ABSTRACT

Most Precambrian dolostones are considered here to represent replacement of original limestone or calcareous sediment, and hence such dolostone sequences can be interpreted in much the same way as Phanerozoic dolostone sequences.

The sequential development of dolomite is recorded in a middle Precambrian carbonate-clastic platform succession in the Belcher islands. The timing of dolomite formation ranges from penecontemporaneous (eogenetic) to late burial (mesogenetic). The platform sequence is divided into three distinctive megacycles: 1) a siliciclastic-dominated phase, wherein pisolitic tufas formed in the upper reaches of a supratidal flat--incipient penecontemporaneous dolomite formed in a setting somewhat analagous to the modern Coorong alkaline lakes; II) a mixed clastic-carbonate and stromatolitic carbonate phase, where penecontemporaneous dolomite formed under conditions of high evaporation rates and high salinity, and is comparable to recent dolomite in sabkha environments; and III) a stromatolite bui dup--slope-basin phase where incipient dolomitization may have taken place within a mixed seawater-freshwater aquifer system.

However, pervasive dolomitization of the platform sequence did not occur until a later stage of burial diagenesis, and postdated the main stage of silica diagenesis. Recrystallization was a late mesogenetic event that resulted in only minor crystal enlargement; stylolitization also produced local crystal growth but was not responsible for pervasive dolomitization. Fracture-filling dolomite accumulated as a result of deformation and uplift during the Hudsonian Orogeny (telogenesis).