Coastal Lithofacies and Biofacies Associated with Syndepositional Dolomitization and Silicification (Draken Formation, Upper Riphean, Svalbard)

Abstract

The Draken Formation (120-250 m) of northeast Spitsbergen (Svalbard) forms part of a thick Upper Proterozoic carbonate platform succession. It consists predominantly of intraformational dolomitic conglomerates, with excellent textural preservation. Six main lithofacies were recognized in the field: quartz sandstones, stromatolitic mats, conglomerates with silicified intraclasts, dolostone conglomerates with desiccated mudrocks, oolitic/pisolitic grainstones and fenestral dolostones. A series of five main gradational biofacies were recognized from silicified (and rare calcified) microfossils. Biofacies 1 represents low-energy subtidal benthos (erect filaments) and plankton (acritarchs and vase-shaped microfossils) whereas biofacies 2 to 5 are microbial mat assemblages (with filamentous mat-builders, and associated dwellers and washed-in plankton) ranging from basal intertidal to high intertidal/supratidal. Colour values (a measure of the lightness of the colour shade) of sawn rock samples were quantified using a Munsell chart, and exhibit a pronounced variation (means of major groups varying from 4.0 to 5.95) across the spectrum of subtidal to supratidal sediments as inferred from other criteria. The lightening in progressively more exposed sediments is related to lowering of organic carbon contents, probably mainly by oxidation. Six types of early cement have been recognized. Calcite microspar (type 1) is common as a subtidal cement in many Proterozoic formations, whereas types 2 (subtidal isopachous fringes), 3 (subtidal hardground dolomicrite) and 4 (intertidal meniscus dolomicrite) are very similar to Phanerozoic examples except for their dolomitic mineralogy. Types 5 and 6 are complex and variable dolomite growths associated with expansion and replacive phenomena. They characterize the fenestral lithofacies and compare with modern supratidal cements. Consideration of diagenetic fabrics and truncation textures of intraclasts indicates that leaching, dolomitization, silicification were all significant syndepositional processes altering the original metastable carbonates. The data set provides evidence for a spectrum of peritidal environments including ooid shoals, protected subtidal, tidal sandflats and protected carbonate mudflats. Different sections show a preponderance of particular facies. The coastal lithofacies continuum was completely dolomitized, unlike offshore to ooid shoal facies of adjacent formations. Dolomitization thus bears a relationship to depositional bathymetry. Although hydrodynamics clearly have a role, the potential importance of whiting precipitation in raising Mg/Ca in marginal marine environments is also stressed.