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Stratigraphic Development from Slope Channels to Slope Gullies During Deposition of Deepwater Mixed Siliciclastic-Carbonates and Carbonates, Isaac Formation, Windermere Turbidite System, Canada


A well-exposed, up to 175 m thick by 1.5-km wide succession of deep-marine Neoproterozoic slope carbonate and siliciclastic strata crops out in rocks of the Windermere turbidite system at Castle Creek, southern Canadian Cordillera. Detailed field analysis reveals two end-member kinds of submarine channels: levee-channels and gullies. Leveed-channel complexes are >1 km wide, up to 20 m thick and composed of stacked, amalgamated channels filled mostly with thick-bedded, quartz-rich and carbonate-clastic sandstone and conglomerate, including common shallow-water extraclasts. Levee deposits consist of thin- to thick-bedded, calcareous sandstone interbedded with mudstone. In contrast, gully complexes comprise multiple isolated, narrow (100-300 m wide), low-relief (up to 2 m deep) channels filled at their base with planar or (single set) dune cross-stratified coarse-grained sandstone overlain sharply by fine-grained upper-division siliciclastic turbidites capped by calciturbidites. Differences in channel architecture are interpreted to reflect changes in relative sea level that caused changes in input variables like sediment flux, size and composition. Channel complexes probably formed during lowered relative sea level when high volumes of coarse siliciclastic sediment bypassed the shelf, and with sediment eroded from the carbonate platform, were fed directly to the slope. Gully complexes most likely formed during or after rises of relative sea level, when carbonate production on the shelf overwhelmed the siliciclastic supply and sediment delivery to the slope became dominated by a mix of carbonate and clastic mud. Locally, however, coarse clastic and carbonate sediments were confined to shallow, narrow gullies. Along their margins and further laterally, finer siliciclastic turbidites accumulated, reflecting a marked segregation of grain size within the depositional currents. Following the abandonment of the local gully system, the area was draped by laterally-continuous, finer-grained carbonate turbidites. In addition to the effects associated with changes of relative sea level and sediment character, the upward change from leveed channels to gullies might also be related an upward steepening of the (continental) slope due to an overall increase in carbonate sediment supply.