Search USGS⌕ Search

Geology topics

D.W. Ramsey

Publications and source records attributed to D.W. Ramsey.

6 recordsLinked to original sources
USGS2002

Morphology, volcanism, and mass wasting in Crater Lake, Oregon

Crater Lake was surveyed nearly to its shoreline by high-resolution multibeam echo sounding in order to define its geologic history and provide an accurate base map for research and monitoring surveys. The bathymetry and acoustic backscatter reveal the character of landforms and lead to a chronology for the concurrent filling of the lake and volcanism within the ca. 7700 calibrated yr B.P. caldera. The andesitic Wizard Island and central-plattform volcanoes are composed of sequences of lava deltas that record former lake levels and demonstrate simultaneous activity at the two vents. Wizard Island eruptions ceased when the lake was ~80 m lower than at present. Lava streams from prominent channels on the surface of the central platform descended to feed extensive subaqueous flow fields on the caldera floor. The Wizard Island and central-platform volcanoes, andesitic Merriam Cone, and a newly discovered probable lava flow on the eastern floor of the lake apparently date from within a few hundred years of caldera collapse, whereas a small rhydacite dome was emplaced on the flank of Wizard Island at ca. 4800 cal. yr B.P. Bedrock outcrops on the submerged caldera walls are shown in detail and, in some cases, can be correlated with exposed geologic units of Mount Mazama. Fragmental debris making up the walls elsewhere consists of narrow talus cones forming a dendritic pattern that leads to fewer, wider ridges downslope. Hummocky topography and scattered blocks up to ~280 m long below many of the embayments in the caldera wall mark debris-avalanche deposits that probably formed in single events and commonly are affected by secondary failures. The flat-floored, deep basins contain relatively fine-grained sediment transported from the debris aprons by sheet-flow turbidity currents. Crater Lake apparently filled rapidly (ca. 400-750 yr) until reaching a permeable layer above glaciated lava identified by the new survey in the northeast caldera wall at ~1845 m elevation. Thereafter, a gradual, climatically modulated rise in lake level to the present 1883 m produced a series of beaches culminating in a modern wave-cut platform, commonly ~40 m wide, where suitable material is present. The new survey reveals landforms that result from intermediate-composition volcanism in rising water, delineates mass wasting and sediment transport into a restricted basin, and yields a more accurate postcaldera history leading to improved assessment of volcanic hazards.

Oregon↗
USGS1999

Fluid migration in a cratonic setting: The fluid histories of two fault zones in the eastern midcontinent

A combined field, petrographic, fluid inclusion, and stable isotope study was undertaken in two fault zones in the eastern midcontinent of the North American craton in order to determine their fluid histories. Because both the Kentucky River fault zone in central Kentucky and the Bowling Green fault zone in northwest Ohio were active intermittently throughout much of the Paleozoic, it was thought that one or both may record the passage of the late Paleozoic brine migration that affected large portions of the eastern midcontinent. Three fluid events were recognized in calcite veins of the Kentucky River fault zone. Each tapped the same dominantly meteoric, low-salinity fluid reservoir, but at different times as the fault zone was cooling (T(h) 110??to 75??C) at relatively shallow depths (<1.0 km). Although the fluid history of the Bowling Green fault zone also reflects a general cooling (T(h) 115??to 60??C) at a shallow depth (<1.5 km), multiple fluid sources were involved. In the first fluid event, brown calcite was precipitated from a methane-rich, aqueous fluid with an immiscible petroleum phase derived from ascending fluids originating in underlying lower Paleozoic or basement units. The second fluid event is similar to the first except it lacks the petroleum phase which resulted in the precipitation of white, rather than brown, calcite. The third event precipitated calcite from a mixture of vertically and horizontally flowing brines. The youngest event resulted in little or no additional mineralization and is recorded by secondary fluid inclusions in preexisting veins. The fluid source is probably meteoric or seawater. From the characteristics of each fluid event, it is concluded that only the Bowling Green fault zone appears to contain evidence for the late Paleozoic regional brine migration. The Kentucky River fault zone either was bypassed by the brines, had an unfavorable orientation, or did not have any permeability at the time of brine migration.

Tectonophysics↗
USGS1999

How useful is landslide hazard information? Lessons learned in the San Francisco Bay region

Landslides, worldwide and in the United States, are arguably the most costly natural hazard. Substantial landslide information is available, but much of it remains underutilized, as a disconnect exists among geologists, decision makers, and the public. The lack of a national landslide insurance policy exacerbates this situation and promotes litigation as the principal recourse for recouping landslide-damage losses. The U.S. Geological Survey's landslide investigation in the San Francisco Bay region of California provides a context for making suggestions on how Earth science information could be used more effectively.

International Geology Review↗
USGS1998

Slope failure and shoreline retreat during northern California's latest El Nino

Surface processes accelerated by severe storms during the 1997–1998 El Niño event scoured hillsides and damaged property across coastal California. Technological advances such as digital mapping, exemplified here for the San Francisco Bay area but applicable elsewhere, have enabled government agencies to better describe, monitor, and predict the effects of shoreline erosion and slope failure.

California↗