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At least 901 records · Page 50Linked to original sources
USGS1996

Differentiating climatic and successional influences on long-term development of a marsh

Comparison of long—term records of local wetland vegetation dynamics with regional, climate—forced terrestrial vegetation changes can be used to differentiate the rates and effects of autogenic successional processes and allogenic environmental change on wetland vegetation dynamics. We studied Holocene plant macrofossil and pollen sequences from Portage Marsh, a shallow, 18—ha marsh in northeastern Indiana. Between 10 000 and 5700 yr BP the basin was occupied by a shallow, open lake, while upland vegetation consisted of mesic forests of Pinus, Quercus, Ulmus, and Carya. At 5700 yr BP the open lake was replaced rapidly by a shallow marsh, while simultaneously Quercus savanna developed on the surrounding uplands. The marsh was characterized by periodic drawdowns, and the uplands by periodic fires. Species composition of the marsh underwent further changes between 3000 and 2000 yr BP. Upland pollen spectra at Portage Marsh and other sites in the region shifted towards more mesic vegetation during that period. The consistency and temporal correspondence between the changes in upland vegetation and marsh vegetation indicate that the major vegetational changes in the marsh during the Holocene resulted from hydrologic changes forced by regional climate change. Progressive shallowing of the basin by autogenic accumulation of organic sediment constrained vegetational responses to climate change but did not serve as the direct mechanism of change.

Ecology↗
USGS2001

Quaternary geology, Cold Bay and False Pass quadrangles, Alaska Peninsula

Recent mapping and interpretation of Quaternary geologic features has improved our understanding of the interaction between volcanic, glacial, and tectonic activity in the Cold Bay and False Pass 1:250,000-scale quadrangles on the Alaska Peninsula. The glacial and volcanic record of the map area strongly suggests that continental-shelf glaciations and two massive volcanic centers were the dominant controls over landscape development during Pleistocene time. Ancestral Morzhovoi and Emmons Volcanoes were major impediments to flow of shelf glaciers during much of the Pleistocene. Our mapping suggests that the area around Emmons Volcano may have also been an important source area for glaciers during this period. Our data further indicate that Frosty Volcano developed late in the Pleistocene, having had no apparent impact on early Brooks Lake glacial advances but serving as a source area for later glacial advances during late Brooks Lake time. We also believe that major Holocene eruptions of Frosty Volcano have yielded multiple debris and ash flows resulting in the construction of a new south summit cone that filled an earlier crater. Frosty Volcano was the source area for multiple Holocene glacial advances, and its flanks preserve the best record of Neoglacial activity in the map area.

Alaska↗
USGS2009

Mineral Resource Assessment of Marine Sand Resources in Cape- and Ridge-Associated Marine Sand Deposits in Three Tracts, New York and New Jersey, United States Atlantic Continental Shelf

Demand is growing in the United States and worldwide for information about the geology of offshore continental shelf regions, the character of the seafloor, and sediments comprising the seafloor and subbottom. Interest in locating sand bodies or high quality deposits that have potential as sources for beach nourishment and ecosystem restoration is especially great in some regions of the country. The Atlantic coast, particularly New York and New Jersey, has been the focus of these studies for the past 40 years with widely varying results. This study is the first attempt at applying probability statistics to modeling Holocene-age cape-and ridge-associated sand deposits and thus focuses on distinct sand body morphology. This modeling technique may have application for other continental shelf regions that have similar geologic character and late Quaternary sea-level transgression history. An estimated volume of 3.9 billion m3 of marine sand resources is predicted in the cape-and ridge-associated marine sand deposits in three representative regions or tracts on the continental shelf offshore of New York and New Jersey. These estimates are taken from probabilistic distributions of sand resources and are produced using deposit models and Monte Carlo Simulation (MCS) techniques. The estimated sand resources presented here are for only three tracts as described below and for Holocene age sand resources contained in cape-and ridge-associated marine sand deposit types within this area. Other areas may qualify as tracts for this deposit type and other deposit types and geologic ages (for example, paleo-stream channels, blanket and outwash deposits, ebb-tide shoals, and lower sea level-stand deltas), which are present on the New Jersey and New York continental shelf area but are not delineated and modeled in this initial evaluation. Admittedly, only a portion of these probable sand resources will ultimately be available and suitable for production, dependent largely on geographic, economic, preemptive use, environmental, geologic and political factors. In addition, offshore sand resources should only be considered if the area is seaward of the active zone of significant nearshore sediment transport, about 10 to 12 m in depth, and in sufficiently shallow water so that sand can be extracted within U.S. dredging equipment limits, currently about 40 m in depth. If the material is to be used for beach nourishment, material must be of an appropriate sediment texture and character (grain size, sorting, shape, and color) to match the native beach and have mineralogical properties important to its use. Extraction of sand can disturb or alter the benthic habitat and seafloor ecology, so these factors and other site-specific effects will need to be evaluated for any intended use. These and other factors are not considered in this report but can be expected to reduce the total net volume of sand resources available for production. The purpose of this report is to describe and present results from a probabilistic mineral modeling technique previously applied to onshore mineral resources. This modeling and assessment procedure is being used for the first time to assess and estimate offshore aggregate resources; this study is part of the U.S. Geological Survey (USGS) Marine Aggregates Resources and Processes Project (http://woodshole.er.usgs.gov/project-pages/aggregates/).

Bulletin↗
USGS1991

Cores from marine geologic features in the western Gulf of Maine

Submerged geologic features located in the western Gulf of Maine between Cape Ann, Massachusetts and New Hampshire (fig. 1), were identified from high-resolution seismic-reflection data collected between 1979 and 1980. The features include the following. (1) A pair of end morained formed during the retreat of the Laurentide Ice Sheet from the Gulf of Maine (Oldale, 1985a). (2) A barrier spit and lagoon complex and (3) a paleodelta, both of which formed during a late Wisconsinan to early Holocene low relative sea-level stand (Oldale and others, 1983; Oldale, 1985b). (4) A wave-cut unconformity that was eroded as the sea transgressed to its present position. (5) A surficial sand depost that formed atop the unconformity in middle Holocene time. Vibracores (fig. 2) were taken in 1984 to corroborate the interpretations from the seismic data, to determine the sedimentary texture and structure of the features, to identify the nature of the unconformity, and to obtain material for 14 C dating,

Maine↗
USGS2002

Hydrostructural maps of the Death Valley regional flow system, Nevada and California

The locations of principal faults and structural zones that may influence ground-water flow were compiled in support of a three-dimensional ground-water model for the Death Valley regional flow system (DVRFS), which covers 80,000 square km in southwestern Nevada and southeastern California. Faults include Neogene extensional and strike-slip faults and pre-Tertiary thrust faults. Emphasis was given to characteristics of faults and deformed zones that may have a high potential for influencing hydraulic conductivity. These include: (1) faulting that results in the juxtaposition of stratigraphic units with contrasting hydrologic properties, which may cause ground-water discharge and other perturbations in the flow system; (2) special physical characteristics of the fault zones, such as brecciation and fracturing, that may cause specific parts of the zone to act either as conduits or as barriers to fluid flow; (3) the presence of a variety of lithologies whose physical and deformational characteristics may serve to impede or enhance flow in fault zones; (4) orientation of a fault with respect to the present-day stress field, possibly influencing hydraulic conductivity along the fault zone; and (5) faults that have been active in late Pleistocene or Holocene time and areas of contemporary seismicity, which may be associated with enhanced permeabilities. The faults shown on maps A and B are largely from Workman and others (in press), and fit one or more of the following criteria: (1) faults that are more than 10 km in map length; (2) faults with more than 500 m of displacement; and (3) faults in sets that define a significant structural fabric that characterizes a particular domain of the DVRFS. The following fault types are shown: Neogene normal, Neogene strike-slip, Neogene low-angle normal, pre-Tertiary thrust, and structural boundaries of Miocene calderas. We have highlighted faults that have late Pleistocene to Holocene displacement (Piety, 1996). Areas of thick Neogene basin-fill deposits (thicknesses 1-2 km, 2-3 km, and >3 km) are shown on map A, based on gravity anomalies and depth-to-basement modeling by Blakely and others (1999). We have interpreted the positions of faults in the subsurface, generally following the interpretations of Blakely and others (1999). Where geophysical constraints are not present, the faults beneath late Tertiary and Quaternary cover have been extended based on geologic reasoning. Nearly all of these concealed faults are shown with continuous solid lines on maps A and B, in order to provide continuous structures for incorporation into the hydrogeologic framework model (HFM). Map A also shows the potentiometric surface, regional springs (25-35 degrees Celsius, D'Agnese and others, 1997), and cold springs (Turner and others, 1996).

Nevada;California↗
USGS2003

Geologic map of the Rio Puerco quadrangle, Bernalillo and Valencia Counties, New Mexico

The Rio Puerco quadrangle is located southwest of Albuquerque in central New Mexico and covers part of the western part of the Isleta Reservation. The U.S. Geological Survey, the New Mexico Bureau of Geology and Mineral Resources, and the University of New Mexico have conducted geologic mapping on the Isleta Reservation and vicinity as part of the Middle Rio Grande Basin Project. The map area contains surficial deposits, calcic soils, fluvial deposits of the Rio Puerco, deposits of the Santa Fe Group, and three volcanic fields. The area is characterized by predominantly north-trending normal faults with generally down-to-the-east movement. Post-Santa Fe Group deposits are composed of surficial deposits (Pleistocene-Holocene) and fluvial deposits of the Rio Puerco (Pleistocene-Holocene). The surficial deposits are divided into eolian, alluvial, colluvial, and landslide deposits. The fluvial deposits of the Rio Puerco consist of four terrace and present channel deposits. The Santa Fe Group is divided into lower and upper parts. The lower part of the Santa Fe Group is exposed near the southwestern corner of the study area where deposits consist of reddish-brown mudstone and sandstone correlated to the Popotosa Formation (Unit 1) of Lozinsky and Tedford (1991). They interpreted deposition of the unit in a basin-floor playa setting. The Popotosa Formation is in fault contact to the east with deposits of the upper Santa Fe Group. The upper Santa Fe Group is derived from major tributary fluvial systems (ancestral Rio Puerco Puerco and possibly the Rio San Jose drainages) draining the adjacent Colorado Plateau and Sierra Nacimiento and correlated to parts of Kelley's (1977) Ceja Formation of the Santa Fe Group and equivalent to Machette's (1978) Sierra Ladrones Formation, Connell's Arroyo Ojito Formation (Connell and others, 1999, and Maldonado's lithofacies of the Isleta Reservation (Maldonado and Atencio,1998a, b). The group also locally includes a fine- grained unit (lower Pleistocene) referred to here as the sand, silt, and clay of Chavez Grant (Qsc). The Ceja Formation of the Santa Fe Group as defined here is divided into the following units in descending stratigraphic order: (1) upper sand and gravel unit (upper Pliocene), (2) middle silt, sand, and clay unit (upper Pliocene), and (3) lower sand and gravel unit (Pliocene). The three volcanic fields in the map area are: (1) basalt of Cat Hills, dated at 98-110 ka and composed of seven lava flows and four cinder cones; the flows overlie calcic soils that overlie the upper sand and gravel unit of the Ceja Formation; (2) lava flow of Cat Mesa, dated at about 3 Ma and interfingers with the upper part of the Ceja Formation; (3) diabase of Mohinas Mountain, dated at 8.3 Ma (Baldridge and others, 1987) and intrudes the Popotosa Formation. Numerous high-angle faults cut the area but are mostly buried. The faults generally trend north but deviate to the northwest and northeast. The major normal faults are the Cat Mesa and Mohinas Mountain faults.

Miscellaneous Field Studies Map↗
USGS2007

Surficial sediment character of the New York-New Jersey offshore continental shelf region: A GIS compilation

Broad continental shelf regions such as the New York Bight are the product of a complex geologic history and dynamic oceanographic processes, dominated by the Holocene marine transgression (>100 m sea-level rise) following the end of the last Pleistocene ice advance ~ 20,000 years ago. The area of the U.S. Exclusive Economic Zone (U.S. EEZ) territory, extending 200 nautical miles seaward from the coast, is larger than the continental U.S. and contains submerged landforms that provide a variety of natural functions and societal benefits, such as: critical habitats for fisheries, ship navigation and homeland security, and engineering activities (i.e. oil and gas platforms, pipeline and cable routes, potential wind-energy-generation sites). Some parts of the continental margins, particularly inner-continental shelf regions, also contain unconsolidated hard-mineral deposits such as sand and gravel that are regarded as potential aggregate resources to meet or augment needs not met by onshore deposits (Williams, 1992). The present distribution of surficial sediment off the northeastern United States is shaped from the deposits left by the last glaciation and reflects the cumulative effects of sediment erosion, transport, sorting, and deposition by storm and tidal processes during the Holocene rise in sea level. As a result, the sediments on the sea floor represent both an historical record of former conditions and a guide to possible future sedimentary environments. The U.S. Geological Survey (USGS) through the Coastal and Marine Geology Program, in cooperation with the University of Colorado and other partners, has compiled extant sediment character and textural data as well as other geologic information on the sea floor from all regions around the U.S. into the usSEABED data system (Reid and others, 2005; Buczkowski and others, 2006; Reid and others, 2006). The usSEABED system, which contains information on sediment grain size and lithology for more than 340,500 stations within the U.S. EEZ. has been developed and populated with data as part of the USGS Marine Aggregate Resources and Processes and the National Benthic Habitats projects in order to provide the base-line data needed to update the current maps of offshore surficial geologic character and sediment distribution. The maps are also used to characterize benthic sea floor environments important for marine ecosystems. U.S. Geological Survey, Data Series 118 (Reid and others, 2005), of the usSEABED data release series, represents the combined efforts of the USGS and several other government agencies to provide a unified resource for accessing and preserving records of U.S. east coast sea floor geologic information and sediment texture data. For this present report, we have chosen to focus on the New York-New Jersey region, an area that has been intensely studied by the USGS for many years to address many complex issues. This report illustrates the uses of the usSEABED database for GIS applications, while offering additional insight into the resources and data available from the USGS and its collaborative institutions. This report is based on data contained in U.S. Geological Survey Data Series 118 (Reid and others, 2005) and shows an assortment of example GIS products that are possible using usSEABED. All data are intended to be GIS-ready and should not require any additional cleanup, formatting, or renaming of fields in order to use the data in a Geographic Information System. This project employs the Environmental Systems Research Institute's (ESRI) ArcView™ software. Many of these maps were made as part of the ongoing USGS study to assess marine aggregate resources offshore New York and New Jersey, but these maps can serve many other purposes. The marine science community, educators, students and others are encouraged to use these data to generate GIS products for their own purposes. The objectives of the Marine Aggregate Resources and Processes project are to produce a series of new geologic maps and reports of the sea floor that will provide scientific insights into the character and geologic development of U.S. continental margins and to use these maps and information to assess the potential availability of offshore sand and gravel resources. The mapping and aggregate resource assessments are being conducted on a national scale using the usSEABED data base as described in Williams and others (2003). Potential uses for these data include: (1) defining the geological variability of the sea floor in relation to benthic habitat diversity; (2) improving our understanding of the processes that control the distribution and transport of bottom sediments and benthic habitats; (3) locating aggregate resources for beach nourishment and industrial applications; and (4) providing a detailed geospatial framework for future marine science research, monitoring, and management activities. The initial assessments are in progress for the New York Bight and Louisiana offshore areas.

New Jersey, New York↗
USGS2007

Antarctic ice-rafted detritus (IRD) in the South Atlantic: Indicators of iceshelf dynamics or ocean surface conditions?

Ocean sediment core TN057-13PC4/ODP1094, from the Atlantic sector of the Southern Ocean, contains elevated lithogenic material in sections representing the last glacial period compared to the Holocene. This ice-rafted detritus is mainly comprised of volcanic glass and ash, but has a significant input of what was previously interpreted as quartz during peak intervals (Kanfoush et al., 2000, 2002). Our analysis of these clear mineral grains indicates that most are plagioclase, and that South Sandwich Islands is the predominant source, similar to that inferred for the volcanic glass (Nielsen et al., in review). In addition, quartz and feldspar with possible Antarctic origin occur in conjunction with postulated episodes of Antarctic deglaciation. We conclude that while sea ice was the dominant ice rafting agent in the Polar Frontal Zone of the South Atlantic during the last glacial period, the Holocene IRD variability may reflect Antarctic ice sheet dynamics.

Open-File Report↗
USGS2007

Preliminary Geologic Map of the Sanchez Reservoir Quadrangle and Eastern Part of the Garcia Quadrangle, Costilla County, Colorado

This geologic map is based entirely on new mapping by Thompson and Machette, whereas the geophysical data and interpretations were supplied by Drenth. The map area includes most of San Pedro Mesa, a basalt covered mesa that is uplifted as a horst between the Southern Sangre de Cristo fault zone (on the west) and the San Luis fault zone on the east. The map also includes most of the Sanchez graben, a deep structural basin that lies between the San Luis fault zone (on the west) and the Central Sangre de Cristo fault zone on the east. The oldest rocks in the map area are Proterozoic granites and Paleozoic sedimentary rocks, which are only exposed in a small hill on the west-central part of the mesa. The low hills that rise above San Pedro mesa are comprised of middle(?) Miocene volcanic rocks that are undated, but possibly correlative with mapped rocks to the east of Sanchez Reservoir. The bulk of the map area is comprised of the Servilleta Basalt, a regional series of flood basalts of Pliocene age. The west, north, and northeast margins of the mesa are covered by extensive landslide deposits that rest on poorly exposed sediment of the Santa Fe Group. Rare exposures of the sediment are comprised of siltstones, sandstones, and minor fluvial conglomerates. Most of the low ground surrounding the mesa is covered by surficial deposits of Quaternary age. The piedmont alluvium is subdivided into three Pleistocene units, and three Holocene units. The oldest Pleistocene gravel (unit Qao) forms an extensive coalesced alluvial fan and piedmont surface that is known as the Costilla Plains. This surface extends west from San Pedro Mesa to the Rio Grande. The primary geologic hazards in the map are are from earthquakes and landslides. There are three major fault zones in the area (as discussed above), and they all show evidence for late Pleistocene to possible Holocene movement. Two generations of landslides are mapped (younger and older), and both may have seismogenic origins.

Open-File Report↗
USGS2009

Logs of paleoseismic excavations across the Central Range Fault, Trinidad

This publication makes available maps and trench logs associated with studies of the Central Range Fault, part of the South American-Caribbean plate boundary in Trinidad. Our studies were conducted in 2001 and 2002. We mapped geomorphic features indicative of active faulting along the right-lateral, Central Range Fault, part of the South American-Caribbean plate boundary in Trinidad. We excavated trenches at two sites, the Samlalsingh and Tabaquite sites. At the Samlalsingh site, sediments deposited after the most recent fault movement bury the fault, and the exact location of the fault was unknown until we exposed it in our excavations. At this site, we excavated a total of eleven trenches, six of which exposed the fault. The trenches exposed fluvial sediments deposited over a strath terrace developed on Miocene bedrock units. We cleaned the walls of the excavations, gridded the walls with either 1 m X 1 m or 1 m X 0.5 m nail and string grid, and logged the walls in detail at a scale of 1:20. Additionally, we described the different sedimentary units in the field, incorporating these descriptions into our trench logs. We mapped the locations of the trenches using a tape and compass. Our field logs were scanned, and unit contacts were traced in Adobe Illustrator. The final drafted logs of all the trenches are presented here, along with photographs showing important relations among faults and Holocene sedimentary deposits. Logs of south walls were reversed in Illustrator, so that all logs are drafted with the view direction to the north. We collected samples of various materials exposed in the trench walls, including charcoal samples for radiocarbon dating from both faulted and unfaulted deposits. The locations of all samples collected are shown on the logs. The ages of seventeen of the charcoal samples submitted for radiocarbon analysis at the University of Arizona Accelerator Mass Spectrometry Laboratory in Tucson, Ariz., are given in Table 1. Samples found in Table 1 are shown in red on the trench logs. All radiocarbon ages are calibrated and given with 2 standard deviation age ranges. Our studies suggest that the Central Range Fault is a Holocene fault capable of producing damaging earthquakes in Trinidad

Open-File Report↗
USGS2015

The relationship between the ratio of strontium to calcium and sea-surface temperature in a modern Porites astreoides coral: Implications for using P. astreoides as a paleoclimate archive

An inverse relationship has been demonstrated between water temperature and the ratio of strontium to calcium (Sr/Ca) in coral aragonite for a number of Pacific species of the genus Porites . This empirically determined relationship has been used to reconstruct past sea-surface temperature (SST) from modern and Holocene age coral archives. A study was conducted to investigate this relationship for Porites astreoides to determine the potential for using these corals as a paleotemperature archive in the Caribbean and western tropical Atlantic Ocean. Skeletal aragonite from a P. astreoides colony growing offshore of the southeast coast of Florida was subsampled with a mean temporal resolution of 14 samples per year and analyzed for Sr/Ca. The resulting Sr/Ca time series yielded well-defined annual cycles that correspond to annual growth bands in the coral. Sr/Ca was regressed against a monthly SST record from C-MAN buoy station FWYF1 (located at Fowey Rocks, Florida), resulting in the following Sr/Ca-SST relationship: Sr/Ca = –0.040*SST + 10.128 (R = –0.77). A 10-year time series of Sr/Ca-derived SST yields annual cycles with a 10–12 degree Celsius seasonal amplitude, consistent with available local instrumental records. We conclude that Sr/Ca in Porites astreoides from the Caribbean/Atlantic region has high potential for developing subannually resolved modern and recent Holocene SST records.

Florida↗
USGS2022

Photomosaics and logs associated with study of West Napa Fault at Ehlers Lane, north of Saint Helena, California

The West Napa Fault has previously been mapped as extending ~45 kilometers (km) from northern Vallejo to southern Saint Helena, California, dominantly running along the western edge of Napa Valley. A zone of fault strands (some previously unmapped) along a ~15-km section of the fault ruptured during the 2014 magnitude 6.0 South Napa earthquake, illustrating the need for further investigation of this little-studied structure. Based on light detection and ranging (lidar) topography and field examination, the fault zone likely extends an additional 10 km or more northward past Saint Helena. In this vicinity, geomorphology suggests two fault strands, one along the range front and another associated with a line of rounded hills that rise 5–10 meters above the middle of the valley. In 2017, we excavated two trenches across an apparent fault scarp on the east side of one elongate hill near Ehlers Lane north of Saint Helena. Examination of the walls revealed three main sedimentary packages. The oldest package, weakly lithified alluvial fan gravels with local sand and silt layers, is tilted 25°–35° to the west. Overlying these tilted strata are two younger sets of strata. On the west side, underlying the crest of the scarp, are alluvial fan gravels with local sand and silt lenses, potentially tilted a few degrees to the west. On the east side, deposited against the scarp, are much finer grained (dominantly fine sand to silt) subhorizontal fluvial strata, likely overbank deposits from the Napa River. We obtained age control on the two younger units through a combination of radiocarbon, infrared-stimulated luminescence, and obsidian hydration dating, establishing that they are latest Pleistocene to modern in age. Although there are no prominent unconformities within the alluvial fan sediments, sample dating indicates there are two generations, one in the 10–20 thousand year (ka) age range and one in the <3 ka age range. Owing to a general lack of well-defined laterally continuous alluvial fan units, it is difficult to distinguish contacts between the two generations except in the immediate proximity of dated samples. The river sediments approximately span the Holocene. No faults were apparent in either trench, indicating that any fault related to the observed surface deformation has not ruptured to the surface at this site during the Holocene and is likely blind.

California↗
USGS1976

A preliminary evaluation of selected earthquake-related geologic hazards in the Kenai Lowland, Alaska

Several major faults exist beneath the Kenai Lowland, and others may be inferred. No surface evidence was found to indicate that any of the faults have been active in Holocene time. The sparse shallow seismicity thus far recorded does not correlate with known faults, nor does it define linear trends suggestive of faulting. Tidal flats at the mouths of the Kenai, Kasilof, and Chickaloon Rivers have been uplifted in the Holocene, however, and the uplift may reflect growing anticlines at depth. Some faults associated with folding may be active, but they are probably too limited in size to generate destructive earthquakes. They could, however, constitute a hazard to manmade structures in their immediate vicinity. Vibrational damage and ground failure during the 1964 earthquake were most extensive north of Tustumena Lake in areas of water-saturated unconsolidated deposits with uneven topography. South of Tustumena Lake, damage resulted mainly from landslides formed along unconfined bluffs and riverbanks, and from submarine sliding and subsidence at Homer Spit. A comparable distribution of surficial effects may be anticipated in future large earthquakes on the Kenai Lowland.

Alaska↗
USGS1977

Paleomagnetic investigation of late Quaternary sediments of south San Francisco Bay, California

Paleomagnetic inclinations of the Late Quaternary sediments of South San Francisco Bay were determined from bore hole samples collected near Dumbarton Bridge. The sediments consist of estuarine muds and nonmarine sand deposits, floored by bedrock of the Mesozoic Franciscan Formation. - Beneath Dumbarton Bridge the entire sedimentary fill is normally polarized; therefore, the fill postdates the Brunhes-Matayama polarity reversal (700,000 y. B.P.). Magnetic time lines such as the Mono Lake excursion (24,000 y. B.P.) and the reversed Blake event (110,000 y B.P.) were not found in this bore hole. In addition to Holocene and modern deposits of San Francisco Bay, an older estuarine unit occurs in the stratigraphic section. The older unit was deposited during a period of high sea level, tentatively correlated with the Sangamon interglacial period. Because evidence of the Blake event is not present in the older estuarine unit, the proposed age of this unit could not be confirmed. Although the Holocene estuarine deposits of South San Francisco Bay carry stable remanent magnetization, a reliable record of geomagnetic secular variation could not be recovered because the water-saturated sdiment was deformed by drilling.

California↗
USGS1977

Geology of the Monterey Bay region

Geophysical data and sea floor samples collected from the continental shelf and slope between Ano Nuevo Point and Point Sur, California indicate that the Monterey Bay region has had a complex late Cenozoic tectonic history. Uplift and depression have produced a succession of regressive and transgressive sedimentary units, while contemporaneous right-slip along faults of the San Andreas system have offset major structural and lithologic elements. This deformation produced three regional and several local unconformities within upper Tertiary rocks and initiated development of a canyon system that today includes the Monterey, Ascension, Carmel, and other large submarine canyons. The Tertiary stratigraphy of the offshore Monterey Bay area is divided into two provinces by a major structural boundary, the north-trending Palo Colorado-San Gregorio fault zone. East of this zone in the offshore are four seismically distinct sequences that can be correlated with major sequences onshore. These sequences comprise (1) pre-Tertiary basement, and (2) middle Miocene, (3) upper Miocene to Pliocene, and (4) upper Pliocene to Holocene sedimentary intervals. Each of the latter three sequences is bounded by unconformities, as is its counterpart on land. Only Neogene sedimentary rocks are present offshore; Paleogene units, if originally present, have been removed completely by pre-middle Miocene erosion. An extensive erosional surface was cut during Zemorrian time into the late Mesozoic granitic basement rocks. Incised into this surface are the ancestral Monterey Canyon and an unnamed canyon. Marine sedimentary rocks of upper Miocene and Pliocene age overlie this unconformably and fill the unnamed canyon. Similar rocks also may have once filled Monterey Canyon. Near shore these strata are covered by terrestrial alluvial and eolian deposits, deltaic deposits, marine canyon fill, landslide and slump deposits, and unconsolidated sediments that range in age from upper Pliocene to Holocene. Monterey Canyon appears to have been filled and exhumed at least twice since its inception in Oligocene time, once in late Miocene and once in Pleistocene time. Three major seismic stratigraphic units are apparent in continuous seismic reflection profiles from the offshore area west of the Palo Colorado-San Gregorio fault zone. These are (1) acoustical basement, and (2) middle Tertiary and (3) late Tertiary to Quaternary sedimentary intervals. Acoustical basement comprises Cretaceous to early Tertiary sedimentary rocks, Mesozoic or older metamorphic rocks, and Cretaceous or Jurassic rocks of the Franciscan assemblage. The middle Tertiary sequence consists of sedimentary rocks of questionable Miocene age. The late Tertiary to Quaternary sequence is composed of Pliocene sedimentary rocks and unconsolidated marine sediments, and submarine landslide and slump deposits. Seismic reflection surveys indicate two major, intersecting, northwest-trending fault zones to be present in the offshore Monterey Bay area. The Palo Colorado-San Gregorio fault zone may be more than 200 km long; it is narrow (approximately 3 km wide) and is represented in most places by one or two faults. This zone appears to connect with faults mapped on land near Ano Nuevo Point and Point Sur. The Monterey Bay fault zone, located in the area between Santa Cruz and Monterey, is a diffuse zone, approximately 10 to 15 km wide, of en echelon faults. Faults within this zone appear to connect with faults on land near Monterey, and the zone appears to be truncated by the Palo Colorado-San Gregorio fault zone west of Santa Cruz. Locations of more than 110 earthquakes (1968-1976) show that the newly mapped fault zones in Monterey Bay are seismically active. Epicenters in the bay form two clusters, one at the intersection of the Monterey Bay and Palo Colorado-San Gregorio fault zones, and the other in a linear belt that trends northwest along the Palo Colorado-San Gregorio fault zone. Faults within th

California↗
USGS1982

Late Cenozoic stratigraphy and structure of the western margin of the central San Joaquin Valley, California

Late Cenozoic Stratigraphy Late Cenozoic deposits in the west-central San Joaquin Valley and adjacent foothills of the Diablo Range consist mainly of unconsolidated, poorly-sorted to well-sorted gravel, sand, silt and clay derived primarily from the Diablo Range and secondarily from the Sierra Nevada. Sedimentary structures, such as channeled contacts, laminated bedding, cross-stratification and clast-imbrication indicate that most of the deposits were transported and laid down by running water. These deposits are described and their facies relationships are illustrated in the 'Late Cenozoic Stratigraphy' section of this report (see Figures 17, and 26, and Table 9). Sediment shed from the Diablo Range accumulated primarily as a complex of coalescing alluvial fans on the piedmont slope of a San Joaquin Valley that at one time extended across the foothill belt to the present margin of the central Diablo Range; and as local fills within stream valleys of the Diablo Range foothills tributary to the San Joaquin Valley. These deposits are well exposed in Interstate-5 roadcuts, California Aqueduct and Delta-Mendota canal cuts, and stream banks along the many ephemeral and intermittent streams draining the Diablo Range. Sediment derived from the Sierra Nevada is confined primarily to the floodbasin of the San Joaquin Valley. It includes arkosic riverine and floodbasin deposits from the San Joaquin River and associated sloughs, as well as local ephemeral and perennial pond, swamp, oxbow-lake and lake deposits. These deposits are well-exposed in stream banks of the San Joaquin River and a few of the larger sloughs such as Salt Slough, Mud Slough and Kings Slough. Well-sorted, fine- and medium-grained, quartzose, cross-bedded sand, presumably derived from the Sierra Nevada, locally interfinger with or underlie fine-grained Coast Range alluvial-fan deposits. The sand probably originated by eolian reworking of Sierran alluvium from the floodbasin of the lower San Joaquin River or from fans of the northeastern San Joaquin Valley. These deposits are locally well exposed in Interstate-5 roadcuts, primarily between Orestimba and Garzas Creeks. The geomorphic character of the alluvium laid down by streams draining the Diablo Range reflects late Cenozoic uplift of the foothills and subsidence of the valley. Within the foothills and near the foothill-valley margin, the deposits form a sequence of inset stream terraces and nested alluvial fans. Valleyward, however, each deposit forms a veneer over older alluvial-fan deposits. Based primarily on geomorphic and pedologic indicators of relative age (see Figure 19 and Table 10), and to a lesser extent on lithologic and absolute age criteria, the late Cenozoic deposits are divided into five stratigraphic units. In order of decreasing age, these include the formally recognized Tulare Formation (Watts, 1894; Anderson, 1905) of late Pliocene and Pleistocene age, and the informally named Los Banos alluvium of middle and late Pleistocene age, San Luis Ranch alluvium of late Pleistocene and early Holocene age, and Patterson alluvium and Dos Palos alluvium of Holocene age. The Los Banos and San Luis Ranch alluvium are further divided into three and two members, respectively. Each of these members ranges in thickness from less than I m up to 15 m and thus represents, at least in part, a distinct period of aggradation. The lithology age and distribution of these units is described in the 'Stratigraphie Divisions' section of this report and is summarized in Figure 25 and Table 11. Plates 1 through 23 show the local distribution of these units on 7.5-minute Quadrangles. Mapping criteria are diagrammatically illustrated in Figure 19 and described in the 'Mapping Criteria' section of this report. Indirect evidence suggests that deposition of these units resulted primarily from climatic change rather than intermittent uplift of the Diablo Range. The units are recognized throughout 1500 Km

Open-File Report↗
USGS1994

Preliminary geomagnetic paleointensities from Long Valley Caldera, California

Paleointensities are being determined in order to characterize geomagnetic intensity variations during Quaternary time. Because the geomagnetic field acts as a shield against cosmic radiation, its intensity directly affects the production rate of radiocarbon (^C) in the upper atmosphere. Coe and others (1978) showed that the variation in dipole moment during Holocene time is nearly proportional to the difference between ^C and tree-ring age. The ^C timescale is currently well calibrated to about 9000 years B.P., and tentatively to about 13,000 years B.P., by comparison with tree-ring and glacial-varve chronologies (Stuiver and others, 1986). However, a comparison of ages determined by the l^C and 234u_230rh methods on coral samples off the island of Barbados (Bard and others, 1990) showed that U-Th ages were consistently older than ^4C ages in material older than about 9,000 years. The discrepancy is so large (3,500 years at 25,000 years B.P.) that Bard and others consider that it can only be due to a significant decrease in geomagnetic intensity during the last glacial period. Dipole field intensity has a similar modulating effect on the production of other cosmogenic nuclides that are used for geochronologic studies and will also affect their calibration curves. Mankinen and Champion (1993a,£) recently established broad trends in geomagnetic paleointensity for latest Pleistocene and Holocene time on Hawaii. The current study is the beginning of an effort to establish a similar record for western North America. Data from North America and Hawaii will eventually be averaged with other data from globally distributed locations to eliminate the effects on nondipole variations so that a true picture of the global (dipole) field can be obtained. The dipole variations can then be used to model accurately the production rate of cosmogenic nuclides as has been attempted by Mazaud and others (1991, 1994) using some relative paleointensity records from marine sediment cores.

California↗
USGS1995

Preliminary map of peak horizontal ground acceleration for the Hanshin-Awaji earthquake of January 17, 1995, Japan - Description of Mapped Data Sets

The Hanshin-Awaji earthquake (also known as the Hyogo-ken Nanbu and the Great Hanshin earthquake) provided an unprecedented set of measurements of strong ground shaking. The measurements constitute the most comprehensive set of strong- motion recordings yet obtained for sites underlain by soft soil deposits of Holocene age within a few kilometers of the crustal rupture zone. The recordings, obtained on or near many important structures, provide an important new empirical data set for evaluating input ground motion levels and site amplification factors for codes and site-specific design procedures world wide. This report describes the data used to prepare a preliminary map summarizing the strong motion data in relation to seismicity and underlying geology (Wentworth, Borcherdt, and Mark., 1995; Figure 1, hereafter referred to as Figure 1/I). The map shows station locations, peak acceleration values, and generalized acceleration contours superimposed on pertinent seismicity and the geologic map of Japan. The map (Figure 1/I) indicates a zone of high acceleration with ground motions throughout the zone greater than 400 gal and locally greater than 800 gal. This zone encompasses the area of most intense damage mapped as JMA intensity level 7, which extends through Kobe City. The zone of most intense damage is parallel, but displaced slightly from the surface projection of the crustal rupture zone implied by aftershock locations. The zone is underlain by soft-soil deposits of Holocene age.

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