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Facies Control on Dolomitisation Within the Neogene Succession of Bonaire, Netherlands Antilles

Abstract

Despite their relative short geological history, the Neogene carbonates of Bonaire record a sequence of important diagenetic events that have overprinted the depositional texture prior to burial. A limited number of Neogene sections have previously been described on the island, and small-scale dolomite bodies identified within the Mio-Pliocene. Recently, field mapping over a large part of the island, combined with digital outcrop models, has revealed dolomitisation is considerably more extensive than previously recognized. The Neogene succession on Bonaire comprises a sequence of units deposited in shallow water on a complex volcanic basement. This has generated a series of four flat-topped terraces up to 70 m elevation which are bounded by steep cliffs. Rather than representing individual episodes of deposition, as previously assumed, these terraces comprise two or more depositional units, reflecting the interaction of different controls. The cliffs provide extensive sections of the Seroe Domi Formation, a Mio-Pliocene bioclastic unit overprinted by intense dolomitisation that has been previously thought on Bonaire to be restricted to inclined beds on the west part of the island. Our cliff sections reveal dolomitisation of reefal and shallow-marine deposits to be widespread, predominantly/preferentially affecting red algae facies. The mostly non-stoichiometric calcian dolomite occurs with sucrosic crystals between 50 to 100 µm with cloudy centers and occasional zoned planar –e textures. Stable isotopes composition of the completely dolomitized red algae facies show a positive to very positive values which varies between +1‰ to +4 ‰ in δ13C and +1.5 ‰ to +5 ‰ δ18O. On the other hand, reefal facies on upper units present a high contrast showing very negative values in carbon and oxygen. Those deposits show a clear contrast with the oldest Miocene sequence located in Goto Meer area North West of the island. Detailed characterization of our type outcrop at Seru Grandi, in the Northern part of the island, shows systematic variations in dolomite abundance. Well-defined tongues of dolomite extend along clinoform surfaces beneath a sequence boundary and overlain by limestone. Such partially dolomitized sequences of early dolomites provide opportunities to unravel the role of individual intrinsic (sedimentary texture and mineralogy) and extrinsic (hydrology and fluid geochemistry) controls on the evolution of dolomite fabrics and porosity prior to burial.