SAUK "SEQUENCE" AND THE KNOX UNCONFORMITIES: PRACTICAL BASE OF THE LAURENTIAN UPPER ORDOVICIAN

A number of caveats are appropriate to application of the Sauk “Sequence” and its upper boundary, the Knox “unconformity,” to geological syntheses and in teaching historical geology. These caveats are evident in a consideration of the magnitude of onlap–offlap events, their timing, and the role of eustatic and epeirogenic activity in developing the Early Paleozoic sequence stratigraphy of Laurentia. Across the Laurentian platform, the Sauk “Sequence” consists of a number of successive depositional sequences. For example, four depositional sequences comprise the traditionally “Lower” Ordovician “Beekmantown Group” in east New York and adjacent Vermont, and these intervals of eustatic highs are represented in east Laurentian continental slope facies of the Taconic allochthons by dysoxic black shales. The “lower” Knox unconformity separates Providence Island Formation peritidal carbonates of the upper “Beekmantown” from basal sandstones of the carbonate-dominated Chazy Group, the lowest unit of Sloss’ Tippecanoe Sequence. Recently recovered Providence Island conodont faunas are upper Middle Ordovician (upper Darriwilian), while international ratification of the Upper Ordovician base at the Nemagraptus gracilus Zone base places most of the Chazy in the Upper Ordovician. Thus, the Sauk–Tippecanoe contact, or “lower” Knox unconformity, in easternmost Laurentia, as well as the Great Basin, approximates the Middle–Upper, not the Lower–Middle, Ordovician boundary. Chazy onlap correlates with dysoxic black shale in the Taconic allochthons and onlap of the Athens Shale in the southern Appalachians. Additional Upper Ordovician Knox unconformities are present in eastern Laurentian. The “middle” Knox is recorded by a local Chazy–Black River Group unconformity and thin overlying quartz sandstone during the generalized onlap of Black River Group carbonates. The dramatic “upper” Knox is tectonic in origin and reflects uplift of eastern Laurentia, down-cutting as low as the Proterozoic basement, and subsequent deposition of Trenton Group carbonates or dysoxic black shales with passage of the Taconic peripheral bulge.