| Abstract/Description or Keywords |
Closely related floodplain and delta sites near Moose Lake, British Columbia (Canadian Rocky Mountains) are examined for evidence of environmental change and to assess the effects of upstream storage and reworking of sediment on downstream sedimentary records. The simultaneous development of composition-based provenance techniques facilitates the interpretation of downstream sediments in terms of source area, sediment production processes and hydroclimatic forcing. Initially, Fraser River bed material is used to characterize upstream source areas and to estimate mixing of source materials downstream. Although sedirnentary rocks provide minimal contrast in ternis of geochemical trace elements, it is possible to distinguish sediments Řrom hnro main watershed source areas based on major elernents associated with carbonate and aluminosilicate bedrock lithologies. Complications arise due to the effects of particle size. hydrodynamic sorting, and transport-related and postdepositional alteration. Currently the Moose River sub-basin contributes a greater proportžon of the total and fine-grained sedirnent load delivered to Moose Lake than expected based on catchment area. The imbalance is related to greater elevations, runoff, and glacier cover in the sub-basin. A substantiat area of the Moose Lake-Fraser River delta-top floodplain derives from the early half of the Holocene, and is characterized by fragmentary. low-discharge paleochannels containing bed materials characteristic of the uppermost Fraser River. Most floodplain development since ca. 4 ka BP records persistently higher sediment loads, aggradation and larger, more active channels. Detailed variations in the composition of laterally and vertically accreted floodplain sediments and of channel morphology reflect regionally documented glacier advance and retreat stages. Except for possible till deposits in the northwest, most valley-fiIl below Moose Lake and the delta is glaciolacustrine in origin. deposited rapidly as ice retreated up-valley at the end of the Wisconsinan. A large proportion of the delta was apparently constructed prior to ca. 10 ka BP and is either paraglacial or deglacial in origin. It is estimated that progradation and channel shifting on the delta surface could account for a graduai increase of up to 21% in distal lacustrine accumulation rates over Neoglacial time. More episodic variations likely occurred due to channel splitting and meander cutoff on the floodplain. |
