CAN ALLUVIAL STRATIGRAPHY BE USED AS A TOOL TO DIFFERENTIATE CLIMATIC FROM TECTONIC CONTROL ON SOIL DRAINAGE FLUCTUATIONS? APPLICATION TO THE PENNSYLVANIAN JOGGINS FOSSIL CLIFFS OF NOVA SCOTIA, CANADA

The UNESCO World Heritage Joggins Fossil Cliffs of Nova Scotia constitute one of the best-known records of the Late Paleozoic Ice Age (LPIA) in an equatorial setting. Major variations are recorded in the drainage of paleosols exposed at Joggins, from well-oxidized and well-drained soils with strongly developed vertic features, to highly reduced, drab mudstones rich in organic remains and suggesting water-saturation for extended periods and overall poor soil development. Such variations can be explained by either climatic or tectonic causes (i.e., halokinetic processes occurring in the substratum during sediment deposition in the Cumberland basin). The fault-bounded Cumberland basin was rapidly subsiding during the Pennsylvanian, and accumulated sediments from two highlands to the NW and SE. Relatively conformable alluvial successions such as at Joggins Fossil Cliffs are particularly suitable for alluvial stacking methodology in order to assess facies alternations in terrestrial environments. Fluvial aggradational cycles (FACs) are m-scale fining-upward successions characteristic of overbank flooding events, commonly with a paleosol weathered into its upper boundary. FACs stack into larger-scale FAC-sets, which are thinning upward, have disconformable lower and upper boundaries, typically show an increase in paleosol maturity and drainage upward, and can, in turn, allow assessment of changes in the rate of sediment accumulation. Because the Cumberland basin is known to have been rapidly subsiding during the Pennsylvanian, accumulating an estimated 4 km of strata in 4 Myr, we argue that changes in the rate of sedimentation at Joggins are closely associated with changes in the rate of subsidence. As a consequence, we propose that short-lived periods of more pronounced sedimentation rate fluctuations are likely associated with episodes of increased salt withdrawal in the substratum, possibly overprinting any climatic causes that could explain soil drainage fluctuations. On the other hand, soil drainage variations during long-lived periods of relatively steady sedimentation rates are most likely explained by long-term climatic fluctuations, likely associated with periods of expansion and contraction of the Gondwanan Ice Sheets. These are provisional hypotheses, still being tested.