Diagenesis of Devonian Reefal Carbonates in the Oscar Range, Canning Basin, Western Australia

ABSTRACT The Oscar Range, part of the extensive Devonian reef complex of the northern Canning Basin, is a large atoll (80 10 km) in which Frasnian and Famennian (Late Devonian) reefs grew around an emergent island of Precambrian metasedimentary rocks. Many sediments in the Oscar Range had high depositional porosities, but this porosity was largely occluded by diagenetic alteration. Diagenesis included marine cementation, dolomitization, pressure solution, burial cementation, and karstification. Field mapping, petrography, cathodoluminescence, and carbon and oxygen isotope analyses have been used to document six major diagenetic episodes that affected Oscar Range sediments. Earliest cementation occurred in the marine environment, where reef-margin and associated subfacies developed abundant non-luminescing radiaxial fibrous and microcrystalline calcite cements. Based on convergent covariant isotopic trends, marine cements had a characteristic Late Devonian oxygen signature of -4.5 (±0.5) (PDB) and a carbon signature of +2.0 (±0.5) (PDB). In well-cemented parts of the reef complex, synsedimentary neptunian fissures formed and were filled with cement and sediment. Cements in these fissures have isotopic ratios that are essentially identical to the values determined for marine cements, thereby suggesting that the fissures acted as conduits f r seawater flow and cementation early in the history of the reef complexes. Marine diagenesis was followed by marine-burial and possible meteoric-phreatic cementation, and dolomitization. Marine cements are overgrown by non-luminescing scalenohedral cements that grade into banded bright- and non-luminescing blocky calcite cements that are common on the platform but are missing in many reef-margin and marginal-slope deposits. Banded luminescence may represent fluctuating reducing and oxidizing conditions in pore waters across the intermittently emergent platform. The reef complexes, however, were generally drowned reefs with little chance for prolonged meteoric exposure. This sedimentologic evidence suggests that banded-luminescing cements may have formed in the marine-burial environment. The next cement generation consists of non-luminescing, non-ferroan bloc y calcites that occur throughout the reef complex. Stratigraphic relationships combined with isotopic results (18O = -6.0 to -9.0; 13C = +2.0) suggest that this spar is a burial cement resulting from Late Devonian through Early Carboniferous burial. The reef complex was exhumed and eroded in the Late Carboniferous. Dedolomite, dolo-moldic porosity, pisolitic calcrete crusts, iron oxide stained blocky cements, and deep (> 200 m) sinkholes formed at the unconformity surface. Non-ferroan, irregularly banded, moderate- to bright-luminescing blocky calcite precipitated throughout the reef complex. Unconformity-related cements, calcrete crusts, and dedolomites have distinctive isotopic signatures that show little variation in oxygen, -7.0 to -9.0, but have a broad range of carbon values, +1.5 to -8.0. Highly negative carbon values are probably related to the interaction of meteoric fluids with isotopically light soil gas at the er sion surface. In post-Carboniferous time, most of the remaining porosity was filled with non-ferroan, bright-luminescing blocky calcite that formed by pressure solution/reprecipitation during burial. Isotopic analyses (18O = -8.0 to -12.0; 13C = +1.5) support a burial origin for this generation of cements. These cements fill primary porosity, tension gashes along stylolites, and hairline fractures along which hematite is leached. A final generation of ferroan, homogeneous, dull- to non-luminescing blocky calcite cement formed in a late-burial event that, according to isotopic evidence (18O = -10.5 to -15.0; 13C = +2.0 to -9.0), was probably related to movement of basinal brines through the reef complexes.