Marsh Creek Volcanics Ryan Fay, Raymond Sullivan Overview

1. Regional Geologic Context 2. Marsh Creek Volcanic units 3. Interpretation and implications 4. Future research

Regional Geologic Perspective

Rhyolitic and dacitic Unit: • Numerous biotite dacite to rhyolite units. • Roughly 7.5 million years age. According to preliminary data from Ar40/Ar39 of biotite • Shallow feeder dikes following the Marsh Creek, Clayton and main detachment fault trends.

ANTSO 34.4 20x magnification Crossed polarizers 0.5 mm • Clinopyroxene rich Andesitic unit: • Occurs as boulders on a topo high ANTSO 16.1 20x magnification Crossed polarizers 0.5 mm • Likely an erosional remainant of a flow • Single occurrence • Age unknown Leg UntitlWrite a description end fored your map. Map

16 TAS Diagram

14 Phonolite

12 Tephriphonolite Trachyte 10 Phonotephrite Rhyolite Trachyandesite 8 Basaltic Basanite Trachy- Antso 33.1

6 Trachy-

Antso 16.1 Dacite 4 Andesite Basaltic Basalt andesite 2 Pirco- basalt

0 36 41 46 51 56 61 66 71 76 SiO2 Mendocino Triple Junction and the Coast

Range Volcanics Mendocino FZ

Sutter Buttes 1.5Ma

Clear lake .01-2Ma

Tolay 8-10.5Ma Sonoma 2.5-8Ma

Mt Diablo Berkeley Hills 8-10 Ma

Quien Sabe 8-10Ma

• Marsh Creek rhyolites and possibly dacites intruded the main piercement/Marsh Creek/Clayton fault trends at 7.5Ma concomitant with passage of the Coast Range Volcanic heating event • Andesite may be related to 5.6Ma Concord Naval Weapons Station ‘Basalt’ and reworked clasts under the 4.8Ma Lawlor tuff ) deg C ( Sample temp in subsurface

150 100 50 0

Future Work

More geochemical data from other volcanics in the area

Magnetometer survey to understand how these volcanics project at depth.

Thank you: Seth Adams and Save Mount Diablo, Stephen Edwards, Alan Deino of UC Berkeley Geochron lab, Ross Wagner, Rex Caufield at East Bay Regional Parks Dept.