HR: 0830h
AN: V51G-0355    [PDF]
TI: Stratigraphy and Structure of Late Oligocene-Early Miocene Ignimbrite-filled Paleovalleys, Northern Sierra Nevada, California
AU: * Rood, D H
EM: dylan@crustal.ucsb.edu
AF: Department of Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AU: Busby, C J
EM: busby@geol.ucsb.edu
AF: Department of Geological Sciences, University of California, Santa Barbara, CA 93106 United States
AU: Wagner, D
EM: dwagner@consrv.ca.gov
AF: California Geological Survey, 801 K St. MS 12-31, Sacramento, CA 95814 United States
AB: Mapping of a Late Oligocene-Early Miocene (31-25 Ma) paleovalley system filled with ignimbrites erupted from calderas in present-day Nevada provides relationships necessary to deduce paleotopographic vs. structural controls on the paleogeography of the Sierra Nevada and its transition into the Basin and Range in Early Tertiary time. A paleovalley filled with five distinctive ignimbrites is well-exposed in the Diamond Mountains about 2 -3 km west of the Honey Lake fault zone, a segment of the northern Walker Lane fault zone. We map a N-S trending paleovalley approximately 7.2 km wide, using ignimbrite distributions and thicknesses of ignimbrites and sedimentary rocks, as well as compaction foliation, cooling joints, welding zonation and lateral variations within each ignimbrite. Paleotopographic relief in the metamorphic and granitic basement reaches approximately 223 m; metamorphic roof pendants form steep-sided paleo-ridges and spires, while the granitic basement forms stepped relief controlled by pre-existing joints. The five ignimbrites are composed of at least nine mappable cooling units that vary laterally, from ca. 260 m thick paleovalley axis deposits to ca. 70 m thick paleovalley wall deposits, with dramatic thickening of vitrophyres toward paleovalley walls. The lowest three ignimbrites are confined to the paleovalley, the fourth passes upward from confined to unconfined, and the fifth is entirely unconfined. All of the ignimbrites are cut by two N-S trending, steeply W-dipping faults; the first, near the axis of the paleovalley, shows maximum 85 m of dip-slip displacement, and the second, near the eastern margin of the paleovalley, shows ca. 40 m of dip slip displacement. These N-S faults parallel the paleovalley and are oblique to the modern (NW-trending) Honey Lake fault zone, suggesting they may be older. Evidence for syndepositional faulting is present along the paleovalley axis fault, where sedimentary rocks between ignimbrites 3 and 4 thicken from several meters to 60 m onto the downthrown block, and along the fault near the eastern paleovalley margin, where ignimbrite 4 thickens from 50 to 90 m onto the downthrown block. We correlate ignimbrite units 3, 4 and 5 of the Diamond Mountains with ignimbrites filling paleovalley erosional remnants near Frenchman Lake (26 km to the SSE), Haskell Peak (52 km to the SSW), and Soda Springs (91 km to the S). At Soda Springs (located 8 km SW of Donner Pass) the N-S trending paleovalley is about 6 km wide. The western paleovalley wall is composed of metamorphic basement rock and is steep, with paleotopographic relief of at least 280 m, whereas the eastern side of the paleovalley is predominantly granitic and is characterized by less dramatic relief. The paleovalley contains seven ignimbrites composed of at least ten cooling units. We correlate units 1, 4 and 7 of the Soda Springs paleovalley with units 3, 4, and 5 of the Diamond Mountains paleovalley, on the basis of petrographic and outcrop characteristics. The ignimbrite sequence is 323 m thick, and has mappable erosional unconformities with relief of up to 30 m between units 4, 5 and 6. The upper three units locally spread beyond the paleovalley walls. We have not recognized any faults in this paleovalley. Late Oligocene-Early Miocene ignimbrite-filled paleovalleys in the Diamond Mountains, Frenchman Lake, Haskell Peak and Soda Springs may represent remnants of a once-continuous valley 117 km long that trended obliquely to, and presumably crossed, the modern range front faults prior to their development.
DE: 1749 Volcanology, geochemistry, and petrology
DE: 8400 VOLCANOLOGY
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting