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1 The Geologic Setting of Iowa The rocks in question, therefore, so far as relates to Iowa, are nothing more than the consolidated sands and muds of old sea bottoms preserving for our inspection samples ofthe life that occupied the seas at the time each successive bed was in process of accumulation. Iowa has passed more time under the ocean than as dry land. Preface to "Report on Geology of Iowa's Counties" (1906; author not indicated but probably Samuel Calvin) Iowa is very very old - as old as the hills and older. So old, in truth, is this fair land that no matter at what period the story is begun, whole eternities of time stretch back to ages still more remote. John Briggs (1920) describing Iowa's antiquity Iowa has a rich record ofthe geologic past, particularly in the form of sedimentary rocks. These strata represent deposits of scores of ancient seas that inundated the heartland ofthe North American continent. However, the state's rock record is not confined to marine deposits alone. The coal beds ofsouthern Iowa are products ofcoastal swamps and deltas. Former streams left telltale signatures in the sandstones of Ledges, Dolliver Memorial, and Wildcat Den state parks. Fossil amphibians document the presence offreshwater habitats in 34o-millionyear -old rocks near Delta in southeastern Iowa. And the sedimentary rock record of Iowa lies on older rocks of even greater variety. These older rocks, assigned to the Precambrian division of the geologic column , consist of a wide assortment of rock types that are representative of the three major rock families (igneous, sedimentary, and metamorphic) that constitute our planet. Over most ofthe state, the rock record is covered and obscured by unconsolidated sediments that were laid down primarily by wind, streams, or glaciers. A geological cross section (fig. 1.1) illustrates the relationship of the unconsolidated material (labeled Pleistocene) to the underlying sedimentary units (labeled Pennsylvanian, Mississippian, Devonian, Silurian, Ordovician, and Cambrian). Iowa lies in a part ofthe United States known as the Stable Interior. In this region , the rocks are generally flat-lying sedimentary rocks that show little or no deformation. In fact, at anyone location in Iowa, the rocks typically appear to be horizontal- as flat as a pancake. On a regional scale, however, the rocks of the NEBRASKA \ IOWA CRETACEOUS THURMAN-REDFIELD STRUCTURAL ZONE IOWA \ ILLINOIS + ZOOO PENN. + 1000 DEVONIAN SILURIAN o Sea Level ORDOVICIAN - 1000 " .:~. - - - - -CAMBRIAN - ----- --- ,.:' '" - 2000 \/ '" , ;' ~EO;A "" " /' " .... " '\.;" , - 3000 DOME PRECAMBRIAN - 4000 Elevation o SO 100 miles Feet ELqa=~~sdMt~~~~1 Scale I • I A geological cross section from near Clinton in eastern Iowa to south ofCouncil Bluffs in southwestern Iowa. As shown, Iowa has a substantial record oflayered sedimentary rocks. These strata rest on older Precambrian rocks ofvaried composition and are generally covered by unconsolidated sediments ofPleistocene age that were laid down by wind, streams, and glacial ice. It is common practice to use different horizontal and vertical scales in order to portray geologic cross sections such as shown here. This produces an exaggeration ofthe vertical dimension. The vertical exaggeration ofthis cross section is 85 times. Adapted from Bunker and Witzke 1988. SW KANSAS IIMISSOURI I IOWA PLEISTOCENE IOWA IMINNESOTA a I PLEISTOCENE Scale NE 100 miles Elevation Feet I" 2000 o Sea Level·2000 -4000 1.2 Iowa's Paleozoic bedrock is inclined gently downward to the southwest. The vertical exaggeration ofthis cross section is 85 times. Adapted from Bunker and Witzke 1988. state display a slight inclination, or tilt. A cross-sectional slice from northeast to southwest (fig. 1.2) shows that Iowa's sedimentary rocks sag downward to form a basin. This structure, termed the Forest City Basin, extends into Missouri, Kansas, and Nebraska. In places, the sedimentary rocks of the Stable Interior arch upward to form geologic structures such as the Keota Dome and the Nemaha Uplift (figs. 1.1 and 1.2). A few hundred barrels of petroleum were obtained from rocks of the Keota Dome in Washington County as a result of drilling explorations in 1963 and 1985. To date, this meager recovery represents the bulk ofthe oil recovered in Iowa. Figure 1.3 shows the distribution of the bedrock units of Iowa. Throughout most of the state the bedrock is covered by unconsolidated deposits such as glacial till, sand gravel, and loess (wind-blown silt). Ifthese unconsolidated materials were removed, it would be possible to see the rock units as they are displayed on the bedrock map (fig. 1.3). Where the unconsolidated deposits are...

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