HOVER, VICTORIA C., DONALD R. PEACOR, and LYNN M. WALTER, University of Michigan, Department of Geological Sciences, Ann Arbor, MI

ABSTRACT: Shale Microstructure and Chemistry in Cratonic Basins: Does Shale Authigenesis Follow the Gulf Coast Paradigm?

The model for shale authigenesis during progressive burial has relied heavily on observations from the Gulf Coast Tertiary section. However, shale diagenesis in intracratonic basins are likely to be influenced by quite different hydrodynamic and geochemical regimes. In this study, we examined clay microstructures and compositions of Upper Devonian Antrim Shale (Michigan Basin) and New Albany Shale (Illinois Basin) by STEM/AEM methods in order to determine the extent and timing of water-rock interaction and to compare results with the Gulf Coast model.

The Antrim and New Albany shale matrix is composed of subparallel intergrowths of illite-rich crystals only 50-200 Angstroms thick some of which appear to have precipitated directly into pore space. Contrast in some lattice fringe images is consistent with ordered illite/smectite (I/S). Electron diffraction patterns indicate a partial disordered 1 M<d> stacking sequence with significant turbostratic stacking. The average composition isK- and Al-deficient relative to muscovite, and also contains significant octahedral Fe and Mg.

These relations are collectively consistent with a single event in which dissolution of detrital smectite was followed by precipitation of immature, authigenic, illite-rich I/S in a relatively open system. Preservation of this immature illite in these 360 Ma shales implies that subsequent to formation of immature illite, no further clay mineral reactions have taken place and these minerals have remained effectively closed to other tectonic overprints or fluid flow events. As such, the formation sequence mirrors that in Gulf Coast shales wherein such immature illite-rich I/S formed over a narrow depth interval, remained unchanged with further burial, and in which shale systems were open with respect to formation fluid up to the point of illite-rich I/S formation.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.