Possible climate variability in the Lower Old Red Sandstone Conigar Pit Sandstone Member (early Devonian), South Wales, UK

摘要

The Early Devonian Conigar Pit Sandstone Member (CPSM) of the Freshwater West Formation (Lower Old Red Sandstone, Lochkovian age) at Freshwater West in south Wales comprises a heterolithic, predominantly alluvial suite (mudstones, fine to coarsegrained sandstones, conglomerates) with varying depositional architecture. A number of discrete facies associations are observed. The fine-grained, inclined- and non-inclined heterolithic association is dominant within the CPSM. It represents dryland sinuous channelized flow (IHS bedsets) and unconfined flow (non-inclined bedsets) at terminal and intermediate floodouts deposited under semi-arid conditions. Mudstones were deposited in two distinct environments associated with this semi-arid river system. Laminated and burrowed, reddish brown mudstones were deposited in shallow semi-permanent lakes or pools on the floodplain. Massive mudstones were deposited as within-channel muddy braid-bars, with sedimentary structures being lost during subsequent compaction. The mudstones exhibit Vertisol pedogenesis reflecting the seasonal wetting and drying associated with the semi-arid climate. Periods of intense desiccation are indicated by deep desiccation cracks and associated rubbly surfaces. Possible, though unproven tidal conditions influenced a small percentage of the heterolithic channels at Freshwater West. The extent of these channels is uncertain. The coarser-grained multi-storey sandstone association was deposited by low-sinuosity rivers with a fluctuating, but perennial discharge. Associated with these laterally extensive sandbodies was a high water table with surface ponding (wetlands) that enabled the preservation of plant detritus. During high discharge events, flow expanded over the contemporaneous floodplain depositing the sheet sandstone association within splay complexes. The interpretation that perennial discharge may have been triggered by basin-wide climate change challenges the long-held view that the Lower ORS climate was continuously semi-arid in nature. The spatial/temporal extent of this climate change is uncertain, as it is possible that the multi-storey sandbodies were deposited by allogenic rivers draining a distant, possibly uplifted source area. A decrease in the observed frequency and maturity of Vertisol profiles, and a corresponding increase in multi-storey sandbodies upward through the succession may reflect a long-term, episodic pattern of climate change.