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Geology topics

D. G. Howell

Publications and source records attributed to D. G. Howell.

31 records · Page 2Linked to original sources
USGS1986

A budget for continental growth and denudation

Oceanic crustal material on a global scale is re-created every 110 million years. From the data presented it is inferred that potential sialic material is formed at a rate of about 1.35 cubic kilometers per year, including hemipelagic volcanic sediments that accumulate at a rate of about 0.05 cubic kilometer per year. It is estimated that the influx of 1.65 cubic kilometers per year of terrigenous and biogenic sediment is deposited on the deep ocean, and this represents continental denudation. Because all this material is brought into a subduction zone, continental accretion rates, which could include all this material, may be as high as 3.0 cubic kilometers per year with a potential net growth for continents of 1.35 cubic kilometers per year.

Science↗
USGS1984

Sedimentary, tectonic, and sea-level controls on submarine fan and slope-apron turbidite systems

To help understand factors that influence submarine fan deposition, we outline some of the principal sedimentary, tectonic, and sea-level controls involved in deep-water sedimentation, give some data on the rates at which they operate, and evaluate their probable effects. Three depositional end-member systems, two submarine fan types (elongate and radial), and a third nonfan, slope-apron system result primarily from variations in sediment type and supply. Tectonic setting and local and global sea-level changes further modify the nature of fan growth, the distribution of facies, and the resulting vertical stratigraphic sequences. ?? 1984 Springer-Verlag New York Inc.

Geo-Marine Letters↗
USGS1984

Miocene Blanca Fan, Northern Channel Islands, California: Small fans reflecting tectonism and volcanism

Blanca fan is a submarine fan composed of Miocene volcaniclastic strata. Parts of the fan system are exposed on Santa Cruz and Santa Rosa Islands, and possibly correlative strata crop out on San Miguel and Santa Catalina Islands. The Blanca fan and underlying breccia reflect regional transcurrent faulting in the California Continental Borderland and development of a system of rapidly subsiding basins and uplifted linear ridges during early and middle Miocene time. Erosion of uplifted crystalline basement rocks followed by the onset of silicic volcanism created linear sediment sources for the alluvial and submarine fans, respectively. ?? 1984 Springer-Verlag New York Inc.

Geo-Marine Letters↗
USGS1984

Ferrelo fan, California: Depositional system influenced by Eustatic sea level changes

Remnants of an Eocene fan system are preserved onshore at San Diego and in the central part of the southern California borderland. Even though faults and erosion have truncated its margins, geophysical data and exploratory wells indicate that remaining parts of the fan extend beneath an offshore area nearly 400-km long and 40- to 100-km wide. Environments representing fluvial, fan-delta, shelf-channel, overlapping inner- to outer-fan, and basin-plain facies are recognized or inferred. Three progradational cycles onshore and two distinct pulses of sand accumulation offshore are attributable to eustatic low sea-level stands rather than to tectonic uplift or shifts in depositional patterns. ?? 1984 Springer-Verlag New York Inc.

Geo-Marine Letters↗
USGS1984

Turbidite facies in an ancient subduction complex: Torlesse terrane, New Zealand

The Torlesse terrane of New Zealand is an ancient subduction complex consisting of deformed turbidite-facies rocks. These are mainly thick-bedded sandstone (facies B and C) with subordinate mudstone (facies D and E), comparable to inner- and middle-fan deposits of a submarine fan. Strata were deposited in trench-floor and trench-slope settings that received sandy sediment from slope-cutting submarine canyons. The dominance of sandstone suggests that some mudstone may have been selectively subducted. Construction of a detailed sediment dispersal model is not possible because tectonic deformation has largely destroyed original facies relationships and paleocurrent patterns. ?? 1984 Springer-Verlag New York Inc.

Geo-Marine Letters↗
USGS1980

Sedimentary masses and concepts about tectonic processes at underthrust ocean margins

Tectonic processes associated with subduction of oceanic crust, but unrelated to the collision of thick crustal masses or microplates, are presumed by many geologists to significantly affect the formation and deformation of large sedimentary bodies at underthrust ocean margins. More geologists are familiar with the concept of subduction accretion , which describes the tectonic attachment of rock and sediment masses to the margin's bedrock framework, than with other noncollision processes—for example, sediment subduction, subduction erosion , and subduction kneading . These are equally important processes controlling the geologic evolution of underthrust margins, and any one of them may predominate at a given place.

Geology↗
USGS1978

Neogene basin formation in relation to plate tectonic evolution of San Andreas fault system, California

More than 90% of the known petroleum accumulations west of the San Andreas fault in California are in strata deposited in areally restricted Neogene basins that formed during a major tectonic reorganization of western California. These deep, localized Neogene basins replaced broad, regionally persistent Paleogene depositional aprons, although some of the Neogene basins in northern and central California had Paleogene precursors. The evolution of each of the Neogene basins is complex, and aspects of the kinematics of each are unique; nonetheless, all can be considered products of an overall right-lateral shear system associated with a sliding margin between the Pacific and North American lithospheric plates. The sliding margin developed in western California about 29 m.y. ago, when the Pacific plate contacted North America after subduction of the intervening Farallon plate. The initial position of the common boundary between the Pacific and North American plates was along the continental margin. Right slip between the Pacific and North American plates gradually shifted eastward to right-slip faults, such as the San Andreas, located farther inland. This shift seems to be documented by relations in the southern California area. About 300 km of right slip has occurred along the San Andreas fault during the past 10 to 15 m.y., and at least several hundred additional kilometers along associated right-slip faults of the San Andreas system. The Neogene basins in southern California began to develop during the interval in which the boundary between the Pacific and North American plates shifted from the continental edge to the San Andreas fault, apparently because the step-by-step switch to different surfaces of weakness caused local extension and compression within a broad zone of right-lateral shear. A major phase of basin formation appears to have been synchronous with a change in azimuth of relative shear between the Pacific and North American plates to a more westerly direction, resulting in extensional strain. This change in motion initiated basin development in offshore central and northern California and affected the ongoing development of basins as a result of right slip along the San Andreas and related faults in other parts of California.

California↗