Geology and structure of a portion of the

Orocopia Mountains, southern California

Amber Johnson-CarrollCalifornia State University Northridge, Northridge, CA 91325

INTRODUCTION

The purpose of my study was to make a geologic map and

cross sections of part of the Orocopia Mountains and to

interpret the geologic events history. My study area is

located northwest of the Salton Sea. The dates of my field

work were six days in March 2009. I was accompanied on this

study by my Field III class from California State University

Northridge. The data we collected included photos,

drawings, strikes, dips, observations of outcrop features,

bearings, and mineralogy. The methods we used to collect

our data included using a Brunton compass and recording data

on our maps (Pl. 1) and in our field notebooks. I collected

78 bedding attitudes (Pl. 4) from my study area.

GEOLOGIC SETTING

The San Andreas fault system passes to the southwest of

the study area (Fig. 1).

The Clemens Wall fault, an

ancestral strand of the San

Andreas fault, bounds the

southwest of our study area

(Ebert K.A., et al. 2007).

Northwest of the study area

are Pliocene-Pleistocene

nonmarine sedimentary rocks

of the Mecca Hills, which are exposed immediately northeast

of the San Andreas fault. North to northeast of our study

area is the Eocene Maniobra Formation, and Mesozoic

granitoids. My study area in the Orocopia Mountains is

located north of the Gulf of California, an extensional

province of the surrounding geology. In my study area we

specifically looked at the Oligocene Diligencia Formation

stratigraphically above the Precambrian igneous and

metamorphic complex.

ROCK UNITS

Figure 1. Index map of southern California showing faulting. Study area in the Orocopia Mountains (A) along the Salton Sea.

Igneous and Metamorphic complex

The structurally lowest unit observed in the study area

is the Precambrian igneous and metamorphic complex, or the

quartzo feldspathic gneiss. This gneiss has a sedimentary

protolith, so is a paragneiss. The composition of the

paragneiss is quartz, feldspar, biotite, and also has a

large percentage of mica. This unit also has a tourmaline

dike. The strength of this unit is friable, and the nature

of the unit exposures is resistant. They create a

noticeable difference in elevation change when the contact

is crossed from this unit to the next observed.

There are metamorphic foliations noticed in this

igneous and metamorphic complex. These foliations are

observed along the mica and biotite compositional banding.

This probably means this high-grade paragneiss went through

plastic deformation with high temperatures and pressures.

In our study area the length of this unit is the west

to east length of the southern most part of our study area.

The contact between this unit and the stratigraphically

upper unit is a fault plane that consistently dips at an

angle of 65° to the south in our study area. This fault

seems to be a reverse fault because it is higher in

elevation than the younger units.

Red Sandstone

The next structurally upper rock type observed in the

study area is the Red Sandstone Conglomerate Unit. This

rock type looks to be a paleosol outcrop which was nonmarine

and fluvial. It contains metamorphic and granitic clasts,

and is matrix supported. It ranges from fine grained, 4.0

to 3.5 phi to cobble-rich (largest cobbles = 3 cm). The

color is grayish red 10R 4/2, to moderate reddish brown 10R

4/6 on fresh and broken surfaces. The strength is weak and

non-friable. The exposures are not very resistant, and

create an elevation change from the stratigraphically lower

igneous and metamorphic complex.

The extent of this unit is half of the east-west length

in the southeast part of the study area. The observed

outcrop has an angular unconformity with the lower rock

unit, the igneous and metamorphic complex. The contact of

these two units is a reverse fault.

Vari-Colored Sandstone

The next oldest rock unit found in the study area is

the vari-Colored sandstone

unit. The composition of

this sandstone is quartz,

plagioclase, and potassium

feldspar. The upper units

are coarser grained and

have clasts ranging from 1

cm to 5 cm. The lower

units have grain size ranging -1.0 to 0.5 phi. The color of

a fresh surface is grayish orange pink 5YR 7/2, and the

weathered surface is pale yellowish brown 10YR 6/2. The

strength is moderately strong, and the exposures were

moderately resistant.

There are ripple marks on the surface of these

sandstone beds which are symmetric and asymmetric (Fig. 2).

These ripple casts are on more than one outcrop and tend to

Figure 2. Ripple marks on the stratigraphically up side of the vari-Colored sandstone. This is in the eastern part of the study area.

have a wavelength of 3.5-4.5 cm on some outcrops, and 20 cm

in others. This indicates a change in water level in the

formation of this unit over time. The smaller wavelengths

correspond with shallower water, and the longer wavelengths

with deeper water.

The extent of this unit was across the east-west

length, and in the southern part of the study area. The bed

thickness is lenticular and is ~1-1.5 m. There are angular

unconformities and truncations observed with various faults

running through this unit in the eastern part of our study

area.

Basalt

The basalt unit is found stratigraphically above the

vari-Colored sandstone unit. The basalt was found to have

amygdules filled in with

sandstone sediment.

Abundant epidote alteration

and veins suggests that

hydrothermal fluids have

altered the basalt. This Figure 3. The sandstone marker unit in the basalt unit. This exposure is in the northeast of the study area.

unit has a marker unit of sandstone within it (Fig. 3). The

color of this marker unit is pale purple 5P 6/2. The

resistance is moderately strong to strong, which means the

outcrops are a bit more pronounced than the surrounding

sandstones. However the marker unit within the basalt is

very resistant which is probably due to the nearby basalt

source.

The basalt and the marker unit within the basalt have

northeast-southwest trending faults truncating them in the

eastern and northeastern parts of the study area which has

caused the basalt and marker unit to have surface outcrops

in the northern part of the study area (Pl. 1). The marker

unit has tabular bedding and is ~6 m thick. The basalt

reaches the western part of the study area, but the marker

unit eventually pinches out in the west, but before the end

of our study area.

Gray Sandstone

The unit which overlies the basalt is a gray-yellow

sandstone. There is gypsum and anhydrite layering in this

unit. This sandstone has occasional pebbly layers, and

occasional layers of siltstone and shale, but the average

grain size is 2.0 to 1.5 phi. These layers of siltstone

have the color light olive gray 5Y 5/2. The color of the

sandstone is yellowish gray 5Y 7/2. The portions of the

sandstone that contain layers of siltstone have joints and

are well sorted with cross-laminations.

This sandstone is also truncated by the same northeast-

southwest trending faults that truncated the basalt. The

gray sandstone extends to the western perimeter of the study

area.

Mudstone

The next oldest unit in our study area is the mudstone

unit. The underlying unit is

the gray sandstone unit which

overlies the basalt. The

grain size on this mudstone

is 3.5 to 4.0 phi and

smaller. There is a volcanic

tuff unit (Fig. 4) in the middle of this mudstone unit, and

so is bounded by mudstone. The color of the mudstone is

grayish orange 10YR 7/2 to pale yellowish brown 10YR 6/2.

The Tuff is pale yellowish green 10 GY 7/2. Because of this

marker tuff the mudstone under the tuff has been deformed to

form tiny folds (Fig. 4), and the color of the Mudstone

stratigraphically above the Tuff is altered to a darker

color. The color of this stratigraphically higher mudstone

is greenish gray 5GY 6/1.

This rock type reacted with HCl. This marker tuff is a

thick unit with no laminations. It has

intermittent beds of orange and greenish

gray mudstone. The contact between the

Figure 4. Folded Mudstone overlain by Volcanic Tuff. This outcrop is in the northeast of the study area.

base of the tuff and the mudstone is abrupt, and has an

orientation of N15E, 19 NW. The outcrops of the mudstone

with the tuff are in the northwest part of the study area.

Many faults and folds, anticlines and synclines, have

truncated and folded the units in this part of the study

area.

STRUCTURE

I mapped 9 longer major faults that continue across

most of the map area, and 4

less continuous faults in

my study area (Pl. 1). The

faults generally trended in

a northeast-southwest

fashion (Fig 6). These

faults may be genetically

related to the Orocopia Mountains detachment fault (Ebert

K.A., et al. 2007). I also observed many anticlines and

synclines in my study area which trended east-west. I

observed and recorded data for two anticlines and two

synclines (Fig. 5). All of the folds in our study area

Figure 5. A syncline fold that was observed in the study area.

range from open to isoclinal. One of these major synclines

is projected into one of my cross-sections, A-A’ (Pl. 1).

Our whole study area however is a syncline (Fig. 7). The

folds in the area have been categorized into two separate

events (Ebert K.A., et al. 2007). The earlier folds are

potentially related to underthrusting, and the younger

structures have been attributed to exhumation. The earlier

fold axes trend northeast-southwest, and the younger ones

are oriented northwest-southeast to north-south.

There are a few foliation data points that I recorded

for the igneous and metamorphic complex. These foliations

all have strikes close to east-west, and dip towards the

south. The dips range from 14°-50°.

GEOMORPHOLOGY

There are three landforms in the study area. The first

type is alluvium which occurs atop ridgelines and upland

areas. This alluvium consists of cemented gravels, and

indicates deposition of a broad alluvial plane, and later

erosion of older sediments. A second type in our study area

is the terrace-forming alluvium above the current streams.

This indicates erosion of sediments younger than the

erosional events of the ridgelines. The third type of

geomorphology in our study area is the drainage system and

streams that have cut through the lower elevations. These

streams show a branching pattern in and around our study

area.

GEOLOGIC HISTORY

Through dynamothermal metamorphism there was a high-

grade paragneiss formed from a sedimentary protolith in the

Precambrian is what is today known as the Orocopia

Mountains. There was gneissic layering, metamorphic

foliations, and compositional banding formed with mica and

biotite. This means that plastic deformation had to have

happened with high temperatures and pressures. This

paragneiss also had igneous influences, such as tourmaline

dikes, and granites intruded into it.

This igneous and metamorphic complex then either had no

deposition on top of it until the Oligocene, or any

deposition was eroded away before the red sandstones were

first deposited. More likely some sediments were deposited,

but most were eroded away, which is the origin of the ridge

alluvium. The fault which is the contact between the

igneous and metamorphic complex and the red sandstones may

also have truncated units deposited between those two.

In the Oligocene the red sandstones were deposited.

Since this unit does have metamorphic clasts, the water

source depositing the sandstones was probably close to a

metamorphic source, possibly the igneous and metamorphic

complex identified in our study area.

The next unit deposited in the Oligocene in the study

area was the vari-Colored sandstones on top of the red

sandstones. This unit has smaller grains which have a

different color and composition. The water source

depositing this sandstone was probably transporting material

from a father distance and source because of these factors.

The next unit in the stratigraphic sequence is the

basalt. After the sandstones were deposited there were

igneous volcanic flows which covered and compacted the

materials below. It seems the basaltic flows were

intermittent because of the sandstone marker unit present in

the middle of the basalt. This sandstone marker unit was

probably deposited from the same water source as the

previous sandstones. The basalt flows then started again,

which covered the sandstone marker unit.

After the basaltic flows stopped, a gray sandstone was

then deposited on top. This also was finer grained so the

source from this sandstone was probably farther away. This

unit also had siltstones and shales present so I would say

this area was under more water than the previously described

units. But since there are also gypsum and anhydrite

layering the environment had to have gone dry often enough

to have evaporates present in the rock record.

The next deposited unit was the mudstone. This is

similar in grain size to the gray sandstone, but was a bit

smaller. This must have been a time in the Oligocene when

water was getting deeper and deeper in the area. But there

is a volcanic tuff unit within the mudstone. This evidence

points to another volcanic eruption nearby. There seems to

be more ash deposits than basaltic flow. This was either a

volcanic source farther away than the source which deposited

the earlier basalt, or the eruption was not as big so as to

cause basaltic flows to cover the area. After the tuff was

initially deposited, mudstone began to form again, but with

some differences. The volcanic tuff caused the mudstone to

turn a black color. This color was due to the carbon which

resulted from living animals and plants dying in mass

numbers due to the volcanic ash. The next process was the

terrace alluvium. This may have been an ancient river bed

where sediments were being eroded from these units

described.

The next event that happened in our study area was the

folding and faulting. The folds in the Orocopias have been

categorized into two separate events (Ebert K.A., et al.

2007). The earlier folds are potentially related to

underthrusting, which has been agreed to have been from

east-dipping low-angle subduction. The younger structures

are from exhumation. Since the fault trends are generally

in the same direction they probably happened in the same

event. The folds were probably also formed together because

of the same general fold axis direction. Since the faults

cut through the folds, (Pl. 1), I assume the folds were part

of the first event. The faults were then part of the second

event in the Orocopia Mountains. The structural stresses

that caused these two events was probably caused by

subduction along the Pacific coast.

The last process that occurred in our study area was

the modern day river channels that seem to have used the old

channels which caused the terrace alluvium.

REFERENCES

Ebert K.A., Grove M., Jacobson C.E., Pedrick J.N., Vucic A.,

2007, Exhumation of the Orocopia Schist and associated

rocks of southeastern California: Relative roles of

erosion, synsubduction tectonic denudation, and middle

Cenozoic extension: Geological Society of America,

special paper 419, p. 1-31.

Intro

-Statement of purpose

-Location of study

-Dates of field work

Geological Setting

-Regional structures nearby

Rock Units

-Orthosgneiss/Paragneiss and Granitoid unit (oldest, first to appear)

-Red S.S. Conglomerate unit

-Vari-colored S.S. unit

-Basalt unit (S.S. marker unit within it)

-Grey/yellowish S.S. unit (overlaying basalt)

-Mudstone unit (youngest)

-Volcanic Tuff unit (last to appear)

Structure

-Faults

-Folds

Geomorphology

-Levels of surface remnants

Geologic History