Sedimentology and diagenesis of Upper Devonian carbonates, Canning Basin, Western Australia

Two Upper Devonian carbonate sequences in the Canning Basin were investigated: a) an outcropping sequence in the Geikie Gorge region on the Lennard Shelf; and b) a subsurface sequence on the Barbwire Terrace. The outcropping Frasnian-Famennian carbonates of the Geikie Gorge region form part of the Devonian \Great Barrier Reef\" of the Canning Basin and consist of a series of platforms and atolls which fringe the basin margin. The platforms have steep reef-fringed margins and the framework lithologies consist predominantly of cyanobacterial framestones. A widespread deeper water stromatolite unit stromatolite unit straddles the Frasnian-Famennian boundary and appears to represent a major halt in reef growth. Stromatactis structures are most common in the reefal-slope subfacies and appear to have formed by soft-body collapse in combination with later internal erosion and sedimentation. The carbonates of the Geikie Gorge region have suffered a long and complex diagenetic history which began in Devonian sea water with extensive marine cementation of platform-margin lithologies. Devonian - Lower Carboniferous burial diagenesis was the most important episode of diagenesis as almost all primary porosity was occluded by calcite cementation (non-luminescent to bright-luminescent to dullluminescent) during this interval. Dolomitization and consequent secondary porosity development also occurred during this interval. Some karstification and calcite cementation took place during Late Carboniferous subaerial exposure. Minor calcite cementation occurred during Permian - Cenozoic burial. Karstification dedolomitization and calcite recrystallization took place during Cenozoic meteoric diagenesis. Recrystallized sparry calcites commonly display cathodoluminescent crystal growth zonations which mimic cementation fabrics. The Barbwire Terrace carbonates consist predominantly of low-energy peritidal and shallow-subtidal lithologies in which most of the clay-free lithologies have been thoroughly dolomitized. The peritidal lithologies resemble those found in the modem arid tidal flats of the Persian Gulf. Cyclic sequences are not well developed in the peritidal lithologies. No true reef-framework lithologies were recognized in the sequence. The dominant synsedimentary diagenetic process was supratidal dolomitization and associated anhydrite precipitation. The `˜í¬•^18O` value for Devonian marine dolomite `{˜í¬•^18O = ~2 ‚ÄövÑ‚àû (PDB)}` was estimated by assuming that the lowest `˜í¬•^18O` values for the supratidal dolomite represent replacement in marine waters. The most important diagenetic process affecting the carbonates was regional dolomitization and this occurred during Devonian - Lower Carboniferous burial diagenesis. Regional dolomitization was not restricted to the peritidal lithologies and thick sequences (up to 600 m) of dolomite with no primary calcite are common. The regionru dolomites have lower `˜í¬•^18O`values than the supratidal synsedimentary dolomite and saddle dolomite cements are common suggesting elevated temperatures of precipitation. Karstification and calcite cementation took place in the dolomites during Late Carboniferous subaerial exposure. Poilcilitic calcite cements were precipitated during Permian - Cenozoic burial diagenesis."