USGS Science⌕ Search

Geology topics

R. J. Anima

Publications and source records attributed to R. J. Anima.

13 recordsLinked to original sources
USGS2003

Budgeting postglacial sedimentation history on the Santa Cruz, California mid-continental shelf

High-resolution seismic reflection profiling and surface texture mapping of the central California continental shelf, reveal a prominent subsurface reflector interpreted as a low stand erosion surface and an overlying mudbelt that covers 421 km 2 of the mid-shelf in depths of 40-90 m. Radiometric and sedimentologic analyses of samples from vibracores taken along the seaward edge of the mudbelt show that initial deposition above the pre-Holocene erosion surface began ca. 14.5 ka. These data and model results of sea-level history, tectonics, and the Monterey Bay littoral sediment budget support the notion that the entire midshelf deposit was formed during the postglacial transgression. An alternative explanation, that <30% of the deposit is Holocene, requires that (1) sediment input is overestimated and/or loss is greatly underestimated, and (2) preservation on the shelf was significant despite deep and active wave scour observed in the form of rapid cliff and bedrock cutting early and late in the transgression. The difference between a basal age of ~14.5 ka and residence time of midshelf sediment (3,273 years), derived from dividing mudbelt volume by modern accumulation rate, implies: (1) significant sediment loss occurred since the mudbelt formed and/or (2) sediment accumulation has varied greatly over time. Although modern sediment budgets are relatively well constrained, it remains uncertain how well we can apply them to the past. An evolving model of sedimentation history explores the likelihood of changes in sediment supply, accumulation patterns, and depositional patterns owing to postglacial sea-level history and human land-use activities while providing important boundary conditions for modeling shoreface evolution.

California↗
USGS2002

Seafloor geology of the Monterey Bay area continental shelf

Acoustic swath-mapping of the greater Monterey Bay area continental shelf from Point An??o Nuevo to Point Sur reveals complex patterns of rock outcrops on the shelf, and coarse-sand bodies that occur in distinct depressions on the inner and mid-shelves. Most of the rock outcrops are erosional cuestas of dipping Tertiary rocks that make up the bedrock of the surrounding lands. A mid-shelf mud belt of Holocene sediment buries the Tertiary rocks in a continuous, 6-km-wide zone on the northern Monterey Bay shelf. Rock exposures occur on the inner shelf, near tectonically uplifting highlands, and on the outer shelf, beyond the reach of the mud depositing on the mid-shelf since the Holocene sea-level rise. The sediment-starved shelf off the Monterey Peninsula and south to Point Sur has a very thin cover of Holocene sediment, and bedrock outcrops occur across the whole shelf, with Salinian granite outcrops surrounding the Monterey Peninsula. Coarse-sand deposits occur both bounded within low-relief rippled scour depressions, and in broad sheets in areas like the Sur Platform where fine sediment sources are limited. The greatest concentrations of coarse-sand deposits occur on the southern Monterey Bay shelf and the Sur shelf. ?? 2002 Elsevier Science B.V. All rights reserved.

Marine Geology↗
USGS2002

Nearshore morphology and late Quaternary geologic framework of the northern Monterey Bay Marine Sanctuary, California

A combination of side-scanning sonar and high-resolution seismic reflection data image seafloor bedrock exposures and erosional features across the nearshore shelf. Sediment-filled troughs incise the inner shelf rock exposures and tie directly to modern coastal streams. The resulting bedrock geometry can be related to its resistance to erosion. Comparison of the depth of the transgressive erosional surface to recently developed sea level curves suggests a period of slow sea level rise during the early stages of post-interglacial marine transgression. The slow rise of sea level suggests an erosional episode that limited the preservation of buried paleo-channels beyond 70 m water depth. Seafloor features suggest that localized faulting in the area may have influenced the morphology of bedrock exposures and the coastline. ?? 2002 Elsevier Science B.V. All rights reserved.

Marine Geology↗
USGS1999

Development of San Leandro synform and neotectonics of the San Francisco Bay block, California

High-resolution, 24-channel seismic-reflection data show a stratified synform beneath south San Francisco Bay. These seismic-reflection data reveal an eastward-dipping bedrock surface that is about 40 m deep (subbottom) beneath the western south bay, and that reaches a maximum observed depth of 500-800 m (subbottom) below the eastern half of the south bay. An angular unconformity cuts both the synform and underlying bedrock. The age of the unconformity is unknown but may be Pleistocene, when these strata forming the synform were presumably exposed subaerially during lowered sea levels. The synformal strata, the unconformity, and some generally flat-lying and overlying strata are folded near the eastern shore of the bay. This folding may result from movement on the Hayward fault (fault interactions and localized strain partitioning) or from compressional deformation in the East Bay Hills related to NE-SW ('fault-normal') convergence between the Pacific and North American plates. In general, reflections from sediment overlying the unconformity are flat lying (except near the eastern shore of the bay), whereas reflections beneath the unconformity dip eastward. The overlying, flat sediment section fills a shallow basin that is coincident with an elliptical residual gravity low. This low appears to be related to the deeper sedimentary, synformal section based on the spatial correlation between the east-dipping reflections and the gravity anomaly. Projecting the east-dipping reflections to the center of the gravity low suggests that the total section of flat-lying and dipping reflections in the synform may exceed 1000 m. Modeling of the gravity low suggests a total low-density section, about 1.5 km thick, at the center of the synform relative to the surrounding bedrock of presumed Franciscan Complex.High-resolution, 24-channel seismic-reflection data show a stratified synform beneath south San Francisco Bay. These seismic-reflection data reveal an eastward-dipping bedrock surface that is about 40 m deep (subbottom) beneath the western south bay, and that reaches a maximum observed depth of 500-800 m (subbottom) below the eastern half of the south bay. An angular unconformity cuts both the synform and underlying bedrock. The age of the unconformity is unknown but may be Pleistocene, when these strata forming the synform were presumably exposed subaerially during lowered sea levels. The synformal strata, the unconformity, and some generally flat-lying and overlying strata are folded near the eastern shore of the bay. This folding may result from movement on the Hayward fault (fault interactions and localized strain partitioning) or from compressional deformation in the East Bay Hills related to NE-SW (`fault-normal') convergence between the Pacific and North American plates. In general, reflections from sediment overlying the unconformity are flat lying (except near the eastern shore of the bay), whereas reflections beneath the unconformity dip eastward. The overlying, flat sediment section fills a shallow basin that is coincident with an elliptical residual gravity low. This low appears to be related to the deeper sedimentary, synformal section based on the spatial correlation between the east-dipping reflections and the gravity anomaly. Projecting the east-dipping reflections to the center of the gravity low suggests that the total section of flat-lying and dipping reflections in the synform may exceed 1000 m. Modeling of the gravity low suggests a total low-density section, about 1.5 km thick, at the center of the synform relative to the surrounding bedrock of presumed Franciscan Complex.

Marine and Petroleum Geology↗
USGS1999

Sedimentary record of anthropogenic and biogenic polycyclic aromatic hydrocarbons in San Francisco Bay, California

Dated sediment cores collected from Richardson and San Pablo Bays in San Francisco Bay were used to reconstruct a history of polycyclic aromatic hydrocarbon (PAH) contamination. The sedimentary record of PAHs in Richardson Bay shows that anthropogenic inputs have increased since the turn of the century, presumably as a result of increasing urbanization and industrialization around the Bay Area. Concentrations range from about 0.04-6.3 ??g g-1. The dominant origin of the PAHs contributing to this modern contamination is from combustion processes. Depth profiles in San Pablo Bay indicate higher concentrations of PAHs since the 1950s than during the late 1800s, also presumably resulting from an increase in urbanization and industrialization. Total PAHs in San Pablo Bay range from about 0.04-1.3 ??g g-1. The ratios of methylphenanthrenes/phenanthrene and (methylfluoranthenes + methylpyrenes)/fluoranthene were sensitive indicators of anthropogenic influences in the estuary. Variations in the ratio of 1,7-dimethylphenanthrene/2,6-dimethylphenanthrene indicate a gradual replacement of wood by fossil-fuel as the main combustion source of PAHs in. San Francisco Bay sediments. The profile of perylene may be an indicator of eroding peat from marshlands.

California↗
USGS1996

Misinterpretation of lateral acoustic variations on high-resolution seismic reflection profiles as fault offsets of Holocene bay mud beneath the southern part of San Francisco Bay, California

We collected high-resolution seismic reflection profiles in the southern part of San Francisco Bay in 1992 and 1993 to investigate possible Holocene faulting along postulated transbay bedrock fault zones. The initial analog records show apparent offsets of reflection packages along sharp vertical boundaries. These records were originally interpreted as showing a complex series of faults along closely spaced, sharp vertical boundaries in the upper 10 m (0.013 s two-way travel time) of Holocene bay mud. A subsequent survey in 1994 was run with a different seismic reflection system, which utilized a higher power source. This second system generated records with deeper penetration (max. 20 m, 0.026 s two-way travel time) and demonstrated that the reflections originally interpreted as fault offsets by faulting were actually laterally continuous reflection horizons. The pitfall in the original interpretations was caused by lateral variations in the amplitude brightness of reflection events, coupled with a long (greater than 15 ms) source signature of the low-power system. These effects combined to show apparent offsets of reflection packages along sharp vertical boundaries. These boundaries, as shown by the second system, in fact occur where the reflection amplitude diminishes abruptly on laterally continuous reflection events. This striking lateral variation in reflection amplitude is attributable to the localized presence of biogenic(?) gas.

Marine and Petroleum Geology↗
USGS1989

Subaqueous grain flows at the head of Carmel Submarine Canyon, California

The head of Carmel Submarine Canyon lies in 15 m of water about 200 m off a coarse-sand beach in the southeast corner of Carmel Bay, California. Very coarse sand is the predominate material on the beach, adjacent shelf, and upper canyon-head slopes, while silt and clay cover the surface below a water depth of about 35 m. Along a shore-normal transect, median grain size decreases between the beach and canyon rim but increases down the upper canyon-head slope. On angle-of-repose slopes in the upper canyon head, downslope-coarsening deposits, which are everywhere there is active sand movement, are similar to a type of sediment gravity flow deposit formed by grain flows (sand avalanches). Using three sand fractions that were dyed different fluorescent colors, scuba divers generated sand avalanches that produced deposits similar to the natural deposits. The dyed-sand deposits, which extended as far as 25 m below the initiation point, were inversely graded and increased in grain size downslope. Inverse grading was well developed within two m of the initiation point, though the thicknesses of the three layers varied in a nonsystematic manner both across and down slope. The minor amount of native material found within the dyed-sand layers showed that entrainment of the underlying sand was minimal.

Journal of Sedimentary Petrology↗
USGS1988

Coarse-sediment bands on the inner shelf of southern Monterey Bay, California

Bands of coarse sand that trend parallel to the shore, unlike the approximately shore-normal bands found in many inner shelf areas, occur in southern Monterey Bay at water depths of 10–20 m, less than 1 km from the shore. The bands are 20–100 m wide and alternate with bands of fine sand that are of similar width. The coarse-sand bands are as much as 1 m lower than the adjacent fine-sand bands, which have margins inclined at angles of about 20°. The mean grain sizes of the coarse and fine sand are in the range of 0.354–1.0 mm and 0.125–0.354 mm, respectively. Wave ripples that average about 1 m in spacing always occur in the coarse-sand bands.

Marine Geology↗
USGS1982

A field study of littoral processes in Estero Bay, California

Estero Bay, which lies on the central California coast, has rocky headlands at both ends and sandy beaches within it. The shoreline of the bay has adjusted to be in equilibrium with the predominant wave climate, which is from the northwest. Because of its present shoreline configuration, the net southward littoral transport found along much of the California coast does not occur within Estero Bay. Instead, the sand primarily moves on- and offshore with a reversing longshore component. This sand transport pattern produces a littoral cell within Estero Bay even though there is no submarine canyon in the area. The primary sand sinks for this cell appear to be the sand spit south of Morro Rock and the entrance to Morro Bay itself, although this opinion was not experimentally verified. Field work during one summer (1978) and the following winter (1979) produced baseline data on the profile of and grain-size distribution across the littoral zone. In the offshore part of the littoral zone we also studied ripple size and type, internal structure, depth of erosion, and mineralogy. Although these data, which were collected along nine transects spaced 2 km apart, are inadequate to yield transport and energy rates, they indicate a northward decrease in wave energy within Estero Bay and a mixing of the sediments in the offshore. Box core and rod height data from grid points in seven meters of water showed that on the order of a meter of erosion occurred in the central part of the bay between the two sampling periods. Offshore, the data were incomplete, but at one station, in 17 m of water, at least 20 cm of erosion occurred.

Open-File Report↗