The influence of basin physiography on the character of deep-marine sedimentation. Lessons from the Neoproterozoic Windermere Supergroup, southern Canadian CordilleraTuesday, November 19, 2019 - 10:00am to 10:20am Theatre Three
Deep marine rocks of the Windermere Supergroup record a several km-thick sedimentary pile that accumulated along the passive continental margin of Neoproterozoic Laurentia (ancestral North America). The succession comprises mostly siliciclastic sedimentary rocks intercalated with carbonate and mixed carbonate-siliciclastic intervals. Observations along a several 100 km-long depositional transect that stretches from upper slope canyons to deep basin floor deposits show a number of systematic changes, but only in the slope part of the transect.
Slope deposits are dominated by levee deposits intercalated with slope channel complexes that range up to >100 m-thick by several km-wide, and exhibit two end member kinds of channel fills: aggradational and laterally accreting. Aggradational channels, which individually are of the order of 10-15 m, exhibit a well-developed upward and lateral fining and thinning of strata. In contrast, laterally-accreting channels, which also are ~10-15 m thick, show little upward or lateral change in grain size that on average is also coarser and better sorted than in aggradational channels. Additionally, laterally-accreting channels are associated with the input of carbonate sediment and evidence of more active mass wasting in the form of thickly developed, areally extensive debrites, slump and slide deposits. Moreover, stromatolite and oolite fragments, in addition to abundant carbonate cemented sandstone and mudstone clasts, indicates the resedimentation of debris sourced from an upslope shallow-water carbonate platform under late transgressive, highstand to possibly early falling stage conditions. The eustatic rise that led to the development of the carbonate platform, is also interpreted to have significantly modified the make-up of the siliciclastic sediment supply, principally in terms of its grain-size distribution, which accordingly controlled the density structure in the through-going turbidity currents, and ultimately the character of the slope channel systems. Central to the systematic change in sediment supply is the presence of a topset (i.e. shelf) -- a physiographic feature that is absent in ramp-style basins, and therefore where modification of the hinterland sediment supply would be minimized, and accordingly its influence on turbidity current structure.
Further basinward, basin floor deposits form a succession of intercalated decameter-thick “sheetlike” sandstone and mudstone layers. Sandstone layers are composed mostly of terminal splay deposits with lesser distributary channel complexes and rare feeder channels. Grain size of the sand is little different from that on the slope. Mudstone layers are dominated by thin-bedded, upper division turbidites. Unlike the slope, the basin floor stratigraphy shows no systematic temporal (i.e. upward) change in architecture. This suggests that the effects of eustatically controlled changes in sediment supply became attenuated and apparently completely filtered as flows descended the continental slope, ultimately resulting in a basin floor stratigraphy largely unaffected by upslope conditions but instead controlled principally by local seabed topography.