A paleohydrologic model for early Proterozoic dolomitization and silicification

Abstract

Primary depositional features observed in the intertidal and supratidal facies of the 2.25 Ga Malmani Dolomite (including tepees and finely laminated domal stromatolites) have equivalents in Holocene salinas on the southern coast of Australia. The hydrological regime, so important in the formation of these recent primary features, is still active and so it has been possible to interpret the palaeohydrological significance of analogous features in the Malmani Dolomite. The palaeoclimate of the Malmani Dolomite shoreline was probably humid to semi-arid and the supratidal zone is interpreted as a zone of groundwater resurgence. Tepees, indurated fenestral limestone sheets, algal mats, small domal stromatolites and intraclast breccias were forming on the supratidal flats in areas of periodically outcropping groundwaters. In the shallow subsurface, the mixing zone between these resurging continental groundwaters and the marine-derived phreatic waters was probably a zone of secondary coarse-grained dolomite precipitation and/or chert nodule formation. This groundwater-controlled mixing zone model explains why such fabrics are restricted to porous and immediately adjacent units which, at the time of diagenesis, were below the regional water table. In the upper supratidal zones, above the regional water table, no secondary dolomites or chert nodules could form, nor could diagenetic fabrics form in the subtidal zone for there the stromatolitic mounds contained no palaeoaquifer.