Contrasting Styles of Chemical Compaction in the Upper Pennsylvanian Dennis Limestone in the Mid-continent Region, U.S.A.

Abstract

ABSTRACT The Upper Pennsylvanian Dennis Limestone of the U.S. Mid-continent region displays a wide variety of pressure dissolution features, the distribution of which was governed largely by patterns of earlier diagenesis. Intergranular compaction in grainstones, measured quantitatively using a newly-developed compaction index, is generally restricted to diagenetic facies that received little cement. Compaction is greatest in the most seaward and stratigraphically lowest facies, where degrading neomorphism, an apparent result of compaction, also occurred despite shallow burial and absence of tectonic deformation. The least compaction in grainstones occurred in the two well-cemented diagenetic facies that are geographically and stratigraphically in the middle of the formation. Intermediate degr es of compaction occurred in the uppermost, most landward diagenetic facies, which was less extensively cemented than the facies just below. Patterns of earlier cementation, and perhaps dissolution of early cement, were more important than porewater chemistry during burial in determining the extent of pressure dissolution in Dennis Formation limestones. Stylolites occur in the most extensively cemented diagenetic facies, where petrographic relationships suggest that stylolitization was initiated by destruction of large voids generated by dissolution of phylloid algae. Dissolution seams, but not stylolites, occur in clay-rich fine-grained dolomitic limestones. Stylolitization, intergranular pressure dissolution, and degrading neomorphism occurred at burial depths of no more than 800 to 1500 m, but these processes may have been promoted by burial temperatures up to 160°C. The style and extent of chemical compaction vary greatly in the Dennis Formation across as little as 5 m vertically and 40 km laterally. This extreme variability within one formation illustrates that compaction and porosity destruction in limestones may often be unpredictable from a purely geographic or stratigraphic perspective, and emphasizes the necessity of understanding earlier diagenetic and hydrologic systems within any given formation.