STRATIGRAPHY AND PETROLOGY OF THE JURASSIC (?) "FORT DODGE BEDS," NORTH-CENTRAL IOWA

by
R.R. Anderson, R.M. McKay, G.A. Ludvigson and B.J. Witzke

Iowa Academy of Science
108th Session
April 26-27, 1996, Simpson College, Indianola, IA
1996 Program Abstracts, p. 19

ABSTRACT


The "Fort Dodge Beds," located in and near the town of Fort Dodge in Webster County, are composed of 6 facies unconformably overlain by Quaternary sediments. In vertical sequence, the facies include (1) a basal conglomerate to sandstone with abundant limestone clasts and reworked Mississippian and Pennsylvanian fossils; (2) a gray siltstone to dark gray shale with small, agglutinated foraminifera (possibly indigenous); (3) a thick, economically important gypsum deposit, containing minor, fine-sand and silt, and abundant palynomorphs; (4) an unconformably-overlying sandy mudstone-dominated sequence that also includes horizontally to thicker lenticular stratified sandstone; (5) a mudstone-dominated interval with discontinuous red, gray, and green mudstones alternating with thin-bedded, fine-to medium-grained red and gray sandstones; and (6) an uppermost sequence of thick, massive, to finely-laminated, fine-grained, yellow to white sandstones separated by thin-bedded sandstones. The upper three facies, 4, 5, and 6, have been informally known as the "Soldier Creek Formation" and are found over a larger geographic area than the underlying units. Bedding within these rocks is laterlly discontinuous, fossils are rare to absent, and sedimentary structures are rare. Maximum thickness of the gypsum facies is approximately 10 m and composition ranges from 89% to 96% CaSO4.2H20, with the major impurities including quartz, clay, and carbonate. The conformable sub-gypsum dark shale and siltstone ranges in thickness from 0 to 7m in thickness and is marked by bright red iron staining along horizontal and vertical fractures. The basal conglomerate, which unconformably overlies Pennsylvanian Cherokee Group rocks, contains abundant limestone clasts, abundant reworked Pennsylvanian forams, brachiopods, bryozoans, and crinoid fragments; large red shale rip-up clasts, and large sand to pebble-size quartz typify this unit.