Date of Graduation

1991

Document Type

Dissertation/Thesis

Abstract

The origin and significance of diamictites in the Late Devonian-Early Mississippian Spechty Kopf and Rockwell Formations is not fully understood. For this study, twelve outcrops in northeastern, central, and southwestern Pennsylvania and three outcrops in western Maryland, which contained the diamictite, were investigated. Petrographic analysis revealed that rocks in this study consisted of nine major lithologies: (1) quartz arenite, (2) sublitharenite, (3) sublitharenite (diamictite), (4) lithic arenite, (5) quartz wacke, (6) lithic wacke (15-40% matrix), (7) lithic wacke (40-60% matrix), (8) lithic wacke (60-75% matrix), and (9) mudstone. Stratigraphic analysis suggested that lithologies were deposited in eight facies: (1) fluvial, (2) estuary, (3) beach-bar, and (4) hyperconcentrated flow, (5) dilute debris flow, (6) viscous debris flow, (7) waning flood, and (8) waning viscous flow. Rocks in the lower member of the Spechty Kopf-Rockwell Formations consist of sandstones with minor amounts of shales and siltstones. All of the rocks in this member were deposited in the fluvial facies except rocks at the LaVale, MD, outcrop, which were deposited in the estuary facies. Rocks in the middle member consist of diamictites with scarce interbedded sandstones of the fluvial and beach-bar facies. Diamictites were formed by hyperconcentrated flow and viscous and dilute debris flows. The upper member sandstones and minor shales were deposited in three facies: (1) fluvial facies in northeastern and central Pennsylvania; (2) beach-bar facies in southwestern Pennsylvania and extreme western Maryland; and (3) estuary facies in western Maryland. Provenance analysis of sandstones from the lower and upper members and framework grains in the diamictite deposits suggested that the source was a recycled orogen consisting of low rank metamorphic and sedimentary rocks. Clasts in the diamictite unit were mostly sedimentary and metamorphic rock fragments, with the exception of volcanic rock fragments in the southern outcrops that suggests a secondary southern source: the Blue Ridge-Shenandoah massif and possibly the Piedmont Chopawamsic terrane. A depositional model for the diamictite sequence includes renewed tectonic activity in the early Mississippian that generated hyperconcentrated flows and debris flows in subaerial alluvial fans and estuarine settings.

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