SEQUENCE STRATIGRAPHY NEAR THE BASE OF THE SAUK SEQUENCE: THE MONTENEGRO MEMBER OF THE CAMPITO FORMATION (LOWER CAMBRIAN) IN THE WESTERN GREAT BASIN

At the beginning of the Sauk Sequence in the early Cambrian, the rifted western margin of Laurentia was the site of an extensive wedge of siliciclastics. The deeper-water, upper part of this wedge has yielded the oldest trilobites and brachiopods in the region. This sedimentary ramp ended with the development of a carbonate bank with archaeocyathid-bearing reefs. The sequence stratigraphy of these siliciclastics provides an insight into the environment of these biologic innovations. The Andrews Mountain Member (CAMM) of the Campito Formation in the White-Inyo, CA-Esmeralda County, NV area is at least 800 m of medium-bedded very fine grained quartzite overlain by dark-colored siltstone of the Montenegro Member (CMM) that increases from about 300 to over 600 m in thickness from south to north. The Sauk Sequence begins with two or more sequences in the CAMM which are relatively deep and dysoxic, generally lacking ichnofossils. Two sequences at the top of CAMM are shallower with a diversity of ichnofossils and, in the upper one, brachiopods and trilobites appear. Small carbonate-bearing pods mark transgressive parasequence boundaries. The lowest sequence in the CMM is dark siltstone and minor sandstone, devoid of fossils for a 40-m interval then trilobites reappear in the transgressive systems tract. The next sequence boundary in the CMM occurs at the base of a shallow, high-energy unit with bioclastic carbonate pods, which is widespread in western North America. The succeeding maximum flooding surface is near the base of a long interval of dark-colored shaly siltstone. Above another sequence boundary, the Poleta Formation begins with ribbon limestone shallowing upward to a variety of carbonate environments which can be recognized along most of the western margin of Laurentia.

This early part of the Sauk Sequence is well developed in a limited basin with individual sequences progressively deeper until the middle of the Montezuman Stage. The major incursion on the Laurentian craton did not occur until late in the Dyeran Stage (Lower Cambrian).