Polygenetic History of Paleosols In Middle-Upper Pennsylvanian Cyclothems of the Illinois Basin, U.S.A.: Part I. Characterization Of Paleosol Types And Interpretation Of Pedogenic Processes

Abstract

The morphology, mineralogy, and stratigraphic and lateral distribution of paleosols in Pennsylvanian (Moscovian-Gzhelian; Atokan-Virgilian) mixed marine and terrestrial coal-bearing strata (i.e., cyclothems) in the Illinois basin (IB) are presented in order to determine the dominant pedogenic processes within a cyclic sedimentary depositional framework. Paleosol morphologies range from weakly developed horizons with evidence of rooting and little to no pedogenic structure, to horizons with well-developed pedogenic slickensides, angular blocky structure, and carbonate nodules and tubules. The majority of paleosols in strata from the basin interior preserve (1) low chroma (gley) colors, (2) identifiable fossil plant organic matter that increases in abundance upward in the profiles toward the interpreted paleo-surface, and (3) calcite, sphaerosiderite, and pyrite cements and nodules. In contrast, paleosols in strata from the northern margin of the basin display high chroma colors and calcite cements and less fossil organic matter, sphaerosiderite, and pyrite cements. These observations indicate that paleosols from the basin interior underwent a multi-phase pedogenic and shallow burial history characterized by an initial better-drained, oxygenated stage under seasonal precipitation, followed by a poorly drained, reducing stage, while paleosols from the northern margin of the basin experienced overall more oxygenated conditions throughout their pedogenic development. X-ray diffraction analysis of the , 2 mm size fraction of paleosol matrix in IB paleosols indicates a mineralogical composition dominated by kaolinite, randomly interstratified (R0) and ordered (R1) illite-smectite (I/S), and subordinate amounts of chlorite. These combined observational and mineralogical datasets indicate that significant changes in paleohydrology were largely responsible for the development of Pennsylvanian soil profiles in the IB.

Wetland-Dryland Vegetational Dynamics in the Pennsylvanian Ice Age Tropics

“…4, 5) mark surfaces of nondeposition or limited deposition. They have characteristics that indicate climatic seasonality of varying degrees (Joeckel 1995(Joeckel , 1999Feldman et al 2005;Catena and Hembree 2012;Rosenau et al 2013aRosenau et al , 2013b. These empirical observations also suggest seasonally dry climates during the falling stages of sea level and during the early phases of lowstand ( fig.…”

“…Almost without exception, they vary from indicative of seasonal dryness in their formative stages to indicative of increasing moisture input under humid climate during their later stages. It was during the latter phases that various clays and minerals, including iron, were translocated down the soil profile, resulting in gleying, indicating both that the soil was still well drained and that weakly acidic conditions, resulting from organic matter accumulation, had begun (converting low-solubility ferric to more soluble ferrous iron; Miller and West 1993;Miller et al 1996;Tabor and Montañ ez 2004;Driese and Ober 2005;Rosenau et al 2013a;fig. 5).…”

“…5). Subcoal paleosols frequently transition to coal beds, indicating that water tables rose to the surface of the soil, most likely driven by increased rainfall, which suppressed decay and permitted the accumulation of organic matter (Cecil et al 2003b;Rosenau et al 2013aRosenau et al , 2013b. It is unlikely that rising sea level would cause a rise in base level of sufficient geographic extent, i.e., across most of the vast Pangean interior continental shelves, to initiate peat formation simultaneously across the region (see point 4; coals are not notably time transgressive), nor can rising sea level account for late-stage soil gleying because it provides no mechanism to translocate soluble iron downprofile or out of the soil entirely.…”

“…Confounding data that such models generally do not address include paleosols indicative of long periods of seasonal drought (figs. 4, 5) located between many coal beds (Tandon andGibling 1994, 1997;Driese and Ober 2005;Rosenau et al 2013aRosenau et al , 2013b or changes in sedimentological patterns linked mechanistically to background climate regimes (Calder 1994;Batson and Gibling 2002;Cecil and Dulong 2003;Theiling et al 2012;Opluštil et al 2013a).…”

“…In the equatorial regions of west-central and western Pangea, there are both empirical evidence and experimental climate model results for the episodic appearance of widespread wetlands separated by time intervals during which conditions suitable for extensive, regionally widespread wetland development were limited or absent. Thus, there does not appear to have been regional-scale spatiotemporal continuity of extensive wetland environments through all phases of a glacial-interglacial cycle (Cecil et al 2003b;Eros et al 2012;Rosenau et al 2013aRosenau et al , 2013b. Thin coals, locally of minable thickness, appear to occur during some interglacials, often forming in deltaic settings during late highstand and sea level fall, but these are separated from lowstand coals by strata that record conditions physically and climatically unsuitable for peat formation.…”