SEDIMENTOLOGY OF TWO MUD MOUND REEFS ON AN UPPER DEVONIAN (FRASNIAN) RAMP, WESTERN ALBERTA, CANADA

Two Upper Devonian (Frasnian) mud mounds in the Mount Hawk Formation crop out side-by-side in the southern Rocky Mountains of west-central Alberta. They grew in a shallow ramp setting seaward of a large carbonate platform to the east. The southern mud-mound is 46.2 m thick and 38.6 m wide at the base, whereas the northern one is 53.3 m thick and 72.2 m wide at the base. Five microfacies are discriminated: (1) bioclastic wackestone, (2) bioclastic wackestone with dense and dark micrite/peloidal micrite matrix, (3) sponge spicular network limestone, (4) stromatolitic limestone, and (5) laminated limestone. Spicules, brachiopods, ostracodes, crinoid ossicles, echinoid spines, foraminifers (Tikhinella, Nanicella, Bisphaera, Evlania), tabulate corals (Thamnopora, Sinopora?), calcimicrobes (Girvanella, Rothpletzella), tentaculites, gastropods, and bryozoans are common. Stromatoporoids (Stachyodes), rugose corals (disphyllids), and calcispheres are also present. Various mechanisms were involved in the formation of the micritic matrix, including cyanobacterial photosynthesis and microbioerosion, bacterial metabolism, and organomineralization of sponges. Microbial components and sponge-related organomineralization and presence of all mound biota in underlying strata suggest a gradual mound initiation instead of a sudden environmental change, and therefore a protracted period of potential mound development waiting for the exact conditions to trigger it. A number of factors have been proposed as potential controllers of mound growth, but only the ramp setting, antecedent sea floor topography and relative sea level probably made a contribution here. Although a few stromatactis cavities are likely related to collapse of uncemented material in early-cementing sponges, most of stromatactis cavities are likely due to excavation of loose sediments because: (1) there are few skeletal remains with original spicule configurations; (2) common appearance of large fossils like erect brachiopod shells inside; (3) distribution of detrital sediments around the framework created by early lithified microfacies; and (4) laminae of calcified cyanobacterial filaments are present on the roofs of some stromatactis.