Distinct lateral and stratigraphic trends in paleosol morphology and clay mineralogy, in conjunction with the stable-isotope composition of sub-millimeter-scale spherulitic siderite (sphaerosiderite) and flint-clay kaolinite in middle–upper Pennsylvanian cyclic coal-bearing strata of the Illinois basin (IB) presented herein provide proxy records of middle–late Pennsylvanian equatorial terrestrial environments. Collectively, these data provide a better understanding of the polygenetic history of ancient soils preserved in cyclic strata and indicate that low-latitude Pennsylvanian IB soil profiles were influenced by a combination of autogenic and allogenic controls.
Lateral variations in paleosol morphology from the interior of the basin to the northern margin indicate that soil development was dominantly influenced by autogenic controls, such as differences in local paleohydrology (i.e., groundwater fluctuations) and subsidence rates across the basin. Conversely, allogenic controls (i.e., glacioeustasy and climate) are interpreted to be principally responsible for the preservation of features in gleyed Protosols, gleyed Vertisols, and gleyed calcic Vertisols that occur in the interior of the IB. These paleosols are characterized by a polygenetic development history that includes (1) an initial period of soil formation in well-drained environments with seasonal wetting and drying of the profile, followed by (2) a subsequent period of waterlogging and development of reducing conditions.
Paleosols that lack evidence for extensive periods of waterlogging are restricted, with the exception of two examples, to the northern margin of the basin, and they become abundant in the stratigraphic record near the middle–upper Pennsylvanian boundary (uppermost Desmoinesian). The stratigraphic distribution of these paleosols, in conjunction with a decrease in the relative abundance of kaolinite in the < 2 µm size fraction of IB paleosols near the Desmoinesian–Missourian (D-M) boundary, is interpreted to reflect a shift in the overall low-latitude regional climate to drier conditions.
The stable-isotope compositions of sphaerosiderite and flint-clay kaolinite in middle–upper Pennsylvanian strata of the IB provide an additional proxy record of past equatorial terrestrial environments and are used to constrain the magnitude of paleoclimate change across the D-M (∼ Moscovian–Kasimovian) boundary. Sphaerosiderite δ18O
The average δ18O