Geology of the Omaha-Council Bluffs Area Nebraska-Iowa HR-112, 1964

(1964) Geology of the Omaha-Council Bluffs Area Nebraska-Iowa HR-112, 1964. Transportation, Department of

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Abstract

The Omaha-Council Bluffs area covers five 7 1/2-minute quadrangles that include parts of Washington, Douglas, and Sarpy Counties, Nebr., and Pottawattamie and Mills Counties, Iowa. The Omaha-Council Bluffs area is a broad loess-mantled up-land till plain into which the valley of the Missouri River is eroded. Terraces locally separate the valley floors from the upland. The highest terrace surface is best preserved in and north of Omaha. Limestone and shale of the Kansas City and Lansing Groups of the Missouri Series of Late Pennsylvanian age make up the principal bedrock exposures in the mapped area. The oldest bed exposed is the Winterset Limestone Member of the Dennis Limestone; the youngest is the South Bend Limestone Member of the Stanton Limestone. All intervening formations and their members occur in normal stratigraphic sequences. Most of the bedrock exposures are in the Olivo, North Omaha Rock and Lime, and Snakirt quarries. The Winterset Limestone Member of the Dennis Limestone, the Cherryvale Formatin, the Drum Limestone, the Chanute Shale, the Iola Limestone, the Lane Shale, the Wyandotte Limestone, and the Bonner Springs Shale, all parts of the Kansas City Group, and the Merriam Limestone Member of the Plattsburg Limestone of the Lansing Group were exposed as of 1957 in these quarries. Isolated outcrops, however, expose other beds in the stratigraphically higher Lansing Group. The uppermost unit of this group, the South Bend Limestone Member of the Stanton Limestone, crops out along the railroad cuts at the base of the Missouri River north of Bellevue, Nebraska, where the Rock Lake Shale Member underlies the South Bend. The Stoner Limestone Member is exposed in a quarry northeast of Council Bluffs along Mosquito Creek. These Upper Pennsylvania limestone, claystones, and silt-stones were deposited in shallow open seas in near-shore waters, or in lagoons and swamps, as indicated by fossils in beds of the Missouri Series. Cyclothem sequences of limestones repeatedly overlain by shaly claystone and siltstone are related to cycles of changing sea levels. Rocks of Cretaceous age are not exposed, but may possibly underlie the Pleistocene rocks west of the mapped area. The Pleistocene deposits in this area include ice-deposited materials as well as those of fluvial, lacustrine, colluvial, and eolian origins. Sand and gravel overlying bedrock in the southeastern part of this area may be equivalent to the David City Formation of early Nebraskan age. Nebraskan till, although not exposed, was penetrated in auger holes along the bluff of the Missouri River. Silt of the Fullerton Formation of late Nebraskan age is the oldest exposed Pleistocene deposit. Unconformably overlying the Fullerton in the Red Cloud Sand and Gravel of early Kansan age. It is a meidum to coarse sand that contains lenses of pebble gravel and irregular inclusions of compact till. The overlying Kansan till is predominantly a heterogeneous mixture of boulders, cobbles, pebbles, and sand in a silt matrix. The lower part of the till is unoxidized and medium to olive gray, whereas the upper part is oxidized and moderate yellowish brown. Medium to coarse sand of hte Grand Island formation of late Kansan age in places unconformably overlies a horizontal surface on the till. Overlying this sand is silt and clayey silt of the Sappa Formation, which was deposited during the last part of the alluvial cycle in late Kansan time. Within the Sappa is the Pearlette Ash Member, a diagnostic stratigraphic marker.