Figure Captions or Annotations

	Figure 1. The oil and gas field map of the Powder River Basin is dominated by fields hosted by NW-SE-trending offshore sandbars, as shown in red.
	Figure 2. Three typical offshore bar fields are illustrated here by sand thickness maps.
	Figure 3. The offshore bars are all hosted by the Pierre Shale. They occur at different levels in the shale and hence are of different ages, and their log signatures are shown here. The different horizons are numbered consecutively from oldest to youngest.
	Figure 4. On the residual magnetic map covering the northern part of the area of offshore bars, the well spots show that sandbars are generally located along magnetic gradients, which correspond to basement faults/shear zones.
	Figure 5. To understand how a magnetic interpretation locates the basement faults, an interpreted residual magnetic map of the western Wind River Basin is shown. The faults are placed along the gradient trends of the magnetic map (A, B, C, etc.) and along truncation lines of anomalies (D, E, etc.).
	Figure 6. The basement faults/shear zones highlighted in red in Figure 5 correspond exactly to the winding, mapped thrust fault extending from Rolfe Lake field on the north to Lander field on the south. This involves four basement faults approximately perpendicular to maximum compressive stress and three cross faults. This example lends credibility to the interpretation that basement faults underlie the sand bars in the Powder River Basin where there is no independent verification of the existence of faults. (Note that not all basement faults become reactivated, at least not to the extent that fault movement can be detected on subsurface maps.)
	Figure 7. Another series of basement faults mapped by magnetics. These lie on the western edge of the Powder River Basin where an underlying thrust fault results in the string of asymmetric anticlines shown here. This example also shows the cross faults that segment the thrust into individual advancing sheets which become anticlines. The fact that movement along basement cross faults cuts a thrust into segments is an important, and new, finding in structural geology.
	Figure 8. Isopach map of the producing sand at Hartzog Draw, a 200 million barrel field, showing its relationship to underlying basement faults.
	Figure 9. Relationship of underlying basement faults to Dead Horse-Barber Creek field, again showing a one-on-one correlation.
	Figure 10. Relationship of an underlying basement fault to Poison Draw field.
	Figure 11. Relationship of Jepson Draw/Holler Draw sand bar to a gradient on the residual magnetic map.
	Figure 12. Heldt Draw-Culp Draw sandbar does not exhibit as obvious a relationship to the magnetic data as previous examples, but the basement fault is no doubt there.
	Figure 13. Triangle “U” field shows that the magnetic gradients that define basement faults can sometimes be subtle and that better defined faults, as the one 3 miles to the southwest and parallel to the field, are not necessarily the ones that were reactivated.
	Figure 14. Kaye field shows that all fields are not necessarily directly over their causative basement faults. There are two faults here, one on each side of the center of the field, following the magnetic gradients.
	Figure 15. House Creek field shows a good correlation with underlying magnetic gradients in the north, but not so good in the south.
	Figure 16. One explanation for the correlation of offshore sandbars with basement faults was presented by Swift and Rice in 1984—the winnowing action of bottom currents on sediments deposited on sea floor topographic highs that formed over basement reverse faults.
	Figure 17. Another explanation for the correlation of offshore sandbars with basement faults has been proposed by John Horne and Dick Inden—transgressive shelf sandstones deposited and preserved on the downthrown side of a reverse fault.
	Figure 18. Another aspect of the offshore bars in the Powder River Basin is that they are almost “dead” parallel to the down-dipping gradient of the basin, as expressed by the structure contours (e.g., Triangle “U” field).
	Figure 19. A similar example (Heldt Draw - Culp Draw field) of an offshore bar parallel to the down-dipping surface of the basin.
	Figure 20. Another example (Poison Draw field) of a sandbar parallel to the down-dipping surface of the basin. An explanation for this is presented in later figures.
	Figure 21. This seismic line and cross-section, by geophysicist Ray and geologist Keefer, explain much of Rocky Mountains geology, in the writer’s opinion. Large mountain-forming thrusts, as the one on the right, have created mountain ranges scattered throughout the region. These mountain ranges depress the crust, forming basins in front of them. In between the ranges, thrusts of lesser throw (“auxiliary” thrusts) created the anticlines that produce oil and gas, and faults of even smaller throw affect the stratigraphy. Note the Madden thrust just in front of the range-forming Owl Creek Mountains thrust.
	Figure 22. Plan view of the relationship between the Owl Creek thrust and the Madden thrust. They are “dead” parallel.
	Figure 23. A section of the geologic map of Wyoming illustrating how the Pinedale thrust occurs just in front of, and parallel to, the Wind River thrust. On the back side of the Wind River thrust block is the Lander trend, which was previously shown in Figures 5 and 6. Here I highlight the underlying basement faults of the Lander trend, which the thrust closely follows. This thrust is not exactly parallel to the Wind River Range, although individual segments of it are. (Why: I don’t know yet.)
	Figure 24. The Uinta Mountains thrust closely follows the area of basement outcrops to the north, and the magnetic data suggest three parallel-trending auxiliary thrusts to the south.
	Figure 25. In the Anadarko Basin in Oklahoma, many sand trends (Late Mississippian – Early Pennsylvanian in age) occur in front of and parallel to the northward-directed Mountain View/Canyon fault system of the Wichita Mountains—quite similar to the situation with the Upper Cretaceous sandbars in the Powder River Basin.
	Figure 26a. West of the Powder River Basin, no large thrust fault has been mapped that raised a mountain range and would have depressed the crust to form the basin, as shown by structural contours on top of Dakota and geologic map.
	Figure 26b. However, a straight line from the easternmost basement outcrops of the Big Horn Mountains on the north to those of the Laramie Range on the south is “dead” parallel to the strike of the down-dipping rocks of the Powder River Basin. Yet, there is no mountain range here and evidently no mapped thrust fault. Where are they?
	Figure 27. Coming back to reality, I have labeled each of the Powder River sandbars with a number corresponding to its age relative to the others, in an attempt to see the timing of compressional faulting. There is, in general, an east-to-west progression, indicating that thrusting proceeded generally away from the missing thrust/mountain range on the west. Also, more late thrusts appear in the south than they do in the north. This somehow relates to regional tectonics, but I’m not sure how.
	Figure 28. The northwestern portion of the Landsat image shows an area of outcropping basement of the Canadian shield that is cut by several zones, such as those occurring under the Powder River Basin. All the Earth’s shields and all the cratons are similarly cut by these long, through-going weakness zones. The eastern and southern parts of this image are covered by Lower Paleozoic rocks of the Ontario Basin, which hide the basement rocks. From Short et al., 1976, p. 194.
	Figure 29. The African shield: Another typical image showing basement shear zones. Three sets of shear zones are readily visible, each set being composed of dozens of parallel structures. Such shear zones occur in Precambrian rocks planet-wide. From Short et al., 1976, p. 384.
	Figure 30. Some potential-field workers use 2nd derivative maps of magnetic data to bring out subtle anomalies, and here I show a 2nd derivative map of the same area of NewMag data in the Powder River Basin as shown in Figure 4. The well spots indicate that the fields essentially lie on the gradients of this map as well. However, NewMag is generally of higher resolution, the reason why I prefer it.
	Figure 31. The Basement Inheritance Chart relates basement faulting to subsequently formed geologic features, resulting from basement fault reactivation. Study of this chart gives one a greater appreciation of importance of basement fault reactivation (“Reactivation Tectonics”). For additional information on basement fault reactivation, the reader is referred to the Applied Geophysical, Inc. website (www.appliedgeophysics.com).
	Figure 32. a. Standard USGS basement map of an 80 by 225 km area in southern Kansas (Sims, 1990) constructed from 35 irregularly distributed data points (black dots); that is, from basement rock types identified from oil well intercepts. b. E-W profile residual aeromagnetic map of same area as in a., flown by Applied Geophysics, Inc., 1982. This map resulted from approximately 70,000 magnetic readings. Note the hundreds of linear anomalies, corresponding to basement fault blocks, the long through-going sutures, identified by letters, and the faulted oval-shaped intrusive (NE quadrant), all major geological features, none of which appear on the USGS map. I show these maps to illustrate why properly processed magnetic maps can be such a tremendous aid in studying and understanding regional geology.

 

References

Anderman, G.G., 1981, Triangle U, in Wyoming Oil and Gas Fields Symposium, Powder River Basin: Wyoming Geological Association, p. 434-435.

Ayers, Mark, 1981, Sussex, in Wyoming Oil and Gas Fields Symposium, Powder River Basin: Wyoming Geological Association, p. 404-406.

Bauder, Janet, 1981, House Creek, in Wyoming Oil and Gas Fields Symposium, Powder River Basin: Wyoming Geological Association, p. 203.

Bayley and Muehlberger, 1968, Basement rock map of the United States, exclusive of Alaska and Hawaii: U.S. Geological Survey, scale 1:2,500,000, 2 sheets.

Berg, William R., 1981, Holler Draw, in Wyoming Oil and Gas Fields Symposium, Powder River Basin: Wyoming Geological Association, p. 198-199.

Campbell, Doug, 1981, Cole Creek South, in Wyoming Oil and Gas Fields Symposium, Powder River Basin: Wyoming Geological Association, p. 94-96.

Geomap, 1989, Executive Reference Map 311.

George, Gene, 1974, Poison Draw Field, Converse County, Wyoming: Wyoming. Geological Association Earth Science Bulletin, v. 7, p. 1-19.

Horne, John, 2005, personal communication, and consultation with Richard Inden, LSSI, Denver.

Lange, Al, 1981, Poison Draw, in Wyoming Oil and Gas Fields Symposium, Powder River Basin: Wyoming Geological Association, p. 303.

Martinsen, Randi S., 1981, Hartzog Draw, in Wyoming Oil and Gas Fields Symposium, Powder River Basin: Wyoming Geological Association, p. 187.

Newcomer, J.E., 1981, Heldt Draw - Culp Draw, in Wyoming Oil and Gas Fields Symposium, Powder River Basin: Wyoming Geological Association, p. 193.

Ray, R.R. and W.R. Keefer, 1985, Wind River Basin, Central Wyoming, in R.R. Gries and R.C. Dyer, eds., Seismic Exploration of the Rocky Mountain Region: Rocky Mountain Association of Geologists, Denver, p. 201-212.

Love, J.D., and A.C. Christiansen, 1985, Geologic Map of Wyoming: Wyoming Geological Survey, 1:500,000 (3 sheets).

Rhoades, R.E., 1981, Teapot (Naval Petroleum Reserve No. 3), in Wyoming Oil and Gas Fields Symposium, Powder River Basin: Wyoming Geological Association, p. 413-415.

Rocky Mountain Map Company (Barlow & Haun, Inc.), 1992, 1998, Structure Contour Map of the West Wind River Basin.

Rocky Mountain Map Company (Barlow & Haun, Inc.), 1987, Structure Contour Map of the Powder River Basin.

Smith, W.H., 1981, Dead Horse Creek—Barber Creek, in Wyoming Oil and Gas Fields Symposium, Powder River Basin: Wyoming Geological Association, p. 111.

Short, Nicholas Short, Paul Lowman, Jr., and Stanley Freden, 1976, Mission to Earth: Landsat Views the World: NASA, New York, p. 194, 384; of 459 p.

Sims, P.K., 1990, Precambrian basement map of the northern midcontinent, U.S.A.; folio of the northern midcontinent area: U.S. Geological Survey Miscellaneous Investigations Series Map, no. I-1853-A, 10 pages, 1 sheet, scale 1:1,000,000.

Smith, W.H., 1981, Dead Horse Creek—Barber Creek, in Wyoming Oil and Gas Fields Symposium, Powder River Basin: Wyoming Geological Association, p. 111.

Smith, W.H., 1981, Sage Spring Creek, in Wyoming Oil and Gas Fields Symposium, Powder River Basin: Wyoming Geological Association, p. 356-357.

Swift, D.J.P., and Rice, D.D., 1984, Sand bodies on muddy shelves- A model for sedimentation in the Western Interior seaway, North America, in Tillman, R.W., and Siemers, C.T., eds., Siliciclastic Shelf Sediments: SEPM Special Publication 34, p. 43-62.

Willibey, Tom, 1981, Kaye, in Wyoming Oil and Gas Fields Symposium, Powder River Basin: Wyoming Geological Association, p. 222-223.

 

Acknowledgments

Appreciation is expressed to Wyoming Geological Association for permission to use parts of its publications of Wyoming oil and gas fields, to Mark Doelger, president of Barlow & Haun, for permission to publish their data shown in Slides (Figures ) 6, 26a, and 26b, and to the Rocky Mountain Association of Geologists for permission to include the seismic line and cross-section in Slide (Figure) 21 from their publication on seismic exploration.

 

Return to top.