Geology topics
Publications and source records attributed to Anne E. McCafferty.
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The area immediately east of Baker Creek, Blaine County, Idaho, is underlain by a thick section of mafic to intermediate lava flows of the Eocene Challis Volcanic Group. Widespread propylitic alteration surrounds a zone of argillic alteration and an inner core of phyllic alteration. Silicified breccia is present along an east-trending fault within the zone of phyllic alteration. As part of a reconnaissance geochemical survey, soils and plants were sampled. Several species of plants (Douglas-fir [ Pseudotsuga menziesii ], mountain big sagebrush [ Artemisia tridentata ssp. vaseyana ], and elk sedge [ Carex geyerii ]) were collected from 10 upland localities and stream sediments, panned concentrates, and aquatic mosses were collected from 16 drainage basin localities all of which were generally within the area of alteration. Geochemical results yielded anomalous concentrations of molybenum, zinc, silver, and lead in at least half of the seven different sample media and of gold, thallium, arsenic, antimony, manganese, boron, cadmium, bismuth, copper, and beryllium in from one to four of the various media. Part of this suite of elements? silver, gold, arsenic, antimony, thallium, and manganese? suggests that the mineralization in the area is epithermal. Barite and pyrite (commonly botryoidal-framboidal) are widespread throughout the area sampled. Visible gold and pyromorphite (a secondary lead mineral) were identified in only one small drainage basin, but high levels of gold were detected in aquatic mosses over a larger area. Data from the upland and stream sampling indicate two possible mineralized areas. The first mineralized area was identified by a grab sample from an outcrop of quartz stockwork that contained 50 ppb Au, 1.5 ppm Ag, and 50 ppm Mo. Although the soil and plant species that were sampled in the area indicated mineralized bedrock, the Douglas-fir samples were the best indicators of the silver anomaly. The second possible mineralized area centers on the fault-controlled silicified breccia that is most likely the source of anomalous silver and molybdenum levels identified in the soils; silver, molybdenum, and manganese in stream sediments; thallium in Douglas-fir; bismuth and silver in concentrates; and gold, silver, arsenic, antimony, and molybdenum and lead in aquatic mosses. An interpretation of regional aeromagnetic data delineated the subsurface extent of shallow, steeply dipping magnetic sources inferred to be shallower parts of an Eocene batholith thought to underlie much of the Baker Creek area. The Eocene intrusive event(s) may have served as the heat source(s) that caused the hydrothermal alteration. Examination of core from a 1,530-ft-deep (466 m) hole drilled in 1982 confirmed a bedrock source for the anomalous silver and base-metal suite at the quartz stockwork location, and indicated subeconomic levels of molybdenum.
The geologic map of the Duncan Canal-Zarembo Island area is the result of a multidisciplinary investigation of an area where an airborne geophysical survey was flown in the spring of 1997. The area was chosen for the geophysical survey because of its high mineral potential, a conclusion of the Petersburg Mineral Resource Assessment Project, conducted by the U.S. Geological Survey from 1978 to 1982. The City of Wrangell, in southeastern Alaska, the Bureau of Land Management, and the State of Alaska provided funding for the airborne geophysical survey. The geophysical data from the airborne survey were released in September 1997. The U.S. Geological Survey conducted field investigations in the spring and fall of 1998 to identify and understand the sources of the geophysical anomalies from the airborne survey. This geologic map updates the geologic maps of the same area published by David A. Brew at 1:63,360 (Brew, 1997a-m; Brew and Koch, 1997). This update is based on 3 weeks of field work, new fossil collections, and the geophysical maps released by the State of Alaska ( DGGS, Staff, and others, 1997a-o). Geologic data from outcrops, fossil ages, radiometric ages, and geochemical signatures were used to identify lithostratigraphic units. Where exposure is poor, geophysical characteristics were used to help control the boundaries of these units. No unit boundaries were drawn based on geophysics alone. The 7200 Hertz resistivity maps (DGGS, Staff, and others, 1997k-o) were particularly helpful for controlling unit boundaries, because different stratigraphic units have distinctive characteristic conductive signatures (Karl and others, 1998). Increased knowledge of unit ages, unit structure, and unit distribution, led to improved understanding of the nature of unit contacts. Northwest- to southwest-directed thrust faults, particularly on Kupreanof Island, are new discovery. Truncated faults and map patterns suggest there were at least 2 generations of thrusting, and that the thrust faults have been folded. Subsequent right-lateral strike-slip NW-SE faults, have offset thrust faults, and these in turn are offset by N-S right-lateral strike-slip faults. Our fieldwork raised as many questions as it answered, and we see this map as a progress report at a reconnaissance level. The main contributions of this map are 1) the greater distribution of Triassic rocks, 2) increased fossil age information, and 3) the identification of thrust faults within and between units.
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This paper documents the result of an integrated analysis of spaceborne radar (SIR-B) and digital aeromagnetic data carried out in the heavily forested Guiana Shield. The objective of the research is to interpret the geophysical data base to its limit to produce a reconnaissance geologic map as an aid to ground work planning in a worst‐case setting. Linear geomorphic features were identified based on the interpretation of the SIR-B image. Digital manipulation of aeromagnetic data allowed the development of a color‐shaded relief map of reduced‐to‐pole magnetic anomalies, a terrace‐magnetization map, and a map showing the location of maximum values of the horizontal component of the pseudogravity gradient (magnetization boundary lines). The resultant end product was a reconnaissance geologic map where broad terrane categories were delineated and geologic faults with both topographic and magnetic expression were defined. The availability of global spaceborne radar coverage in the 1990s and the large number of existing digital aeromagnetic surveys in northwestern Brazil indicate that this approach can be potentially useful for reconnaissance geologic mapping elsewhere in the Guiana Shield.
As part of the expedition GANOVEX VI 1990/91, the Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), the Alfred Wegener Institute for Polar and Marine Research (AWI), and the United States Geological Survey (USGS) carried out an airborne magnetic survey over the northwestern Ross Ice Shelf and McMurdo Sound between Ross Island, the Transantarctic Mountains, and Minna Bluff. The area planned for the aeromagnetic survey connects directly with the GANOVEX IV survey area (BGR & USGS 1987, BACHEM et al. 1989a) which terminated along an approximately cast-west line at about the latitude of Cape Bird. The GANOVEX IV survey covered the Victoria Land Basin - which is the westernmost of the three north/south striking basins in the Ross Sea - from its northern end in the Terra Nova Bay region to its known southern end at Ross Island. A possible southem extension of the Victoria Land Basin and its central graben structure (the Terror Rift as found from seismic surveys (COOPER et al. 1987) and what has been referred to as Victoria Graben in the aeromagnetic interpretation of the GANOVEX IV data (BOSUM et al. 1989) was one of the major targets for this survey. The survey was laid out in the form of blocks. A 135 km wide transect from the Transantarctic Mountains to about 175° W formed the focal block of the survey (Fig. 1) in which all major scientific objectives could be covered. Other survey blocks were planned to the south of this. During the course of the survey it turned out that in the time available no further area could be covered. Indeed, the unfavourable weather conditions made it impossible to complete even the above described main section fully. In detail, the lay-out of the survey area was closely following that of the GANOVEX IV survey to assure compatibility and direct continuation between the two data sets. The line spacing was chosen again to 4.4 km with a tic-line separation of 22 km. The survey altitude of 2,000 ft (610 m) corresponds to that of the Ross Sea section of the GANOVEX IV survey to which this survey area is directly adjacent. The profile lines were aligned approximately E-W, the tie-lines N-S. As part of the survey area is covered by the maps of the USGS Antarctica Reconnaissance Series 1:250,000 the Lambert conformal conic projection used for these maps was also used for the preparation of a flight line grid.
This report is an analysis of regional gravity and aeromagnetic data that was carried out as part of a Conterminuous United States Mineral Assessment Program (CUSMAP) study of the Joplin 1° X 2° quadrangle, Kansas and Missouri. It is one in a series of reports representing a cooperative effort between the U.S. Geological Survey, Kansas Geological Survey, and Missouri Department of Natural Resources, Division of Geology and Land Survey. The work presented here is part of a larger project whose goal is to assess the mineral resource potential of the Paleozoic sedimentary section and crystalline basement within the quadrangle. Reports discussing geochemical, geological, and various other aspects of the study area are included in this Miscellaneous Field Studies Map series as MF-2125-A through MF-2125-E. Geophysical interpretation of Precambrian crystalline basement lithology and structure is the focus of this report. The study of the crystalline basement is complicated by the lack of exposures due to the presence of a thick sequence of Phanerozoic sedimentary cover. In areas where there are no outcrops, the geologist must turn to other indirect methods to assist in an understanding of the basement. Previous investigations of the buried basement in this region used available drill hole data, isotope age information, and regional geophysical data (Sims, 1990; Denison and others, 1984; Bickford and others, 1986). These studies were regional in scope and were presented at state and multistate scales. The work documented here used recently collected detailed gravity and aeromagnetic data to enhance the regional geologic knowledge of the area. Terrace-density and terrace-magnetization maps were calculated from the gravity and aeromagnetic data, leading directly to inferred physical-property (density and magnetization) maps. Once these maps were produced, the known geology and drill-hole data were reconciled with the physical-property maps to form a refined structural and lithologic map of the crystalline basement.
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