Origin and modification of Cambrian dolomites (Red Heart Dolomite and Arthur Creek Formation), Georgina Basin, central Australia

Abstract

ABSTRACTThe Early to Middle Cambrian Red Heart Dolomite and lower Arthur Creek Formation of the southern portion of the Georgina Basin, Australia, is an entirely dolomitized succession of shallow‐water evaporitic mudflat and deeper‐water subtidal lithologies. Three types of dolomite have been identified and are interpreted as: (1) syndepositional dolomite; (2) regional replacement dolomite; and (3) void‐filling dolomite (cement). Syndepositional dolomite, derived from saline pore fluids developed in a sabkha environment, is a minor dolomite type with very fine crystal mosaics and has a mottled, non‐zoned cathodoluminescence. The widespread regional replacement dolomite ranges from fine‐ to medium‐crystalline forming mainly planar‐s and non‐planar‐a crystal mosaics, and displays blotchy, mottled, non‐zoned cathodoluminescence. Void‐filling dolomite commonly forms planar‐s to planar‐e, medium to very coarse crystal mosaics. Rare non‐planar‐c, very coarsely crystalline saddle dolomite also exists. Void‐filling dolomite has a successively zoned cathodoluminescence pattern from non‐, to brightly, to dully luminescent.Geochemically, the syndepositional dolomite has δ18O (PDB) values ranging between − 5.3 and − 8.6%o. Regional replacement dolomites exhibit a wide range of δ18O values from − 3.3 to − 10.9%o whereas void‐filling dolomite has δ18O values ranging from − 10.8 to − 14.3%o. All three dolomite types have similar δ13C (PDB) values, in the range between +1.7 and −1.7%o.Three initial dolomitization episodes are interpreted: (1) a sabkha stage, forming the syndepositional dolomite and dolomitizing the evaporitic mudflat lithologies; (2) a brine‐reflux stage, replacing the subtidal lithologies; and (3) a burial stage, forming the void‐filling dolomite type. Final dolomite stabilization occurred during burial, at elevated temperatures, in the presence of basinal fluids, resulting in progressive recrystallization and stabilization of the earlier‐formed syndepositional and replacement dolomites. Both textural and geochemical evolution should be taken into account when studying the origin of dolomites, based on their present geochemical composition.Sulphates are represented by very fine‐crystalline syndepositional anhydrite in association with the syndepositional dolomite, and coarse to very coarse anhydrite cement. Evaportic mudflat (sabkha) and burial environments are inferred for the origin of the former and the latter anhydrite types, respectively. Evaporite dissolution breccias, indicative of the former presence of evaporites, are common throughout the succession.