Duncan Mackay1 and Dr. Robert W. Dalrymple2
1 Shell Canada, Calgary, AB
2 Queen's University, Kingston, ON
In the Peace River area of Alberta, tide-dominated deltaic and tide-dominated estuarine facies of the Gething and Bluesky formations (Lower Cretaceous) comprise a sand-dominated, heterolithic succession that reaches 30-40 m thick. Deltaic environments observed include: delta-plain tidal-channel bars, interdistributary-bay fills, delta-front tidal bars and prodelta. Estuarine environments observed include: middle and outer estuary tidal-channel bars, outer estuary sand flats, and outer estuary tidal bars.
Depositional processes and sedimentary facies in tide-dominated deltas and estuaries are remarkably similar. Tidally deposited sedimentary features common to both in the Peace River deposit include: mud-couplets, bi-directional cross-strata, repetitive patterns of heterolithic lamination and bedding, and ubiquitous brackish-water trace-fossil assemblages. Despite the similarities in sedimentary facies between tide-dominated deltaic and estuarine systems, differences in longer-term processes – mainly regressive vs. transgressive conditions – result in significantly different facies associations and stratigraphic architecture.
The tide-dominated estuary deposits show coarsening and cleaning in a palaeo-seaward direction due to the combination of efficient trapping of mud in landward areas and inefficient landward transport of coarser sand during transgression. Conversely, the efficient seaward transport of mud and the inefficient export of coarser fractions results in deltaic deposits that fine and become progressively muddier towards their seaward terminus. In vertical profiles, successions in both systems show cleaning and coarsening-upward. Fining-upward is locally observed near the base of the tide-dominated estuarine succession, where tidal channels scoured and incorporated coarser material from older deposits. Typically estuarine successions are much cleaner and have better sorting than deltaic successions.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005