Quaternary Geology
of the Great Basin
Inglewood, California to Salt Lake City, Utah
June 27-July 8, 1 9 8 9
Field Trip Guidebook T 1 1 7
Leaders:
George I. Smith, Larry Benson
and Donald R. Currey
American Geophysical Union, Washington, D.C.
COVER View toward the south-southwest of the Mono Craters, the
south shore of Mono Lake (foreground), and the Sierra Nevada
(background). Panurn Dome is the nearest crater; diameter of sharp
crest of tephra rim is about 500 m. "Navy Beach" (Stop 4-4) is part
of the left half of the shoreline.
Photograph by Roland E. von Huene.
Leaders :
George I. Smith
U.S. Geological Survey
345 Middlefield Rd. - MS 902
Menlo Park, CA 94025
Larry Benson
U.S. Geological Survey
Denver Federal Center - MS 403
Lakewood, CO 80225
Donald R. Currey
Department of Geography
University of Utah
Salt Lake City, UT 84112
Copyright 1989 American Geophysical Union
2000 Florida Ave., N.W., Washington, D.C. 20009
ISBN: 0-87590-651-6
Printed in the United States of America
IGC FIELD TRIP T117:
QUATERNARY GEOLOGY OF THE GREAT BASIN
George I. Smith
U.S. Geological Survey, Menlo Park, California
Larry Benson
U.S. Geological Survey, Denver, Colorado
Donald R. Currey
University of Utah, Salt Lake City, Utah
This pre-meeting t r i p i s designed t o present a
v a r i e t y of Quaternary geological studies from a
p a r t of t h e American Southwest.
Specialties
within t h e broad scope of "Quaternary geology"
t h a t will be discussed include archaeology,
climatology and paleoclimatology, geochemistry,
geomorphology,
paleoecology,
paleontology,
pedology, sedimentology, stratigraphy, tectonics,
and volcanology. After t h e first day, all a r e a s
visited a r e in-or along t h e margins of t h e G r e a t
Basin subprovince of t h e Basin and Range Province
(fig.
- I). Much of t h e G r e a t Basin i s characterized
by northerly-trending mountain ranges s e p a r a t e d
by wide valleys t h a t have internal drainage. T h e
G r e a t Basin is bounded on t h e west by t h e Sierra
Nevada (in California and Nevada) which will b e
within view during days 2 t o 7, and on t h e east by
a n uplifted a r e a expressed by t h e Wasatch Range
(in Utah) which adjoins t h e a r e a s visited on days 8
t o 10. The c l i m a t e in all of t h e s e a r e a s is arid t o
semi-arid, promoting generally good exposures of
rocks and sediments, but also causing warm t o hot
s u m m e r days in t h e lower or even i n t e r m e d i a t e
elevations.
(Note: Persons using this field-trip
guide for independent summer-time study should
be cautious in t h e warmest areas; use reliable
vehicles, c a r r y ample water, and notify someone
a b o u t where you a r e traveling and when you
e x p e c t t o return.)
Most of t h e geologic and other phenomena t o be
observed a r e t h e results of one or both of t w o
natural
phenomena--tectonic
processes
and
c l i m a t e change. Tectonic processes a r e directly
responsible for t h e high mountains t h a t bound t h e
G r e a t Basin on i t s eastern and western edges, for
t h e linear mountain ranges and wide and d e e p
basins t h a t characterize t h e internal part of t h e
region, and for t h e Quaternary volcanism.
Tectonism is also indirectly responsible for t h e
present-day semi-arid t o arid c l i m a t e because a s
much a s 90 percent of t h e w a t e r contained by air
masses moving eastward from t h e Northeast
Pacific is condensed a s a result of orographic
uplift and cooling as i t travels over t h e 2500 m- t o
4400 m-high Sierra Nevada and o t h e r mountains,
depriving t h e interior of t h e region of most of t h e
rain a n d snow expected at these latitudes. The
marked climate changes t h a t occurred during
Pleistocene t i m e in this a r e a and other parts of
OREGON
FIGURE 1 Map of western United States showing
state boundaries, route t o be followed by trip T117 (dotted line); stopping points at ends of Days 1
t o 10 indicated by numbers. Boundaries of the
Mojave Desert and Great Basin are shown as heavy
solid lines; these areas are subprovinces of the
Basin and Range province whose boundary is shown
by heavy dashed lines (labeled BARP) where it
extends outside of the Great Basin. Also shown are
locations of Sierra Nevada and Wasatch Range
(heavy, short lines), Great Salt Lake (GSL), Lake
Tahoe (LT), Utah Lake (UL),and state capitals or
large cities.
t h e globe required fundamental meteorological
changes t h a t were apparently driven by varying
combinations of earth-surface and astronomical
influences. Most of t h e paleoclimatic phenomena
t o be observed during this t r i p r e f l e c t differences
between t h e present a r i d c l i m a t e and t h e previous
more-moist climates t h a t characterized much of
Quaternary t i m e in t h e s e regions.
Descriptions of t h e geology visible at stops and
along t h e routes t h a t connect t h e m a r e necessarily
abbreviated so t h a t t h e r e a d e r will not "drown" in
details. For t h e s a m e reason, r e f e r e n c e s t o d a t a
sources a r e limited. Many of t h e c i t e d d a t e s on
rocks a r e also rounded from t h e a c t u a l laboratory
results.
Dates in t h e range of "thousandst' a r e
generally expressed numerically whereas d a t e s in
t h e range of "millions" a r e so expressed (example:
"2.1 million"); for t h e s a k e of consistency, t h e s e
conventions have also been applied t o t h e quoted
material, without indicating a n y change. T o s a v e
space, t h e following abbreviations a r e generally
used: "ft" (feet), "m" (meters), "mm" (millimeters),
"mi" (miles), "km" (kilometers), "sq" (square), "CU"
(cubic), "yrs" an ears), "B.P." (before t h e resent),
"N" (north), "S" (south), etc.
The remaining parts of this guidebook a r e
divided i n t o daily segments.
Each day's
description was written by t h e field-trip leader
primarily responsible for t h a t day's agenda. Days
1 t o 4 w e r e planned by Smith, Days 5 t o 7 by
Benson, and Days 8 t o 10 by Currey.
DAY 1
En r o u t e t o S t o p I- 1
A
Route. As shown by t o d yfs r o u t e (fig. 21, we
f i r s t travel north via 1-405 -, then east via 1-10,
and finally northeastward via 1-15 (approximately
90 mi, 145 km, normally about 2 hrs). Turn off I1 5 about 1.2 mi (1.9 km) past t h e junction with I215, t o "old R o u t e 66", follow i t north for about 4
mi (6.4 km), and s t o p about 500 m north of t h e
second of two overhead high-voltage transmission
lines. A d i r t road on t h e right can be used for
par king.
Commentary. This r o u t e crosses t h e floors of
t h r e e valleys, t h e Los Angeles Basin, t h e San
Gabriel Valley, and t h e San Bernardino Valley.
Q u a t e r n a r y deposits in t h e Los Angeles basin
reveal a structurally complex history, and deposits
a s much a s 1,000 m thick a r e reported from
subsurface data; m a r i n e deposits commonly g r a d e
e a s t w a r d i n t o non-marine s e d i m e n t s along a line
t h a t lies 1 0 t o 20 km east of t h e present coast.
E a s t of 1-405, n e a r t h e Culver C i t y exit, t h e
upward-faulted-and-warped
Pleistocene m a r i n e
and non-marine sediments of t h e Baldwin Hills rise
120 m above t h e s u r f a c e of t h e Q u a t e r n a r y t e r r a c e
gravels t h a t underlie this p a r t of t h e basin. In
1/
Highways in t h e United S t a t e s a r e mostly
numbered and preceded (in this guidebook) by
l e t t e r s indicative of t h e system t o which t h e y
belong:
I- = I n t e r s t a t e highways (nation-wide,
limited access, ususally divided); US- = United
S t a t e s highways (also nation-wide, usually good
quality, not necessarily divided); CA- = California,
NV- = Nevada, UT- = Utah (state-wide, varying
quality); various other l e t t e r s a r e used for county o r
local roads.
FIGURE 2 Map showing route to be followed on
Day 1 (roads indicated by parallel dotted lines).
Names of geographic features mentioned in text
also shown. Arrows on fault lines indicate relative
direction of movement.
1963, f a i l u r e of a reservoir dam built on t h e c r e s t
of t h e Baldwin Hills caused m o r e t h a n $12 million
in property damage; failure was a t t r i b u t e d t o
subsidence caused by production of oil from an
a d j a c e n t field (Castle and Yerkes, 1976).
For t h e f i r s t 5 t o 1 0 km of e a s t w a r d travel on I10, t h e c i t i e s of Beverly Hills a n d Hollywood-traditional homes of t h e U.S. motion picture
industry--are on t h e l e f t (N). About 8 km north of
t h e L a C i e n e g a Blvd. exit, t h e east end of t h e
S a n t a Monica Mountains a r e visible. This p a r t of
t h e range, which is relatively low, exposes
Mesozoic rocks overlain by extensively deformed
middle T e r t i a r y m a r i n e s e d i m e n t a r y and volcanic
rocks.
To t h e west, t h e r a n g e i s composed of
Miocene-age
volcanic
rocks
and
marine
sedimentary rocks, c r e a t i n g peaks t h a t approach
1,000 m elevation. Exposed and concealed faults
along t h e south edge of t h e s e mountains a r e
responsible for t h e i r uplift during l a t e T e r t i a r y and
Quaternary time.
Continuations of t h e s e f a u l t
zones t o t h e west of t h e s e g m e n t visible from 1-10
a r e responsible f o r t h e east-west t r e n d of this p a r t
of t h e California coast and t h e existence of
several of t h e Channel Islands; extensions of t h e s e
f a u l t s t o t h e east control t h e position of t h e south
edge of t h e Transverse Ranges b e t w e e n this point
and t h e San Andreas fault (fig. 1).
Immediately a f t e r passing t h e t a l l buildings of
c e n t r a l Los Angeles and t h e i n t e r c h a n g e between
1-10 and 1-5, 1-10 e n t e r s t h e R e p e t t o Hills. Marine
sedimentary rocks of Miocene and Pliocene ages
crop o u t in this northwest-trending topographic
f e a t u r e which generally has less than 100 m of
relief.
The rocks a r e extensively f a u l t e d and
folded.
Between e x i t s t h a t lead t o t h e "Long Beach
Freeway" (1-710) and t h e "San Gabriel River
Freeway" (1-6051, t h e San Gabriel Mountains a r e
visible (most of t h e time!) a b o u t 10 km t o t h e
north. This s e g m e n t of t h e Transverse Ranges h a s
numerous peaks between 1,500 and 3,000 m
elevation, e x t r e m e l y s t e e p slopes, a n d deeply
dissected
canyons.
c r y s t a l l i n e rocks -of
Precambrian and Mesozoic a g e s m a k e up most of
t h e range. A complex s e r i e s of high-angle, and
north-dipping low-angle, faults a r e exposed along
t h e s o u t h edge of t h e range, with fresh-appearing
s c a r p s indicating Q u a t e r n a r y displacements.
Notably l a r g e alluvial f a n s have been developed t o
t h e south of t h e s e faults, with much of t h e 1-10
r o u t e lying a b o u t half way down t h e i r slopes. T h e
San Gabriel River, a f t e r which t h i s valley is
named, is crossed just prior t o t h e 1-605 exit; i t
c a r r i e s w a t e r from t h e l a r g e s t of t h e San Gabriel
Mountains drainage areas.
About 4 km e a s t of t h e "West Covina" exit, t h e
f r e e w a y begins t o climb i n t o t h e low-elevation San
J o s e Hills. Marine s t r a t a and volcanic rocks of
middle Miocene a g e c r o p o u t in this a r e a because
t h e y w e r e deformed i n t o east-northeast folds
during l a t e Cenozoic time.
Between t h e San J o s e Hills and t h e junction with
1-13, t h e r o u t e continues east a c r o s s t h e San
Bernardino Valley which is underlain by alluvial
f a n s t h a t e m e r g e from t h e south side of t h e San
Gabriel Mountains.
The Ontario-Upland exit,
a b o u t 8 km south of t h e range front, lies a b o u t on
t h e c e n t e r l i n e of a l a r g e and s y m m e t r i c a l alluvial
f a n t h a t e x t e n d s south a t o t a l of 1 0 t o 1 5 km.
Turning n o r t h e a s t on 1-15, t h e r o u t e climbs t h e
alluvial f a n toward Cajon Pass which divides t h e
San Gabriel Mountains t o t h e west f r o m t h e San
Bernardino Mountains t o t h e east, a n d t h e San
Bernardino Valley t o t h e south f r o m t h e t h e
Mojave Desert t o t h e north. F a n s and t e r r a c e
deposits become m o r e deeply dissected a s t h e
mountain f r o n t is approached, providing a preview
of t h e very complex Quaternary t e c t o n i c and
s e d i m e n t a r y history of this area. The f i r s t close
approach t o bedrock on t h e l e f t (NW) side of t h e
highway allows a view of t h e e a s t end of a lowangle northwest-dipping f a u l t t h a t c u t s Q u a t e r n a r y
deposits, s o m e s e g m e n t s of which displace alluvial
deposits t h a t may be a s young a s 1,000 yrs (hlorton
and Matti, 1987).
S t o p 1-1
This s t o p is within 100 m of t h e San Andreas
fault, and displacement on t h i s f a u l t during t h e
1857 e a r t h q u a k e was t r a c e d f r o m i t s epicenter,
a b o u t 50 mi (80 km) northwest, t o a b o u t this
point. The Q u a t e r n a r y geology of this a r e a (fig. 3)
i s described by Weldon (1987), and e x t r a c t s of his
paper follow:
"A 3 mi (5 km) s t r e t c h of t h e San
Andreas f a u l t , c e n t e r e d at Cajon C r e e k ,
* * * contains e x c e l l e n t e x a m p l e s of t h e
style of faulting and t h e geomorphology
associated with t h e San Andreas fault.
Abandoned t r a c e s of t h e San Andreas
fault, evidence for l a t e r a l and v e r t i c a l
deformation across t h e f a u l t zone, and
l a t e Q u a t e r n a r y o f f s e t river t e r r a c e s ,
landslides, and s a g ponds can all b e
found in this small area.
The
relationships seen h e r e have been used
t o c h a r a c t e r i z e t h e uplift of t h e
Transverse Ranges a n d t h e Q u a t e r n a r y
slip r a t e and r e c u r r e n c e interval of
earthquakes on t h e San Andreas fault.*
* *.
"Near Cajon Creek, t h r e e major
f a u l t s of t h e San Andreas system a r e
well exposed in deep, cross c u t t i n g
canyons; t h e y a r e t h e San Andreas f a u l t ,
t h e North Branch (of t h e San Andreas
fault), and t h e Punchbowl fault. * * *
These f a u l t s juxtapose d i f f e r e n t rock
types, but only t h e San Andreas f a u l t
has experienced significant l a t e r a l
displacements
during
the
late
Quaternary. * * *
"Late Q u a t e r n a r y deposits in t h e
a r e a a r e right-laterally o f f s e t up t o 0.9
mi (1.4 km) by t h e San Andreas f a u l t and
up t o 30 f t (10 m) vertically by t h e
North Branch and s e v e r a l o t h e r unnamed
dip-slip f a u l t s n o r t h e a s t of t h e San
Andreas fault.* * *
"The Holocene record a t t h e s i t e is
preserved in a flight of t e r r a c e s c u t i n t o
(middle Quaternary-age alluvium) or
bedrock * * *; minor fluvial deposits,
s a g pond sediments, small landslides,
and colluvium c o m p l e t e t h e Holocene
sedimentary record. * * *It
"A
Holocene
history
of
sedimentation and t e r r a c e formation h a s
been combined with t h e numerous
o f f s e t s of geomorphic f e a t u r e s t o
c a l c u l a t e an a v e r a g e slip r a t e of 24.5
2 3.5 mm/yr on t h e San Andreas fault."
Between S t o p 1-1 and S t o p 1-2
Route.
Continue north on "old r o u t e 66"
highway, rejoin 1-15, and t r a v e l north and e a s t for
a t o t a l d i s t a n c e of a b o u t 8 3 m i (134 km). T h e
r o u t e passes through t h e c i t i e s of Victorville and
Barstow. About 37 miles (60 km) a f t e r leaving
Barstow, t u r n off a t t h e e x i t t o A f t o n Canyon.
Turn right and drive a s h o r t d i s t a n c e t o a point
where exposures of t h e dissected lake beds can be
viewed.
Commentary. After rejoining 1-15, t h e route
continues t o climb up through strongly-deformed
crystalline rocks, some or all of which a r e
Mesozoic in age, into outcrops of Miocene, then
Pliocene, and finally (near Cajon Summit)
Pleistocene
continental
sandstones
and
conglomerates.
These sedimentary rocks dip
northeast up t o 20°, documenting a large vertical
component of t h e tectonic activity associated with
t h e San Andreas fault in this region.
An a r e a near Cajon Pass will probably be
selected for t h e Day-1 lunch stop. The drill site
for t h e Deep Observation and Sampling of t h e
Earths's Continental Crust Program (DOSECC),
sponsored by t h e National Science Foundation, is
near t h i s stop. The drill hole is scheduled t o reach
a t o t a l depth of 5 km. A main objective of this
t e s t is t o measure t h e h e a t flow characteristics of
this a r e a and to reconcile any discrepancies
between them and h e a t flows calculated from
known slip r a t e s on t h e San Andreas fault.
Numerous other geophysical and geologic studies
a r e also scheduled a s part of this study.
Following lunch, t h e t r i p continues northeast on
1-15.
This segment crosses t h e Mojave Desert
diagonally t o the northeast, passing through t h e
cities of Victorville and then Barstow. Both cities
a r e adjacent t o the Mojave River which flows
north out of t h e San Bernardino Mountains and
then curves east. Most of t h e time, t h e Mojave
River flows in very small volumes (especially in
recent years because t h e flow is now controlled
upstream by a normally-filled reservoir and a
nor mall y-dry flood-control basin). In extreme1y
wet years, though, floodwaters reach Soda and
Silver Lakes, about 175 km downstream from t h e
point where we first cross t h e river, near
Victorville.
The topography of t h e Mojave Desert can be
characterized a s a low-relief western half and a
higher-relief eastern half.
The elevations of
valley floors and nearby peaks in t h e western half
a r e typically 700 m and 1400 m, respectively;
those-elevations in t h e eastern half of t h e Mojave
a r e commonly 500 m and 2,000 m, respectively.
Our route enters t h e Mojave Desert a t about t h e
division between these two terrains. The mountain
2000,
P
meters
contour ~nterval 4 0 fee!
FIGURE 3 ( A ) Geologic map of area surrounding Stop 1-1. San Andreas fault shown trending from
NW to SE. Symbols: gn, gneiss; psg, schist; Tu, Tertiary rocks; Qoa-d, Pleistocene alluvium (ca.
55,000 yrs); Qoa-c, late Pleistocene alluvium (14,800 to 12,400 yrs); Qoa-a, Holocene alluvium
(1705-275 yrs); Qhf, Holocene fanglomerate; Qc, colluvium; Qhs, Holocene swamp deposits; Qal,
modern alluvium; Qm, man-made deposits. Heavy dots indicate position of east edge of bedrock
channel prior to 350-m offset which occurred before deposition of Qoa-c. (Profiles A-A' and B-B'
not reproduced here.) From Weldon and Sieh (1985, fig. 9). (B)Schematic cross-section through
Cajon Creek, parallel to, and south of, San Andreas fault. Qoa-e, older terrace deposits (Qoa-N),
position of 0.73 million yr magnetic reversal within this unit); flights of terraces (Qt-1, Qt-2, etc.)
W r e cut on Qoa-c and Qoa-a by Cajon Creek, and dates of their abandonment are shown, based on
C dates (boxes) or on geomorphic cmd stratigraphic criteria (open). Channel containing Qoa-a
and present stream bed was cut SE of channel containing Qoa-c as a result of that much offset
along fault. From McFadden and Weldon (1987).