TECTONIC COMPLICATIONS IN CORRELATING THE SAUK-TIPPECANOE MEGASEQUENCE BOUNDARY BETWEEN WESTERN AND EASTERN LAURENTIA: CLUES FROM CONODONT BIOSTRATIGRAPHY

The unconformity at the top of the Knox Supergroup frequently has been identified as the Sauk–Tippecanoe megasequence boundary in eastern Laurentia, where overlying strata contain conodonts of the Phragmodus polonicus Zone. Coeval strata in west-central Utah contain conodonts of several Histiodella zones, including a new H. labiosa Zone and the rarely preserved H. kristinae Zone, the P. polonicus Zone, and the basal Chazyan Cahabagnathus friendsvillensis Zone. Our conodont collections from Utah document an exceptionally complete stratigraphic succession that lacks any obvious unconformities.

Regressive quartz arenites of the Watson Ranch Formation include interbedded limestones containing mid-Whiterockian conodonts. The overlying Crystal Peak Formation comprises offshore carbonates that record transgression and contain conodonts of the Phragmodus polonicus Zone. Thus, the Watson Ranch Formation is coeval with the upper part of the hiatus recorded by the Knox unconformity in eastern Laurentia, and the Crystal Peak Formation is coeval with strata deposited above that unconformity. The most likely position of the Sauk–Tippecanoe megasequence boundary in western Laurentia is in the overlying Eureka Quartzite.

Maximum regression is expressed in the Eureka Quartzite, the base of which is within the Cahabagnathus friendvillensis Zone near the Utah–Nevada border. There, the Sauk–Tippecanoe megasequence boundary is in strata younger than the interval that has been identified traditionally as containing the boundary in eastern Laurentia. The different ages of what appear to be the terminal-Sauk regression in eastern versus western Laurentia is most likely related to the onset of tectonic activity in eastern Laurentia. Strata above the post-Knox unconformity in eastern Laurentia correlate with the transgressive Crystal Peak Formation in western Laurentia. The subsequent tectonic deepening event along the eastern active margin of Laurentia corresponds to a dramatic shallowing on the western passive margin. Our reinterpretation highlights the complexity of sequence correlation between areas with very different tectonic histories.