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What WAS Going on in the Late Devonian?

Donald L. Woodrow1 and John B. Richardson2

1 Emeritus Professor of Geology, Hobart and William Smith Colleges, 41 Idaho St., Richmond, CA 94801, [email protected]

2Department of Paleontology, Natural History Museum, Cromwell Road, London, 5BDSW7, England, [email protected]

Late Devonian/Early Mississippian rock units in central Pennsylvania and western Maryland include distinctive rock types and sequences some of which are found nowhere else in the Appalachians. Diamictite-bearing sequences exist in the Spechty Kopf (PA) and Rockwell Formation (MD) while in the Huntley Mountain Formation (PA) there are conglomerates with exotic pebbles and quartz-rich sandstones marked by large-scale, water-release structures capped by what appear to be sand volcanoes.

These distinctive sequences occupy a small part of the Retispora lepidophyta – Indotriradites explanatus (LE) palynozone in the upper part of the Famennian Stage. Sparse, preliminary palynological data suggest that the same palynozone includes all or part of the Murrysville Sandstone of southwest PA.

The variety, wide distribution and apparent contemporaneity of these rocks strongly constrain any origin scenario proposed for them. Falling sea-level and cooling climate provide less obvious constraints. The diamictites require sudden delivery of massive amounts of poorly sorted sediment (including exotic pebbles) to the deposites. The quartz-rich sandstones with water-release structures also require sudden deposition. Deposition of the gravels with exotic pebbles found in the Huntley Mountain Formation, strongly cross-bedded gravels in the Knapp and the Murrysville sands likewise may have been deposited rapidly.

Sedimentary processes effective over a large area for a short period of time are required to explain the origin of these rocks. Processes related to orogeny, climate, major storms, sea-level change, and impact events must be considered.

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York