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.

INTRODUCTION

Submarine canyons and channels that extend across the shelves and slopes of continental margins facilitate source-to-sink sediment transfer from terrestrial catchments to the deep sea (Normark et al., 1993). As such, these conduits are important conveyors and sinks for significant volumes of sediment and chemicals (e.g., organic carbon, pollutants), especially for large sediment-routing systems that persist for millions of years (Clift et al., 2001; Lyons et al., 2002; Galy et al., 2008; Fildani et al., 2016, 2018; Blum et al., 2018).

Submarine channels have multiphase evolutionary histories that include periods of erosion, sediment bypass, and deposition (e.g., Deptuck et al., 2003, 2007; McHargue et al., 2011; Fildani et al., 2013; Hubbard et al., 2014; Hodgson et al., 2016; Covault et al., 2016; Casciano et al., 2018). These processes are recorded in deepwater deposits and result in stratigraphic products that are characterized by diachronous erosional surfaces and heterogeneous fill (Fig. 1; Sylvester et al., 2011; Gamberi et al., 2013; Bain and Hubbard, 2016; Hodgson et al., 2016). Identifying the phases that make up submarine-channel evolution and their temporal significance from ancient deposits can be challenging, particularly considering the inherently biased and fragmentary nature of the sedimentary record. Submarine channel deposits are typically dominated by coarse-grained channelform units that record channel-filling processes (Hodgson et al., 2011; Hubbard et al., 2014). However, as demonstrated in many depositional environments, preserved deposits often account for proportionately small fractions of time compared to gaps more cryptically represented in the stratigraphic record (Barrell, 1917; Ager, 1981, 1993; Miall, 2014; Durkin et al., 2018; Straub and Foreman, 2018; Vendettuoli et al., 2019). Sedimentological and architectural observations at a range of scales provide abundant evidence for multiphase incision and sediment bypass that precedes the filling of submarine channels. Although more subtly expressed in the stratigraphic record, sediment bypass is considered an important and potentially prolonged component of submarine-channel evolution (Mutti and Normark, 1987; Hubbard et al., 2014; Stevenson et al., 2015).

Despite increased recognition of the protracted nature of submarine-channel evolution, the timing and duration of different evolutionary phases are poorly constrained. Quantification of time represented in slope-channel deposits may provide insight into the relative importance of sedimentary processes over the lifespan of a channel system and their degree of preservation in the stratigraphic record. In addition, temporal constraints on submarine-channel evolution are fundamentally important to understanding the timing and magnitude of downstream sediment and chemical transport to deep-sea fans versus deposition within submarine-slope channels. For example, evolutionary phase durations could be applied to predict how long large, unfilled slope systems (such as the Congo Canyon, offshore southwestern Africa) will transfer material to downslope submarine fans, which currently act as large sinks for sediment, organic carbon, and/or debris (e.g., microplastics) that impact ecological communities (Van Cauwenberghe et al., 2013; Stetten et al., 2015; Kane and Clare, 2019).

Numerical dating using detrital-zircon geochronology provides a powerful tool to determine the timing of sedimentation from deep-time strata (e.g., Dickinson and Gehrels, 2009; Spencer et al., 2012; Schwartz et al., 2017; Daniels et al., 2018; Englert et al., 2018; Malkowski et al., 2018). We use detrital- zircon geochronology data to identify and temporally constrain the stratigraphic evolution of a long-lived (>106 yr) Cretaceous submarine slope-channel system that crops out on Hornby and Denman Islands in British Columbia, Canada. We hypothesize that periods of erosion and sediment bypass dominate long-term channel-system evolution, supporting that submarine channels primarily function to transport, rather than accommodate, sediment. Results of our integrated geochronologic-stratigraphic study provide insight into the evolutionary history of long-lived deepwater sediment routing, which is relevant to understanding the timing and longevity of sediment dispersal in other voluminous, large-scale deepwater systems (e.g., Bengal [offshore India], Indus [offshore Pakistan], Congo submarine canyon-channel-fan systems).

STUDY AREA

Outcrops of the Cretaceous Nanaimo Group, present on the western coast of British Columbia, record the filling of the Nanaimo forearc basin west of the Coast Plutonic Complex (Fig. 2; England, 1989; Mustard, 1994). On Hornby and Denman Islands, the upper Nanaimo Group is continuously exposed in northeast-dipping strata and consists of six lithostratigraphic formations that alternate between predominantly coarse- and fine-grained siliciclastic units (Rowe et al., 2002; Katnick and Mustard, 2003). Initial mapping efforts invoked large faults to explain the lateral juxtaposition of coarse-grained strata of the DeCourcy, Geoffrey, and Gabriola Formations with fine-grained strata of the Cedar District, Northumberland, and Spray Formations (Fig. 2B; Muller and Jeletzky, 1970). Subsequent studies reinterpreted these faults as stratigraphic contacts (Fig. 2B; Katnick and Mustard, 2003) and the entire succession to reveal a 20-km-wide, 1500-m-thick approximately depositional strike–oriented cross-section through a large-scale, submarine slope-channel system (Figs. 2B and 2D; Bain and Hubbard, 2016).

Hornby and Denman Islands are characterized by channelform bodies composed of conglomerate and sandstone that are bounded by concordant and/or chaotically bedded discordant packages of thin-bedded sandstone and mudstone (Fig. 2D; Bain and Hubbard, 2016). Channelform bodies represent composite slope channel-fill deposits that are 80–120 m thick and 1–4 km wide (cf. Beaubouef, 2004; Greene and Surpless, 2017; Li et al., 2018). Adjacent thin-bedded units are interpreted to be out-of-channel deposits formed by hemipelagic settling or overbank deposition from turbidity currents where bedding is concordant, with subsequent slumping and sliding where bedding is discordant or chaotic (Piper and Normark, 2001; Hickson and Lowe, 2002; Deptuck et al., 2003; Kane and Hodgson, 2011). The succession exposed on the islands transitions upward from laterally offset to vertically aligned channelform bodies (Bain and Hubbard, 2016); this pattern is comparable to channelform stacking observed in multiphase submarine channel systems worldwide (Fig. 1; Covault et al., 2016; Jobe et al., 2016).

Previous paleomagnetic (Enkin et al., 2001) and paleontological studies (Muller and Jeletzky, 1970; Sliter, 1973; Ward, 1978; McGugan, 1979, 1982; Ward et al., 2012; McLachlan and Haggart, 2018; McLachlan et al., 2018) constrain the age of Nanaimo Group strata on Hornby and Denman Islands to range from the middle Campanian to latest Maastrichtian (Fig. 3). However, these investigations relied on low-quality paleomagnetic data and the restricted presence of micro- and macrofossil assemblages, found only in fine-grained deposits of the Cedar District and Northumberland Formations (Enkin et al., 2001; Ward et al., 2012). As a result, poor age control exists for coarse-grained units within the succession, as well as younger fine-grained strata of the Spray Formation which lack fossils (Fig. 2D; Katnick and Mustard, 2003). Englert et al. (2018) used detrital zircons in the Nanaimo Group to constrain the timing of Geoffrey Formation deposition on Hornby Island and other localities in the southern Nanaimo Basin to approximately the Campanian-Maastrichtian boundary. As demonstrated by this recent study, detrital zircon geochronology provides the opportunity to precisely date sandstone-prone units throughout the Nanaimo Group succession on Hornby and Denman Islands and hone previous correlations based on paleomagnetic and paleontological data sets.

METHODS

Sedimentary basins adjacent to magmatic arcs (e.g., forearc and retroarc basins) typically contain abundant near-depositional-age zircons (Dickinson and Gehrels, 2009; Cawood et al., 2012). In these settings, maximum depositional ages (MDAs) calculated from detrital zircon populations can approximate the true depositional age of strata and are useful for constraining the timing and rates of depositional processes (Dickinson and Gehrels, 2009; Schwartz et al., 2017; Daniels et al., 2018). Several studies have demonstrated that large detrital zircon data sets containing ≥300 grains are preferred for detrital zircon geochronology analyses and necessary for maximizing the accuracy of a calculated MDA (Coutts et al., 2019), identifying low-abundance age populations (Pullen et al., 2014), and quantitatively comparing detrital zircon samples (Saylor and Sundell, 2016). An abundance of near-depositional-age zircons in the Nanaimo Basin supplied by the adjacent magmatic arc combined with a large-count (300) measurement method inspires confidence in the identification of a near-depositional age population of detrital zircons and reliable MDAs for Nanaimo Group deposits (Englert et al., 2018).

In this study, we use detrital zircon dates first reported by Matthews et al. (2017) to constrain the age of Nanaimo Group deposits on Hornby and Denman Islands. Dates for 300 zircons per sample were acquired via laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) techniques (Matthews and Guest, 2016) and were previously used by Englert et al. (2018) to determine that Geoffrey Formation channel deposits on Hornby Island spanned 72.8 ± 1.2 Ma to 71.2 ± 1.0 Ma. Building on that work here, we calculate MDAs and uncertainties for 12 samples on Hornby and Denman Islands by taking a weighted mean of zircon dates from the youngest grain cluster, defined as three or more grains with overlapping dates at 2σ uncertainty (YC2σ) (Dickinson and Gehrels, 2009; see Supplemental Files S1–S31 for acquisition parameters, details of the uncertainty propagation, complete data sets, and MDA weighted-average graphs). This is a conservative method, shown to produce an MDA younger than the true depositional age of the sample <0.5% of the time (Coutts et al., 2019). These new ages are integrated with previously published paleontological and paleomagnetic data sets to temporally constrain the timing of deposition within the channel-system succession.

Following the approach of Daniels et al. (2018), we calculate the duration, sedimentation rate (linear and two-dimensional [2-D]), and respective uncertainties for distinct depositional intervals using Monte Carlo simulations. For each interval, a synthetic population of 10,000 ages is generated from the probability distribution of the bounding MDAs. Duration and sedimentation rates are then determined for each pair of ages within the synthetic population, with the mean and standard deviation of the resulting duration or sedimentation rate distributions defining the duration or sedimentation rate and its uncertainty for the interval (see Supplemental File S4 [footnote 1] for synthetic distributions and calculations).

RESULTS

All detrital zircon samples analyzed from Nanaimo Group strata on Hornby and Denman Islands contain several young grains (3–41 grains or 2%–25% of each sample) that result in MDAs with 2σ uncertainties <2 m.y. between 81.8 ± 1.8 Ma to 63.0 ± 1.7 Ma (2σ absolute certainty, including all random and systematic uncertainties; Table 1; Figs. 3 and 4A). The stratigraphic evolution recorded by the succession includes a change in channelform stacking patterns from horizontally offset to vertically aligned channelforms (Bain and Hubbard, 2016). Integration of new temporal constraints with stratigraphic architecture and sedimentological characteristics allows further subdivision of deposits on Denman and Hornby Islands into phases of submarine-channel evolution based on channel-process interpretations.

Horizontally Aligned Channelform Succession

Description

Horizontally aligned channelform bodies make up a broad, elongate succession (19 km wide, 675 m thick) including deposits on Denman Island and the southeastern corner of Hornby Island (Fig. 4B). Associated detrital zircon samples (samples 1–6) are dominated by Mesozoic age populations that yield MDAs ranging from 81.8 ± 1.8 Ma to 72.3 ± 1.1 Ma (Fig. 4). Macrofossils (Muller and Jeletzky, 1970; Ward et al., 2012) and paleomagnetic samples (samples 33 and 34; Enkin et al., 2001) from Denman Island, at the base of the succession near the lighthouse and along the west side of the same island (Fig. 2D), place these units in the middle Campanian Metaplacenticeras cf. pacificum biozone and C33N chron (Fig. 3). Microfossil and macrofossil samples from units on the southeastern corner of Hornby Island, above channelforms correlated to the DeCourcy Formation on Denman Island, indicate a late Campanian age (Figs. 2D and 3; Sliter, 1973; McLachlan and Haggart, 2018).

The base of the channel system is represented by an ∼50–250-m-thick interval of thin-bedded turbidites (cf. Bain and Hubbard, 2016) containing a few horizontally aligned channelform deposits between detrital zircon samples 1 and 2. In places, including the lowermost exposures by the lighthouse on Denman Island, largely fine-grained deposits are characterized by undulating erosion surfaces overlain by pebble conglomerate layers, abundant intrabasinal ripup clasts, and backset stratified scour fill (Figs. 5A–5D; Bain and Hubbard, 2016). The overlying stratigraphic unit is 625 m thick and contains channelform bodies composed of conglomerate and sandstone (including detrital zircon samples 2–6) that form laterally offset, erosion surface–bound composite deposits with MDAs that overlap at 2σ uncertainty (Figs. 4 and 5E).

Interpretation

Biostratigraphic and paleomagnetic samples from the base of Denman Island succession and the MDA from the detrital zircon sample at the transition in channelform stacking patterns on Hornby Island (sample 7) constrain deposition of the horizontally aligned channelform package to span 7.4 ± 1.9 m.y. between ca. 79 Ma and 71.6 ± 1.9 Ma (Figs. 4 and 6). The MDA from the lowest detrital zircon sample on Denman Island (sample 1, 81.8 ± 1.8 Ma) is slightly older than suggested by paleomagnetic (C33N chron, ca. 80–74.3 Ma) and ammonite samples (M. cf. pacificum biozone, ca. 79.0–76.3 Ma; Figs. 3 and 4A). We use the age range suggested by Ward et al. (2012) as the lower bounding age on Denman Island stratigraphy because, by nature, MDAs may be older than the true depositional age of a unit, particularly considering our conservative calculation method (Dickinson and Gehrels, 2009; Coutts et al., 2019).

A large portion of time (3.4 ± 1.5 m.y.) is encompassed by the comparatively thinner, siltstone-dominated deposit at the base of Denman Island (Figs. 6 and 7). Lower sedimentation rates and abundant fine-grained units are typically associated with periods of reduced sediment delivery; however, these deposits provide evidence for erosion and sediment-transfer processes within the submarine channel system (Fig. 5). Thin gravel layers in siltstone-dominated strata are interpreted to represent interbedded coarse-grained lag and fine-grained tail deposits from largely bypassing turbidity currents (Figs. 5C and 5D; Mutti and Normark, 1987; Grecula et al., 2003; Stevenson et al., 2015). The presence of scours and upslope-migrating bedforms are indicative of high-energy, Froude-supercritical flow transitions (Figs. 5A and 5B; e.g., Cartigny et al., 2014; Covault et al., 2017; Hage et al., 2018). Thus, we interpret that these deposits record the passage of energetic flows that transported sediment downslope and represent an initial phase of channel development dominated by erosion and sediment bypass. The presence of bypass deposits at the stratigraphically lowest point on Denman Island coinciding with the location of ammonites assigned to the M. cf. pacificum biozone supports the interpretation that these strata were deposited within the channel system, not within underlying slope sediments, and can be used to date channel processes (Figs. 2D). The width of the deposit, spanning the southwestern shore of Denman Island, suggests that erosion and bypass occurred through punctuated lateral migration of submarine channels and resulted in a wide composite bounding surface and diachronous basal fill (e.g., McHargue et al., 2011; Hodgson et al., 2011).

Horizontally aligned channelforms in the overlying strata (including samples 2–6) are interpreted to be nested erosional channel remnants formed by lateral channel migration during phases of relatively limited channel aggradation (e.g., Deptuck et al., 2003; Hodgson et al., 2011). Although MDAs do not differ significantly, the presence of nested erosion surfaces and laterally offset deposits (dashed red lines and encompassing channelform deposits in Fig. 4B) support that this interval formed by multiple episodes of cut and fill that occurred too quickly for their chronology to be resolved by the detrital zircon dating method used here. Fine-grained deposits on the southeastern corner of Hornby Island above sample 4 appear older than some samples on Denman Island (samples 5 and 6) and provide further support for a history of reincision and lateral channel migration (Figs. 2D and 3). The oldest and youngest MDAs that bound this interval (samples 2 and 7) indicate deposition of a much thicker deposit (625 m) than the underlying basal unit over a comparable time period (4.0 ± 2.4 m.y.; Figs. 6 and 7).

Vertically Aligned Channelform Succession

Description

Vertically aligned channelform bodies define a narrower and thicker succession (8.5 km wide, 950 m thick) on Hornby Island compared to the underlying succession of horizontally aligned channelform bodies. These units are also dominated by Mesozoic detrital zircon populations; however, they contain an additional zircon population (>30%) that yields dates >300 Ma (mainly Proterozoic; Fig. 4A). MDAs for samples on Hornby Island (samples 7–12) range from 71.6 ± 1.9 Ma to 63.0 ± 1.7 Ma (Fig. 4A). Several ammonite and foraminifera samples have been recovered from out-of-channel deposits of the Northumberland Formation and, along with paleomagnetic samples, suggest that the deposits span from the late Campanian to early Maastrichtian (Sliter, 1973; McGugan, 1979; Enkin et al., 2001; Ward et al., 2012; McLachlan and Haggart, 2018). Carbon isotope stratigraphic analyses identified a δ13C excursion at the top of the Northumberland Formation attributed to the Campanian- Maastrichtian boundary event, which is also consistent with paleontological results (Hasegawa et al., 2014). Previous interpretation of paleomagnetic samples along the northeastern shore of Hornby Island place these units within the Maastrichtian and correlate the top of the succession to the base of or within the C29R chron at or around the Late Cretaceous–Paleocene boundary (Enkin et al., 2001; Ward et al., 2012); however, with no biostratigraphic age constraint, this age assignment is tenuous.

MDAs in the vertically aligned channelform succession generally decrease upward apart from sample 9, which is located stratigraphically 500 m below sample 8 but is almost 6 m.y. younger (Fig. 4). Sample 9 derives from below the base of a channelform deposit at Downes Point (Hornby Island), which overlies a succession characterized by chaotically bedded and/or discordant strata as well as thinly bedded sandstone and siltstone associated with broad undulatory erosion surfaces (Fig. 8; Bain and Hubbard, 2016). The age gap between samples 8 and 9 temporally separates two 650–725-m-thick packages of vertically aligned sandstone and conglomerate channelform deposits, each containing MDAs that overlap at 2σ uncertainty (Figs. 4 and 8).

Interpretation

The oldest and youngest MDAs, corresponding to samples 7 and 10, respectively, are used to constrain the timing of deposition within the vertically aligned channelform succession to between 71.6 ± 1.9 Ma and 63.0 ± 1.7 Ma, spanning 8.6 ± 2.5 m.y. (Figs. 4 and 6). Four samples (samples 9–12) from the northeastern side of Hornby Island each contain 3–7 grains with dates <66 Ma that yielded Paleocene MDAs significantly younger than previously interpreted based on paleomagnetic analyses. We use these MDAs to temporally constrain the top of the succession because MDAs were calculated using a conservative approach (YC2σ method) that is unlikely (<0.5%) to yield ages younger than the true depositional age of a unit (Dickinson and Gehrels, 2009; Coutts et al., 2019). Based on similar reasoning, sample 10 was used as the upper bounding age despite its position stratigraphically below samples 11 and 12.

Successions of vertically stacked channelforms record phases of channel aggradation (e.g., Deptuck et al., 2003; Hodgson et al., 2011), in which deposition of 725 m and 650 m of strata occurred rapidly over 2.0 ± 2.2 m.y. (between samples 7 and 8) and 0.4 ± 2.1 m.y. (between samples 9 and 10) respectively (Figs. 6 and 7). The large overlap in MDA uncertainties limits the use of detrital zircon geochronology for determining accurate durations for these short time intervals. The presence of units that are 6.2 ± 1.6 m.y. younger yet ∼500 m lower in the section define a phase of deep incision that separated the two rapid depositional episodes (Fig. 6). Chaotic and thin-bedded scour deposits below Downes Point provide additional support for erosion, mass wasting, and sediment transfer prior to the second phase of channel aggradation and deposition (Fig. 8). These observations indicate that a ∼500-m-high terrace once existed on the slope landscape, which is a common geomorphic element of submarine-channel systems described in seismic and modern seafloor data sets but more difficult to distinguish in outcrop (e.g., Deptuck et al., 2003; Babonneau et al., 2004; Alves et al., 2009; Sylvester and Covault, 2016; Hansen et al., 2017).

INTEGRATION OF RESULTS: SUBMARINE CHANNEL-SYSTEM EVOLUTION

The Nanaimo Group succession exposed on Hornby and Denman Islands documents a large-scale, long-lived submarine channel system that persisted on the North American continental margin for 16.0 ± 1.7 m.y. (Fig. 4). The stratigraphic architecture, sedimentology, and age of deposits on Hornby and Denman Islands define five phases of submarine-channel evolution (Fig. 9). The horizontally aligned channelform succession primarily on Denman Island can be divided into two phases characterized by: (1) an initial phase of incision, sediment bypass, and limited deposition (samples 1–2); followed by (2) a phase of deposition (samples 2–6) associated with laterally migrating channels (Figs. 4, 6, and 9). These processes suggest initial conditions of relatively low aggradation, leading to the self-cannibalization and reworking of deposited channel-fill units, creation of a wide composite erosion surface, and abundant downslope transport of sediment (Hodgson et al., 2011; Covault et al., 2016). Vertically aligned channelform deposits on Hornby Island record three additional phases of submarine-channel evolution including: (3) a short phase dominated by vertical channel aggradation (samples 7–8); (4) a long phase of deep incision (between samples 8 and 9); and (5) a final rapid phase of vertical channel aggradation (samples 9–12; Figs. 4, 6, and 9).

In some cases, the durations associated with phases of submarine-channel evolution exceeded the limit resolvable by LA-ICP-MS detrital zircon geochronology dating methods (∼4 m.y. for Late Cretaceous strata; Daniels et al., 2018). This resulted in statistically indistinguishable bounding MDAs, and thus unrealistic duration and sediment accumulation rate distributions with large ranges from Monte Carlo simulations (Figs. 4A, 6, and 7). Although the absolute values of calculated parameters for these phases may not be meaningful, their comparison is still useful for differentiating phases of relatively short or long durations and high or low sedimentation rates. Calculated linear and 2-D sedimentation rates increase upwards in the succession for each depositional phase of submarine-channel evolution (Fig. 7).

A transition from laterally offset to vertically aligned internal channelform bodies, comparable to stacking patterns on Hornby and Denman Islands, commonly characterize submarine slope-channel system deposits and is interpreted to represent an evolution from early channel incision and bypass to downslope depocenters followed by increased deposition on the slope during channel lateral migration and late-stage aggradation (Fig. 1; Deptuck et al., 2003, 2007; Posamentier and Kolla, 2003; Mayall et al., 2006; Sylvester et al., 2011; Hodgson et al., 2011, 2016; McHargue et al., 2011; Janocko et al., 2013; Covault et al., 2016; Jobe et al., 2016). The chronostratigraphic framework and sedimentological characteristics established here are generally consistent with the evolution and sediment dispersal patterns typically associated with this globally prevalent stratigraphic pattern (Fig. 9). The succession on Hornby Island includes an additional and significant phase of deep incision, bypass, and terrace formation (phase 4). Although reincision during overall channel aggradation is documented elsewhere (Fig. 1A; e.g., Samuel et al., 2003; Deptuck et al., 2007; Catterall et al., 2010; Stevenson et al., 2015), it is unclear whether extensive periods of erosion during late-phase submarine-channel evolution are common or reflect an external perturbation more specific to this system.

DISCUSSION

Age and Chronology of Submarine Channel Deposits on Hornby and Denman Islands

High-resolution MDAs calculated from large-n (≥300) detrital zircon data sets in this study refine the age of the Nanaimo Group on Hornby and Denman Islands. Differences between calculated MDAs and age ranges established from paleontological and paleomagnetic data sets occur at the base and top of the succession where MDAs indicate a longer duration of channel-system activity. Variations are also observed between ages determined for fine-grained out-of-channel deposits and adjacent coarse-grained channel fill. While some inconsistencies can be explained by considering the sedimentological characteristics of associated units, others designate new constraints on fossil-barren units and provide necessary calibration for correlations to the magnetic polarity time scale.

Our MDA results suggest that deposition spanned a longer time period than previously interpreted, encompassing at least 16.0 ± 1.7 m.y. (Figs. 3 and 4). The MDA from the base of the succession on Denman Island (sample 1) indicates that deposition began earlier in the Campanian than suggested by ammonite and paleomagnetic samples (Muller and Jeletzky, 1970; Ward et al., 2012) at other positions along the shoreline (Figs. 2D and 3). Although we cannot rule out the possibility that the MDA is older than the true depositional age, it is perhaps not surprising that samples yield different ages considering the significant length of time spanned by this broad and thin stratigraphic interval (phase 1). If accurate, this age would suggest a more extensive period of bypass, incision, and limited deposition spanning 6.2 ± 2.4 m.y. and can be considered to represent a potential upper limit on the duration of the initial phase of submarine-channel evolution (Fig. 4). MDAs from the northeastern shore of Hornby Island are significantly younger than previously considered based on paleomagnetic analyses (Figs. 2D and 3; Enkin et al., 2001; Ward et al., 2012); our results confirm earlier speculations that Nanaimo Group deposition continued into the Paleocene (e.g., Mustard, 1994). New absolute age constraints provide a basis for correlation to the magnetic polarity time scale at the top of the Nanaimo Group succession, previously viewed as problematic (Ward et al., 2012), and indicate that these units should be recorrelated to the C27R or C26R chron (Fig. 3).

Less-apparent differences exist between ages determined using detrital zircon geochronology for coarse-grained channel fill and the ages of adjacent fine-grained, out-of-channel units constrained by other methods. Discrepancies occur when comparing deposits at the base of the Northumberland Formation on the southeastern corner of Hornby Island to younger DeCourcy Formation channel fill on Denman Island (samples 5–6). Moreover, age ranges for the Northumberland Formation (latest Campanian to earliest Maastrichtian) on the northwestern shore of Hornby Island are generally older than the range of MDAs (within the Maastrichtian) from the base and top of the adjacent Geoffrey Formation channel-fill succession (samples 7–8; Figs. 2D and 3). Although some overlap does exist with the 2σ uncertainty range for sample 7, sample 8 is distinctly younger (Fig. 3). The Northumberland Formation consists of siltstone-prone units containing significant mass-wasting deposits and is interpreted to represent slope strata subsequently incised by submarine channels and/or channel-overbank strata (Katnick and Mustard, 2003; Bain and Hubbard, 2016). Both the incision of older stratigraphic units and mass-wasting of proximal, older deposits into the channel system could explain slightly differing ages between these adjacent out-of-channel and channel-fill successions.

MDAs also establish a more precise chronologic framework for deposits on Hornby and Denman Islands, which assisted with stratigraphic mapping by Bain and Hubbard (2016). For example, MDAs informed the delineation of a stratigraphic erosion surface between samples 8 and 9, effectively identifying a large terrace in outcrop (Fig. 4). This erosion surface was previously interpreted as a faulted (Muller and Jeletzky, 1970) or interfingered contact (Katnick and Mustard, 2003) and would be difficult to recognize without temporal constraints due to poor exposure in the island interior (Figs. 2B and 8A). The identification of 500 m of incision between channel-fill deposits of significantly different ages is nontrivial and could be indicative of important submarine-channel self-maintenance processes and/or responses to external forcings that are commonly attributed to terrace formation (e.g., Samuel et al., 2003; Heiniö and Davies, 2007; Sylvester and Covault, 2016; Hansen et al., 2017). Detrital-zircon geochronology, as shown here, provides important age control for coarse-grained channel-system deposits and is useful for deciphering the record of long-lived deepwater sedimentary systems.

Temporal Constraints and Preservation of Submarine-Channel Evolutionary Phases

Integration of geochronology with detailed stratigraphic analysis provides insight into the relative importance of sedimentary processes over the lifespan of submarine slope channels and their preservation in sedimentary deposits. The stratigraphic evolution of the channel system exposed on Hornby and Denman Islands can be divided into temporally constrained phases dominated by incision, bypass, lateral channel migration, and vertical channel aggradation (Fig. 10A). The presence of numerous erosion surfaces and composite deposits suggests that each phase contains more frequent and shorter intervals of erosion, bypass, and deposition (cf. Hubbard et al., 2014; Stevenson et al., 2015; Vendettuoli et al., 2019) than are resolvable by detrital zircon geochronologic methods (Fig. 4B). We focus on the timing of processes that occur and dominate slope channel-system evolution over the long time scales (106 yr periods) we are able to measure.

Temporal constraints indicate that stratigraphic intervals dominated by bypass, erosion, and limited deposition, such as during early channel evolution (phase 1) and the later phase of deep incision (phase 4), constitute at least a combined 9.6 ± 3.1 m.y., or 60% (12.4 ± 4.0 m.y., or 66% including detrital zircon sample 1) of the total time spanned by the succession (Figs. 10A and 10C). Conversely, periods associated with thick deposits, such as those recording channel lateral migration and aggradation, together encompass 6.4 ± 6.7 m.y., or up to 40% of total elapsed time (Figs. 10A and 10C). Thus, calculated durations indicate that sediment-transport and erosion as opposed to channel-filling processes account for the majority of time recorded by the submarine slope- channel system.

Temporal constraints in addition to stratigraphic observations (e.g., erosion surfaces, lags) from the channel system on Hornby and Denman Islands reveal a variably complete sedimentary succession (Fig. 10B). Stratigraphic intervals characterized by lower sedimentation rates contain an increased proportion of erosional surfaces and bypass indicators suggesting a more incomplete succession with missing time that is not recorded by deposits in stratigraphy (Figs. 10A and 10B). As such, a preservation bias is evident in the stratigraphic record when comparing the cross-sectional area of deposits within the succession to the amount of time they represent (Fig. 10C). At time scales discernible by MDAs, phases dominated by erosion and sediment bypass represent >50% (up to 67%) of elapsed time; however, they are incomplete and poorly represented by thin, predominantly fine-grained successions and composite erosion surfaces that make up a cumulative 17% of the submarine channel-system deposit (Fig. 10). The remaining 83% of the stratigraphic record is represented by packages of coarse-grained channelform units, which account for <50% of submarine channel-system evolution. Durations and sedimentation rates for these phases indicate that the products of channel filling record relatively rapid, short-lived depositional events that are better preserved and disproportionately represented in the submarine slope-channel system (Fig. 10).

Implications for Deepwater Sediment Transfer and the Role of Submarine Slope Channels

Our results indicate that more than half of the history of the Nanaimo Group submarine slope-channel system is captured in an incomplete and poorly preserved record of incision and bypass, during which sediment was ultimately transported downslope. That is not to say channel-filling processes did not occur, but that most deposits were subsequently removed by erosion over these time periods. Our temporal constraints support the hypothesis that submarine channels function primarily as conduits for sediment to the deeper ocean and are incredibly efficient at eroding and bypassing material on continental slopes (Mutti and Normark, 1987; McHargue et al., 2011; Hubbard et al., 2014; Stevenson et al., 2015). The role of submarine slope channels in transferring large volumes of sediment has been well established from sedimentological and architectural observations of their deposits and the presence of voluminous downslope sediment accumulations (i.e., submarine fans). This example suggests that episodes of sediment throughput are not only present, but can dominate submarine slope-channel evolution over long time scales. Long-lived phases of sediment transfer can characterize early submarine slope-channel evolution (e.g., phase 1) but can also occur during the later stages (e.g., phase 4).

The findings of this study corroborate observations from modern deepwater sediment-routing systems. Repeat bathymetric surveys and direct monitoring provide evidence for multiphase sediment-transport processes in many active unfilled canyons and slope-channel systems (Conway et al., 2012; Gales et al., 2019; Vendettuoli et al., 2019). For example, local deposition of sediment by regular turbidity currents is commonly observed in slope channels but these deposits are eventually flushed downslope during less-frequent, larger-magnitude flow events (Jobe et al., 2018; Paull et al., 2018; Stacey et al., 2019). In some cases, such net-transport conditions can persist for millions of years (e.g., 5 m.y. in the Congo Canyon; Babonneau et al., 2004; Ferry et al., 2004; Anka et al., 2009; Dennielou et al., 2017). Our results support the prevalence of these geomorphic features on the seafloor as conduits for sediment over long time scales (up to millions of years) and suggest ultimate, relatively rapid filling of submarine canyon-channel systems.

Implications for the Interpretation of Submarine Slope-Channel System Deposits

Understanding the degree of completeness of sedimentary successions and how well they capture the geological past in different depositional environments is fundamentally important for interpreting the stratigraphic record. Temporal constraints on the succession exposed on Hornby and Denman Islands demonstrate how submarine slope-channel system deposits are inherently incomplete and can include large time gaps in deposition (spanning millions of years) and nested units of significantly different ages as a result of channel incision, bypass, lateral channel migration, and mass-wasting processes. Consideration of the likely diachronous and discontinuous nature of slope-channel deposits may be critical for accurately interpreting and correlating deepwater successions. For example, temporal gaps are problematic for analyses such as magnetostratigraphy, which assume fairly steady deposition and a preconceived understanding of the relative age of units (e.g., Opdyke and Channell, 1996; Enkin et al., 2001; Langereis et al., 2010).

Missing time not recorded by a preserved deposit also introduces some uncertainty when interpreting submarine-channel evolution. Incomplete stratigraphic intervals could include periods of erosion, complete bypass, hiatus, or deposition of units that were subsequently reworked over long time periods (Sadler and Strauss, 1990; Straub and Foreman, 2018). Even phases recorded by deposits were undoubtedly accompanied by some degree of unrecorded bypass and/or subsequent erosion of the finer-grained fraction of flows not observed in coarse-grained channelforms (Stevenson et al., 2015). The strike-oriented perspective of exposures on Hornby and Denman Islands provides a more faithful record of submarine-channel processes than any one vertical section (Straub and Foreman, 2018). However, without a more complete three-dimensional perspective including downslope correlative components, it is difficult to fully decipher what occurred during poorly preserved intervals and quantify the amount of bypassed and reworked sediment.

Our data set sheds light on the fidelity of the stratigraphic record and also provides temporal context for the commonly recognized stratigraphic evolution of submarine channels. Horizontally to vertically aligned channelform stacking patterns are observed globally over a wide range of temporal and spatial scales (spanning distances up to 20 km laterally and >1000 m vertically, over time scales ranging from 104 to 106 yr; Jobe et al., 2018; Sansom, 2018). Although calculated durations may not be directly applicable to systems of varying longevity, their relative comparison is informative and potentially useful for evaluating the temporal significance of this pattern in other deposits.

For example, in the two seismic-reflection profiles in Figure 1, our results suggest that between 20% and 33% of time could be captured in erosion surfaces and thin units at the base of the section, representing <20% of stratigraphy (phase 1 equivalent; Fig. 10). Overlying horizontally and vertically aligned channelform packages that make up most of the succession might represent proportionately less time, where horizontally aligned channelforms (phase 2 equivalent) can encompass twice as much time as vertically aligned channelforms (phase 3 and 5 equivalents). Moreover, incisional surfaces between nested packages of vertically stacked channelforms may represent temporally significant periods (e.g., Fig. 1A). The greater geochronological control combined with detailed outcrop characterization of the channel system exposed on Hornby and Denman Islands provides a new understanding of time represented in preserved stratigraphy that is widely relevant to the interpretation of slope-channel deposits.

CONCLUSION

Our integrated geochronologic-stratigraphic analysis reveals the timing of evolution of a submarine slope-channel system, characterized by horizontally to vertically aligned channelform architecture, exposed on Hornby and Denman Islands, British Columbia, Canada. MDAs derived from detrital zircon data sets indicate that this channel system persisted on the Nanaimo Basin margin for at least 16.0 ± 1.7 m.y., from the middle Campanian into the Paleocene. Temporal constraints combined with detailed sedimentologic and stratigraphic analysis (Bain and Hubbard, 2016) of exposed deposits demonstrate a multiphase history of erosion, bypass, and fill; stratigraphic products include nested sedimentary units as well as composite stratigraphic surfaces and thin successions that encompass significant time spans (i.e., millions of years). From the succession, we document an initial period (phase 1) of long-lived erosion, sediment transfer, and lateral channel migration, reflected in lower sedimentation rates consistent with sediment reworking and bypass downslope. Later submarine-channel evolution is characterized by increasingly shorter phases of more rapid deposition on the slope by laterally migrating (phase 2) and aggrading (phases 3 and 5) channels, as well as potentially long periods of deep incision (phase 4). Calculated durations for identified phases of evolution indicate that erosion and sediment-transfer processes dominate long-term slope-channel system evolution (60%), but, by their nature, these periods are not well represented in submarine slope-channel system deposits. Quantifying the exact duration of sediment bypass and amount of transferred sediment is limited in this study by the resolution of MDAs and the 2-D cross-sectional extent of the outcrop. The temporal evolution and relative durations of evolutionary phases documented in this study are important for understanding the significant role and longevity of submarine slope channels as conduits for sediment and are applicable for interpreting submarine channel evolution in ancient and modern environments worldwide.

ACKNOWLEDGMENTS

The funding for this research was generously provided by a Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant (RG-PIN/341715-2013) to S. Hubbard. R. Englert received support from a NSERC Canada Graduate Scholarship Award and the University of Calgary (John D. Petrie, QC Memorial Graduate Scholarship and Queen Elizabeth II Graduate Scholarship). This manuscript benefited from early discussions with Heather Bain. The drone photo in Figure 8F was generously provided by Sebastian Kaempfe. Additionally, we would like to thank Ian Kane and Luke Pettinga for helpful reviews of this manuscript.

1Supplemental Files. File S1: Laser and mass spectrometer acquisition settings and details of uncertainty determination. File S2: U-Pb zircon geochronology results determined by LA-ICP-MS at the University of Calgary Centre for Pure and Applied Tectonics and Thermochronology. File S3: Weighted-average graphs displaying dates with 2σ uncertainty of the youngest 50 concordant grains for each sample. Grains used in the MDA calculations are indicated. File S4: Synthetic distributions generated by Monte Carlo simulations used to determine phase durations and sedimentation rates, and their uncertainties. Please visit https://doi.org/10.1130/GES02091.S1 or access the full-text article on www.gsapubs.org to view the Supplemental Files.
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