ASPECTS OF PENNSYLVANIAN SANDSTONE DIAGENESIS, CENTRAL APPALACHIAN BASIN: QUALITATIVE AND QUANTITATIVE ANALYSIS

Pennsylvanian lithic and quartz-rich sandstones from different stratigraphic intervals and depositional environments of the central Appalachian Basin were sampled from three cores to evaluate controls on sandstone diagenesis. The preliminary compositional multivariate data set generated from point counts was reduced using Correspondence Analysis (CA), an exploratory, non-parametric method.

Ordination plots demonstrate that upper Pennsylvanian samples are predominantly lithic arenites whereas lower Pennsylvanian samples are mostly quartz-rich. General paragenetic trends, that can be resolved petrographically, reveal that siderite rims and iron oxide coatings on detrital grains, especially quartz, are the earliest diagenetic products. Calcite cements/replacements occupy distinct zones and, locally, are replaced by ankerite. The most volumetrically significant burial cement is quartz.

Bivariate plots show a number of patterns reflecting stratigraphic, depositional environment, and framework grain composition controls on sandstone diagenesis. Sixty-three times more siderite is present in lower Pennsylvanian samples than in upper Pennsylvanian samples. Conversely, 121 times more iron-oxide/oxyhydroxide occurs in the upper Pennsylvanian samples. The distribution of these early cements may reflect the second-order paleoclimatic signature recognized through the Upper Carboniferous, in which early Pennsylvanian time was marked by everwet and potentially reducing conditions, whereas the late Pennsylvanian climate was more seasonal and possibly characterized by oxidizing surface and near-surface conditions. Calcite cementation/replacement appears to be controlled, in part, by depositional environment because most calcite occurs near the top of each sand body where evidence of marine influence exists in most upper Pennsylvanian sandstones. Quartz cement is most abundant in the lower Pennsylvanian quartz-rich sandstones. This is attributed to less compaction-related loss of primary porosity than in the more lithic, upper Pennsylvanian sandstones. In view of the volume of quartz cement, the most likely source of silica-rich fluids is basinal brines mobilized by thrust loading.