ABSTRACT

During the Late Paleozoic Ice Age, the fault-bounded equatorial Cumberland Basin of Nova Scotia experienced rapid subsidence, accumulating kilometer-thick fluvial sedimentary units derived from two highlands to the northwest and southeast. Major variations are recorded in the paleosols exposed at the Joggins Fossil Cliffs, ranging from oxidized and well-drained paleosols with recognizable vertic features to highly reduced organic-rich paleosols. These different soil lithologies suggest alternating conditions between well-drained floodplain environments and water saturation associated with overall poor soil development. Although halokinetic subsidence of the Cumberland Basin is known to have been operative during deposition of these units, previous research favored glacio-eustatic processes as the primary forcing mechanism of sedimentation. A total of 474 fluvial aggradational cycles were identified within a kilometer-thick interval and show a fluctuating accommodation history with a very abrupt nature. The series of fluvial aggradational cycles was used to develop threshold autoregressive models based on 1) their thickness, 2) their paleosol thickness, 3) their sandstone content, and 4) their paleosol-to-sandstone ratio. For each model, results suggest no evidence of statistically significant cyclicity, contradicting the hypothesis that fluvial sedimentation was mainly driven by glacio-eustatic cyclothems. Additionally, a total of 7 lithologies were recognized through 1,655 beds. Evaluation of 8 spherical semivariograms suggests no evidence for cyclicity in the frequency, order, or distribution of the data based on lithologies, although some covariance was found at distances between 550 and 750 m suggesting similar processes controlling sedimentation in the lower and upper Joggins Formation. The Cumberland Basin is known to have been rapidly subsiding, mainly because of ductile deformation of salt deposits in the deeper basinal units. Our results suggest that Joggins records tectonically induced ponding of a part of the sedimentary basin, allowing more extensive preservation of abundant coal and organic-rich units, as well as still-standing fossil forests exposed along the cliffs. These new results suggest that tectonic subsidence of the Cumberland Basin during the Late Paleozoic Ice Age was a more important driver of fluvial sedimentation than previously thought. This novel application of the TAR methodology provides a mathematical description of the sediment accumulation history of terrestrial basins when applied to conformable sedimentary successions, along with the means of linking paleosol development to climatic processes.

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