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M.R. Hudson

Publications and source records attributed to M.R. Hudson.

24 records · Page 2Linked to original sources
USGS1992

Paleomagnetic data bearing on the origin of arcuate structures in the French Peak-Massachusetts Mountain area of southern Nevada

The eastern margin of the Yucca Flat basin, in southern Nevada, is bounded by north-northwest-striking tilted fault blocks of the Halfpint Range whose strikes curve as much as 90° clockwise into east-northeast strikes in the French Peak-Massachusetts Mountain (FPMM) area. This pattern of arcuate structures has been attributed to clockwise drag along a postulated northwest-trending, right-lateral shear zone. The flexure model implies that rocks within the FPMM area were rotated strongly clockwise about a vertical axis. Directions of remanent magnetization of the middle Miocene Ammonia Tanks and Rainier Mesa Members of the Timber Mountain Tuff and of the Topopah Spring Member of the Paintbrush Tuff indicate no systematic vertical-axis rotation in the FPMM area and disprove the flexure model. After tilt correction, declinations of 29 site means obtained from the three ash-flow sheets in the FPMM area are not systematically different than declinations of 17 site means from the Halfpint Range or declinations of 16 site means from little-deformed mesa areas to the west. The paleomagnetic data thus indicate that structures in the FPMM area initiated with arcuate trends and were not originally straight elements that were rotated by right-lateral drag. The structures probably formed under the influence of spatially variable stress fields. The FPMM area lies in an accommodation zone between domains of oppositely tilted extensional fault blocks. Interaction between stress fields associated with propagating normal-fault zones may have been responsible for the arcuate structures in the FPMM area.

Nevada↗
USGS1991

Sources of aeromagnetic anomalies over Cement oil field (Oklahoma), Simpson oil field (Alaska), and the Wyoming-Idaho-Utah thrust belt

Geochemical and rock magnetic studies, undertaken to determine the causes of magnetic anomalies over Cement oil field (Anadarko basin, Oklahoma), Simpson oil field (North Slope basin, Alaska), and the Wyoming-Idaho-Utah thrust belt, have revealed different magnetic sources developed under different sedimentologic, geochemical, and structural settings.At Cement, ferrimagnetic pyrrhotite (Fe 7 S 8 ), typically intergrown with more abundant, nonmagnetic pyrite (FeS 2 ), formed as a result of hydrocarbon seepage. Sulfur isotopic data indicate that sulfur in the Fe-S minerals was probably derived from two different sources: (1) isotopically heavy, thermochemical H 2 S in petroleum, and (2) isotopically light H 2 S generated by sulfate-reducing bacteria that derived metabolic energy from leaking hydrocarbons or organic compounds derived from hydrocarbons. Although pyrrhotite may make a minor contribution to the reported total magnetic field anomalies at Cement, the measured anomalies are probably mostly caused by man-made features such as buried well casings and pipelines. Magnetite, found in well cuttings from Cement and previously considered the source of the anomalies, is contamination from drilling.At Simpson, ferrimagnetic greigite (Fe 3 S 4 ) is concentrated locally in Upper Cretaceous beds. A preliminary geochemical study reveals a complicated picture in which early diagenetic chemical and mineralogic changes are variably overprinted by later epigenetic alterations, perhaps related to hydrocarbon seepage. The greigite probably formed at different times from sulfate reduction by bacteria that used either organic compounds derived from hydrocarbons or detrital organic matter, or both, as food sources. Magnetic-property studies suggest the natural remanent magnetization of greigite-bearing rocks may contribute to the magnetic anomalies.In the thrust belt, the Middle Jurassic Preuss Sandstone has magnetizations as much as 0.76 A/m (average 0.14 A/m) west of the Absaroka fault where aeromagnetic anomalies locally correspond to exposures of the Preuss and much lower magnetizations (average 0.44 X 10 (super -2) A/m) east of the Absaroka fault. The magnetization is carried by rounded titanium-bearing detrital magnetite, commonly concentrated along heavy-mineral laminations. Carbon isotopic compositions of calcite cement (-2.4 to +1.5 per mil) reflect an absence of organic carbon in the calcite and thus suggest that hydrocarbon had no role in the preservation of the detrital magnetite.Our results show that abiologic and biologic mechanisms can generate different magnetic sulfide minerals in zones of sulfidic hydrocarbon seepage. More commonly, sulfidic seepage could either diminish magnetization by replacement of detrital magnetic minerals with nonmagnetic sulfide minerals, or it would have no effect on magnetization if such detrital minerals were originally absent. An important negative result is the absence of abundant secondary (diagenetic) magnetite in the seepage environments. Although secondary magnetite occurs in some biodegraded crude oils, concentrations of such magnetite capable of producing aeromagnetic anomalies have not been documented.

Geophysics↗