THE JURASSIC(?) FORT DODGE FORMATION IN NORTH-CENTRAL IOWA

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

The Geological Society of America
30th Annual North-Central Section Meeting
Iowa State University, Ames, IA, May 2-3, 1996
1996 Abstracts with Programs, v.28, no.6, p.26

ABSTRACT


The Upper Jurassic (?) Fort Dodge Formation, is an areally restricted sequence located in and near the town of Fort Dodge in Webster County, Iowa. These units are composed of 6 facies that unconformably overlie Mississippian and Pennsylvanian strata, and are, in turn, overlain by Quaternary sediments. The facies, in vertical sequence, 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 includes thin, horizontally stratified and thicker, lenticular sandstones; (5) a mudstone-dominated interval with discontinuous red, gray, and green mudstones (possibly paleosols) 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, are informally known as the "Soldier Creek beds" and are found over a larger geographic area than the underlying units. Bedding within the "Soldier Creek beds" is laterally discontinuous, fossils are rare to absent, and sedimentary structures are rare. In total, the "Soldier Creek beds" reach a maximum observed thickness of about 12.5 m. The gypsum facies, apparently deposited from marine brines, reaches a maximum thickness of approximately 10 m and ranges in composition from 89% to 96% CaSO4.2H20, with the major impurities including quartz, clay, and carbonate. The conformable sub-gypsum claystone ranges in thickness from 0 to 7 m. The basal conglomerate 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. The basal facies reaches a maximum observed thickness of about 1 m.