SAUK SEQUENCE SEQUENCES (SOUTHERN GREAT BASIN)

Historically, the off-craton stratigraphic interval of Sloss´ (1963) Sauk sequence has been subdivided into second-order sequences (Sauk I, Sauk II, etc.) using the concept of biomeres. In our opinion, however, such sequences should be defined on physical evidence in a sequence-stratigraphic framework. Biostratigraphy should guide the sequence boundary correlations, but should not be the primary defining criterion.

In the southern Great Basin, we use major surfaces across which there are indications of abrupt sedimentological change that record the major accommodation and base-level changes manifested in platform-wide exposure and subsequent flooding. Here the off-craton expression of the Sauk Sequence commences with deposition of the Lower Cambrian middle member Wood Canyon Formation across a regional disconformity (base Sauk I) and terminates at the regional unconformity at the top of the Middle Ordovician (Mohawkian) Eureka Quartzite (top Sauk IV). Both surfaces reflect the times of maximum freeboard on the Cambro-Ordovician Laurentian craton.

Internally, the end of Sauk I is the regional disconformity at the top of the Zabriskie Quartzite, and Sauk II begins with the major marine flooding event represented by the Lower Cambrian Carrara Formation. The top of Sauk II is the regional disconformity at the top of the early Late Cambrian Bonanza King Formation, and Sauk III commences with the Late Cambrian flooding event expressed by the Dunderberg Shale. The top of Sauk III is the regional disconformity at the top of the Lower Ordovician (Ibexian) Goodwin Formation, Pogonip Group, and Sauk IV commences with the regional flooding event recorded by the Ninemile Shale.

This subdivision provides a model for comparison with other platform-wide transects whereby the anatomy of the Sauk Sequence in the off-craton domain can be more fully assessed.