FACIES CHANGE ALONG THE MARGIN OF A DEEP-MARINE TERMINAL SPLAY, NEOPROTEROZOIC WINDERMERE SUPERGROUP, BRITISH COLUMBIA, CANADA

Many deep-marine turbidite systems transition downflow from an expansive distributive network of channels into sand-rich splays, collectively forming a depositional lobe. While the gross morphology of lobes are well imaged on seismic, details of their lithological composition and distribution are still incompletely understood. This is particularly true for sand-rich splays, which owing to a paucity of internal seismic reflections suggest little apparent internal lithological variability. Needed, therefore, are examples from the ancient deep-marine sedimentary record where splay deposits are well exposed, and details of not only their sand-rich internal stratigraphy, but also how they terminate laterally along their margins, can be observed.

Well-exposed basin floor deposits crop out in the Upper Kaza Group at Castle Creek (Windermere Supergroup) in the southern Canadian Cordillera. Here, a 1 km wide section that varies laterally from 17 to 9 m thick was studied in detail. Exposed in the southeastern basal part of the section are two several meter-thick, sand-rich distributary channel fills. Toward the northwest these strata are terminated abruptly along the base of a sand-rich terminal splay, which in turn is overlain by a second sand-rich splay that spans the width of the outcrop. The margins of both splays are well exposed in the northwest part of the study area.

Within the study area four facies are recognized: (1) amalgamated sandstone, (2) classical (Bouma) turbidites, (3) muddy turbidites, and (4) matrix rich sandstones. In ~700 m of the section, splay deposits consist of high net-to-gross, amalgamated, dm-thick, coarse- and medium-grained sandstone. Laterally, however, these strata transition into a complexly intercalated succession of thinner (cm- to dm-thick) bedded Facies 1-4 strata that are continuous with the amalgamated part of the splay. Particularly important is that this transition occurs over a distance of about 300 m and is marked by a 26% decrease in sandstone content (specifically, 16% mud, 10% matrix-rich beds, and 74% sandstone). These latter strata are interpreted to be the lateral margins of sand-rich splays, and importantly illustrate the rapid changes in facies and reservoir characteristics along the margins of otherwise areally-extensive, sand-rich terminal splay deposits.