Dale A. Leckie

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.

Is There Evidence for Geostrophic Currents Preserved in the Sedimentary Record of Inner to Middle-Shelf Deposits?

ABSTRACT Unidirectional, oscillatory and combined-flow paleocurrent data from many ancient wave-dominated coastlines indicate that sediment transport was directed offshore at angles of 70 to 90° from the inferred local paleoshoreline. Solemarks and parting lineations from turbidite-like storm beds and solemarks from hummocky beds indicate that flows transporting sediment offshore were orthogonal to the local shoreline. The shore-normal orientations of solemarks, parting lineation and asymmetric ripples on hummocky cross-stratification and their orthogonal relationship with respect to wave-ripple crest orientation indicate that the dominant direction of wave approach at the sediment-water interface was also orthogonal to the shoreline. The formation of hummocky cross-stratification by an al ng-shore flow component superimposed on oscillatory wave motion is not supported by paleocurrent data. Rather, much of the evidence from the ancient record suggests that hummocky cross-stratification forms under high-energy, oscillatory-flow with a weak, shore-normal, combined-flow component. The orientation of wave-ripple crests from the lower shoreface to offshore-transitional zone of wave-dominated coasts can be used to approximate the local paleoshoreline trend. The paleocurrent data from the ancient examples suggest a strong predominance of offshore-directed sediment transport, as opposed to the nearly shore-parallel orientation of currents observed in modern geostrophic flows.

Wave-Formed, Coarse-Grained Ripples and their Relationship to Hummocky Cross-Stratification

ABSTRACT Wave-formed, coarse-grained ripples (CGR) form in water depths ranging from 3 to 160 m on transgressive surfaces, in upper and lower shoreface sediments and in finer-grained offshore-transition deposits. CGR have crest spacings of 0.25-3.0 m with amplitudes of 5-35 cm. Internal stratification is rarely preserved, but where present, crossbeds dip en echelon away from the ripple crest and individual laminae feather out away from the crest. Waves that form CGR commonly have wavelengths on the order of 2-5 m with periods of 8 to 14 sec. These are not large storm events, especially on a geological scale. CGR commonly occur in close association, laterally and vertically with hummocky cross stratification. It is postulated that if sediment is medium to very coarse-grained sand,or pebbly sand then two-dimensional CGR will be formed from oscillatory or oscillatory-dominant flow. If, however, sediment is very fine to fine-grained sand, then three-dimensional hummocky cross-stratification will be formed. The wave conditions under which CGR form are similar to those which would form hummocky cross-stratification; it is only the grain size that is different.Reported and calculated wave conditions for hummocky cross-stratification are almost identical to those for CGR. Modern CGR crests trend parallel to the local shoreline and bathymetry; as such, ancient ripple crests are a useful tool for basin reconstruction.

Estuarine Deposits of the Albian Paddy Member (Peace River Formation) and Lowermost Shaftesbury Formation, Alberta, Canada

ABSTRACT Outcrop exposures of the Peace River Formation in northwestern Alberta contain evidence of significant relative sea-level fluctuations that occurred during the middle to late Albian. These include Paddy Member channels that incised into the Cadotte Member shoreline deposits and the subsequent estuarine-frill. The upper Cadotte Member contact is irregular and siderite-cemented; it has been scoured into and infilled by sandstone of the Paddy Member. The lower sandstone of the Paddy Member is crossbedded, with abundant comminuted, carbonaceous debris. Channels cut into the Cadotte Member are up to 5 m deep and are locally overlain by in situ coal and roots. The channels are infilled with alternating couplets of moderately to intensely bioturbated sand and mud which form inclined h terolithic stratification: mud plugs are also present. Internally these sands are ripple and parallel cross-laminated. This part of the Paddy Member is interpreted as estuarine fill that resulted from a relative sea-level rise. The channel fill is overlain by intensely bioturbated, locally rooted, finely interbedded sandstone, siltstone and shale, with minor reworked bentonites, interpreted as tidal flat deposits. Tidal flat sediments are overlain by 2 to 3 m of scoured, parallel-laminated sandstone and capped by 1.5 m of planar-tabular crossbedded sandstone interpreted as shoreface and transgressive, estuary-mouth deposits, respectively. Evidence for tides throughout the Paddy Member includes inclined heterolithic stratification, mud couplets, reactivation surfaces, reversing paleoflows and compound crossbeds. The incised channels and related fill are correlative with a sequence of paleosols 300 km southwest in the Boulder Creek Formation. A brackish influence is indicated by the presence of a few species of peridinioid dinoflagellates occurring in abundance at certain levels and the trace fossil assemblage. The Paddy Member is overlain by marine mudstone of the Shaftesbury Formation. A 20 to 30 cm thick, wave-rippled layer of fish teeth, fish bones and pebbles, near the base of the Shaftesbury Formation, can be traced along the Peace River for at least 75 km. The layer of fish remains is interpreted as transgressive lag.

Canterbury Plains, New Zealand--Implications for Sequence Stratigraphic Models

The Canterbury Plains, New Zealand, bounded by the Southern Alps and the Pacific Ocean, are traversed by four large gravel rivers. The coastline is wave dominated and microtidal (<2 m) with high rates of longshore sediment transport. The Canterbury coast is subdivided into southern and northern portions separated by the Banks Peninsula. The southern coastline, which is subjected to large oceanic swell originating as far away as 2000 km, is retrogradational and has wave-cut cliffs up to 25 m high. Coastal erosion at about 1 m/yr steepens the gradient, causing the rivers to incise 1.5-4.2 mm/yr during the present sea level highstand and accompanying transgression. Coastal erosion is caused by the extreme wave energy and efficient longshore sediment transport. River mouth are incising into the regional flood plain, with the amount of incision decreasing inland from the coast. The fluvial headwaters in the Southern Alps are rising tectonically and isostatically, causing incision that decreases seaward. Thus, fluvial incision takes place in the west due to mountain uplift and in the east, along the coast, due to a retreating shoreline during marine transgression. A zone of minimal valley incision occurs 8-15 km from the coast. In contrast to this observation, sequence stratigraphic models suggest that downcutting should occur during falling sea level, not during transgression. The northern coastline progrades about 1 m/yr and is largely sandy in its southern reaches. Thus, the Canterbury coastline is at one locale progradational and elsewhere retrogradational. The thickness of material being removed by erosional shoreface retreat during the present transgression is about 40 m. Although sea level plays a role, more important controls on progradation, retrogradation, and valley incision on the Canterbury Plains are the extreme wave energy and longshore drift.

Use of micromorphology for palaeoenvironmental interpretation of complex alluvial palaeosols: an example from the Mill Creek Formation (Albian), southwestern Alberta, Canada

Abstract Field observations are often not sufficient for process-based interpretations of palaeosols, particularly where they form parts of thick aggradational pedocomplexes within alluvial successions. Under such conditions micromorphology provides genetic, temporal and spatial information on soil-forming processes that is critical to an understanding of past environmental conditions. Thick alluvial successions of the Albian Mill Creek Formation contain abundant evidence of pedogenesis, but few well-developed palaeosol profiles, and therefore, provide an ideal case study in which to demonstrate the usefulness of micromorphological data for palaeoenvironmental interpretation. The micromorphological features of greatest interpretive value are types of clay coatings and ferruginous segregations, structure and fabric. Papules, evidence of bioactivity and ferruginous concretions provide information on geomorphic surface stability and assist in reconstructing temporal changes in drainage conditions. While individual features can provide some palaeoenvironmental information, the relationships of features to one another and assemblages of features provides additional information when analysed hierarchically to establish a sequence of sedimentologic and pedogenic events. A common, recurring sequence of palaeoenvironmental events, subject to local variations, can be recognized throughout the Mill Creek Formation. The presence of illuvial clay requires that water percolated through the soil and that the soil periodically dried out so that the translocated clay was retained. Dark reddish clay coatings indicate clay illuviation under freely drained conditions, while pale-yellow and silty clay coatings suggest that phases of free drainage alternated with phases of poorly drained or saturated soil conditions. The presence of iron-depletion coatings, iron nodules and quasiferrans indicates that these units were at least periodically saturated, and the occurrence of multiple, overlapping phases within single thin sections demonstrates that redox conditions fluctuated, strongly suggesting development in the vadose zone. Recent soils containing similar assemblages of features develop under warm temperate seasonal climates. Alternating phases of well-drained and saturated conditions on the Mill Creek floodplains are attributed to changing sediment supply and local palaeogeomorphology rather than to any major regional climate change. This type of process-based, micromorphological analysis should have broad application in other complex, pedogenically modified alluvial successions and similar studies would lead to a more detailed understanding of ancient palaeoenvironments.

Sedimentological and petrographical characteristics of Cretaceous strandplain coals: a model for coal accumulation from the North American Western Interior Seaway

Abstract Coal seams formed on many Cretaceous wave-dominated strandplain sediments in North America are characterized by great lateral continuity (tens to hundreds of kilometres), substantial thicknesses (up to 12 m), relatively low ash and sulphur contents. The coals formed behind an active shoreline in areas undergoing subsidence due to shale compaction and dewatering. The zone of peat accumulation was remote from the shoreline and storm/tidal inundations and generally protected from fluvial flooding. If the rate of subsidence was too great, lakes formed and peat did not accumulate. Statistical evaluation of petrographic properties, by correspondence analysis, of the Lower Cretaceous coals show that the strandplain coals form distinctive petrographic groups that are characterized by relatively low vitrinite contents and high inertinite contents. Semifusinite and inertodetrinite dominate in the inertinite maceral group. Liptinite contents are negligible. Tissue Preservation Indices and Gelification Indices indicate for the strandplain coals a forest-type paleodepositional environment in which a relatively low water table allowed the accumulation of oxidized and partly oxidized components (fusinite and semifusinite). Significant amounts of detrital components, such as inertodetrinite and vitrinite B, are diagnostic that some transportation of the organic material took place prior to deposition. Comparison of coal facies and depositional environments from Permian coals of Australian show that the Lower Cretaceous strandplain coals have petrographic similarities to coals that were formed under regressive back-barrier conditions in the Permian. Due to differences in nomenclature, previously interpreted regressive back-barrier environments may be similar to the strandplain environments discussed here.

Organic-rich, radioactive marine shale; a case study of a shallow-water condensed section, Cretaceous Shaftesbury Formation, Alberta, Canada

ABSTRACT Organic-rich radioactive shales are a common regional feature resting on Cretaceous transgressive surfaces in western Canada. The basal shale in the Shaftesbury Formation (Late Albian) from the Peace River area of northern Alberta is characterized by high gamma-ray, high resistivity, and low neutron wireline log signatures. Three facies, in ascending order, are present within the basal Shaftesbury Formation: 1) a brackish-water estuarine shale; 2) a restricted, marginal-marine shale which is radioactive; and 3) an open-marine, normal salinity shale. The radioactive shale contains an abundance of large, lenticular algal cysts (cf. Lancettopsis lanceolata Madler 1963) which are rare in overlying and underlying shale. The algal cysts and high organic content may be the locus of th radioactivity. The total organic carbon content ( 6%) and sulphur content ( 3.4%) of the radioactive shale also are higher than the shale above and below, with a different mineralogy as well. The radioactive portion of the basal Shaftesbury shale has the characteristics of a condensed section; it is directly above a ravinement surface and transgressive-lag deposit which, in turn, locally overlie estuarine sediments deposited within an incised valley. Other characteristics include evidence of low oxygen values, low concentrations of benthonic foraminifera, and evidence of a slow sedimentation rate. Palynological, micropaleontological, and geochemical results indicate that the radioactive s ale was deposited in restricted, marginal marine conditions and that overlying shale shows a progressive deepening to nearshore, open-marine conditions. This radioactive shale does not represent the deepest water sediments of the transgression but is a condensed section deposited in relatively shallow water.

Emplacement and reworking of Cretaceous, diamond-bearing, crater facies kimberlite of central Saskatchewan, Canada

In central Saskatchewan, Canada, kimberlites were emplaced into Cretaceous marine and nonmarine clastic sediments. Core recovered from one drill hole that intersects kimberlite (Smeaton FAC/UK core 169/8) was selected for an integrated study involving sedimentology, volcanology, mineralogy, geochemistry, palynology, micropaleontology, organic petrology, and radiometric age determination. Only crater facies kimberlite has been observed; there is no indication of the locations of feeder dikes. Four varieties of kimberlite occur, all originating from subaerial volcanism: (1) fluvial-reworked kimberlite; (2) diamondiferous kimberlite lapillistone air-fall deposits; (3) kimberlite olivine crystal-tuff air-fall deposits; and (4) diamondiferous marine wave-reworked kimberlite. Within the multiple primary eruptive phases of the kimberlite air-fall deposits, the volcanic style changed upward with time, from violent Strombolian to more explosive volcanism. The bulk of the volcanism formed conformable, air-fall deposits on terrestrial sediments of the Cantuar Formation, resulting in the development of positive-relief tephra cones. Subsequent marine transgression associated with the Westgate Formation partially beveled the top of the cone. The kimberlite air-fall deposits contain microdiamonds, 5 to 25 μm in diameter. The maximum temperature and vitrinite reflectance values of coaly matter in the kimberlites indicate that these deposits, although originally derived from magma at high temperatures, did not thermally affect entrained surficially derived clasts or the country rock during emplacement. The chemical content of intrakimberlite shale clasts is markedly different from the marine and nonmarine shales and indicates significant synemplacement and postemplacement fluid movement through the volcanic pile. At least two episodes of kimberlite volcanism occurred in the middle and late Albian (paleontologically assigned). A U-Pb perovskite radiometric age of 101.1 ± 2.2 Ma from a kimberlite lapillistone from the younger episode of volcanism is internally consistent with biostratigraphic studies that constrain the kimberlite volcanism as post–middle Albian and pre–late Albian to late Albian.

Late Cretaceous (Cenomanian to Campanian) paleoenvironmental history of the Eastern Canadian margin of the Western Interior Seaway: bonebeds and anoxic events

Abstract Upper Cretaceous strata in the Pasquia Hills of the northern Manitoba Escarpment, eastern Saskatchewan, Canada provide a detailed paleoenvironmental and sea-level record of the eastern margin of the Western Interior Seaway. Sediments deposited during the Cenomanian/Turonian Greenhorn marine cycle are dominantly black mudstones deposited in a stratified water column, with bottom-water anoxia recurrently reaching into the photic zone. A middle Cenomanian sea-level lowstand event followed by transgression left a series of bonebeds within the Belle Fourche Member of the Ashville Formation, indicating a sedimentary environment starved of coarse siliciclastics. Maximum sea level resulted in the formation of limestone beds within the Favel Formation, further favoured by reduced terrigenous sediment input compared to the western margin. Limestone sedimentation was followed by a phase of increased freshwater input under lower sea level conditions, and reducing zoo- and phytoplankton diversities. During final Greenhorn regression, eastern Saskatchewan probably turned into a restricted basin severely limiting marine circulation. Poor or absent benthic foraminiferal assemblages and biomarker analysis suggest prevailing watermass stratification throughout the Cenomanian/Turonian transgressive/regressive cycle. This was caused either by a freshwater lid, stratification of Boreal and Tethyan-derived watermasses, or both, to various intensities affected by changing sea level. Basin oxygenation during Niobrara time varies between localities along the eastern margin as documented by presence/absence of benthic and planktic foraminifera.