An early inhabitant of Illinois - the trilobite

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Bedrock geology of Dixon East Quadrangle, Lee County, Illinois

Two 30" × 35" map sheets; sheet 1 contains map and legends; sheet 2 includes descriptive text with 2 figures, descriptive stratigraphic column, and cross section

The Trilobite-- an early inhabitant of Illinois

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Lithostratigraphy and depositional environments of the Maquoketa Group (Ordovician) in northern Illinois

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Structural framework of the Mississippi embayment of southern Illinois

Bibliography: p. 26-27

A New Species of Bothriocidaris (Echinoidea) from the Cincinnatian Maquoketa Group of Iowa

A new species of echinoid , Bothriocidaris maquoketensis, has been discovered in the Cincinnatian (Upper Ordovician) Fort Atkinson Formation, Maquoketa Group, of northeastern Iowa. The new species is characterized by two widely spaced primary perforate tubercles on opposite sides of the peripodia and by numerous paired interambulacral plates. This occurrence extends the geographic and stratigraphic range of Bothriocidaris in North America

Characterization of karst terrain and regional tectonics using remotely sensed data in Jo Daviess County, Illinois

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Ordovician K-bentonites in the Argentine Precordillera: relations to Gondwana margin evolution

Ordovician K-bcn Ionites have now been recorded from >20 localities in the vicinity of the Argentine Prccordillera. Most occur in I he eastern thrust belts, in the San Juan Limestone and the overlying the Gualcamayo Formation, but a few ash beds are known also from the central thrust belts. The oldest occur in the middle Arenig t. victorias iunatus grnplolite (Oe. evae conodont) Zone, and the youngest in the middle Llanvirn P. etegans (P. .tuecicas) Zone. Mineralogical characteristics, typical of other Ordovician K-bentonites, include a matrix of illite/smectite mixed-layer clay and a typical felsic volcanic phenocryst assemblage: biotite, beta-form quartz, alkali and plagioclase feldspar, apatite, and zircon, with lesser amounts of hornblende, clinopyroxene, tilanilc and Fe-Ti oxides. The proportions of the mineral phases and variations iii their crystal chemistry are commonly unique to individual (or small groups of) K-benlonite beds. Glass melt inclusions preserved in quartz are rhyolitic in composition The sequence is unique in its abundance of K-benlonite beds, but a close association between the Precordillera and other Ordovician sedimentary basins cannot be established.Theash distribution is most consistent with palacogeographical reconstructions in which early Ordovician drifting of the Precordillera occurred in proximity to one or more volcanic arcs, and with eventual collision along the Andean margin of Gondwana during the mid-Ordovician Ocloyic event of the Famalinian orogeny. The Puna-Famatina terrane northeast of the Precordillera might have served as the source of the K-bcnlonite ashes, possibly in concert with active arc magmatism on the Gondwana plate itself.Centro de Investigaciones Geológica

Bedrock geology of Illinois

This map is a revision of the 1:500,000 Geologic Map of Illinois compiled by Willman et al. (1967)."/ Includes a source map and ancillary map: "Areas mapped since 1967."/ "3M - 8/05."/ On verso: Two cross sections, 2 stratigraphic columns, wireline logs, references, and ancillary maps: Structural features of Illinois -- Configuration of top of Precambrian rocks -- Sub-Tippecanoe geologic map -- Sub-Kaskaskia geologic map -- Quaternary geology of Illinois -- Sub-Absaroka (sub-Pennsylvanian) geologic map./ Includes bibliographical references.Ope

Bedrock Geology of Grand Detour Quadrangle, Ogle and Lee Counties, Illinois

USGS STATEMAPpublished or submitted for publicatio