PALEOCLIMATIC IMPLICATIONS OF LATE DEVONIAN GLACIAL DEPOSITS FROM THE EASTERN UNITED STATES

An episode of significant climatic cooling is recorded in Late Devonian strata of the Appalachian basin. An extensive interval of diamictite, varvite, and braided fluvial sandstone is preserved over an area of more than 16,000 km2 and extending for more than 400 km along depositional strike. This lithologic association is preserved in the Spechty Kopf Formation of northeastern Pennsylvania, Rockwell Formation of south-central Pennsylvania and western Maryland, and upper Hampshire Formation of east central West Virginia. Although the diamictite is variable within and among exposures, it is typically matrix-supported, unbedded mudstone containing subangular to rounded clasts up to 2 m in diameter. The enclosed clasts are both locally derived and exotic; some clasts exhibit striations, faceting, and polish. At many locations the diamictite is overlain by laminated varvite containing isolated sand, pebbles, or cobbles, and rarely boulders. Typically, the diamictite/varvite interval is overlain by a coarse-grained, trough to planar cross-bedded sandstone interval that contains abundant quartz gravel layers. This depositional package is interpreted as a glacial advance and retreat sequence with the diamictite (ice contact facies) overlain by glaciolacustrine varvite with dropstones, and topped by glaciofluvial braided stream outwash sandstone and gravels, and sandy lacustrine deposits. The interpreted glacial deposits are correlative with changes in the character of laterally equivalent paleosols. Stratigraphic changes from red, calcic paleo-Vertisols in strata below the diamictite to gray, non-calcic Spodosols and coal beds in the diamictite interval suggest that the climate became much wetter during the deposition of the diamictite. Following diamictite deposition the climate again became drier as indicated a return of red soils above the diamictite interval. In western Pennsylvania and eastern Ohio, rocks that are contemporaneous to the glacial sequence record a sharp drop in sea level.

The Appalachian Late Devonian interval of climatic cooling coincides with glacial deposits in South America, suggesting that this cooling event was global in its effects. This Late Devonian global cooling event is interpreted to be responsible for the extinction episode at the Devonian/Carboniferous boundary.