GEOLOGIC AND GEOCHRONOLOGIC RECONNAISSANCE OF THE SOUTHEAST OWLSHEAD MOUNTAINS IN SOUTHERN DEATH VALLEY, CALIFORNIA

We present field and geochronological data for the southeastern Owlshead Mountains located at the confluence of the active Garlock and southern Death Valley fault systems, California. Mapping covers about 16 km², provides regional correlation of basement rocks, and reveals a new stratigraphic section for the area. We dated three volcanic and one plutonic rock by laser ablation ICPMS geochronology for ²⁰⁶Pb/²³⁸U zircon ages.

Older basement rocks are massive white to light gray marble occurring as pendants and screens with steeply dipping contacts within granitic plutonic rocks. The marbles are correlated to parts of the Mesoproterozoic Crystal Spring Formation. Intruding granitic rocks are mostly alkali-feldspar porphyritic quartz monzonite. A sample of this unit yielded an age of 144.8 +0.9/-1.5 Ma. Granitic rocks are similar in composition, texture, and age to the distinctive Manly Peak quartz monzonite of the southern Panamint Range. Miocene rocks overlie basement units nonconformably, and consist of volcanic lava flow units intercalated with pyroclastic, epiclastic, and sedimentary rocks. Volcanic rocks cover the majority of the map area and consist of ridge-forming andesitic flows, recessive intermediate to felsic lithic tuffs, and resistant basalt flows. The andesites yielded ages from 13.1 to 13.4 Ma. The volcanic section is overlain unconformably by a cliff-forming megabreccia unit that is probably a landslide deposit lithologically similar to the underlying basement rocks, and an areally extensive conglomerate. The conglomerate contains abundant clasts of extremely well rounded alkali-feldspar-rich granite and mafic to intermediate plutonic rocks, as well as porphyritic to aphanitic volcanic rocks of intermediate to felsic composition. This unit is probably Pliocene in age.

Basement rocks fit well with the geologic framework of the adjacent Panamint Range. Volcanic rocks are similar in age and composition to most of the Owlshead Mountains. The youngest units give some additional insights into the regional development. Although the megabreccia unit appears locally derived, the conglomerate clasts are exotic to the area and were probably sourced from Miocene units with similar clasts in the Avawatz Mountains, indicating transport across the Garlock fault in Pliocene time.