Temporal Constraints on Submarine Slope Channel System Evolution, Late Cretaceous Nanaimo Group, British Columbia, Canada

Abstract

Submarine canyon-channel systems commonly transect the slopes of basin margins, facilitating the transfer of terrestrially-derived detritus from shallow marine environments to the deep-sea. These conduits convey sediment to some of the largest sediment accumulations on Earth (submarine fans) and can, themselves, fill with significant volumes of coarse-grained material that serve as prolific hydrocarbon reservoirs. The stratigraphic record of submarine slope channels includes heterogeneous, composite deposits that provide evidence for erosion, sediment bypass, and deposition. However, the timing and duration of these processes is poorly constrained over geologic timescales. We apply an integrated geochronologic-stratigraphic approach to temporally constrain the stratigraphic evolution of a long-lived channel system deposit within the Nanaimo Group, British Columbia, Canada.

We investigate a 20 km wide, 1500 m thick channel system deposit exposed on Hornby and Denman islands. The succession consists of coarse-grained channel fills that transition upward from horizontally offset to vertically aligned channelform stacking patterns. Twelve detrital zircon samples (n=300/sample) from throughout the succession were used to calculate maximum depositional ages, which span 16.0±1.7 m.y., from ~79-63 Ma. The stratigraphic architecture, sedimentological characteristics, and age of deposits define 5 phases of submarine channel evolution. We document a prolonged period of incision, sediment bypass, and limited deposition (phase 1) during early channel system evolution. This is followed by increasingly shorter and more rapid phases of deposition on the slope by laterally migrating (phase 2) and aggrading channels (phase 3 & 5), as well as an abrupt long period of deep incision and sediment bypass (phase 4).

Our results indicate that phases of incision and sediment bypass encompass ~60% of the evolutionary history of the submarine channel system, where sediment was ultimately transported downslope. However, evidence for these phases is poorly preserved in the stratigraphic record and only recorded by erosion surfaces and thin deposits that make up <5% of outcropping stratigraphy. Our findings are applicable to interpreting submarine channel system evolution in ancient and modern settings worldwide and fundamentally important to understanding the timing and nature of sediment delivery to the deep-sea.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019