--> Preservation of Upper-Flow-Regime Bedforms and Their Recognition in the Stratigraphic Record: Comparisons From Modern and Ancient Active Margin Settings

AAPG ACE 2018

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Preservation of Upper-Flow-Regime Bedforms and Their Recognition in the Stratigraphic Record: Comparisons From Modern and Ancient Active Margin Settings

Abstract

Upper-flow-regime bedforms (e.g., cyclic steps, antidunes) are commonly identified in modern deep-water environments, particularly on high-gradient, tectonically active margins. Recent studies have increasingly recognized the importance of these bedforms, and their relevance to the initiation and maintenance of submarine channels. However, identification and interpretation of associated upper-flow-regime bedform deposits in the rock record remains challenging due to limited established recognition criteria. Upper-flow-regime bedform deposits contain numerous bounding and internal stratigraphic surfaces, often resulting in successions of lenticular or wedge shaped sedimentary bodies. This study investigates the origin and geometry of sedimentary bodies constructed from repeat bathymetric surveys of modern active-margin deep-water settings (Squamish, British Columbia and Monterey Bay, California). These results are used to aid interpretation of Late Cretaceous submarine conduit strata of the Nanaimo Group exposed on Gabriola Island, British Columbia.

Upstream migrating, crescent-shaped bedforms in submarine channels of the Squamish prodelta and Monterey Canyon have been previously associated with supercritical flows. Along-channel and across-channel bathymetric profiles indicate significant reworking of bedform deposits between survey time intervals. The resulting stratigraphic products are highly composite, consisting of bedform remnants that are 5–30 m long, <30 m wide, and 0.5–2 m thick. Geometrically and dimensionally comparable, scour-surface bound units are identified in Nanaimo Group deposits. These units are defined by laminated siltstone that locally overlies basal erosion surfaces, which are primarily overlain by structureless sandstone. Backset stratification is observed locally within packages of 2–4 backset beds, each of which are up to 60 cm thick, up to 15 m long (along dip), and commonly share composite basal scour surfaces. Numerous sedimentary units are documented over thin successions (<4 m), suggesting limited aggradation and preservation of bedforms. Evidence for upper-flow-regime bedforms in Nanaimo Group stratigraphy supports interpretation of a high-gradient setting dominated by supercritical flow conditions. Our results help to establish geometries, dimensions, and architecture of upper-flow-regime bedform deposits, which have implications for hydrocarbon reservoir prediction and characterization in channelized deep-water deposits.