Geological controls on the geometry of incised-valley fills: Insights from a global dataset of late-Quaternary examples

Incised valleys that develop due to relative sea-level change are common features of continental shelves and coastal plains. Assessment of the factors that control the geometry of incised-valley fills has hitherto largely relied on conceptual, experimental or numerical models, else has been grounded on case studies of individual depositional systems. Here, a database-driven statistical analysis of 151 late-Quaternary incised-valley fills has been performed, the aim being to investigate the geological controls on their geometry. Results of this analysis have been interpreted with consideration of the role of different processes in determining the geometry of incised-valley fills through their effect on the degree and rate of river incision, and on river size and mobility. The studied incised-valley fills developed along active margins are thicker and wider, on average, than those along passive margins, suggesting that tectonic setting exerts a control on the geometry of incised-valley fills, likely through effects on relative sea-level change and river behaviour, and in relation to distinct characteristics of basin physiography, water discharge and modes of sediment delivery. Valley-fill geometry is positively correlated with the associated drainage-basin size, confirming the dominant role of water discharge. Climate is also inferred to exert a potential control on valley-fill dimensions, possibly through modulations of temperature, peak precipitation, vegetation and permafrost, which would in turn affect water discharge, rates of sediment supply and valley-margin stability. Shelves with slope breaks that are currently deeper than 120 m contain incised-valley fills that are thicker and wider, on average, than those hosted on shelves with breaks shallower than 120 m. No correlation exists between valley-fill thickness and present-day coastal-prism convexity, which is measured as the difference in gradient between lower coastal plains and inner shelves. These findings challenge some concepts embedded in sequence stratigraphic thinking, and have significant implications for analysis and improved understanding of source-to-sink sediment route-ways, and for attempting predictions of the occurrence and characteristics of hydrocarbon reservoirs

Tectonic, eustatic and climatic controls on marginal-marine sedimentation across a flexural depocentre: paddy member of peace river formation (late albian), western Canada foreland basin

In north-central Alberta and adjacent British Columbia, clastic strata of the middle to late Albian Peace River and Shaftesbury formations were deposited in alluvial to shallow-marine environments across the foredeep of the Western Canada Foreland Basin. A high-resolution, log and core-based allostratigraphic framework for the Paddy Member of the Peace River Formation established nine allomembers, PA to PI, bounded by flooding surfaces and apparently equivalent non-marine surfaces. Within the estimated 2 Myr. duration of the Paddy, allomembers allow the evolving palaeogeography and changing relationship between accommodation and sedimentation rates to be analysed on timesteps on the order of 105 years. Paddy strata fill an arcuate depocentre ca 300 km wide, across which the rocks thin eastward from 125 m to ca 5 to 10 m. The northern part of the basin is occupied by muddy, offshore marine deposits that pass abruptly southward into a linear, WSW-ENE-trending body of sandstone deposited in a wave-dominated barrier-strandplain, at least 350 km long. Extending >200 km to the south of the strandplain was a region of shallow brackish to freshwater lagoons and lakes that graded to the SW into alluvial facies. Within the lagoon region, few-m thick, elongate and patchy sandstones represent river-dominated deltas. In allomembers PA to PG, these sandstones are concentrated in the west and south, implying supply from the western Cordillera. In allomembers PH and PI, sandstones are mainly in the east and have a distinctive, quartz-rich composition. They can be correlated eastward into the coeval Pelican Formation, and were sourced probably from the Canadian Shield on the opposite side of the basin. In the western foredeep, alluvial rocks comprise aggradational, unconfined floodplain deposits with ribbon sandstones, dissected, on at least nine separate levels, by palaeovalleys that are confined to the proximal foredeep. Valleys are 10 to 30 m deep, few km wide, and filled with multi-storey channel-bars of pebbly coarse sandstone or conglomerate. Valleys cut down from well-developed interfluve palaeosols that record a falling and then rising water table. Alternating aggradation and degradation, and advance and retreat of the alluvial gravel front is attributed to cycles of varying rainfall intensity, rather than tectonism or eustasy. Apparently, coeval transgressive-regressive successions in the lagoon and marine regions are attributed to few-m scale eustatic changes. On the NE margin of the basin, tidal sandstone fills a northward-opening estuary cut on the basal PaddyGnesioceramus comancheanus (Cragin), proving contemporaneity with at least part of the marine Joli Fou Formation to the east. Paddy allomembers change shape upward from short blunt wedges, through more acutely tapered wedges, to sheets. This change reflects initially rapid flexural subsidence, attributed to active thickening of the adjacent orogenic wedge. A waning rate of deformation permitted wider dispersal of sediment across the basin, driving broad isostatic subsidence beneath increasingly sheet-like rock bodies. A major hiatal surface, VE3, records non-deposition or subtle erosion attributed to erosional unloading and uplift of the adjacent orogen. A subsequent marine transgression is attributed to renewed thickening of the tectonic wedge that triggered deposition of marine mudstone that thickens westward from 0 to >110 m over 300 km. A postulated Milankovitch-band climatic control on both local gravel supply (via fluctuating rainfall), and shoreline movement (via ?Antarctic glacio-eustasy or groundwater storage), might account for cycles of alternating incision and aggradation in the alluvial realm. The same mechanism may also explain why shallow-marine units such as the Cretaceous Viking and Cardium formations contain abundant conglomerate in lowstand shoreface deposits (higher river discharge), yet have highstand shorelines dominated by sandstone (lower river discharge)

A new stratigraphic framework for the Upper Colorado Group (Cretaceous) in southern Alberta and southwestern Saskatchewan, Canada

Extensive marine shales and shallow-marine sandstones of the Cretaceous Upper Colorado Group represent one of the last thick, informally named, stratigraphic intervals in the southern part of the Western Canada Sedimentary Basin. Two regionally-mappable formations in southern Alberta and southwestern Saskatchewan are introduced: The Carlile Formation (Turonian) overlying the Second White Specks Formation, and the Niobrara Formation (Coniacian-Santonian) underlying the Milk River Formation. Both names are extensions of lithologically-similar and laterally-equivalent strata in adjacent parts of the Interior Seaway in Canada and the United States. The boundary between the Carlile and Niobrara formations is recognized at a distinct zone (3 to 15 m thick) of 17 bentonites in the lower part of the Niobrara Formation, whereof two are argon-argon dated to 89.19 (± 0.51) and 89.40 (± 0.31). Regional variations in lithology, petrophysics and geochemistry of the two new formations make it possible to further subdivide these into formal subunits. The Carlile Formation is subdivided into the informal lower, middle and upper units. The Niobrara Formation is formally subdivided into three mappable members in ascending order: the shaly, non-calcareous Verger Member, the sandstone-rich Medicine Hat Member, and the shaly calcareous First White Specks Member. An outcrop reference section of the Carlile Formation is designated at Deer Creek (east of West Butte) in the Sweetgrass Hills of north-central Montana. The single core cut from the Carlile Formation in southern Alberta and southwestern Saskatchewan is located at 13-20-17-7W4 and includes twelve metres of the upper part of the formation. It is used as a reference section for the boundary between the Carlile and Niobrara formations. A reference core of the Niobrara Formation is located at 4-16-22-15W4, which also is designated as the type section of the Verger, Medicine Hat and First White Specks Member. An outcrop section at the Ghost River Dam Spillway, west of Calgary, serves as the outcrop reference section for the Medicine Hat Member and is correlated to the subsurface using wireline logs and foraminiferal biostratigraphy

An outcrop of the Albian viking formation and a southerly extension of the Hulcross/Harmon interval in west-central Alberta

An outcrop of Albian-aged sediments located on Fall Creek, in the Rocky Mountain Foothills, contains an interval of the Viking Formation. The significance of this locality, which was studied by integrating lithostratigraphy and foraminiferal content, is threefold. This outcrop, which can be considered as a reference section, is the first documented occurrence of the Viking Formation. The Viking Formation at this locality is not underlain by the early late Albian Joli Fou Formation, such as in the central plains, but rather by marine sediments of the middle Albian Hulcross Formation/Harmon Member. The distribution of marine sediments associated with the Hulcross Sea are much more extensive than previously documented

Revised stratigraphy of the lower Colorado Group (Albian to Turonian) , western Canada

The sedimentology, biostratigraphy and geochemistry of the Albian to middle(?) Turonian interval of the Colorado Group in Western Canada indicate that four, regionally mappable lithostratigraphic units are present in this marine shale succession. In ascending order, these are: the Westgate Formation, the Fish Scales Formation, the Belle Fourche Formation and the Second White Specks Formation. -from Author

Paleoenvironmental changes in the Cretaceous (Albian to Turonian) Colorado group of western Canada: Microfossil, sedimentological and geochemical evidence

Paleoenvironmental interpretations presented here for a portion of the Cretaceous Colorado Group marine shale succession in western Canada are based on the synthesis of biofacies, sedimentological and geochemical data. Vertical and lateral variations in foraminiferal, coccolith and dinoflagellate assemblages, in sediment fabric, structures and grain size, and in organic matter abundance and composition indicate shale deposition in a dynamic and variable basin setting. The upper Albian to middle Turonian Colorado Group shales were deposited during an overall eustatic sea-level rise punctuated by local, tectonically-induced, relative sea-level drops and variable circulation patterns. The upper Albian Westgate Formation was deposited during the initial stage of Mowry Sea transgression under a dominantly low-salinity, cool, Boreal watermass. Up to three coarsening-up cycles identified within this unit indicate local sea-level fluctuations or changes in sediment supply and/or distributio

Upper Cretaceous Medicine Hat Formation and First White Speckled Shale in southeastern Alberta: Evidence for localized shallow water deposition

Marine sediments of the Medicine Hat Formation and First White Speckled Shale in the subsurface of southeastern Alberta reflect the influence of relative sea-level fluctuations during the Santonian Stage of the Upper Cretaceous in the Western Canadian Sedimentary Basin. A pause, or minor regression, within the overall transgression associated with the Upper Colorado Group resulted in deposition of coarsening-upward Medicine Hat Formation sandstone units as shallow shelf, sand bodies. As the transgression resumed, finer grained marine siltstone and mudstone of the First White Speckled Shale was deposited. Within this unit a previously unnamed sandstone interval (Sweetgrass Member) was identified in part of the study area. Paleontological data show that this sandstone is younger than the Medicine Hat sandstone with which it has been previously correlated. The Sweetgrass Member is a result of a minor shallowing episode during the overall sea-level rise and deep-water