Geology topics

T.A. Brown

Publications and source records attributed to T.A. Brown.

4 recordsLinked to original sources

USGS2017

Late Quaternary distal tephra-fall deposits in lacustrine sediments, Kenai Peninsula, Alaska

Tephra-fall deposits from Cook Inlet volcanoes were detected in sediment cores from Tustumena and Paradox Lakes, Kenai Peninsula, Alaska, using magnetic susceptibility and petrography. The ages of tephra layers were estimated using 21 14 C ages on macrofossils. Tephras layers are typically fine, gray ash, 1–5 mm thick, and composed of varying proportions of glass shards, pumice, and glass-coated phenocrysts. Of the two lakes, Paradox Lake contained a higher frequency of tephra (0.8 tephra/100 yr; 109 over the 13,200-yr record). The unusually large number of tephra in this lake relative to others previously studied in the area is attributed to the lake's physiography, sedimentology, and limnology. The frequency of ash fall was not constant through the Holocene. In Paradox Lake, tephra layers are absent between ca. 800–2200, 3800–4800, and 9000–10,300 cal yr BP, despite continuously layered lacustrine sediment. In contrast, between 5000 and 9000 cal yr BP, an average of 1.7 tephra layers are present per 100 yr. The peak period of tephra fall (7000–9000 cal yr BP; 2.6 tephra/100 yr) in Paradox Lake is consistent with the increase in volcanism between 7000 and 9000 yr ago recorded in the Greenland ice cores.

Quaternary Research↗

USGS2008

Modeled tephra ages from lake sediments, base of Redoubt Volcano, Alaska

A 5.6-m-long lake sediment core from Bear Lake, Alaska, located 22 km southeast of Redoubt Volcano, contains 67 tephra layers deposited over the last 8750 cal yr, comprising 15% of the total thickness of recovered sediment. Using 12 AMS 14C ages, along with the 137Cs and 210Pb activities of recent sediment, we evaluated different models to determine the age-depth relation of the core, and to determine the age of each tephra deposit. The selected age model is based on a mixed-effect regression that was passed through the adjusted tephra-free depth of each dated layer. The estimated age uncertainty of the 67 tephras averages ??105 yr (95% confidence intervals). Tephra-fall frequency at Bear Lake was among the highest during the past 500 yr, with eight tephras deposited compared to an average of 3.7/500 yr over the last 8500 yr. Other periods of increased tephra fall occurred 2500-3500, 4500-5000, and 7000-7500 cal yr. Our record suggests that Bear Lake experienced extended periods (1000-2000 yr) of increased tephra fall separated by shorter periods (500-1000 yr) of apparent quiescence. The Bear Lake sediment core affords the most comprehensive tephrochronology from the base of the Redoubt Volcano to date, with an average tephra-fall frequency of one every 130 yr. ?? 2007 Elsevier Ltd. All rights reserved.

Quaternary Geochronology↗

USGS1999

Potentiometric levels and water quality in the aquifers underlying Belvidere, Illinois, 1993–96

In 1992, the U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency (USEPA), began a study of the hydrogeology and water quality of the aquifers underlying the vicinity of Belvidere, Boone County, Ill. Previously, volatile organic compounds (VOC's) and other constituents of industrial origin were detected in one or more ground-water samples from about 100 of the approximately 700 monitoring and water-supply wells in the area, including the 8 municipal wells in Belvidere. A glacial drift aquifer underlies at least 50 percent of the 80-square-mile study area; bedrock aquifers that underlie virtually all of the study area include the Galena-Platteville, St. Peter Sandstone, Ordovician, and Cambrian-Ordovician aquifers. During 1993, water levels were measured in 152 wells and water-quality samples were collected from 97 wells distributed throughout the study area. During 1994–96, similar data were collected from 31 wells. Potentiometric levels in the glacial drift and Galena-Platteville aquifers are similar and range from about 750 to 900 feet above sea level. The potentiometric surfaces of the aquifers are subdued representations of the land surface. Horizontal ground-water flow in the aquifers primarily is towards the Kishwaukee River, which flows through the central part of the study area, and its principal tributaries. Vertical ground-water flow appears to be downward at most locations in the study area, particularly in the urbanized areas affected by pumping of the Belvidere municipal wells and upland areas remote from the principal surface-water drainages. Flow appears to be upward between the Galena-Platteville and glacial drift aquifers where ground water discharges to the Kishwaukee River and its principal tributaries. All water samples were analyzed for VOC's. Selected samples also were analyzed for trace metals, cyanide, semivolatile organic compounds, or other constituents. VOC's were detected in samples from 50 wells (52 percent of total wells sampled). Twenty-seven specific VOC's were identified in the samples. Samples were collected from six municipal wells in use during the study; two wells were not in use because one or more VOC's exceeded maximum contaminant levels (MCL's). Two VOC's were detected in one of the samples at concentrations below MCL's established by the USEPA for protection of public-water supplies. Samples from 21 wells had at least one VOC detected at a concentration above MCL's. The VOC's detected above MCL's and their maximum concentrations were 1,2-dichloroethene (total), 470 micrograms per liter; trichloroethene (TCE), 360 micrograms per liter; tetrachloroethene (PCE), 82 micrograms per liter; benzene, 53 micrograms per liter; and vinyl chloride, 11 micrograms per liter. TCE and PCE were the most frequently detected VOC's and generally had the highest concentrations. VOC's with concentrations above MCL's were detected in samples from 15 wells open to the glacial drift aquifer and 6 wells open to the Galena-Platteville aquifer. Generally, the concentrations of VOC's were higher, and number and type of VOC's detected were greater in the glacial drift aquifer than in the Galena-Platteville aquifer and the deeper bedrock aquifers. The high concentrations and spatial distribution of VOC's in the glacial drift aquifer usually were related to nearby sources of contamination. Except in the immediate vicinity of a known hazardous-waste site, possible sources of VOC's in the bedrock aquifers were difficult to identify in the study area; VOC concentrations at most locations in the bedrock aquifers were below 5 micrograms per liter. Most locations where VOC's were detected in the glacial and bedrock aquifers were within about 1,000 feet of the Kishwaukee River. Hydrogeologic factors that affect the distribution of VOC's in the aquifers include ground-water flow through (1) the glacial drift aquifer with discharge to the nearby Kishwaukee River; and (2) the weathered-surface deposits, bedding-plane partings, and fractures in the Galena-Platteville aquifer. One bedding-plane parting intersecting wells that represent an area of about 1.5 square miles has a horizontal hydraulic conductivity as high as 220 feet per day. Pumping of high-capacity wells may contribute to the widespread distribution of VOC’s at low concentrations in the bedrock aquifers.

Illinois↗

USGS1997

Rock-stratigraphic nomenclature, lithology, and subcrop area of the Galena-Platteville bedrock unit in Illinois and Wisconsin

The Galena-Platteville bedrock unit is a dependable source of ground water for many private well owners and some municipal-water-supply systems in northern Illinois (Hackett, 1960) and in Wisconsin. The carbonate lithology of the unit contributes to the availability of ground water and also to the susceptibility of the unit to ground-water contamination. Susceptibility to contamination is greatest in areas where the unit is overlain by only a thin layer (less than 50 feet) of soil or unconsolidated glacial deposits. Within the study area in Illinois and Wisconsin (fig. 1), volatile organic compounds and other contaminants have been detected in groundwater samples from various sites (Kay and others, 1989; Mills, 1993a, 1993b; Kay and others, 1994). Known and suspected sources of contaminants are numerous, including landfills and industrial facilities. To determine the possible effects of contamination on the ground-water supply, an understanding of the regional hydrogeologic framework of the Galena-Platteville bedrock unit is needed. Published map and point data describing the geologic and hydrologic properties of the Galena-Platteville bedrock unit are available from many sources. The U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency, Region 5, has selected and compiled pertinent data. The objective of this study is to publish these data in a series of concise map reports and a bibliographic report listing available sources of information by county for the Galena-Platteville bedrock unit. Investigators involved in site-specific studies within the subcrop area will be able to utilize these reports to design effective site investigations. This report presents the rock-stratigraphic nomenclature of the lithologic units that make up the Galena-Platteville bedrock unit (fig.2) and provides a brief, generalized description of the lithologic characteristics of each unit. Sources with more detailed descriptions of lithology can be found below in SELECTED REFERENCES. Figure 3 is a map, created from published maps of various scales, showing the areal extent of the Galena-Platteville subcrop and major known geologic structural features in Illinois and Wisconsin. The subcrop area of the Galena-Platteville bedrock unit is that area where the unit crops out, or is the uppermost bedrock unit and is overlain by soil or glacial deposits. The unit is present at depth under younger bedrock units south and east of the subcrop area and is absent north and west of the subcrop area. Data sources used to prepare the map are included in SELECTED REFERENCES.

Illinois, Wisconsin↗