Age, composition, and areal distribution of the Pliocene Lawlor Tuff, and three younger Pliocene tuffs, California and Nevada

Geosphere
By: , and 

Links

Abstract

The Lawlor Tuff is a widespread dacitic tephra layer produced by Plinian eruptions and ash flows derived from the Sonoma Volcanics, a volcanic area north of San Francisco Bay in the central Coast Ranges of California, USA. The younger, chemically similar Huichica tuff, the tuff of Napa, and the tuff of Monticello Road sequentially overlie the Lawlor Tuff, and were erupted from the same volcanic field. We obtain new laser-fusion and incremental-heating 40Ar/39Ar isochron and plateau ages of 4.834 ± 0.011, 4.76 ± 0.03, ≤4.70 ± 0.03, and 4.50 ± 0.02 Ma (1 sigma), respectively, for these layers. The ages are concordant with their stratigraphic positions and are significantly older than those determined previously by the K-Ar method on the same tuffs in previous studies.

Based on offsets of the ash-flow phase of the Lawlor Tuff by strands of the eastern San Andreas fault system within the northeastern San Francisco Bay area, total offset east of the Rodgers Creek–Healdsburg fault is estimated to be in the range of 36 to 56 km, with corresponding displacement rates between 8.4 and 11.6 mm/yr over the past ∼4.83 Ma.

We identify these tuffs by their chemical, petrographic, and magnetic characteristics over a large area in California and western Nevada, and at a number of new localities. They are thus unique chronostratigraphic markers that allow correlation of marine and terrestrial sedimentary and volcanic strata of early Pliocene age for their region of fallout. The tuff of Monticello Road is identified only near its eruptive source.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Age, composition, and areal distribution of the Pliocene Lawlor Tuff, and three younger Pliocene tuffs, California and Nevada
Series title Geosphere
DOI 10.1130/GES00609.1
Volume 7
Issue 3
Year Published 2011
Language English
Publisher The Geological Society of America
Contributing office(s) Geology and Geophysics Science Center
Description 30 p.
First page 599
Last page 628
Country United States
State California, Nevada
Online Only (Y/N) N
Additional Online Files (Y/N) N