More Evidence of the Devonian Shift From Hot House to Ice House Global COnditions

Sequence stratigraphic hierarchy of the Upper Devonian Foreknobs Formation, central Appalachian Basin, USA: Evidence for transitional greenhouse to icehouse conditions

Authors:

1. Wilson S. McClung (a)
2. Kenneth A. Eriksson (b)
3. Dennis O. Terry Jr. (c)
4. Clifford A. Cuffey (a)

Affiliations:

a. Chevron USA Inc, 15 Smith Rd, Midland, TX 79705

b. Department of Geosciences, Virginia Tech, Blacksburg, VA 24061

c. Department of Earth and Environmental Science, Temple University, Philadelphia, PA 19122

Abstract:

The Foreknobs Formation (Upper Devonian; Upper Frasnian to basal Famennian) comprises the uppermost marine strata of the progradational “Catskill clastic wedge” of the south-central Appalachian Mountains (Virginia-West Virginia; USA). The Foreknobs Formation consists of 14 lithofacies arranged in four facies associations which record the following depositional settings: 1) storm-dominated distal to proximal offshore to shoreface (facies association A); 2) sharp-based conglomeratic shoreface (facies association B); 3) fluvial redbed (facies association C); and 4) incised-valley fill (IVF; facies association D). Vertical juxtaposition and stacking patterns of lithofacies and facies associations permits recognition of a hierarchy of three scales of cyclicity. Up to 70 short-term 5th-order cycles, each averaging ~ 65 Kyr, consist of coarsening-upward parasequences of storm-dominated offshore marine facies in the distal setting which correspond to high frequency (unconformity bound) sequences (HFS) of fluvial redbed strata overlain by offshore marine strata in the proximal setting. These facies relationships are a consequence of 10 – 15 m of sea-level fluctuations. Up to 12 intermediate-term 4th-order cycles, each averaging ~ 375 Kyr, consist of stacked 5th-order cycles. The 4th-order cycles are bounded by regressive surfaces of marine erosion (RSME) at the base of sharp-based conglomeratic shoreface sandstones in the distal setting that correspond with paleosols in the proximal setting. In some cases, the 5th-order cycles within each 4th-order cycle exhibit stacking patterns indicative of increasing or decreasing accomodation space. These facies relationships are a consequence of 25 – 35 m of sea-level fluctuations. Three complete and portions of two additional 3rd-order cycles, each averaging ~ 1.12 Myr, consist of stacked 4th-order cycles. The 3rd-order sea-level trends reflected in the Foreknobs Formation are nearly identical to published eustatic sea-level curves. Incised-valley fills are present at one of the 3rd-order cycle boundaries and are a consequence of a 35 – 45 m sea-level fluctuation. The amplitudes of the inferred sea-level fluctuations are comparable to the expansion and contraction of ice volumes within the current Greenland and Antarctic ice sheets which suggests glacioeustasy was the primary control on sea-level fluctuations and cyclicity within the Foreknobs Formation. Such an interpretation is consistent with knowledge of Devonian climate, transitioning from Middle Devonian greenhouse to Late Devonian icehouse, as indicated by evidence of glaciation during parts of the Late Devonian in South America and the Appalachians.