KEY TEPHROCHRONOLOGIC CONTRIBUTIONS TO THE LATE CENOZOIC HISTORY OF THE WESTERN BASIN AND RANGE: EARTHQUAKES, PLUVIAL LAKES, AND THE BIRTH OF A RIVER

For nearly 30 years the tephrochronology database has provided an invaluable tool for solving many of the complexities in our studies of late Cenozoic stratigraphy. Here, we showcase several key contributions that Andrei and other tephrochronologists have made to our understanding of important geologic events in the western Basin and Range.

The central Nevada seismic belt (CNSB) is an unprecedented concentration of historical surface-rupturing events (nine) in the western Basin and Range, yet until recently the relevance of the CNSB to the overall seismic hazard of the region was poorly understood. Based in large measure on key structural relations and a suite of late Quaternary (0.5-90 ka) Mono Crater and Mazama tephra beds, we now know that 1) the CNSB is not a zone of elevated geologic strain; and 2) the ages of the penultimate events and slip histories for the historical faults are dissimilar, thereby precluding a similar seismic belt. Importantly we found that the 1954 Fairview Peak fault was pre-Holocene in age—older than the 35.4 ka Wilson Creek tephra.

The history of pluvial Lake Lahontan is well established, but our recent studies in the Pyramid Lake basin have revealed new stratigraphic age relations provided by key tephra beds. The occurrence of the Wono, Trego Hot Springs, Mount St. Helens, and Marble Bluff tephra beds together with 14C ages indicates that the late glacial ascension of Lake Lahontan to above 1250 m began between 30-50 ka, somewhat earlier than previously believed. The lake then rapidly receded from the 13 ka 1332 m highstand to near the historical 1170-m level and the present-day Truckee River Canyon was cut by Mazama ash time (6.8 ka).

Tephra beds in late Cenozoic deposits along the lower Colorado River at and south of Laughlin, NV help constrain the time of birth of the river and the cutting of the Grand Canyon at between 5.5 and ~3.3 Ma. The lacustrine Bouse Fm overlies the 5.5 Ma Connant Creek tephra and contains the 5-6 Ma basal Bouse tephra. A thick, overlying sequence of Colorado River sand and gravel derived from excavation of the Grand Canyon contains the 3.6-4.2 Ma lower Nomlaki tephra. Major river alluviation had ceased by the time of deposition of the 3.3 Ma upper Nomlaki tephra bed. Late Quaternary fluvial episodes are also dated beginning with the 0.65 Ma Lava Creek B (?) tephra in the Chemehuevi Fm.