CONODONT BIOSTRATIGRAPHY ACROSS A CONFORMABLE SAUK–TIPPECANOE MEGASEQUENCE BOUNDARY, WESTERN CENTRAL UTAH

The Sauk–Tippecanoe Megasequence boundary is a major unconformity in cratonic Laurentia and in the Appalachians. In contrast, the western margin of Laurentia in west-central Utah preserves a thick, continuously deposited record of the Whiterockian Regression at the top of the Sauk Megasequence (Middle Ordovician). Relevant strata include the Lehman Formation (limestone, shale), Watson Ranch Quartzite (sandstone, limestone, shale), and Crystal Peak Dolomite (dolomite, limestone).

Prominent facies changes occur in the Watson Ranch Quartzite, which is all sandstone near Ibex (southern Barn Hills), is sandstone with sandy carbonate and carbonate interbeds 12 miles SW near Crystal Peak (southern Confusion Range), and is limestone with sandstone and shale interbeds 12 miles farther SW at the Desert Range Experiment Station. Equivalent strata are shaly limestone farther west in the southern Snake Range, easternmost Nevada. The final stages of the Whiterockian Regression are preserved in the Watson Ranch Quartzite and record westward progradation of craton-derived sand that intertongued with more offshore carbonates. The overlying Crystal Peak Dolomite records the eastward onlap of basal carbonates of the Tippecanoe Megasequence.

Conodonts are common to very abundant in limestones, dolomites, and sandy carbonates of the Lehman, Watson Ranch, and Crystal Peak formations. Taxa in the lower Lehman Formation are diagnostic of the Histiodella labiosa Zone. Middle to upper Lehman strata and carbonate interbeds in the Watson Ranch Quartzite contain taxa diagnostic of the Histiodella holodentata Zone. All of the Crystal Peak Dolomite near Ibex and in the type section at Crystal Peak plus ~75% of the formation at the Desert Range Experiment Station yielded taxa that we interpret to represent a Phragmodus harrisi Zone. At the Desert Range Experiment Station the highest ~25% of the Crystal Peak Dolomite has taxa diagnostic of the Phragmodus flexuosus Zone.

We place the Sauk–Tippecanoe Megasequence boundary in west-central Utah at the Watson Ranch Quartzite–Crystal Peak Dolomite boundary and at the Histiodella holodentata Zone–Phragmodus harrisi Zone boundary.