CROSS-BEDDED WOODY DEBRIS FROM A PLIOCENE FORESTED RIVER SYSTEM IN THE HIGH ARCTIC: BEAUFORT FORMATION, MEIGHEN ISLAND, CANADA

摘要

Cross-bedding, the inclined internal stratification that records the migration of certain transverse sedimentary bedforms, is nearly ubiquitous in current-transported bedload sediments. Although examples of the structure are known from inorganic clastic sediments and sedimentary rocks from practically all depositional environments and intervals of geologic history, here we report cross-bedded lenses that are composed wholly or significantly of woody debris, in Pliocene alluvium of the Beaufort Formation in the Canadian High Arctic. The uniqueness of cross-bedded woody debris has hitherto been overlooked, but we demonstrate that, in the entire Phanerozoic record, it is apparently restricted to alluvium deposited during a warm climatic interval that permitted the growth of boreal-type forests within 10u latitude of the North Pole. The marked spatiotemporal restriction of cross-bedded woody debris implies that there may be environmental factors, unique to polar forests, which promote the subaqueous transport of large amounts of fine woody debris as fluvial bedload. We propose a nonuniformitarian conceptual model for the formation of cross-bedded woody debris in forested polar rivers whereby an exceptional abundance of woody debris could accumulate, and become saturated and denser than water, due to reduced decomposition on forest floors that were subject to prolonged periods of darkness and subzero temperatures.