THE GEOLOGY OF THE FORT DODGE FORMATION WEBSTER COUNTY, IOWA

by
R.R. Anderson and R.D. Cody

Iowa Department of Natural Resources, Geological Survey Bureau
Guidebook Series No. 9, 1996, 74 p.

INTRODUCTION


The rocks of the Fort Dodge Formation (Upper Jurassic) are the most intensely exploited of the bedrock units in Iowa. The formation includes a basal conglomerate, overlying very dark gray shale, a thick gypsum unit, and an upper suite of sandstones and mudstones locally referred to as the 'Soldier Creek beds.' The formation has an extremely restricted areal extent, with its only known occurrences in central Webster County, Iowa. The thick and exceptionally high quality gypsum unit that dominates the formation has been mined for a variety of economic products since the mid-1800's. The formation was the target of several early geologic investigations in the late 1800's and early 1900's. While these early studies discussed many details of the lithology, structure, and economic features of these strata, especially the gypsum beds, they failed to adequately answer many basic questions, such as the age, depositional environments, and the mechanics of their preservation. The high quality gypsum in the formation has been intensely mined for over 125 years, and recently the finite nature of this very limited resource has prompted a renewed interest in trying to better-understand this geologic unit before it is consumed by mining activity. In the last decade a number of geology students at Iowa State University have investigated various aspects of the formation as a part of their thesis and dissertation programs. Last year the Iowa Department of Natural Resources Geological Survey Bureau embarked on a detailed investigation of the formation which will culminate in a report on the geology of the Fort Dodge Formation later this year. The hosting of the 30th Annual Meeting of the North-Central Section of the Geological Society of America by the Department of Geological and Atmospheric Science of Iowa State University in Ames provides an excellent opportunity to show this rock unit to regional geologists, and to encourage them to take part in this, perhaps the final detailed investigation of the Fort Dodge Formation before the majority of the formation is mined away.