Along-strike variations in sediment provenance within the Nanaimo basin reveal mechanisms of forearc basin sediment influx events

Abstract

The along-strike variability in sediment provenance within the Nanaimo basin is important for understanding the tectonic evolution of North America’s Late Cretaceous Pacific margin, providing context for paleogeographic reconstructions. Here, we provide 35 point-counted sandstone samples and 22 new detrital zircon samples from the Nanaimo basin. These new detrital zircon samples compose a portion of a basin-wide data set (N = 49, n = 10,942) that is leveraged to discern spatio-temporal changes in sediment provenance. Provenance data demonstrates that the majority of Nanaimo basin strata were sourced from regions within and east of the Coast Mountains Batholith, while only the southernmost Nanaimo basin, exposed in the San Juan Islands, was supplied sediment from the North Cascade thrust system. Additionally, near-identical age modes and synchronous changes in detrital zircon facies are used to hypothesize a correlation between the Nanaimo Group and the protolith of the Swakane Gneiss. These observations, along with previously identified events in the Cordillera, are used to define two basin-wide events that affected the Nanaimo basin: the first at 84 Ma and the second at 72 Ma. The first event is correlated to the onset of Kula-Farallon spreading, which affected basin subsidence, introduced Proterozoic detrital zircon to the central and southern Nanaimo basin, and uplifted the North Cascade thrust system. The second basin-wide event, which is speculated to have been driven by increased rates of subduction and obliquity, resulted in localized high-flux events in the arc, increased exhumation of the Cascade Crystalline Core, underplating of the Swakane Gneiss, and coarse-grained sedimentation across the basin. The data presented here provides added context for the evolution of the basin and provides insight into the protracted geodynamics of forearc basins undergoing oblique subduction.

Tectonic Underplating and Dismemberment of the Maclaren‐Kluane Schist Records Late Cretaceous Terrane Accretion Polarity and ~480 km of Post‐52 Ma Dextral Displacement on the Denali Fault

“…Second, the recognition of Cenozoic slip on the modern Denali fault allows for larger cumulative margin‐parallel displacement of the Wrangellia composite terrane relative to inboard terranes since the Early Cretaceous. Recently published geology‐based data sets bear on long‐standing hypotheses that the Cordilleran terranes experienced large magnitude (>1,500 km) northward transport along dextral fault systems in the mid‐ and Late Cretaceous (Coutts et al., 2020; Garver & Davidson, 2015; Matthews et al., 2017; Rusmore et al., 2013; Sauer et al., 2019). Our review of slip in the Denali fault system reveals that the oft‐cited ~370–400 km of slip that was inferred to have taken place since the Early Cretaceous is not an accurate assessment of Denali fault slip.…”

Late Middle Miocene Emergence of the Olympic Peninsula Shown by Sedimentary Provenance

“…The maximum depositional ages of 16.5 and 16:7 ± 0:5 Ma in samples #26 and #27 of the coastal OSC indicate that the OSC was fed by Cascade-derived sediment until at least that time. (Figure 7; [77,78]). The age clusters at 90 Ma, 1380 Ma, and 1650-1800 Ma found in upper Nanaimo samples have been interpreted as derived from Idaho [79] and the younger ages from the Coast Mountains Batholith.…”

Vesiculation in the dinoflagellate cyst Cannosphaeropsis franciscana Damassa, 1979 across the K/Pg boundary (Vancouver Island, Canada) with implications for spiniferate gonyaulacacean taxonomy and ecophenotypy

“…Mudstones of the Haslam Formation also crop out along the banks of the Oyster River 1.5 km upstream and to the east of the Oyster Bay Formation exposures at the terminus of Lorna Lane. Dated as late Santonianearly Campanian in age (e.g., Haggart and Graham, 2018;Coutts et al, 2020), beds of the Haslam Formation unconformably overlie the massive sandstones of the Comox Formation described by Muller and Jeletzky (1970) factor in subsequent broad-scale movement of these terrains, but in the absence of plutonic bodies in close proximity to these beds as evidenced by the low thermal maturation of the indigenous palynomorphs.…”

“…3.1) has revealed this unit to encompass the K/Pg boundary interval based on age-diagnostic dinoflagellate cysts and a detailed survey of the marine palynological assemblages therein . The Oyster Bay Formation unconformably overlies Nanaimo Group rocks of the Santonian-Campanian Haslam Formation (e.g., Haggart and Graham, 2018;Coutts et al, 2020) and lower-upper Campanian Cedar District Formation (e.g., Enkin et al, 2001;Ward et al, 2012). The Haslam Formation crops out along the banks of the Oyster River 1.5 km upstream and to the east of the uppermost Oyster Bay Formation beds at the terminus of Lorna Lane and the Cedar District Formation forms an expansive coastal outcrop at Shelter Point, long renowned for its marine invertebrate fossils (Richards, 1975;Ward, 1978) 1.9 km north of the basal Oyster Bay Formation exposures.…”