Abstract

The Lower Cretaceous Bluesky sandbodies at Edson and Pine Creek trend southwest-northeast, parallel to the barrier and lowstand shoreline sandbodies of the Glauconite Member at Hoadley and Pembina/Bigoray. However, the Edson sandbodies have erosive channellized bases that cut down into brackish water ostracod-bearing shales and sandstones and into the Gething Formation. Six separate incised-valley sandbodies have been mapped at Edson/Pine Creek, based upon the nature of the incised valley fills and the way in which the various valleys cut into each other. Valley fill 1 is the oldest. It is up to 23 m thick and consists mostly of mudstones that have a limited number of small trace fossils, suggesting a stressed environment. There are some sandstones at the base of Valley 1, oriented parallel to the trend of the valley. Valley fills 2 through 5 are believed to be of approximately the same age. Both Valleys 2 and 3 cut into Valley 1, establishing relative ages. The fills of Valleys 2 through 5 are up to 33, 25, 8 and 20 m thick, respectively, and consist of crossbedded sandstones (almost devoid of trace fauna) at the southwestern ends and mudstones with a stressed trace fauna at the northeastern ends; these mudstones backstep on top of the sandstones southwestward. Valley 6 cuts into Valleys 2 through 5, establishing Valley 6 as the youngest. It is filled with up to 15 m of thinly interbedded sandstones and mudstones (with a stressed trace fauna) at the southwestern end, but mudstones are more prevalent at the northeastern end. All six valley fills are truncated by a major bounding discontinuity, overlain by a thin coarse lag, in turn overlain by open marine mudstones of the Wilrich Formation. The combination of deep channelling, sandstones in the southwest and mudstones with a stressed trace fauna in the northeast suggests estuarine depositional environments. The crossbedded sandstones are interpreted to have formed in the proximal (less marine) part of a bay head delta, whereas the interbedded sandstones and mudstones in Valley 6 are interpreted as the deposits of the marine part of the bay head delta. The mudstones are interpreted as central estuary deposits. There is little record of possible marine sandstones from the marine end of the estuary. There is no preserved evidence of tides and the entire depositional system appears to fit closely with a tripartite wave-dominated estuary model. This proposed estuary trends southwest-northeast, implying a shoreline that trends roughly at right angles: north-west-southeast. Such a shoreline would lie at 90 degrees to the trends of the Hoadley and Pembina/Bigoray barrier and shoreline sandbodies, implying a major change in paleogeography between deposition at Pembina/Bigoray and deposition at Edson. The interpretation of Edson/Pine Creek as a set of six nested estuarine channels also influences interpretations of the paleogeography north of Pine Creek.

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