Pedogenic-phreatic carbonates on a Middle Devonian (Givetian) terrigenous alluvial-deltaic plain, Gilwood Member (Watt Mountain Formation), northcentral Alberta, Canada

In the Muskeg Trough of northcentral Alberta the Gilwood Member contains wi despread carbonate deposits that formed within terrigenous mudstone and san dstone hosts. Stratigraphic, depositional and petrographic relationships in dicate that these carbonates represent calcretes and dolocretes. Calcretes, observed best with cathodoluminescence, display microcrystalline alpha fab rics, circumgranular cracks, root networks, displacive growth fabrics, elon gate channel voids and rare coloform growths with flower spar. Similarly, d olocretes have microcrystalline alpha fabrics, brecciation, gradational con tacts with host mudstones, extensive layered nodular horizons and are assoc iated with anhydrite and pyrite. delta(13)C values range between -7 parts p er thousand to +1 parts per thousand and -6 parts per thousand to +3 parts per thousand for calcretes and dolocretes, respectively. Oxygen isotopes ar e more variable and differ with host lithologies. delta(18)O of calcretes r anges between -11 parts per thousand to -8 parts per thousand for sandstone s and -8 parts per thousand to -3 parts per thousand for mudstones, whereas delta(18)O of dolocretes ranges between -3 parts per thousand to 1 parts p er thousand for marine mudstones and -6 parts per thousand to -2 parts per thousand for pedogenic mudstones. Regional mapping indicates that calcretes thicken towards the deepest parts of the Muskeg Trough. Widespread dolocre tes extend beyond the eastern and western limits of Muskeg Trough and are u seful marker intervals for regional correlations. Dolocretes of restricted lateral extent are found within gleyed palaeosol mudstones next to calcreti zed channel sandstones. Calcrete isotopic values are interpreted as indicative of carbonate precipi tation from waters with meteoric water input. However, the higher delta(18) O values in dolocretes are indicative of a contribution from an isotopicall y heavier source such as seawater. Stratigraphically, calcretes are most co mmon along the western and northern edges of Muskeg Trough; thus, calcrete accumulation was further controlled by meteoric water in-flow from the high land to the west and sluggish groundwater flow in Muskeg Trough. In contras t, regionally widespread dolocrete horizons appear to have formed from mixi ng of fresh waters derived from the highland to the west and seawaters intr oduced from the east. Regionally restricted dolocretes which are found next to channel sandstones formed from groundwater out-flow from the permeable channel sandstones which resulted in calcretization in channel proximal mud stones and dolomitization in channel distal mudstones.