The Stratigraphic Architecture of Laterally-Accreting and Laterally-Offset Ancient Sinuous Channel Fills, Lower Isaac Formation (Windermere Supergroup), British Columbia, Canada

Abstract

Highly sinuous channels are important architectural elements in deep-marine sedimentary systems. Seismic reflectors on the inner-bend of these channels, termed lateral accretion packages (LAPs), are thought to resemble, at least in morphology, point bar deposits in better known fluvial channels. However, the paucity of accessible, well-exposed outcrop analogues has impeded efforts to test this correlation and therefore the link between highly-sinuous channel (lateral) migration and deposition remain equivocal. Well-exposed channel fills in the Lower Isaac Formation (Neoproterozoic Windermere Supergroup, British Columbia), however, show well-developed, laterally-offset inclined stratal packages, here termed LADs, that are interpreted to represent deposition on the inner-bend of laterally-migrating sinuous channels. As such this provides an ideal opportunity to study the stratigraphic details of laterally migrating sinuous deep-marine channels. Detailed sedimentological logs were measured and important stratal surfaces mapped in three different channel fill complexes that range up to 40m thick. In their basal part channels are filled with amalgamated, medium- to very thickly-bedded, upper medium sandstone to granule conglomerate. Obliquely upward these strata transition into equally coarse, but inclined sandstones that can be traced for ∼20-30m before thinning abruptly and then pinching out. These, then, are over- and underlain by thinly- and very-thinly bedded, ripple cross-laminated, mostly fine-grain sandstone turbidites that terminate obliquely downward as adjacent LADs amalgamate. Inclined strata are interpreted to represent deposition on the upper inclined (inner bend) surface of a laterally-migrating sinuous channel, and here termed a lateral accretion deposit (LAD). Obliquely downward these strata merge with amalgamated deposits that accumulated lower on the same accretion surface. Intercalated fine-grained strata, then, are the inner-bend levees on which the LADs onlap. Along the top of each of the three channels LADs form common dm-thick, finger-like projections that locally extend into inner-bend levee deposits and occur across the width of each channel fill (100s m). LADs, which in all channels are generally lithologically and stratigraphically similar, are too numerous to be associated with individual channel fills. Instead, LADs, whether they be associated with the initiation or lateral migration of a channel are indistinguishable.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014