NEW CONSTRAINTS ON THE AGE OF THE WASATCH MONOCLINE, CENTRAL UTAH

The Wasatch monocline (WM) is a broad, S-shaped fold extending for 89 km through central Utah and forming the western front of the southern Wasatch Plateau. The WM plays a key role in the Mesozoic-Cenozoic tectonic framework of the region; therefore, its timing is of particular interest. Previous workers have suggested the monocline formed at different times, ranging from the Middle Eocene through the Early Miocene or even younger. This project enhances previous research in central Utah by further constraining the timing of WM flexure.

New data obtained to determine the timing of flexure includes paleocurrents, stratigraphic field relationships, and ⁴⁰Ar/³⁹Ar dating. These data bracket the timing of flexure by providing maximum and minimum ages. Paleocurrents were collected from two units of distinct ages (Early Eocene Colton Fm and Middle Eocene Crazy Hollow Fm), and they indicate a general flow pattern from the south-southeast to the north-northwest through the area where the WM presently stands. Evidence of similar paleocurrent patterns suggests that the WM did not affect paleoflow during deposition of either unit and, therefore, was not a feature of high topographic relief at the time of Crazy Hollow deposition.

Spatial and geometric relationships between units near the southern end of the WM suggest the presence of an unconformity between the Crazy Hollow Fm and the Fm of Aurora. Analysis of bedding dips indicates a 15° dip difference between the units. The Aurora progressively thins from west to east, suggesting onlap against the limb of the WM. These relationships indicate that WM flexure had initiated after deposition of the Crazy Hollow but prior to deposition of an ash flow tuff located just above the base of the Aurora in this region. Biotites from this ash flow tuff yielded consistent and concordant ⁴⁰Ar/³⁹Ar dates of 38.0 ± 0.2 Ma from both incremental heating and direct, single-grain laser fusions. Because initiation of flexure is slightly older than the base of the ash flow tuff, which was deposited and truncated against the limb of the WM, this 38 Ma radiometric date gives a minimum age for flexure.