Abstract

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_V-PDB and δ¹³C_V-PDB values range from −3.6‰ to −0.5‰ and −12.8‰ to −3.2‰, respectively. In particular, sphaerosiderite δ¹⁸O values from the B horizons of paleosols display an approximate −1.5‰ shift across the D-M boundary. This stratigraphically short isotopic shift occurs across the D-M boundary, on the order of one cyclothem (∼ 400 kyr), and is followed by a subsequent shift on the order of one cyclothem, back to more positive siderite δ¹⁸O values.

The average δ¹⁸O_V-SMOW and δD_V-SMOW values of kaolinite isolated from the < 0.2 µm size fraction of a latest Desmoinesian flint-clay breccia are +21.0‰ and −46.5‰. These values correspond to a latest Desmoinesian crystallization temperature of 27 ± 2°C and a meteoric-water δ¹⁸O_V-SMOW value of −3.1‰. Combination of sphaerosiderite δ¹⁸O values and co-occurring flint-clay kaolinite δD and δ¹⁸O values suggest a possible warming of up to 6°C across the D-M boundary, corresponding to an approximate temperature change of siderite crystallization from 27 to 33°C. This temperature increase, in conjunction with distinct stratigraphic trends in the clay mineralogy and morphology of IB paleosols, is consistent with previous paleoclimate interpretations, and it suggests that low-latitude Pennsylvanian paleoclimate became drier and warmer across the D-M boundary.