103 Sediments, Carbonates

Monday, 8 November 2004
1:30 PM - 5:30 PM

STROMATOLITES AND MICROBIALITES IN SUBTIDAL CARBONATES OF THE NEOARCHEAN CARAWINE FORMATION, HAMERSLEY GROUP, WESTERN AUSTRALIA

Paper number: 103-6
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

The Neoarchean Carawine Formation, Hamersley Group, Western Australia, and the contemporaneous Monteville and Reivilo formations, Transvaal Supergroup, South Africa, represent the development of 2.6 Ga carbonate platforms on early continental crust. The distribution of facies within the platforms reflects carbonate production relative to evolving depositional environments. Three previously undescribed facies in the Carawine Formation include shallow subtidal bioherms, below wave-base fenestrate microbialites, and subtidal, flat-laminated mats with dewatering structures. The bioherm facies is subdivided into bedded and unbedded bioherms. Bedded bioherms contain uniform stromatolites distributed in individual beds; bioherms are separated by wavy and planar laminated grainstones, sometimes with dewatering structures. Irregularly distributed conical and domal stromatolites of various sizes form the unbedded bioherm subfacies. These bioherms are separated by planar laminated, current and wave ripple cross laminated, and wavy laminated grainstones. The bioherm facies is comparable to bioherm-grainstone facies in many Proterozoic carbonate platforms, but has not been observed in the Monteville and Reivilo formations.

The fenestrate microbialite facies contains diverse fenestrate microbialites composed of vertically oriented support structures, draping and laminated mats, and roll-up structures; this facies also includes planar laminae with interbeds of roll-ups structures. This facies is similar to slope facies of the Reivilo Formation. In the Carawine Formation, pseudomorphed aragonite fans are present within some planar laminated zones, suggesting the water depth was shallow enough for abundant aragonite precipitation.

Laminated mat with large-scale dewatering structures forms a subtidal facies that also includes possible cuspate fenestrate microbialites. This facies lies within the same fault block as the fenestrate microbialite facies and may be depositionally associated with it. Bedded bioherms overlie the dewatering facies in some areas suggesting that they represent neighboring depositional environments, with fenestrate microbialite and laminated mat facies representing deeper water environments.