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Robert G. Bohannon

Publications and source records attributed to Robert G. Bohannon.

At least 19 recordsLinked to original sources
USGS2017

Geologic map of the upper Arkansas River valley region, north-central Colorado

This 1:50,000-scale U.S. Geological Survey geologic map represents a compilation of the most recent geologic studies of the upper Arkansas River valley between Leadville and Salida, Colorado. The valley is structurally controlled by an extensional fault system that forms part of the prominent northern Rio Grande rift, an intra-continental region of crustal extension. This report also incorporates new detailed geologic mapping of previously poorly understood areas within the map area and reinterprets previously studied areas. The mapped region extends into the Proterozoic metamorphic and intrusive rocks in the Sawatch Range west of the valley and the Mosquito Range to the east. Paleozoic rocks are preserved along the crest of the Mosquito Range, but most of them have been eroded from the Sawatch Range. Numerous new isotopic ages better constrain the timing of both Proterozoic intrusive events, Late Cretaceous to early Tertiary intrusive events, and Eocene and Miocene volcanic episodes, including widespread ignimbrite eruptions. The uranium-lead ages document extensive about 1,440-million years (Ma) granitic plutonism mostly north of Buena Vista that produced batholiths that intruded an older suite of about 1,760-Ma metamorphic rocks and about 1,700-Ma plutonic rocks. As a result of extension during the Neogene and possibly latest Paleogene, the graben underlying the valley is filled with thick basin-fill deposits (Dry Union Formation and older sediments), which occupy two sub-basins separated by a bedrock high near the town of Granite. The Dry Union Formation has undergone deep erosion since the late Miocene or early Pliocene. During the Pleistocene, ongoing stream incision by the Arkansas River and its major tributaries has been interrupted by periodic aggradation. From Leadville south to Salida as many as seven mapped alluvial depositional units, that range in age from early to late Pleistocene, record periodic aggradational events along these streams that are commonly associated with deposition of glacial outwash or bouldery glacial-flood deposits. Many previously unrecognized Neogene and Quaternary faults, some of the latter with possible Holocene displacement, have been identified on lidar (light detection and ranging) imagery which covers 59 percent of the map area. This imagery has also permitted more accurate remapping of glacial, fluvial, and mass-movement deposits and aided in the determination of their relative ages. Recently published 10 beryllium cosmogenic surface-exposure ages, coupled with our new geologic mapping, have revealed the timing and rates of late Pleistocene deglaciation. Glacial dams that impounded the Arkansas River at Clear Creek and possibly at Pine Creek failed at least three times during the middle and late Pleistocene, resulting in catastrophic floods and deposition of enormous boulders and bouldery alluvium downstream; at least two failures occurred during the late Pleistocene during the Pinedale glaciation.

Colorado↗
USGS2013

Geologic map of the Mount Sherman 7.5' quadrangle, Lake and Park Counties, Colorado

The Mount Sherman 7.5- minute quadrangle is located along the crest of the Mosquito Range in between Leadville and Fairplay, Colorado. There are eleven 13,000-foot peaks and one fourteener, Mount Sherman, within the quadrangle. General elevations range from 10,400–14,036 feet (3,200–4,278 meters). The western half of the quadrangle primarily consists of Proterozoic granitic rocks reverse faulted over Paleozoic sedimentary rocks during the Laramide orogeny of late Cretaceous and Paleocene time. Coeval to this contractional event, sills and laccoliths of the White porphyry group (which probably includes rocks equivalent to the Pando Porphyry) were emplaced in the surrounding country rocks. Igneous activity continued into the Late Eocene with the emplacement of the Sacramento Porphyry (about 43.9 Ma) and the Gray porphyry group (about 36.7 Ma), and as young as 29 Ma to the north within the Climax quadrangle. With the inception of the Rio Grande rift within the region, the Paleozoic sedimentary rocks and Late Cretaceous to early Oligocene igneous rocks were extensionally faulted and tilted to the east. This resulted in the present 20–30 degree dip-slope of these rocks on top of Proterozoic basement rocks within the eastern half of the quadrangle. This extensional regime has continued well into the Pliocene. Within the southwestern quadrant, suspicious lineaments, alignment of springs, and continuous, measureable escarpments provide reasonable evidence for Quaternary tectonic activity along the western flank of the range. Pleistocene glaciers have dramatically sculpted the region, providing exceptional exposure of the region’s bedrock and structure.

Colorado↗
USGS2011

Geologic map of the Chisos Mountains, Big Bend National Park, Texas

The Chisos Mountains form some of the highest ground in Texas, second only to Guadalupe Peak near the New Mexico border. The northern half of the range is mostly above 5,500 feet with Emory Peak the high point at 7,825 feet. The mountains are centrally located in Big Bend National Park between Panther Junction and Punta de la Sierra. Big Bend National Park lies near the diffuse border between the Great Plains Province to the northeast and the Sonoran section of the Basin-and-Range structural province to the west and southwest. These geologically unique regions are distinguished from one another by large differences in their landscape and by the amount and style of internal structural deformation. The Great Plains Province is characterized by flat-lying or gently dipping sedimentary strata, low topographic relief, shallow stream valleys, and by a general lack of faulting. Very little active deposition is occurring on the plains, except in the bottoms of active stream valleys. In southwestern Texas the plains stand at average elevations of 2,000 to 3,300 feet and slope gently east toward the Mississippi River and the Gulf of Mexico. The Great Plains have remained relatively unchanged for the last 65 million years, except that they have been uplifted to their present height from lower elevations probably in the last 5 million years. The Basin-and-Range province is characterized by linear parallel mountain ranges, deep sediment-filled valleys, and high structural and topographic relief. The eastern part of the province is at a slightly higher average elevation than the plains. The province is known for its complex patterns of Cenozoic faulting. Today it bears little resemblance to the way it was during the Paleocene when the entire Trans-Pecos region was a simple lowland that was near or slightly below sea level.

Scientific Investigations Map↗
USGS2011

Geologic map of Big Bend National Park, Texas

The purpose of this map is to provide the National Park Service and the public with an updated digital geologic map of Big Bend National Park (BBNP). The geologic map report of Maxwell and others (1967) provides a fully comprehensive account of the important volcanic, structural, geomorphological, and paleontological features that define BBNP. However, the map is on a geographically distorted planimetric base and lacks topography, which has caused difficulty in conducting GIS-based data analyses and georeferencing the many geologic features investigated and depicted on the map. In addition, the map is outdated, excluding significant data from numerous studies that have been carried out since its publication more than 40 years ago. This report includes a modern digital geologic map that can be utilized with standard GIS applications to aid BBNP researchers in geologic data analysis, natural resource and ecosystem management, monitoring, assessment, inventory activities, and educational and recreational uses. The digital map incorporates new data, many revisions, and greater detail than the original map. Although some geologic issues remain unresolved for BBNP, the updated map serves as a foundation for addressing those issues. Funding for the Big Bend National Park geologic map was provided by the United States Geological Survey (USGS) National Cooperative Geologic Mapping Program and the National Park Service. The Big Bend mapping project was administered by staff in the USGS Geology and Environmental Change Science Center, Denver, Colo. Members of the USGS Mineral and Environmental Resources Science Center completed investigations in parallel with the geologic mapping project. Results of these investigations addressed some significant current issues in BBNP and the U.S.-Mexico border region, including contaminants and human health, ecosystems, and water resources. Funding for the high-resolution aeromagnetic survey in BBNP, and associated data analyses and interpretation, was from the USGS Crustal Geophysics and Geochemistry Science Center. Mapping contributed from university professors and students was mostly funded by independent sources, including academic institutions, private industry, and other agencies.

Texas↗
USGS2011

Geologic map of the Bailey 30' x 60' quadrangle, North-Central Colorado

The Bailey, Colo. 1:100,000-scale quadrangle lies within two physiographic and geologic provinces in central Colorado: 1) the Front Range and 2) South Park. Most of the Front Range is composed of Proterozoic rocks ranging in age from 1,790 Ma to 1,074 Ma. Along the eastern flanks and within the Denver Basin, sedimentary rocks ranging from Pennsylvanian to Cretaceous are deformed and steeply tilted to the east. Upper Cretaceous through Paleocene rocks were deposited in the foreland (that is, the Front Range eastern flank) and hinterland (that is, South Park) of this thrust and reverse fault system developed during the Late Cretaceous to Paleocene Laramide orogeny. Within South Park, rocks range in age from Pennsylvanian to Miocene with Quaternary deposits indicating tectonic subsidence of the basin. These rocks record five major geologic episodes: 1) the Paleozoic Anasazi uplift that formed the Ancestral Rockies, 2) the Late Cretaceous to Paleocene Laramide orogeny, 3) widespread Eocene to Oligocene volcanism, 4) Oligocene-Quaternary tectonics, and 5) Quaternary glacial episodes.

Colorado↗
USGS2010

The morphology, processes, and evolution of Monterey Fan: a revisit

Long-range (GLORIA) and mid-range (TOBI) sidescan imagery and seismic-reflection profiles have revealed the surface morphology and architecture of the complete Monterey Fan. The fan has not developed a classic wedge shape because it has been blocked for much of its history by Morro Fracture Zone. The barrier has caused the fan to develop an upper-fan and lower-fan sequence that are distinctly different from one another. The upper-fan sequence is characterized by Monterey and Ascension Channels and associated Monterey Channel-levee system. The lower-fan sequence is characterized by depositional lobes of the Ascension, Monterey, and Sur-Parkington-Lucia systems, with the Monterey depositional lobe being the youngest. Presently, the Monterey depositional lobe is being downcut because the system has reached a new, lower base level in the Murray Fracture Zone. A five-step evolution of Monterey Fan is presented, starting with initial fan deposition in the Late Miocene, about 5.5 Ma. This first stage was one of filling bathymetric lows in the oceanic basement in what was to become the upper-fan segment. The second stage involved filling the bathymetric low on the north side of Morro Fracture Zone, and probably not much sediment was transported beyond the fracture zone. The third stage witnessed sediment being transported around both ends of Morro Fracture Zone and initial sedimentation on the lower-fan segment. During the fourth stage Ascension Channel was diverted into Monterey Channel, thereby cutting off sedimentation to the Ascension depositional lobe.

California↗
USGS2009

Geologic Map of the Sulphur Mountain Quadrangle, Park County, Colorado

The main structural element in the Sulphur Mountain quadrangle is the Elkhorn thrust. This northwest-trending fault is the southernmost structure that bounds the west side of the Late Cretaceous and early Tertiary Front Range basement-rock uplift. The Elkhorn thrust and the Williams Range thrust that occurs in the Dillon area north of the quadrangle bound the west flank of the Williams Range and the Front Range uplift in the South Park area. Kellogg (2004) described widespread, intense fracturing, landsliding, and deep-rooted scarps in the crystalline rocks that comprise the upper plate of the Williams Range thrust. The latter thrust is also demonstrably a low-angle structure upon which the fractured bedrock of the upper plate was translated west above Cretaceous shales. Westward thrusting along the border of the Front Range uplift is probably best developed in that area. By contrast, the Elkhorn in the Sulphur Mountain quadrangle is poorly exposed and occurs in an area of relatively low relief. The thrust also apparently ends in the central part of the quadrangle, dying out into a broad area of open, upright folds with northwest axes in the Sulphur Mountain area.

Scientific Investigations Map↗
USGS2008

Geologic Map of the Elkhorn Quadrangle, Park County, Colorado

The Elkhorn thrust is defined by the juxtaposition of Early and Middle Proterozoic metamorphic and igneous rocks against Mesozoic and Tertiary rocks. Within the mapped area, an imbricate frontal thrust system juxtaposes Upper Cretaceous rocks against Paleocene rocks of the South Park Formation. In the southeastern section of the quadrangle, Middle Proterozoic igneous rocks are thrust over the South Park Formation. Syntectonic conglomerates (Txc) are preserved both on the hanging wall and footwall of the Elkhorn thrust. North of the map area we have identified normal faulting of probable Quaternary age.

Scientific Investigations Map↗
USGS1999

Cruise report for A1-98-SC southern California Earthquake Hazards Project

The focus of the Southern California Earthquake Hazards project, within the Western Region Coastal and Marine Geology team (WRCMG), is to identify the landslide and earthquake hazards and related ground-deformation processes that can potentially impact the social and economic well-being of the inhabitants of the Southern California coastal region, the most populated urban corridor along the U.S. Pacific margin. The primary objective is to help mitigate the earthquake hazards for the Southern California region by improving our understanding of how deformation is distributed (spatially and temporally) in the offshore with respect to the onshore region. To meet this overall objective, we are investigating the distribution, character, and relative intensity of active (i.e., primarily Holocene) deformation within the basins and along the shelf adjacent to the most highly populated areas (see Fig. 1). In addition, the project will examine the Pliocene-Pleistocene record of how this deformation has shifted in space and time. The results of this study should improve our knowledge of shifting deformation for both the long-term (105 to several 106 yr) and short-term (<50 ky) time frames and enable us to identify actively deforming structures that may constitute current significant seismic hazards.

California↗
USGS1995

Multichannel seismic-reflection profiling on the R/V Maurice Ewing during the Los Angeles Region Seismic Experiment (LARSE), California

This report describes the acquisition of deep-crustal multichannel seismic-reflection data in the Inner California Borderland aboard the R/V Maurice Ewing, conducted in October 1994 as part of the Los Angeles Regional Seismic Experiment (LARSE). LARSE is a cooperative study of the crustal structure of southern California involving earth scientists from the U.S. Geological Survey, Caltech, the University of Southern California, the University of California Los Angeles, and the Southern California Earthquake Center (SCEC). During LARSE, the R/V Ewing's 20- element air gun array, totaling 137.7 liters (8470 cu. in.), was used as the primary seismic source for wide-angle recording along three main onshore-offshore lines centered on the Los Angeles basin and the epicenters of the 1933 Long Beach and 1994 Northridge earthquakes. The LARSE onshore-offshore lines were each 200-250 km long, with the offshore portions being between 90 and 150 km long. The nearly 24,000 air gun signals generated by the Ewing were recorded by an array of 170 PASSCAL REFTEK recorders deployed at 2 km intervals along all three of the onshore lines and 9 ocean bottom seismometers (OBSs) deployed along two of the lines. Separate passes over the OBS-deployment lines were performed with a long air gun repetition rate (60 and 90 seconds) to minimize acoustic-wave interference from previous shots in the OBS data. The Ewing's 4.2-km, 160-channel, digital streamer was also used to record approximately 1250 km of 40-fold multichannel seismic-reflection data. To enhance the fold of the wide-angle data recorded onshore, mitigating against cultural and wind noise in the Los Angeles basin, the entire ship track was repeated at least once resulting in fewer than about 660 km of unique trackline coverage in the Inner Borderland. Portions of the seismic-reflection lines were repeated up to 6 times. A variety of other geophysical data were also continuously recorded, including 3.5 kHz bathymetry, multi-beam swath Hydrosweep bathymetry, magnetics, and gravity data. In this report, we describe the equipment and procedures used to acquire multichannel seismic-reflection and other geophysical data aboard the Ewing, provide a detailed cruise narrative, discuss the reduction of the data, and present near-trace constant offset seismic sections of the acquired profiles.

California↗
USGS1982

Kinematic evolution of the junction of the San Andreas, Garlock, and Big Pine faults, California

If the San Andreas fault with about 300 km of right slip, the Carlock fault with about 60 km of left slip, and the Big Pine fault with about 15 km of left slip are considered to have been contemporaneously active, a space problem at their high-angle junctions becomes apparent. Large crustal masses converge in the area of the junctions as a result of the simultaneous large displacements on the faults. We present here a model in which an early straight north-northwest–trending San Andreas deforms to its present bent configuration in response to a westward displacement of crust north of the Garlock fault. During this deformation, the crust north of the Garlock in the vicinity of the junction undergoes north-south shortening, while the fault junction migrates along the trace of the San Andreas fault to the southeast relative to its original position. As a result of this migration, the Mojave area is displaced to the east relative to the original junction position. We suggest a similar history in mirror image for the Big Pine fault and the areas of crust adjacent to it.

California↗
USGS1982

Mineral resource potential map of the Muddy Mountains Wilderness Study Area, Clark County, Nevada

The Muddy Mountains Wilderness Study Area (WSA 050-0229), Clark County, Nevada, has a high potential for mineral deposits of calcium borates and lithium. The known and potential mineral deposits are concentrated in the east-central and south-central parts of the study area (see map). Zeolites (in particular clinoptilolite) are present in some tuff beds throughout much of the study area, and this resource potential is probably moderate to high. Stream-sediment sampling suggests that the Muddy Mountains area has little potential for mineral deposits of metals (other than lithium). Clay minerals are mined at one locality in the (!rea (see map). Building stone and silica sand have moderate to low potential in some places. Oil and gas potential within the study area is low, but complete evaluation of its potential is not possible without drilling.

Nevada↗