure circulation. The atypical humidity of Triassic time probably resulted from the monsoonal circulation created by the geography of Pangaea. This circulation is predicted to have been at a maximum in the Triassic and was likely to have been powerful enough to draw moisture along the equator from the ocean to the west.

western United States↗