IDENTIFICATION OF HUNTER MOUNTAIN BATHOLITH CLASTS IN THE FURNACE CREEK FORMATION, DEATH VALLEY, CALIFORNIA

The 156-180 Ma Hunter Mountain monzonite-leucomonzogabbro batholith (HMb) in the Cottonwood Mountains, west of Death Valley, is regionally unique for both its age and composition. The sedimentary clasts eroded from this pluton are traceable to this source and form a key aspect of tectonic reconstruction and paleodepositional environments in the central Basin and Range. For example, it was suggested that HMb clasts in the 11-15 Ma Eagle Mountain Formation alluvial fan deposits indicates 80-104 km of the northwest-southeast-directed extension since the Miocene. An alternate interpretation is that the Eagle Mountain Formation is fluvio-lacustrine deposit and does not constrain palinspastic reconstructions. Key aspects in these reconstructions are 1) identification of HMb clasts, 2) determining the depositional environment and 3) age of the deposits. In this study, we examine three deposits of the Furnace Creek Formation containing HMb clasts at Gower Gulch, East Coleman Hills (ECH) and Salt Creek. Geochemical data from all three sites show the clasts are monzonite (54-64% SiO₂). In the lower Furnace Creek Formation near Gower Gulch, rounded monzonite clasts are found in a clast-to-matrix supported, pebble to boulder, rounded to subrounded conglomerate. Zircons from these clasts yielded an age of 177 ± 2 Ma. The upper Furnace Creek Formation in the ECH contains subrounded to angular monzonite boulders (3 m in long dimension) in a matrix-supported conglomerate have an age of 173 ± 5 Ma. At Salt Creek, 3-m-long monzonite boulders are in a matrix-supported conglomerate. Whole-rock K-Ar ages on overlying basalt flows provide minimum ages of 5.8 and 4.6 Ma for Gower Gulch and Salt Creek, respectively. We interpret the Gower Gulch deposits, which are over 40 km from HMb, as fluvial. In contrast, the ECH and Salt Creek deposits, which are 40 km and 20 km, respectively, from HMb, are alluvial fan deposits. At this time, we speculate that these deposits are not equivalent to the 11-15 Ma Eagle Mountain Formation and represent a fluvial connection to the HMb significantly younger than previously known. Detrital sanidine and redating of basalt flows may shed some light on tectonic reconstructions.