No abstract

Submarine channel systems convey terrestrially derived detritus from shallow-marine environments to some of the largest sediment accumulations on Earth, submarine fans. 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 time scales. We integrate geochronology with detailed stratigraphic characterization to temporally constrain the stratigraphic evolution recorded by horizontally to vertically aligned channel-fill stacking patterns in a Nanaimo Group channel system exposed on Hornby and Denman Islands, British Columbia, Canada. Twelve detrital zircon samples (n = 300/sample) were used to calculate maximum depositional ages, which identified a new age range for the succession from ca. 79 to 63 Ma. We document five phases of submarine-channel evolution over 16.0 ± 1.7 m.y. including: an initial phase dominated by incision, sediment bypass, and limited deposition (phase 1); followed by increasingly shorter and more rapid phases of deposition on the slope by laterally migrating (phase 2) and aggrading channels (phase 3); a long period of deep incision (phase 4); and a final rapid phase of vertical channel aggradation (phase 5). Our results suggest that ∼60% of the evolutionary history of the submarine channel system is captured in an incomplete, poorly preserved record of incision and sediment bypass, which makes up <20% of outcropping stratigraphy. Our findings are applicable to interpreting submarine channel-system evolution in ancient and modern settings worldwide and fundamentally important to understanding long-term sediment dispersal in the deep sea.

Submarine channel-systems are globally prevalent on continental margins and their deposits record the transfer of significant volumes of sediment from continental catchments to deep-water environments. These deposits contain signals of past events that can provide critical insight into the geological history of an area, including the long-term (>1 Ma) controls on sedimentation. Well-exposed outcrops of the Late Cretaceous Nanaimo Group, British Columbia, Canada, provide an opportunity to investigate long-term sediment-routing in a tectonically active setting. This study combines detrital zircon geochronology with stratigraphic analysis to establish a robust, basin-wide palaeogeographic framework that constrains the timing and distribution of coarse-grained sediment transport and deposition in the Nanaimo forearc basin. Three south-west trending submarine conduits are documented along a 135 km long strike-oriented transect, parallel to the north-west/south-east trending basin. Composite conduit deposits are 350 to 700 m thick, 4 to 16 km wide, and record substantial delivery of coarsegrained detritus (up to boulder-sized clasts) within submarine channel-lev ee systems. Maximum depositional ages derived from detrital zircon datasets constrain contemporaneous sediment transfer and deposition at each conduit location, which spanned between 73Á5 AE 1Á3 Ma to 69Á1 AE 1Á1 Ma around the Campanian-Maastrichtian boundary. Submarine conduits are spaced at 40 km and 70 km, and were likely connected to fluvial drainage systems sourced in uplifted catchments. Widespread, coarse-grained deposits suggest that surface uplift associated with concurrent regional events (such as deformation and/or magma emplacement) along the North American margin may have promoted sediment delivery to the deep-water basin. Comparisons to modern and ancient analogues support palaeogeographic interpretations, as well as the interpretation that pervasive coarse-grained deep-water sediment delivery was linked to tectonic activity. The integrated stratigraphicgeochronological approach used provides unique insight into the influence of regional tectonics on continental margin physiography and the nature of deep-water sediment dispersal.

No abstract

The development of the Cordilleran orogen of western North American is disputed despite a century of study. Paleomagnetic observations require large‐scale dextral displacements of crustal fragments along the western margin of North America, from low latitudes to moderate latitudes during the Cretaceous‐Paleogene. A lack of corroborating geological evidence for large‐scale (>1500 km) displacements has prevented the widespread integration of paleomagnetic data into most contemporary tectonic models for the margin. Here we use detrital zircons from the Nanaimo basin, southwestern British Columbia, Canada as an independent test of its Late Cretaceous paleogeographic position. We compare 4310 detrital zircon U/Pb dates from 16 samples to potential source areas in western North America to test hypothesized northern and southern Late Cretaceous paleogeographic positions. Our detrital zircon data suggest that sediment in the Nanaimo basin derives from either a geographically restricted portion of the Belt‐Purcell basin or the Mojave‐Sonoran region of southwestern North America. A paleogeographic position for the basin adjacent to the Mojave‐Sonoran region is preferred as it is consistent with the paleomagnetic results, but further geological, isotopic, or geophysical data are required to rule out a Belt‐Purcell source.