NEW ISOTOPIC AGES FROM THE EARLY AND MIDDLE JURASSIC OF UTAH: RESOLVING THE AGE OF THE GYPSUM SPRING AND TEMPLE CAP FORMATIONS

New isotopic ages from the Early and Middle Jurassic of Utah have been obtained using both ⁴⁰Ar/³⁹Ar and U-Pb systems. The most surprising results are ages from the westernmost exposures of the Gypsum Spring Member of the Twin Creek Limestone and from the Temple Cap Formation. Near Croydon at Devil’s Slide in northern Utah we have obtained an ⁴⁰Ar/³⁹Ar age on sanidine of 184.6 ± 0.2 Ma (1σ) and a U-Pb zircon age of 184.3 ± 2.3 Ma from an ash bed in the Gypsum Spring Member that is about 40 meters above the contact with the Nugget Sandstone. This age is the first published isotopic age on the Gypsum Spring and suggests that it is, at least in part, Early Jurassic (Toarcian/Pliensbachian) in age. The overlying Sliderock Member of the Twin Creek has produced ages ranging between 168-172 Ma (Bajocian), which agree with the age of fossils from this member. The age gap between the Sliderock and the Gypsum Spring Members appears to be substantial (probably 10+ Ma) and argues for removal of the Gypsum Spring Member from the Twin Creek Limestone and elevation of this member to formation status.

In southern Utah ash beds from the westernmost exposures of the Temple Cap Formation have been dated from the Pine Valley Mountains. An ash low in the formation produced ⁴⁰Ar/³⁹Ar ages of 176.7 ± 0.6 Ma on biotite and 178.9 ± 0.8 Ma on sanidine. From the same locality a second ash bed higher in the formation gave an ⁴⁰Ar/³⁹Ar age of 171.8 ± 1.4 Ma on biotite and a U-Pb age of 173.2 ± 2.5 Ma on zircon. These ages place at least the lower part of the Temple Cap Formation into the Early Jurassic (Toarcian) and give it an age range of about 171-179 Ma.