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Magnetostratigraphy of the Ludlow Member of the Fort Union Formation (Lower Paleocene) in the Williston Basin, North Dakota

D.J. Peppe^1,, D.A.D. Evans¹ and A.V. Smirnov²

¹ Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA
² Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, Michigan 49931, USA

View larger version (47K): [in this window] [in a new window]   Figure 1. (A and B) Geologic map of study area in Williston Basin (WB), northern Great Plains, United States. Dashed black line indicates line of section measured and sampled through the Ludlow Member of the Fort Union Formation. Khc—Hell Creek Formation, Tlm—Ludlow Member, Ttr—Tongue River Member, Tsb—Sentinel Butte Member, Tgv—Golden Valley Formation. (C) Generalized stratigraphy of Paleocene and Eocene section of the Williston Basin in southwestern North Dakota. BB—Bighorn Basin; DB—Denver basin; Eoc.—Eocene; PRB—Powder River Basin; WRB—Wind River Basin.

View larger version (34K): [in this window] [in a new window]   Figure 2. Representative Zijderveld diagrams and equal-area plots for each subset of data. (A) Demagnetization trajectory of reversed polarity sample from C29r (representative of ~25% of the data). (B) Demagnetization trajectory of normal polarity sample from C28n (representative of ~35% of the data). (C) Reversed polarity sample with demagnetization direction characterized as a great circle from C27r (representative of ~20% of the data). (D) Normal polarity sample with demagnetization direction characterized as a great circle from C28n (representative of ~5% of the data). (E) Reversed sample clustered around one point from C28r. Characteristic remanence component was calculated by selecting at least four consecutive points (representative of ~5% of the data). (F) Representative of ~10% of the data that yielded no useable data. NRM—natural remanent magnetization.

View larger version (22K): [in this window] [in a new window]   Figure 3. (A) Equal-area plot of characteristic magnetization directions calculated from all lines measured in this study. (B) Mean normal and reversed polarity directions from lines, plotted with present-day field direction, the expected middle Paleocene direction, and the antipode to the expected middle Paleocene direction. Circle around mean direction represents 95% confidence limit (Fisher, 1953). (C) Equal-area plot of all site mean directions. (D) Mean normal and reversed directions of site means, plotted with present-day field direction, the expected middle Paleocene direction, and the antipode to the expected middle Paleocene direction. Circle around mean direction represents 95% confidence ellipse (Fisher, 1953).

View larger version (33K): [in this window] [in a new window]   Figure 4. Polar equal-area projection of Ludlow Member (chrons 29r through 28n) paleomagnetic poles computed from quality-filtered lines and sites (Tables DR3 and DR4, see text footnote 1) compared with relevant early Paleocene paleomagnetic poles from interior North America. Edmonton Group pole (chron 29n/r boundary mean) is from Lerbekmo and Coulter (1985); Montana intrusions pole (61–67 Ma) is from Diehl et al. (1983); Besse and Courtillot pole (65 Ma, sliding window) is from Besse and Courtillot (2003).

View larger version (61K): [in this window] [in a new window]   Figure 5. Magnetostratigraphic sections and equal-area plot for Bald Butte, Never-Ending Butte, John's Nose, and Upper John's Nose. Sections are shown in approximate geographic alignment from southwest to northeast. Inclination versus stratigraphic level is plotted for all magnetostratigraphic sections. Interpreted polarity for each section is overlain on inclination plot (gray indicates normal, white is reversed). MAD—mean angle of deviation.

View larger version (56K): [in this window] [in a new window]   Figure 6. Magnetostratigraphic sections and equal-area plot for Lonesome Bull, Bug Butte, and Three V Butte. Sections are shown in approximate geographic alignment from southwest to northeast. Inclination versus stratigraphic level is plotted for all magnetostratigraphic sections. Interpreted polarity for each section is overlain on inclination plot (gray indicates normal, white is reversed). MAD—mean angle of deviation.

View larger version (35K): [in this window] [in a new window]   Figure 7. Magnetostratigraphic sections and equal-area plot for Three V Amphitheater and Far Butte. Sections are shown in approximate geographic alignment from southwest to northeast. Inclination versus stratigraphic level is plotted for all magnetostratigraphic sections. Interpreted polarity for each section is overlain on inclination plot (gray indicates normal, white is reversed). MAD—mean angle of deviation.

View larger version (25K): [in this window] [in a new window]   Figure 8. Composite polarity stratigraphy for all Little Missouri sections (black is normal, white is reversed). Composite stratigraphy is related to geomagnetic polarity time scale of Ogg and Smith (2004) on far right of figure.

View larger version (19K): [in this window] [in a new window]   Figure 9. Characteristic J/J0 plots of paleomagnetic samples. (A and B) Typical J/J0 plots of reversed polarity samples. (A) Sample P05BB04A shows a large drop in magnetic intensity between 100 and 150 °C and a gradual drop in intensity between 150 and 325 °C. (B) Sample P05NB07A shows a gradual drop in magnetic intensity between natural remanent magnetization (NRM) and 425 °C. Zijderveld diagrams and equal-area plot for this sample are shown in Figure 2A. (C) Typical J/J0 plot of anomalous normal polarity samples. Sample P05BB13D shows a significant drop in magnetic intensity between 100 and 150 °C, suggesting the presence of goethite. (D) Typical Zijderveld diagram and equal-area plot of anomalous normal polarity samples. Sample P05BB13D shows random demagnetization behavior above 125 °C, which indicates that the dominant detrital magnetic mineral is goethite, and only a normal polarity overprint direction was determined.

View larger version (12K): [in this window] [in a new window]   Figure 10. (A and B) Typical dependences of low-field magnetic susceptibility versus temperature for samples P05NB09D (A) and P05NB05D (B). The arrows show the direction of temperature change, and the numbers show the order in which the heating and cooling runs were measured. Both samples manifest irreversible behavior. Sample P05NB09D (A) shows a slight inflection at ~180–200 °C (indicated by gray arrow) and a dominant Curie temperature at ~550 °C (heating curve 2). No clear Curie temperature is defined in heating curves 1 or 2 for sample P05NB05D (B). Both samples manifest a Curie temperature at ~550–580 °C on cooling (curves 3).

View larger version (10K): [in this window] [in a new window]   Figure 11. Typical isothermal remanent magnetization (IRM) acquisition curves measured for carbonaceous siltstone (A), siltstone (B), and sandstone (C). The curves are normalized by the maximum IRM value (IRMmax). All samples show nonsaturation by a magnetic field exceeding 100 mT.

View larger version (17K): [in this window] [in a new window]   Figure 12. Sedimentation rates of the Ludlow Member of the Fort Union Formation. Slight change in slope at the base and top of C28r most likely represents the transition from predominately terrestrial to marginal marine sedimentation then back to terrestrial sedimentation. All data for plots are given in Table DR5 (see text footnote 1). (A) Ogg and Smith (2004). (B) Cande and Kent (1995). (C) Preissinger et al. (2002).