科研之友
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摘要
The supposition that the Late Devonian Gondwanan glaciation should be recorded in more temperate regions by black shales, incised valleys, and lowstand wedges is tested with reference to the Foreknobs Formation (late Frasnian to early Famennian) of the central Appalachian Basin (USA). The upper (early Famennian) portion of the Foreknobs Formation in a proximal strike belt in eastern West Virginia contains an erosionally based, coarse, conglomeratic, incised valley fill (IVF) that is underlain and overlain by strata deposited in a relatively deep-marine, ramp setting. A signal of sea-level drawdown is in the form of conglomeratic, hummocky cross stratified (HCS) storm event beds underlying the IVF, suggesting the proximity of a gravely fluvial point source up-paleoslope. Within a more distal outcrop 66 km (41 miles) to the west, and at about the same stratigraphic horizon, the Foreknobs Formation exhibits a relatively thin succession of black shale that was deposited as sea-level fell, resulting in the concentration of nutrients that caused anoxic conditions. Overlying the black shale are several erosionally based lowstand wedge deposits comprised of cyclic conglomeratic, braided-stream facies which fine-upward to dark gray shales deposited within estuarine environments. The genetically related proximal IVF and the lowermost distal lowstand wedge formed during a major 3rd-order eustatic sea level fall (35-45 m) consistent with published 3rd-order sea-level curves. Lowering of sea-level (base-level) resulted in steeper stream gradients which caused incision and transportation of gravel from the wedge-top depozone of the foreland basin down to the basinward-migrated shoreline. Inferred depositional environments of the distal lowstand wedge are consistent with interpretations from regional subsurface mapping west of the most distal outcrop belt. From inferred magnitudes of sea-level falls and the physics of ice-sheets, calculations of areal extents of ice over Gondwana suggest that an early Famennian inferred sea-level fall of 35-45 m translates to an area of ice which is only half of the reported area of late Famennian ice. The data are consistent with step-wise increases in magnitude of sea-level fluctuations during deposition of the Upper Devonian Foreknobs Formation within a period of time transitional between Middle Devonian greenhouse conditions and the later icehouse conditions of the Late Paleozoic.
- 出版日期2016-3-15
