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Filion, Evan

Fluvial sedimentology and architecture of two Latest Devonian Lower Huntley Mountain Formation outcrops, north-central Pennsylvania, USA

Thick successions of river deposits accumulated in the north-central Pennsylvania region of the Appalachian foreland basin during Late Devonian time (~380-360 Ma). The properties and morphologies of these paleorivers are not well characterized. Latest Devonian tectonic, climatic, and eustatic controls on river dynamics and basin infilling also remain unclear. This study assesses the sedimentology, facies architecture, paleochannel depths, and grain size of a 133 m thick section of fluvial strata at two outcrops mapped as Lower Huntley Mountain Formation near Blossburg, Pennsylvania. Field-based lithofacies observations, high-resolution panoramic photography, terrestrial lidar scanning, and laser particle size analysis were used to build a stratigraphic column, map fluvial architecture, and estimate river paleoslope.

Channel facies primarily consist of cross- and horizontally-stratified fine- to medium-grained ribbon and sheet sand bodies or amalgamated multistory sand bodies with bar surfaces and scours. Proximal floodplain facies consist of thinly-bedded gray crevasse splay sandstones and massive levee siltstones and are more abundant upsection. Distal floodplain facies chiefly consist of red relict-bedded and fissile mudstone paleosols (with rootlets, slickensides, caliche nodules) and are more abundant downsection. Paleochannel depths (0.6-3.5 m, median = 1.6 m) and bar deposit grain sizes (median diameters 99-221 μm) increase significantly upsection. Estimated river paleoslopes   (2.47 x10-5 – 4.22 x10-4) do not change significantly over the studied interval.

Three hypotheses are explored to explain the observed upsection changes. An increase in sandiness, channel depth, and grain size could be generated by a prograding distributive fluvial system. Under this model, an unchanging paleoslope would suggest a decrease in uplift rates; however, age constraints on Latest Devonian tectonics must be improved to test this hypothesis. Though the increase in channel facies proportion and sand body connectedness is consistent with a decrease in basin accommodation creation rate, these architectural indicators are considered unreliable, and the effects of concurrent subsidence and sea level regression are uncertain. The similar age between Blossburg West strata and glacial deposits in the Spechty Kopf and Rockwell Fms. (LN biozone) indicates potential contemporaneous deposition. An increase in channel depth and grain size could be explained by increased discharge from higher precipitation and pro-glacial outwash; however, none of the associated evidence for increased sediment supply, higher channel braiding, or rapid drop in sea level during this time is present at Blossburg West. It is therefore likely that Blossburg West strata were deposited before the Appalachian glacial advance. An increase in precipitation could also be linked to continental drift of paleo North America into subtropical latitudes, yet Blossburg palynology data and high variability in paleogeographic reconstructions do not definitively support this hypothesis.

Check out slides from my honors thesis defense presentation (similar to what the Kalman presentation would have been) at the link below:

https://www.slideshare.net/secret/y6wbIlmSsymmiJ

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