Stratigraphic Architecture of an Ancient Deep-Marine Channel-Lobe Transition Zone, Windermere Turbidite System, Cariboo Mountains, B.C.

Abstract

The slope-basin transition typically includes the channel-lobe transition zone (CLTZ), which in deep-water turbidite system separates slope channels from more basinward depositional lobes. Although the seismic imaging of modern CLTZs has much improved, it still provides only limited insight into the lithological make up, stratigraphic architecture and stratigraphic evolution of this important zone of stratigraphic and oceanographic transition. In this study, a well exposed ~250 m-thick by 1200 m-wide outcrop separates basin floor “sheets” of the uppermost Kaza Group from leveed slope channels of the Isaac Formation, and therein provides an excellent opportunity to analyze the spatial and temporal development of a well-constrained CLTZ. Here the transition zone comprises a complex assemblage of small and large scours, proximal and distal distributary channels, terminal splays, and fine-grained sheets, and at its top, a leveed slope channel. Mass-wasting deposits (debrites and slides), erosional channels, crevasse splays and hydraulic-jump bars are also documented. Unique to the transition zone are scours, whose fills show little vertical or lateral lithological change. Small scours are typically 1–4 m deep and extend laterally for 10- >600 m, and are generally filled with thick- to medium-bedded, structureless or planar-laminated, very coarse- to coarse-grained sandstone. Less commonly, small scours are filled with thin-bedded turbidites, debrites, granule conglomerate, or mudstone-clast breccia. A rare large scour is recognized. It is up to 32 m deep by >600 wide, and is divided in two parts: a lower unit with several conglomerate- and sandstone-rich, small scour-fills that locally are amalgamated; and an upper unit composed of multiple sandstone-filled small scours typically encased in thin-bedded turbidites. Detailed analyses along the Kaza-Isaac transition zone reveal two different architectural assemblages that stack vertically: a lower unit of a large scour that overlies a succession dominated by distributary channel-fills, and an upper unit of alternating small scours and basin-floor elements (e.g. distributary channels and terminal splays). Such changes in architectural styles coincide with changes in sediment supply, namely sediment volume and caliber, which are interpreted to be linked with major changes in relative sea level. This large-scale architecture may be comparable to other turbidite systems and has implications for reservoir connectivity.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015