WOODY DEBRIS LENSES AS PALEOENIVORNMENTAL ARCHIVES

Allochthanous woody debris lenses (WDL) are found with remarkable preservation within the unconsolidated fluvial deposits of the mid-Pliocene Beaufort Formation on northern Banks Island in Arctic Canada. WDL were measured and classified according to sedimentology, woody debris sorting, and morphology to understand variation in paleodeposition and infer rudimentary stream dynamics. Fossil wood (pristine and reworked) and pollen collected from WDL were analyzed to reconstruct the paleoenvironment of the surrounding floodplain. Two types (A and B) of WDL are discernable within the Beaufort Formation. We classified 44% of WDL as “Type A” lenses which contain poorly sorted debris, often with randomly oriented medium to large pieces of wood, that rest upon fining upward sediment sequences, suggesting likely deposition in a river bend. “Type B” lenses comprise 33% of WDL and are elongate lenses composed of well-sorted fine debris often underlain by massive sandy sediment. The remaining 22% of WDL do not display any consistent characteristics amongst themselves or that relate to types A or B and thus are anomalous and classified as “Type C”. Fossil wood recovered from “Type A” WDL show remarkable heterogeneity in preservation and indicate that some fossil wood was substantially reworked. We identified 4 types of fossil wood in the WDL referable to the following genera: Picea, Larix, Pinus, and Abies. All four types were found in pristine condition. 48% of the samples were Pinus, 30% were Picea and Larix and 22% were Abies. Nearly two thirds of the reworked wood was Picea and Larix with the balance (33%) being Pinus. Abies was not found as reworked wood. Fossil wood growth rates (inferred from fossil ring widths) on average were largest (0.86 mm) in pristine wood samples and smaller in reworked wood samples (0.61 mm). Among the samples analyzed to date we have found no consistent relationship between growth rate and wood taxonomy. WDL description and classification suggests two distinct paleodepositional environments within a braided river setting; at the heads of small islands and bars and the outside perimeter of channel bends. The fossil wood taxonomy and wood growth rates are consistent with trees growing in a sub-boreal floodplain environment.