Diagenesis of the Purington Shale in the Illinois Basin and Implications for the Diagenetic State of Sedimentary Rocks of Shallow Paleozoic Basins

The clay minerals, micas, and feldspars of the Pennsylvanian‐age Purington Shale have been more diagenetically active than generally recognized. They have undergone diagenetic changes comparable to those of Cenozoic and Mesozoic sedimentary rocks buried three to four times as deeply and heated to approximately twice the temperature. The Purington Shale on the Western Shelf of the Illinois Basin has experienced maximum burial of ≤1.0 km and was never heated to more than 60°C, except for a very brief time at about 80°C. The illite/smectite (I/S) of the Purington Shale has ≥90% illite. The chemical compositions of the feldspar assemblage in the 0.09–0.063‐mm fraction differ from most modern detrital suites of feldspars but are similar to diagenetic sequences described by others from much greater burial depths and are extensively, but not completely, albitized. The simplest interpretation of the apparently advanced stage of diagenesis is that the diagenetic processes have been operating at less pressure and lower temperature but for a longer time; an example of low‐temperature, time‐dependent diagenesis. By similar reasoning, the apparently anomalously advanced maturity of all of the sedimentary rocks in the relatively shallow Illinois Basin is explained. This generalization should be considered for all shallow Paleozoic basins and should influence (1) exploration for petroleum, (2) use of modal and chemical analyses for determining provenance, and (3) use of chemical composition of shales as proxy for crustal evolution.