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 Mesoz oic sedimentary rocks buried three to four times as deeply and heated to ap proximately twice the temperature. The Purington Shale on the Western Shelf of the Illinois Basin has experienced maximum burial of less than or equal to 1.0 km and was never heated to more than 60 degrees C, except for a ver y brief time at about 80 degrees C. The illite/smectite (I/S) of the Puring ton Shale has greater than or equal to 90% illite. The chemical composition s 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 sequence s described by others from much greater burial depths and are extensively, but not completely, albitized. The simplest interpretation of the apparentl y advanced stage of diagenesis is that the diagenetic processes have been o perating at less pressure and lower temperature but for a longer time; an e xample of low-temperature, time-dependent diagenesis. By similar reasoning, the apparently anomalously advanced maturity of all of the sedimentary roc ks in the relatively shallow Illinois Basin is explained. This generalizati on should be considered for all shallow Paleozoic basins and should influen ce (1) exploration for petroleum, (2) use of modal and chemical analyses fo r determining provenance, and (3) use of chemical composition of shales as proxy for crustal evolution.