Classification and Environmental Interpretation of the Cryptalgal Organosedimentary "Molar-Tooth" Structure from the Late Precambrian Belt-Purcell Supergroup

Abstract

The late Precambrian "molar-tooth" structure of the Belt-Purcell Supergroup of western North America is subdivided on the basis of geometry into vertical and horizontal structures. Subtypes of the vertical structures include ribbons and blobs, whereas the horizontal structures occur as mats, lenses, and pods. On the basis of (1) transitional relationships among vertical structures, horizontal structures, and algal stromatolites; (2) stratigraphic relationships of the structures within cyclical sedimentary sequences; and (3) published data, the vertical and horizontal structures are interpreted as cryptalgal organosedimentary structures. Like the algal stromatolites, their geometry is considered to be a function of prevailing environmental factors. Facies interpretation of the major rock types suggests that all of the structures formed in very shallow to intertidal environments, the ribbons being characterized by moderate wave or current action and rapid deposition; the blobs, by low wave or current action and slow deposition; and the horizontal structures, by high wave or current action and slow deposition.