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Isopach of the Devonian Prairie Evaporite Edward C 100K Series: Crsb - ssg North Dakota Geological Survey Isopach of the Devonian Prairie Evaporite Edward C. Murphy, Acting State Geologist Crosby 100K Sheet, North Dakota o ' Crosby 100K Sheet, North Dakota Adjoining 100K Maps 104 00 o ' 103 00 R. 102 W. R. 101 W. R. 100 W. R. 99 W. R. 98 W. R. 97 W. R. 96 W. R. 95 W. o ' µ o ' R. B. Burke 48 00 49 00 80 . N ¤£85 4 !(42 !(40 2005 6 1 . T GENERAL INFORMATION ABOUT THE PRAIRIE EVAPORITE The Prairie evaporite is the salt unit of the Middle Devonian Prairie Formation. 100 130 The Prairie Formation consists of argillaceous dolostone, shale, and salts that represent a minor regressive phase of deposition in the overall transgression of the Middle Devonian 120 sea into North Dakota. The majority of the formation is composed of the salt mineral halite. Predominantly normal marine carbonates of the Middle Devonian Winnipegosis 0 0 0 4 1 10 " 1 0 150 Formation conformably underlie the Prairie and represent the transgressive marine phase Ambrose of deposition. The top of the Prairie Formation is the top of the argillaceous Second Red beds (Heck and Burke, 1991). The marine carbonates of the lower Dawson Bay . N Formation conformably overlie the Prairie Formation and represent renewed 3 6 1 transgression of the Devonian sea into North Dakota (Pound, 1985). T The Prairie evaporite unit consists of anhydrite, halite and potash lithologies. These lithologies were deposited during a forced regression resulting from restriction of circulation of normal marine seawater into the North Dakota portion of the Devonian Elk Point Basin, recognized then as the Williston sub-Basin. Restriction resulted from a 0 6 1 combination of earlier reef growth during Winnipegosis time and a relative drop in sea 85 Fortuna !(42 ¤£ " level. This combination of events physically reduced the inflow of seawater but did not Crosby !(5 !(5 " stop the influx of seawater into the Williston sub-Basin. Very arid conditions existing in this region at this time resulted in high evaporation rates that concentrated sea salts from ¤£85 !(42 !(40 the reduced inflow to the point of precipitation of large volumes of a variety of salt 120 !(5 !(5 minerals. The Prairie evaporite is principally composed of halite salt with lesser amounts " of potassium salts (potash) such as sylvite and carnallite. Over 198 meters (650 feet) of Noonan salt were deposited in Burke County, North Dakota, just east of this map sheet. THE CROSBY SHEET The Prairie evaporite in the mapped area is composed primarily of halite and potash salts. In general, evaporite thickness doubles from west to east from about 91 0 0 1 meters (299 feet) to over 160 meters (525 feet) although some anomalously thin (81 m or 266 ft) occurrences are present within the thick salts on the eastern side of the map. In . general, the top of the Prairie evaporite is about 2,750 to 3,350 meters (9,000 to 11,000 N 2 0 6 11 feet) below the surface. Pressures at these depths are great enough to make salt mobile 1 . 15 T 0 and is one reason that open hole mining of salt from the surface in not practical. Abrupt change in evaporite thickness is seen in T160W R96W where the salt 100 thins from the regional background of about 120 meters (394 feet) to less than 90 meters (295 feet). The thinning is the result of dissolution of salt. Dissolution of Prairie salt has occurred numerous times throughout geologic time and is an ongoing process in some 140 portions of the Williston Basin (Anderson, S. B. and Swinehart, R.P., 1979). REFERENCES Anderson, S. B. and Swinehart, R.P., 1979. Potash Salts in the Williston Basin, Economic Geology, 74(2): 358-376. Heck, T.J. and Burke, R.B., 1991. Devonian Stratigraphy of North Dakota from 0 9 Wireline-log Sections. North Dakota Geological Survey Report of Investigation No. 90, pages 8, plates 28. Pound, W.R., 1985. The geology and hydrocarbon potent ial of the Dawson Bay 0 10 Formation carbonate unit (Middle Devonian), Williston Basin North Dakota. Unpublished M.S. Thesis, University of North Dakota, 320 p. 100 . N 1 6 1 EXPLANATION . T 1 30 ISOPACH CONTOURS (intervals in meters) ¤£85 70 - 80 120 !(42 80 - 90 90 - 100 100 - 110 100 0 9 110 - 120 . N 0 120 - 130 6 1 . 130 T 130 - 140 140 - 150 150 - 160 DIVIDE COUNTY 160 - 170 0 WILLIAMS COUNTY 1 1 !(50 " GEOLOGIC SYMBOLS Grenora 100 Isopath Contour (in meters) Hash Marks point to Thins . N 9 5 1 50 OTHER FEATURES . !( T 0 0 10 2 1 " Alamo 42 " Town Sites County Boundary ¤£85 !( !(50 !(50 State Highway Township Boundary !(50 90 ¤£85 U.S. Highway The North Dakota Geological Survey compiled this map according to conventional cartographic standards, using what is thought to be the most reliable information available. The North Dakota Geological Survey does not guarantee freedom from errors or inaccuracies and disclaims any legal responsibility or liability thereon. N 1:100,000 Scale 8 5 1 0 1 2 3 4 . T Miles Mercator Projection 1927 North American Datum o ' 100 Standard parallel 48o 30' Central meridian 103 30 ¤£85 o ' o ' 48 30 48 30 o ' o ' 104 00 103 00 R. 103 W. R. 102 W. R. 101 W. R. 100 W. R. 99 W. R. 98 W. R. 97 W. R. 96 W. Cartographic Compilation: Elroy L. Kadrmas !(73 !(73 1804 1804 1804 1804 1804 1804 1804 1804 1804 1804.
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