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Title:Paleogeomorphology Affecting Early Pennsylvanian Floras in Rock Island County, Illinois
Author(s):Leary, Richard Lee
Department / Program:Geology
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:Plant fossils preserved in basal Pennsylvanian rocks of western Illinois are significant because composition of many of the paleofloras differs from those of more commonly preserved and better studied coal swamp floras. Each Early Pennsylvanian (Namurian - Westphalian A) flora of the Rock Island County area differs from the others to some extent; most are different from a typical Pennsylvanian coal swamp flora of comparable age. Differences are attributed to environmental factors; some floras were restricted to uplands, others grew on alluvial or delta plains.
Indirect evidence exists for the presence of a thin, terra rossa-like soil on the unconformity prior to earliest Pennsylvanian sedimentation. The thinness, well-drained nature, and terra rossa character of the soils possibly were sufficient to affect the content of the upland floras.
Floras dominated by Megalopteris and Lesleya and containing Noeggerathiales and Cordaites grew on well drained uplands. Uplands are inferred here to have been possibly no more than 5 m above local base level. These uplands were often underlain by carbonate rocks which permitted soils to be well drained and which provided source materials for terra rossa-type soils.
Subsurface data and exposures of the pre-Pennsylvanian unconformity in Rock Island County, Illinois, reveal a surface of low relief with scattered steep-sided valleys as much as 60 m deep and hills up to 20 m high. Several quarries expose nearly level erosion surfaces on Devonian limestone. Relief in single exposures, as much as 1.5 km long, generally is only 2 to 3 m. Narrow, steep-sided channels 4 to 6 m deep and 8 to 20 m wide are incised in these flat surfaces. Orientation of channels and major valleys in Rock Island County parallels, and probably was controlled by, jointing in underlying limestone.
A karst topography developed on Devonian limestone in the Rock Island County area. Recognizable karst features include caves, cutters, and solutionally produced channels. Subsurface data suggest possible blind valleys, incised river valleys, and uvalas. Lithologies of strata expose at the paleosurface influenced topographic development.
Basal Pennsylvanian strata in the Rock Island County area provide evidence for several cycles of erosion and deposition. The channels and valleys probably developed during Late Mississippian and Early Pennsylvanian. Earliest Pennsylvanian sediments are those of isolated channels and localized depressions. Channel fills are probably lacustrine or similar quiet-water deposits. Deeper valleys appear to have been filled later by Caseyville strata. The Babylon Sandstone Member of the Abbot Formation is the oldest unit representing deposition of widespread, mappable Pennsylvanian strata on the northwest margin of the Illinois Basin.
Paleoland forms, smooth bedrock surfaces, and lack of limestone debris at the pre-Pennsylvanian unconformity (post-Late Devonian - pre-Westphalian B) surface indicate a warm, humid climate in earliest Pennsylvanian. Presence of several buried hills which are similar to those of cone karst may also indicate a period of warm, humid climate. Possible presence of dolines, uvalas, blind valleys, and incised river valleys in the subsurface may represent a subsequent interval of uplift and rejuvenated erosion.
Issue Date:1980
Description:153 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1980.
Other Identifier(s):(UMI)AAI8108575
Date Available in IDEALS:2014-12-14
Date Deposited:1980

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