Lateral and Vertical Changes in Deep Marine Levee Deposits — Where Intuition and Geological Reality Diverge: Neoproterozoic Isaac Formation, Windermere Supergroup, B.C., Canada

Abstract

Channel-levee complexes are a principal component of deep-marine systems. Previous work, particularly in the ancient sedimentary record, has tended to focus on the channels rather than the levees, mostly because these strata are typically poorly exposed in outcrop and their less than Dm characteristics are below the resolution of even high resolution industry seismic. At the Castle Creek study area, however, glacially-polished, vegetation-free levee deposits are 100% exposed over an area that is more than 250 m wide and 40 m thick. This, then, provides an unparalleled opportunity to describe details of the lateral and vertical lithological changes in channel-associated levee deposits and therein provide important details about their reservoir geometry and stratal continuity. Levee deposits are generally assumed to have a gull-wing or wedge-shaped geometry caused by the thinning of beds (laterally) away from the channel. Additionally they are typically reported to fine, have a decreasing sandstone: mudstone ratio, and exhibit a change from Ta-e turbidites to Td-e turbidites laterally. In this study detailed logs spaced 5-10 m apart indicate that the observed lateral trends are not so predictable, and typically exhibit two end-member styles of lateral change: beds abruptly thin and then remain uniform in thickness; or beds rhythmically thicken and thin. Associated with variations in bed thickness are changes in grain size and sedimentary structures. Over lateral distances of several 10s of meters, sandstone beds decrease in thickness from ∼60 cm to ∼5 cm while simultaneously transitioning from upper medium-grained Tbc turbidites to fine-grained Tc turbidites. These changes occur abruptly, sometimes over lateral distances of as little as 1 m. Moreover, superimposed bedsets do not consistently thin or thicken in any single direction, but instead stack compensationally, with changes in the thickness of Tbc and Tc beds accounting for most of the lateral variability. Stratigraphically upward beds generally fine and thin, and in the upper 25 m of the section consist mostly of fine-grained Tc sandstone that show negligible lateral change in grain size or sedimentary structures. Bedsets however, like those in the lower part of the section, stack compensationally over lateral distances of several 10s of meters, and collectively suggest the persistent muting of topography by local changes in the patterns and nature of sediment deposition on the levee.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016