Oolitic/pisolitic dolostones from the Late Precambrian of south Spitsbergen: their sedimentary environment and diagenesis

Abstract

The oolitic/pisolitic dolostones of the Late Precambrian Dunoyane Member (Höferpynten Dolomite Formation) in south Spitsbergen reveal a set of features attributable to a physico-chemical accretion of the ooids/pisoids, the enormous size of which (attaining 9.7 mm in diameter for simple ooids, and up to 15 mm for complex ooids) is assumed to have resulted from a profuse ,carbonate precipitation, possibly of primary dolomite. Under such conditions, the accretionary bodies were prevented more effectively from abrasion, and were kept in suspension longer than in present-day calcium-carbonate oolitic environments. The environmental model of the Dunoyane oolites is reconstructed as comprising a pattern of facies from the shallow subtidal through the supratidal zones, the latter being evidenced by the formation of half-moon ooids that owed their structure to a dissolution of some envelopes, composed of soluble salts, presumably calcium sulphates (anhydrite or gypsum). The final sedimentation occurred in a subtidal zone, to which all the recognized inter- or supratidal deposits underwent redeposition. This facies pattern is compared to the present-day Bahamian oolite facies, and a conclusion as to the composition of its Precambrian equivalents is presented. The study of diagenetic deformations in the Dunoyane ooids shows them as being mostly confined to pressure solution (pitted, cracked, snouted, and distorted ooids). The commonly known distorted ooids appear to result from subsequent deformations of the pressure-solution contacts in ooids if the latter were pinch-and-swelled and contorted under conditions typical of sedimentary boudinage.