ESTUARINE VALLEY FILLS IN THE LOWER CRETACEOUS BLUESKY FORMATION, EDSON AREA, ALBERTA

The Lower Cretaceous Bluesky sandbodies at Edson and Pine Creek trend southwest-northeast, parallel to the barrier and lowstand shoreline sa ndbodies of the Glauconite Member at Hoadley and Pembina/Bigoray. Howe ver, the Edson sandbodies have erosive channellized bases that cut dow n into brackish water ostracod-bearing shales and sandstones and into the Gething Formation. Six separate incised-valley sandbodies have bee n mapped at Edson/Pine Creek, based upon the nature of the incised val ley 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 mos tly of mudstones that have a limited number of small trace fossils, su ggesting a stressed environment. There are some sandstones at the base of Valley 1, oriented parallel to the trend of the valley. Valley fil ls 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 fil ls of Valleys 2 through 5 are up to 33, 25, 8 and 20 m thick, respecti vely, and consist of crossbedded sandstones (almost devoid of trace fa una) at the southwestern ends and mudstones with a stressed trace faun a at the northeastern ends; these mudstones backstep on top of the san dstones southwestward. Valley 6 cuts into Valleys 2 through 5, establi shing 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 northea stern end. All six valley fills are truncated by a major bounding disc ontinuity, overlain by a thin coarse lag, in turn overlain by open mar ine mudstones of the Wilrich Formation. The combination of deep channe lling, 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 proxi mal (less marine) part of a bay head delta, whereas the interbedded sa ndstones and mudstones in Valley 6 are interpreted as the deposits of the marine part of the bay head delta. The mudstones are interpreted a s central estuary deposits. There is little record of possible marine sandstones from the marine end of the estuary. There is no preserved e vidence of tides and the entire depositional system appears to fit clo sely with a tripartite wave-dominated estuary model. This proposed est uary trends southwest-northeast, implying a shoreline that trends roug hly at right angles: northwest-southeast. Such a shoreline would lie a t 90 degrees to the trends of the Hoadley and Pembina/Bigoray barrier and shoreline sandbodies, implying a major change in paleogeography be tween deposition at Pembina/Bigoray and deposition at Edson. The inter pretation of Edson/Pine Creek as a set of six nested estuarine channel s also influences interpretations of the paleogeography north of Pine Creek.