Precambrian Cyclic Carbonate Facies, Western Montana: ABSTRACT

Abstract

At least 5 main facies are recognizable in the Helena Formation (of the Precambrian Belt Supergroup) and its equivalents of western Montana: (1) stromatolite-ribbon beds dominate the shallow-water shelf facies; (2) dark-gray to black argillaceous pod carbonate facies, of slightly deeper water origin; (3) green-gray and green argillite-dolomite and tan dolomite facies; (4) dark shale and platy dolomite facies of the central basin; and (5) a more highly clastic facies derived from a probable western source. The Helena facies are markedly cyclic, with individual cycles expressed in several ways depending on horizontal and vertical stratigraphic position in the overall depositional complex of the basin. A characteristic cycle in the shelf belt includes, from base upward, (1) a stromatolitic dark limestone overlying an eroded, scour surface at the top of a dark, brown-weathering massive silty dolomite; (2) a massive bed; (3) a dark argillite or argillaceous pod carbonate thinly bedded unit that may be green gray or green in some cycles; and (4) a black or dark-gray, brown-weathering massive dolomite unit, in places containing scattered ribbon organic structures. The brown-weathering dolomite in almost all places shows a prominent scour surface of v ried relief at the top and overlain by a prominent stromatolitic structure. Reasonable interpretations for each of these rocks and facies can be made to fit the environmental provinces of a normal epicontinental basin. However, a complete analysis of cycle and basin facies genesis must face the question of versus nonmarine origin of Belt sediments. Most, if not all, of the marine sedimentary features of the Helena beds can fit the broad sedimentary patterns of an extensive lacustrine basin. This possibility needs further study, and conceivably could help to explain problems related to Early Cambrian stratigraphy and faunas. End_of_Article - Last_Page 540------------