Steven D. Sheriff

Paleomagnetic study of thrust-sheet motion along the Rocky Mountain front in Montana

Near Wolf Creek, Montana, the fold and thrust belt intersects the central Montana trough at the northern edge of the Helena salient. Remanent magnetizations from 18 of 31 paleomagnetic sites in the Late Cretaceous Two Medicine Formation (∼79 Ma) document counterclockwise rotations of thrust-bound rocks along the margin of the salient. Results from 12 reliable sites in the upper of two thrust sheets gives a mean direction of Dec = 328.4°, Inc = 67.9°, α 95 = 5.63°. Results from 6 reliable sites in the underlying thrust sheet yield Dec = 321.9°, Inc = 69.3°, α 95 = 11.87°. The difference in these directions is not significant at the 95% confidence level. The thrust sheets show no relative rotation. Combining all 18 sites gives an average direction of Dec = 326.3°, Inc = 68.4°, α 95 = 4.94°. This average direction is 25.2° ± 14.0° counterclockwise from a Late Cretaceous reference direction (Dec = 351.5°, Inc = 69.3°) established from the adjacent Adel Mountain volcanic rocks (sim;76 Ma). Comparison to an older (∼88 Ma) pole established from the Niobrara Formation indicates 11.7° ± 11.4° of counterclockwise rotation. The paleomagnetic results, with other geologic evidence, show that both thrust sheets interacted with an oblique, lateral foreland ramp.

Paleomagnetic evidence for en echelon crustal extension and crustal rotations in western Montana and Idaho

The Bitterroot metamorphic core complex is one of many Eocene core complexes in the northern Cordillera. A middle Eocene dike swarm cuts the Bitterroot metamorphic core complex and its hanging wall, the Skalkaho slab. A 26° change in strike of the dikes across the core complex suggests either a refracted stress field or rotation during Eocene or younger extension. The nearby Bitterroot Valley and Big Hole Basin also indicate tectonic rotation. To distinguish between the hypotheses of refracted stress and tectonic rotation, we collected paleomagnetic data from 28 dikes. Nineteen sites provide reliable results. Eight sites from the hanging wall east of the mylonite zone give Dec = 46°, Inc = 69°, α95 = 13°; 11 dikes to the west yield Dec = 334°, Inc = 64°, α95 = 6°. The declinations diverge from the expected direction; the inclinations do not. The hanging wall rocks are rotated 58° ± 32° clockwise at the 95 % confidence level. The footwall shows 14° ± 11° of counterclockwise rotation. Both normal and reversed polarity dikes are present in each area, indicating that the dike swarms may adequately average paleosecular variation. Vertical contacts reveal that the dikes have not tilted since emplacement. These paleomagnetic results and available geochronologic data show that the Bitterroot dome was pulled from beneath the Skalkaho slab in the Eocene. As the dome slid toward the northwest, it experienced a slight counterclockwise rotation, while the upper plate moved clockwise around a vertical axis near its northern edge. Rotation of the Skalkaho slab occurs because the slab lies between en echelon extensional detachments of the core complex and the next basin to the east, the Big Hole Basin. The Boehls Butte area, north central Idaho, occupies a step in the Lewis and Clark fault zone and is probably a core complex related to right-lateral motion on the fault system. Similarities in both the style and timing of extension suggest that Eocene ductile/brittle extension occupies a swath from British Columbia, past the Bitterroot metamorphic core complex, to the Pioneer core complex in south central Idaho.

Forward modeling of electrical sounding experiments using convolution and a spreadsheet

Abstract For many geologic applications, programming a graphics interface or display takes much more code than the actual calculations. Usually, one can bypass this bottleneck by using the high-level language, and built in graphics, provided by commercial spreadsheets. As an example, calculating expected apparent resistivities, using linear filtering and convolution, is rapid and convenient with a commercial spreadsheet. The spreadsheets immediate access to screen graphics facilitates an intuitive grasp of electrical sounding and saves much programming time.

Fishing Bridge Point (48YE381): A Stratified Prehistoric Site at Yellowstone Lake, Wyoming

Abstract Excavations at the Fishing Bridge Point site (48YE381) at Yellowstone Lake, Wyoming reveal stratified Early Archaic, Middle Archaic, Late Archaic, and Late Prehistoric period occupations. Results of specialized studies such as lithics, X-ray fluorescence, protein residue, pollen, ethnobotanical, and sub-surface imaging contribute to existing information regarding prehistoric human use of high-elevation lake basins within the Greater Yellowstone Ecosystem. Among the significant findings are the remains of the only radiocarbon-dated Early Archaic hearth feature excavated within Yellowstone National Park. Lithic raw material use patterns of Native Americans in the Early Archaic occupation greatly contrast those from the nearby Late Paleoindian Osprey Beach site, suggesting changing material use patterns at the beginning of the Holocene at Yellowstone Lake. Late Prehistoric lithic material use patterns also contrast those of the preceding Middle and Late Archaic periods.