Paper No. 6
Presentation Time: 9:40 AM
A GLACIOEUSTATIC ORIGIN FOR THE UPPER DEVONIAN FOREKNOBS FORMATION, SOUTH-CENTRAL APPALACHIAN BASIN, WEST VIRGINIA-VIRGINIA
The Foreknobs Formation (Upper Frasnian to basal Famennian) comprises the uppermost marine strata of the progradational “Catskill clastic wedge.” The Foreknobs Formation consists of four facies associations recording the following depositional settings: 1) storm-dominated offshore to shoreface; 2) sharp-based conglomeratic shoreface; 3) fluvial redbed; and 4) incised-valley fill. Stacking patterns of facies associations allows recognition of a hierarchy of three scales of cyclicity. Up to 70 short-term 5th-order cycles, each averaging ~65 Kyr, consist of shoaling-upward parasequences of storm-dominated marine facies in the distal setting which correlate to high frequency (unconformity bound) sequences of fluvial redbed strata overlain by offshore marine strata in the proximal setting - a result of sea-level fluctuations in the range of 10-15 m. Up to 12 intermediate-term 4th-order cycles, each averaging ~375 Kyr, consist of stacked 5th-order cycles. These 4th-order cycles are bounded by regressive surfaces of marine erosion at the base of sharp-based conglomeratic shoreface sandstones in the distal setting which correspond with paleosols in the proximal setting - a result of sea-level fluctuations in the range of 25-35 m. The 5th-order cycles within each 4th-order cycle commonly exhibit stacking patterns indicative of increasing or decreasing accomodation space. These 4th-order cycles stack to form three complete and portions of two additional 3rd-order cycles, each averaging ~1.12 Myr. An incised-valley fill is present at one of the 3rd-order cycle boundaries and is a consequence of a 35-45 m sea-level fluctuation. The inferred 3rd-order sea-level trends of the Foreknobs Formation are nearly identical to previously published eustatic sea-level curves. The inferred amplitudes of the sea-level fluctuations are comparable to those which would be produced by melting the current Greenland or Antarctic ice sheets suggesting glacioeustasy was the primary control on cyclicity within the Foreknobs Formation. This interpretation is consistent with knowledge of Devonian climate transitioning from Middle Devonian greenhouse to Late Devonian icehouse as well as evidence of glaciation during portions of the Late Devonian in South America and in the Appalachians.