Mixed-Layer Illite-Smectite in Pennsylvanian-Aged Paleosols: Assessing Sources of Illitization in the Illinois Basin

Julia A Mcintosh,Nicholas A Rosenau,Neil J Tabor

doi:10.3390/min11020108

Julia A Mcintosh, Nicholas A Rosenau + Show 1 more

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https://doi.org/10.3390/min11020108

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Abstract

Mixed-layer illite-smectite (I-S) from a new set of Pennsylvanian-aged Illinois Basin underclays, identified as paleosols, are investigated to assess the impact of (1) regional diagenesis across the basin and (2) the extent to which ancient environments promoted illitization during episodes of soil formation. Interpretations from Reichweite Ordering and Δ° 2θ metrics applied to X-ray diffraction patterns suggest that most I-S in Illinois Basin paleosols are likely the product of burial diagenetic processes and not ancient soil formation processes. Acid leaching from abundant coal units and hydrothermal brines are likely diagenetic mechanisms that may have impacted I-S in Pennsylvanian paleosols. These findings also suggest that shallowly buried basins (<3 km) such as the Illinois Basin may still promote clay mineral alteration through illitization pathways if maximum burial occurred in the deep past and remained within the diagenetic window for extended periods of time. More importantly, since many pedogenic clay minerals may have been geochemically reset during illitization, sources of diagenetic alteration in the Illinois Basin should be better understood if Pennsylvanian paleosol minerals are to be utilized for paleoclimate reconstructions.

Highlights

Mixed-layered, interstratified, or interlayed clay minerals possess two or more layer types or mineral components that are vertically stacked along the direction perpendicular to (001) [1,2]
By assessing the Reichweite ordering and % illite in I-S superstructures of illitesmectite mineralogical intergrades from Illinois Basin (IB) paleosols, this study attempts to understand the post-Pennsylvanian history and diagenetic impact on clay minerals originally formed during pedogenesis
This study finds that deep-burial diagenesis is a more probable source of illitization in IB paleosols than a pedogenic origin for these mineralogical phases

Summary

Introduction

Mixed-layered, interstratified, or interlayed clay minerals possess two or more layer types or mineral components that are vertically stacked along the direction perpendicular to (001) [1,2]. The most ubiquitous evidence of these reactions in sedimentary basins is mixed-layered illite-smectite (I-S) [2,3,6,14] and may be used to assess maximum burial and a basin’s thermal evolution This was demonstrated in studies of pelitic sediments from the U.S Gulf of Mexico region, which recognized patterns of diagenesis in offshore wells by observing decreasing abundances of potassium-feldspar and smectite and increasing abundances of I-S and discrete illite with increasing burial depth [6,7,15,16,17,18,19,20]. Smectite illitization is characterized by the collapse of the smectite 2:1 expansible interlayer into a 12-fold coordination of basal tetrahedral oxygen anions around a K+ interlayer cation as other, exchangeable, hydrated cations (e.g., Mg2+ , Ca2+ , Na+ ) are expelled from the interlayer to form illite (Figure 1) [26]

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