A Detailed Analysis of Crevasse and Overbank Splay Units in the Isaac Formation (Windermere Supergroup), of the Southern Canadian Cordillera

Crevasse and overbank splay deposits have been studied in detail within the Isaac Formation. Crevasse splay units (CSU) range from 10 – 20 m thick and extend the length of the study area (> 4 km). Strata within CSU comprise medium to coarse sandstone Ta-c turbidites that range from 0.2 – 1 m thick and are interstratified with thinly-bedded tc-e turbidites. In addition, distinctively matrix-rich sandstone form up to 7 m thick packages intercalated with the turbidites. CSU are interpreted to have been deposited outboard of a crevasse channel that most likely breached the outer-bend channel margin. Episodically a number of high-energy expanding flows went through a hydraulic jump and deposited the matrix-rich sandstone. At other times, or at least locally, flows remained subcritical and deposited the more common turbidites.

OSU are common in the outer-bend distal levee setting of a significant channel-levee complex and encased by thinly-bedded turbidites. OSU consist of amalgamated normally graded to planar and cross-stratified medium-bedded, medium sandstone turbidites. OSU are 2 – 4 m thick and extend up to 1 km laterally, however, individual beds within units are laterally discontinuous. Lateral discontinuity is likely the result of rapid flow thinning, filling of topography on the distal levee, and to a lesser extent erosion. As a result, OSU comprise a complex arrangement of sandstone turbidites that commonly offlap and onlap one another locally creating high N:G within the predominantly muddy distal levee. OSU were deposited by turbidity currents that overtopped the channel margin without breaching the levee. These overspilling flows bypassed the proximal levee area because of the steep slope on the backside of the levee, and deposited much of their sediment on the reduced slope over the more distal levee.