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Abstract
Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 66 (1996)No.
3. (May), Pages 519-530
Stem/Aem Evidence for Preservation of Burial Diagenetic Fabrics in Devonian
Shales: Implications for Fluid/rock Interaction in Cratonic Basins (U.S.A.)
Victoria C. Hover, Donald R. Peacor, Lynn M. Walter
ABSTRACT
Fabrics, microstructures, and compositions of authigenic illite-rich clays
in Devonian intracratonic basin shales were examined by X-ray diffraction
(XRD), scanning electron microscopy (SEM), and scanning transmission electron/analytical
electron microscopy (STEM/AEM) methods in order to relate the extent of
clay diagenesis to the timing of mudstone-system closure and to reconcile
conflicting radiogenic and paleomagnetic ages obtained from these shales
and associated limestones and bentonites. Authigenic illite in these Devonian
shales is similar to post-transition illite-rich mixed-layer illite/smectite
(I/S) in Gulf Coast and other young basin mudstones that have formed through
replacement of detrital smectite or smectite-rich I/S without subsequent
diagenetic modification. The clay matrix is composed of subparallel intergrowths
of illite-rich crystals (30-200 A thick) with low-angle grain boundaries
and defects at layer terminations. Contrast in some lattice fringe images
is consistent with R1-, R2-, and R3-ordered, mixed-layer I/S. Electron
diffraction patterns indicate partially disordered (1Md) stacking
sequences with significant turbostratic stacking and mixed layering. The
illite-rich material is K- and Aldeficient, and Si-enriched, relative to
ideal muscovite.
Preservation of burial diagenetic, authigenic, mixed-layer I/S microstructures
in these Devonian shales implies that they have remained effectively closed
systems following the smectite-to-illite transformation. Later Alleghenian
(ca. 300 Ma) tectonic or fluid-flow events recorded by authigenic minerals
in bentonites and carbonate lithologies throughout the U.S. Midcontinent
are apparently not recorded in these Devonian shales. Preservation of diagenetic
fabrics indicates that radiogenic isotope dating of cratonic basin shales
should yield syndepositional to early burial diagenetic ages corresponding
to the timing of I/S authigenesis.
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