Roles of Provenance, Eustasy, and Paleoclimates in Determining Diagenetic Alterations in Pennsylvanian Sandstones, Forest City Basin, Mid-Continent, U.S.A.

BRENNER, ROBERT L., University of Iowa, Iowa City, IA, ROLAND SCAL, University of New Hampshire, Durham, NH, and GREG A. LUDVIGSON, Iowa Department of Natural Resources, Geological Survey Bureau, Iowa City, IA

Pennsylvanian quartz arenites and subarkoses in the Forest City basin have a variety of potential reservoir qualities that hinge on changes in siliciclastic provenance and diagenetic processes. Upward increases in the amounts of unstable grains and grain angularity record the unroofing of cratonic sedimentary rocks overlying crystalline basement rocks. Superimposed over this trend was a cyclic pattern of climatic changes controlled by eustatic sea-level and plate tectonic movements that profoundly affected early diagenetic processes. For example, plagioclase feldspars were altered as a result of the circulation of low pH, organic-rich waters through a moderately open hydrologic system. Coal deposits, paleoflora, and paleosols suggest that from Morrowan through middle Desmoinesian, sed mentation occurred under wet-humid to ever-wet tropical conditions conducive to unstable silicate mineral degradation.

Upper Desmoinesian and Missourian climates were drier and thus more conducive to preservation of such minerals during transportation and to reduction in the amount of weathering after deposition. Upper Desmoinesian and Virgilian units were deposited during periods of increased marine influence within eustatically controlled cyclic sedimentary packages. These units were covered quickly by relatively impermeable marine shales and limestones, and reacted with modified marine fluids whose pH were buffered by carbonate equilibria.

Drier climates in the Late Pennsylvanian resulted in reduced acidic meteoric pore water, leading to preservation of detrital apatite, reduced etching of garnet, and better preservation of feldspars. Feldspars in the Virgilian strata are much fresher, showing an increase in well-preserved grains and an overall reduction in the intensity of etch solution.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)