Stephen M. Hubbard

Seismic geomorphology and sedimentology of a tidally influenced river deposit, Lower Cretaceous Athabasca oil sands, Alberta, Canada

The bitumen of the Lower Cretaceous McMurray Formation in Alberta arguably represents one of the most important hydrocarbon accumulations in the world. In-situ development relies on heat transfer through the reservoir via horizontal steam injection wells placed 4 to 6 m (13–20 ft) above horizontal producers near the base of the sandstone reservoirs. Given this technology, understanding the distribution of the resource is paramount for a successful development program. Sedimentary facies provide a direct control on bitumen distribution and recovery. Most facies models developed to describe and predict sedimentary units of the McMurray Formation consider fluvial, estuarine, and/or deltaic depositional settings. In-situ development, however, requires a particularly high-resolution sedimentologic interpretation. High-quality three-dimensional seismic reflection data and extensive drill cores from acreage located approximately 50 km (31 mi) south of Fort McMurray provide important insights into the sedimentologic organization of reservoir and nonreservoir deposits in the upper one third (40 m [131 ft]) of the reservoir interval. Geomorphologic characteristics of the strata observed in seismic time slices reveal that a fluvial depositional setting was prevalent. Ichnologic and palynologic data, as well as sedimentary structures suggestive of tidal processes, indicate a marine influence in the upper reaches of a fluvial system characterized by channels that were 390 to 640 m (1280–2100 ft) wide and 28 to 36 m (92–118 ft) deep. The complex stratigraphic architecture consists of a mosaic of large-scale depositional elements, including abandoned channels or oxbow lake fills, point bars associated with lateral accretion, point bars associated with downstream accretion, counter point bars, and sandstone-filled channels. Reservoir deposits are primarily associated with point bars and sandstone-filled channels.

Sediment transfer and deposition in slope channels: Deciphering the record of enigmatic deep-sea processes from outcrop

The processes within deep-sea sediment-routing systems are difficult to directly monitor. Therefore, we rely on other means to decipher the sequence and relative magnitude of the events related to erosion, sediment bypass, and deposition within channels that crosscut the seascape, and in particular, continental slopes. In this analysis, we examine the nature of slope channel fill in outcrop (Cretaceous Tres Pasos Formation, southern Chile) in order to evaluate the geological evidence of the full channel cycle, from inception to terminal infill with sediment, and we attempt to provide insight into the enigmatic deep-sea processes that are critical for a comprehensive understanding of Earth surface dynamics. In the stratigraphic record, slope channel fills are typically represented by sandstone- or conglomerate-dominated deposits that define channelform sedimentary bodies tens of meters thick and hundreds of meters across. Despite the prevalence of coarse-grained sediment, key information is recorded in the fine-grained deposits locally preserved within the channelform bodies, as well as a breadth of scours or internal channelform stratal surfaces. These characteristics preserve the record of protracted sedimentary bypass and erosion. In many instances, the life of a slope channel is dominated by sedimentary bypass, and the stratigraphic record is biased by the products of shorter-lived channel filling and abandonment.

Sediment dispersal in an evolving foreland: Detrital zircon geochronology from Upper Jurassic and lowermost Cretaceous strata, Alberta Basin, Canada

The Alberta foreland basin is a classic example of a retro-arc foreland basin, yet the early stages of its development remain poorly understood. Several contrasting hypotheses have been proposed to explain the source areas and dispersal patterns of sediment in western Canada during the Late Jurassic initiation of the foreland basin. Here, we use detrital zircon uranium-lead (U-Pb) geochronology, sandstone petrography, paleocurrent measurements, and regional correlations to reconstruct the early basin evolution, including sediment provenance and depositional history. These data indicate sediment in the early foreland basin was delivered via two principal sedimentary systems: a south-to-north axial river system, and transverse fluvial systems that emanated from the adjacent Cordillera. Accordingly, sandstones of the Jurassic foreland, associated with the Minnes Group and equivalent Kootenay and Nikanassin formations, are divided into two informal groups, type 1 and type 2. Type 1 sandstones are mature quartz arenites, present along the entire north-south length of the Alberta Basin, and generally at the base of the succession. Type 1 sandstones have zircons with age populations between 980 and 2000 Ma, similar to sediments of Jurassic and Lower Cretaceous strata in the western United States. These deposits are interpreted to have been derived from southern sources and transported axially to the north along the earliest foredeep of the Cordilleran foreland basin. Type 2 sandstones by contrast, are less mature, containing higher quantities of chert and lithic fragments, and are dominated by 1765–2100 Ma zircons with a smaller population at 2500–2800 Ma. The zircon age populations of type 2 sandstones are similar to populations recorded in the Neoproterozoic to Triassic miogeocline strata of the adjacent fold-and-thrust belt. Type 2 sandstones are common in the western, orogenic side of the basin, but they extend eastward across the basin in fluvial sediments in the upper portion of the succession. Changes in provenance and sediment composition are associated with the evolution of paleodrainages and the increasing importance of Cordilleran erosion to the sediment budget. The progressively greater influx of orogen-derived material relative to subsidence displaced the axial fluvial system toward a more cratonward-position. The collected data support the hypothesis that much of the sediment was initially transported northward by an axial drainage network, followed by Cordilleran-sourced sediments fed by transverse river systems. The present study attempts to unravel predictable patterns of sediment dispersal in evolving foreland basins, while testing whether clear changes in sediment composition of the first clastic pulse of sediment are related to hinterland exhumation, changing drainage divides, weathering processes, or varied provenance.

Deep Burrows in Submarine Fan-Channel Deposits of the Cerro Toro Formation (Cretaceous), Chilean Patagonia: Implications For Firmground Development and Colonization in the Deep Sea

Abstract The Glossifungites ichnofacies recognized in Cretaceous strata (Cerro Toro Formation) of the Magallanes foreland basin in southern Chile represents an important discovery in that it extends the stratigraphic utility of firmground trace-fossil suites into thick-bedded, gravity-flow deposits of submarine fan-channel environments. The trace-fossil suite consists of atypically large Diplocraterion, Skolithos, and Arenicolites, which may reach an inferred length of 7 m. The burrows penetrate muddy, matrix-supported conglomeratic deposits dewatered and consolidated as a result of burial and subsequently exhumed by erosive turbidity currents. In a stratigraphic succession dominated by coarse-grained facies >350 m thick, the burrows are abundant at one stratigraphic horizon correlatable up to 200 km2. This horizon is interpreted as a stratigraphic discontinuity associated with a long-term cessation of coarse-grained, sediment-laden gravity flows into the basin. The colonized surface is the only marker horizon traceable across much of the Magallanes basin study area.

An Outcrop Example of Large-scale Conglomeratic Intrusions Sourced from Deep-water Channel Deposits, Cerro Toro Formation, Magallanes Basin, Southern Chile

Large-scale vertical to subvertical clastic intrusions (as much as 67 m [219 ft] wide and 100 m [330 ft] high) are present in Cretaceous strata (Cerro Toro Formation) of the Ultima Esperanza district, southern Chile. The injectites emanate from the margins of submarine-channel deposits that accumulated at water depths of 1000–2000 m (3300–6600 ft) in the Magallanes foreland basin. The remobilized sediment is very coarse, consisting of sandy matrix conglomerate, muddy matrix conglomerate, and poorly sorted sandstone. The injectite bodies sometimes bifurcate upward and are circular in plan view and, thus, are geometrically analogous in many respects to numerous injection features mapped seismically in the North Sea Basin. The remobilization of coarse sediment was likely induced after the burial of the parent deposit to at least a few hundred meters. The controlling factors on injection are difficult to discern; however, it is probable that the highly energetic process involved gas charging of the source body and, potentially, a seismic event trigger associated with the uplift of the Patagonian Andes.

The stratigraphic expression of decreasing confinement along a deep-water sediment routing system: Outcrop example from southern Chile

The products of sediment-laden turbidity currents that traverse areas of decreasing confinement on submarine slopes include erosional and depositional features that record the inception and propagation of deep-sea channels. The cumulative stratigraphic expression and deposits of such transitions, however, are poorly constrained relative to depositional settings dominated by end-member confined (i.e., submarine channel fill) and unconfined (i.e., lobe) deposits. Upper Cretaceous strata of the Magallanes foreland basin in southern Chile are characterized by a variety of stratigraphic architectural elements in close juxtaposition both laterally and vertically, including: (1) low-aspect-ratio channelform bodies attributed to slope channel fills; (2) high-aspect-ratio channelform bodies interpreted as the deposits of weakly confined submarine channels; (3) lenticular sedimentary bodies considered to represent the infill of laterally coalesced scours; (4) discontinuous channelform bodies representing isolated scour fills; and (5) a cross-stratified, positive-relief sedimentary body, which is interpreted to record an upslope-migrating depositional bedform. These elements are interpreted to have formed at a submarine sediment routing system segment characterized by a break in slope, and an accompanying decrease in confinement. The various architectural elements examined are interpreted to record a unique stratigraphic perspective of turbidite channels at various stages of development, from early-stage discontinuous and isolated scour fills to low-aspect-ratio channel units.

Chapter 20 – Slopes

Slope settings are influenced by numerous factors, including sediment bypass, mass wasting, incision by canyons and gullies, and/or widespread suspension settling of fine-grained particulate debris. Significant deposition of coarse-grained detritus may occur on slopes, particularly associated with ponding in channels and minibasins. Ichnology has been underutilized in assessing slope strata, mainly owing to erroneous perceptions that slope trace-fossil suites are (1) relatively homogeneous and exclusively assigned to the Zoophycos Ichnofacies or (2) largely indistinguishable from basin-floor assemblages. Organisms residing on the slope respond to varied parameters such as current energy, sedimentation, slope instability, substrate consistency, oxygen and food availability, and water turbidity, which in turn, are controlled by factors such as basin circulation, water stratification, shelf width, and sediment supply (e.g., point vs. line source). Depending upon the influence of these parameters on a subenvironment, slope trace-fossil suites can be assigned to the Zoophycos, Cruziana, Skolithos, Glossifungites, or Nereites ichnofacies.

Sandstone provenance and insights into the paleogeography of the McMurray Formation from detrital zircon geochronology, Athabasca Oil Sands, Canada

The Lower Cretaceous McMurray Formation of northeastern Alberta hosts most of the bitumen resources of the Athabasca Oil Sands. Despite its importance, the sedimentary provenance and corresponding Early Cretaceous paleodrainage system associated with these fluvial deposits remain poorly understood. We collected 18 sandstone samples from five cored wells drilled in the McMurray Formation and analyzed these for detrital zircon uranium–lead (U–Pb) geochronology. Together, these samples yield detrital zircon U–Pb age populations of less than 250, 300–600, 1000–1200, 1800–1900, and 2600–2800 Ma. Almost all of the samples contain detrital zircons with ages of 300–600 and 1000–1200 Ma, which were originally derived from the Appalachian and Grenville provinces, respectively, of eastern North America. Lowermost strata of the McMurray Formation are characterized by relatively small fluvial channel deposits and detrital zircon ages of 1800–1900 and 2600–2800 Ma, which suggest a limited paleodrainage area that includes the adjacent Canadian shield. In contrast, channel deposits in the middle–upper part of the formation are relatively large and contain abundant Appalachian- and Grenville-derived detrital zircons. These data suggest that the paleodrainage system of the McMurray Formation evolved over time, increasing in size between deposition of the lowermost units and the middle–upper deposits. Detrital zircons from the Appalachian and Grenville regions may have been transported directly to western Canada during the Cretaceous or recycled multiple times prior to their deposition. Detrital zircons from the Cordillera (<250 Ma) are restricted to the northern part of the study area, which suggests that a tributary may have joined the main trunk fluvial system in this area.

Timing of deep-water slope evolution constrained by large-n detrital and volcanic ash zircon geochronology, Cretaceous Magallanes Basin, Chile

Deciphering depositional age from deposits that accumulate in deep-water slope settings can enhance understanding of shelf-margin evolutionary timing, as well as controlling mechanisms in ancient systems worldwide. Basin analysis has long employed biostratigraphy and/or tephrochronology to temporally constrain ancient environments. However, due to poor preservation of index fossils and volcanic ash beds in many deepwater systems, deducing the timing of slope evolution has proven challenging. Here, we present >6600 new U-Pb zircon ages with stratigraphic information from an ~100-kmlong by ~2.5-km-thick outcrop belt to elucidate evolutionary timing for a Campanian– Maastrichtian slope succession in the Magallanes Basin, Chile. Results show that the succession consists of four stratigraphic intervals, which characterize four evolutionary phases of the slope system. Overall, the succession records 9.9 ± 1.4 m.y. (80.5 ± 0.3 Ma to 70.6 ± 1.5 Ma) of graded clinoform development punctuated by out-of-grade periods distinguished by enhanced coarse-grained sediment bypass downslope. Synthesis of our results with geochronologic, structural, and stratigraphic data from the basin suggests that slope evolution was largely controlled by an overall decline in basin subsidence from 82 to 74 Ma. In addition to providing insight into slope evolution, our results show that the reliability of zircon-derived depositional duration estimates for ancient sedimentary systems is controlled by: (1) the proportion of syndepositionally formed zircon in a strati-

How to recognize crescentic bedforms formed by supercritical turbidity currents in the geologic record: Insights from active submarine channels

Submarine channels have been important throughout geologic time for feeding globally significant volumes of sediment from land to the deep sea. Modern observations show that submarine channels can be sculpted by supercritical turbidity currents (seafloor sediment flows) that can generate upstream-migrating bedforms with a crescentic planform. In order to accurately interpret supercritical flows and depositional environments in the geologic record, it is important to be able to recognize the depositional signature of crescentic bedforms. Field geologists commonly link scour fills containing massive sands to crescentic bedforms, whereas models of turbidity currents produce deposits dominated by back-stepping beds. Here we reconcile this apparent contradiction by presenting the most detailed study yet that combines direct flow observations, time-lapse seabed mapping, and sediment cores, thus providing the link from flow process to depositional product. These data were collected within the proximal part of a submarine channel on the Squamish Delta, Canada. We demonstrate that bedform migration initially produces back-stepping beds of sand. However, these back-stepping beds are partially eroded by further bedform migration during subsequent flows, resulting in scour fills containing massive sand. As a result, our observations better match the depositional architecture of upstream-migrating bedforms produced by fluvial models, despite the fact that they formed beneath turbidity currents.