Levee Asymmetry in a Deep-Water Channel Bend: Example from the Isaac Formation (Windermere Supergroup), Southern Canadian Cordillera

Zishann Khan, Lilian Navarro, and Bill Arnott
Earth Sciences, University of Ottawa, Ottawa, ON, Canada

Isaac Channel 3 is a rare outcrop example of a perpendicular cut through a sinuous deep-water channel in which levee deposits formed on opposite sides of the channel are well exposed. Important differences between strata flanking the outer- versus the inner-bend margin of the channel have been noted, and include: lithofacies, architecture, and association with channel-fill strata. Proximal outer-bend levee deposits are sand-rich (N:G up to 70%) and comprise medium- to thick-bedded, Ta-c turbidites interstratified with thinly-bedded, Tc-e turbidites (TBT). In contrast, inner-bend levee deposits are mud-rich (N:G as low as 20%) and consist predominantly of TBT.

Erosion associated with lateral migration of the channel axis produced a sharp contact along the outer-bend channel margin and as a consequence coarse-grained channel-fill deposits are in erosional contact with levee deposits. This suggests that the crest of the outer-bend levee was elevated above the channel floor and produced a channel margin upon which channel-fill strata onlapped. Positive topography was most likely developed by “flow-stripping”, which deposited abundant sand on the outer-bend levee while the majority of the flow continued through the channel bend and bypassed to areas further downslope.

In contrast, thick-bedded, amalgamated channel-fill deposits in the axial channel area grade laterally over 150 m into TBT on the inner-bend levee. The lack of channel-fill onlap relationships imply that topography along the inner-bend was sufficiently subtle that flows expanded laterally into the overbank area without being separated from the main throughgoing channel flow. Accordingly, coarse-grained strata in the channel axis are laterally continuous with muddier and thinner-bedded strata deposited on the inner-bend levee, and hence contrast markedly stratal conditions observed on the outer-bend levee.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas