Contrasting Dolomite Textures of Miocene Carbonate Platforms in Central Luconia, Sarawak, Malaysia

Dolomite is common in the Miocene carbonate platforms of Central Luconia, Sarawak but so far few studies have addressed the dolomitization processes in these isolated platforms. This paper presents the results of an investigation of dolomite horizons and the origin of the dolomite in two Miocene platforms of Central Luconia. 65 thin sections and core plugs were selected from two carbonate platforms, located in the southern and northern part of the province. Polarized light and cathodoluminescence petrographic analyses, SEM investigations, stable isotope and elemental composition analyses were carried out to reconstruct the succession of diagenetic events. Two distinct dolomite textures characterize the two platforms, namely mimetic replacement fabricpreserving) in the northern platform, and fabric-destroying with sucrosic dolomite texture in the southern platform. Dolomite crystal size ranges from < 10 μm to 100 μm in both cases. There are also indications of overdolomitization, mainly in pore-lining and pore-filling dolomite cement. In cathodo-luminescence, all dolomites are generally dull-red to extinct, although a few samples from the northern platform show occurrences of bright luminescence in the outer dolomitic cement rim. Both platforms have undergone diagenesis associated with the mixing zone. Dolomoldic porosity and geopetal structures suggestive of subaerial exposure and karstification, are mostly developed in the northern platform, whereas intercrystalline porosity occurs together with dolomoldic porosity in the southern platform. Dolomite seems to form as an early replacive phase. Late sparry calcite occurs in the northern platform and poikilitic calcite in the southern platform, suggesting (shallow?) burial diagenesis. Other types of cement indicative of early marine diagenesis, freshwater phreatic to burial diagenetic realm are also present (e.g.: dog tooth cement, isopachous cement, micritic cement, syntaxial overgrowth and drusy calcite spar). Stable isotope compositions suggest that the dolomite could have formed from slightly depleted seawater or fluids that have been diluted by meteoric water in both platforms. The δ18O values in the northern platform average -3.06 ‰ (V-PDB) compared to -2.71‰ in the southern platform. Calcite cement in both<br>platforms are more depleted in δ18O (-7.12 ‰ in the north and -6.37 ‰ in the south), probably due to a high intensity of meteoric water penetration in a late stage of cementation during a period of subaerial exposure. δ13C values of dolomite showed very little influence of carbon derived from soils during exposure.