Stratigraphic Implications of Lower Ordovician Conodonts from the Munising and Au Train Formations at Pictured Rocks National Lakeshore, Upper Peninsula of Michigan

Abstract

Early Ordovician conodonts have been recovered from the upper Munising Formation and the lower part of the Au Train Formation at Pictured Rock National Lakeshore in the Upper Peninsula of Michigan. Two trilobites from the lower Au Train probably represent the Symphysurina Zone ( S. woosteri Subzone) and confirm the Early Ordovician age of the Au Train. The presence of middle Late Cambrian (Franconian) trilobites in the underlying Munising Formation led to traditional assumption of a major hiatus between the Munising and the Au Train. An unconformity between these formations has been difficult to recognize in outcrops because upper Munising sandstones grade upward into weathered, reworked beach sands deposited during Ordovician transgression. The reworked sands traditionally have been assigned to the Munising, with the base of the Au Train placed at the lowest occurrence of carbonate rocks in the sequence. Conodonts from the uppermost Munising and lower Au Train are characteristic of the Rossodus manitouensis Zone (upper Skullrockian Stage, Millardan Series, Lower Ordovician). The conodont and trilobite data permit correlation of the rocks of this interval with sequence stratigraphic units in the Burnout Canyon Member of the House Limestone in the Ibex area of Utah and with equivalent strata in other parts of Laurentia.