GENESIS OF THE QUATERNARY TERRACES OF THE EASTERN SIERRA EL MAYOR, NORTHERN BAJA ,

An Undergraduate Thesis Presented to The Faculty of California State University, Fullerton Department of Geological Sciences

In Partial Fulfillment of the Requirements for the Degree Bachelor of Science in Geology

By Rene Perez 2003

Phil Armstrong, Faculty Advisor

Genesis of the Quaternary Terraces of the Eastern Sierra El Mayor,

Northern , Mexico

A Thesis Presented to the Faculty of California State University, Fullerton

In Partial Fulfillment of the Requirements for the Degree of Bachelor of Science in Geology

By: Rene Perez, Department of Geological Sciences, California State University, Fullerton Thesis Advisor: Dr. Phil Armstrong, Department of Geological Sciences, California State University, Fullerton

TABLE OF CONTENTS

ABSTRACT...... 1 INTRODUCTION...... 2 TERRACES AS INDICATORS OF GEOLOGIC ACTIVITY ...... 6 REGIONAL GEOLOGY...... 9 Geology of the Sierra Cucapa and Sierra El Mayor ...... 9 Faults in the Sierra Cucapa and Sierra El Mayor...... 12 Colorado River Delta and Ancient Lake Deposits...... 14 GEOLOGY OF TERRACES...... 16 Clay Units ...... 18 Silt Units ...... 18 Sand Units...... 18 Gravel Units...... 20 Capping Gravel Units...... 20 GEOMORPHIC ANALYSIS...... 21 Data Collection...... 21 Data analysis and Results...... 22 DISCUSSION ...... 34 Hypothesis for terrace genesis...... 38 CONCLUSION ...... 39 ACKNOWLEDGEMENTS...... 40 REFRENCES CITED ...... 41 APPENDIX ...... 43

TABLES

TABLE 1: TABLE SHOWING CORRECTED LOCATIONS...... 22 TABLE 2: MAXIMUM AND MINIMUM ELEVATIONS ...... 32 TABLE 3: SLOPE ANALYSIS RESULTS ...... 33

FIGURES

FIGURE 1: LOCATION MAP...... 3 FIGURE 2: PHOTO SHOWING TERRACE ...... 5 FIGURE 3: SCHEMATIC CONFIGURATION OF RIVER TERRACE ...... 7 FIGURE 4: HISTORIC SEIMICITY ...... 10 FIGURE 5: GEOLOGIC MAP ...... 11 FIGURE 6: COLORADO RIVER DISCHARGE...... 14 FIGURE 7: MAP OF CALIFORNIA-MEXICO BORDERLAND ...... 15 FIGURE 8A: PICTURE SHOWING INTERLAYERING WEDGE DEPOSITS ...... 17 FIGURE 8B: PICTURE OF DEPOSITS EXPOSED AT DISTAL END OF WEDGE ...... 17 FIGURE 9A: PHOTO SHOWING TRENCH ...... 19 FIGURE 9B: PICTURE SHOWING DESERT PAVEMENT AND VARNISH ...... 19

- i -

FIGURE 10: OBLIQUE AERIAL PHOTOGRAPH...... 23 FIGURE 11A: DIGITAL ELEVATION MODEL ...... 24 FIGURE 11B: DIGITAL ELEVATION MODEL WITH 1M CONTOURS...... 24 FIGURE 12: MAP OF TERRACE SURFACES ...... 26 FIGURE 13: CORRELATION OF TERRACE SURFACES ...... 27 FIGURE 14: TOPOGRAPHIC PROFILES...... 28 FIGURE 15: SURFACE PROFILES OF ELEVATION VS DISTANCE...... 30 FIGURE 16: GRAPH SHOWING DISTRIBUTION OF TERRACE SURFACE PROFILES ...... 31 FIGURE 17: SEA LEVEL CHANGES...... 35 FIGURE 18: LANDSAT 7TM ...... 38

- ii - Genesis of the Quaternary Terraces of the Eastern Sierra El Mayor,

Northern Baja California, Mexico

By: Rene Perez, Department of Geological Sciences, California State University, Fullerton Thesis Advisor: Dr. Phil Armstrong, Department of Geological Sciences, California State University, Fullerton

Abstract

A series of terrace strath surfaces are located along the eastern margin of the Sierra El Mayor in Northern Baja California, Mexico. They are located at distal extents of Colorado River Delta, near the area where the delta converges with the Sea of Cortez. The terrace sequences are only found on the eastern base of the northern Sierra El Mayor. The Sierra El Mayor is bound to the west by the faults of the Laguna Salada/Cañada David Fault Zones, which are believed to be the southern extents of the Elsinore Fault. The Laguna Salada exhibits a slip rate of 2 to 3 mm/yr with a reoccurrence interval ranging from 1000-2000 yr.

The terraces consist of an older pediment surface and four 1 to 3m high terrace straths that are carved into a wedge of interbedded clays, silts, sands, and gravels deposited at the base of the Sierra El Mayor, which slope and thin northeastward toward the basin. These wedge deposits are the result of the intermingling of the coarser alluvial deposits derived form the Sierra El Mayor (sands and gravels) and the finer deltaic flood plain deposit (silts and clays) from the Colorado River. The straths are capped by a veneer of coarse, subangular to angular gravel that is up to 1 m thick. All the gravels on the strath surfaces exhibit desert pavement, which is most evolved on the highest pediment surfaces and lessens on the lower terrace straths. Also the resistant quartzite clasts on the capping gravel exhibit desert varnish.

The terrace surfaces slope NNW and are punctuated by incised channels that form four erosional arm-like ridges. Some surfaces extend up to 100 m along the side of the drainages as thin strands caused by dissection of surfaces. The terraces slopes range locally from 0.4 to 8.0 degrees, but generally fall within l and 2 degrees. The total vertical relief from the top pediment surface to active flood plain ranges from 13 to 16 m depending on distance from the range front.

The terraces formed due to the interaction of the uplift of the Sierra El Mayor along the Laguna Salada and Cañada David Faults and erosional process of the Colorado River during large flooding events, which carved the straths at different base levels. Due to the presence desert pavement and varnish, I believe the terraces were formed during the late Pleistocene-early Holocene during times when flow down the Colorado River was greater. Using previously derived slip rates the terraces would have formed within 8000 years of each other.

- 1 -

Introduction

A series of terrace flights are present along the eastern margin of the Sierra El

Mayor in northern Baja California. The terrace surfaces are located at the edge of an active flood plain of the Colorado River delta, and above the high tide region of the northern Sea of Cortez. The terraces consist of four distinct steps and an upper pediment surface. They are cut into fine-grained Colorado River Delta deposits and are capped by coarse alluvial deposits (Carter, 1977). The area east to the terraces contains numerous levees that control water flow to the area. When the levees are breached, flooding occurs at the base of the lower most terrace surface. A number of questions can be asked regarding the genesis of the terraces found in this geologically diverse area. Are these terraces the result of the interaction of the Colorado River with deposits at the base of the

Sierra El Mayor due to uplifting of the range? Were the terraces formed by high sea level stands of the Sea of Cortez? Or could they have formed on the shores of ancient pluvial lakes?

The system transitions from a transform fault to an active spreading center in and northern Baja California. Geothermal plants both in Imperial County, California and , Baja California are evidence of the active spreading centers in the vicinity (Fuis and Kohler, 1984). The Sierra El Mayor mountain range is located in Northern Baja California at the western extent of the

Colorado River delta (Figure 1). The Sierra El Mayor and Sierra Cucapa form a northwest-trending mountain range that separates the Colorado River delta from the

Laguna Salada Basin to the west.

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Figure 1: Location Map (base map: INEGI Digital Elevation Model, 60-meter gird in southeast corner, remaining areas are 90-meter grid).

-3- The northwest trending Elsinore Fault in Southern California is thought to continue along the western margin of the Sierra Cucapa and Sierra El Mayor as the

Laguna Salada Fault (Figure 1), which last ruptured in 1892 (Mueller and Rockwell,

1995).

Studies related to the Sierra El Mayor and Sierra Cucapa have focused mainly on the western side of the mountain range along the Laguna Salada Basin. Mueller and

Rockwell (1995) studied faulted alluvial fans and bajadas along the Laguna Salada Fault and concluded that the fault has been active in the Holocene. They also were able to derive slip rate and calculate reoccurrence intervals for the fault. Axen and Fletcher

(1999), studied scarps along the west-central Sierra El mayor, and concluded that this area was bounded by an active low-angle detachment fault (Axen and Fletcher, 1999).

Carter (1977) studied the area around the south and southwestern ends of the

Sierra El Mayor. In particular, he studied a terrace that resembled dissected remnants of abruptly truncated alluvial fans (Carter, 1977). He also noticed that the fill in these deposits were not all colluviums or alluvium, but alluvium interbedded with well-bedded sands, silts and clays of either deltaic or marine origin. He concluded that these pediment surfaces were formed by the interaction of episodic uplift with a steady-state slope that formed three terrace surfaces and segmented alluvial surfaces with the youngest located at the farthest extent of the alluvial deposits. The terraces on the east side may also be records of fault activity.

The terrace sequence, which is the focus of this study, is located on the eastern base of the Sierra El Mayor and is relatively well defined in the northern ~5 kilometers of the Sierra El Mayor (Figure 2). In this study I map and describe a set of terraces surfaces

- 4 - that covers an approximate area of about 0.5 km2. Detailed descriptions of the rock units that comprise the terrace surfaces were obtained in order to interpret their origin.

Available maps for this area are at a scale of 1:50,000, which are too large a scale to be used to map the terrace surfaces. I surveyed the terraces in order to define the boundaries of the terrace surfaces and obtain useful relative elevation data. The study also integrated

Global Positioning System (GPS) and Geographical Information System (GIS) to better define and analyze the terrace surfaces. Evolution of these terraces may help decipher the recent uplift record of the Sierra El Mayor and its links to active faulting in the area.

Figure 2: Photo showing terraces (facing southwest). Sierra El Mayor is in background.

- 5 - Terraces as Indicators of Geologic Activity

“Terraces are relatively level bench or step-like surfaces breaking the continuity

of a slope” (Bates, 1984). Terrace formation can be broken down into two processes: aggradational or degradational. Aggradational terraces form as a result of streambed load, which is left on a surface at different base levels. Aggradational terraces can also form as a result of infilling an incised river channel with bed load material, which is then incised and refilled at lower base level. Aggradation terraces usually form along stream or rivers.

Degradational terraces are formed by erosional processes whereby terrace straths are cut into older material. If the base level is lowered, the older upper surfaces are preserved and another strath is cut into deposits at the lower elevation, thus forming degradational terraces (Burbank and Anderson, 2001). Figure 3 shows some examples of degradational terraces straths cut along streams or rivers. Terraces do not have to form by just one of these processes alone. They can be the result of a combination of both aggradational and degradational processes (Figure 3c) (Burbank and Anderson, 2001).

Terraces are geomorphic markers that can be used to measure uplift caused by tectonic processes, surface water level change caused by climate change, or a combination of these processes. In order to optimize the data retrieved from terraces they must meet the following criteria: 1) initial undeformed geometry must be known; 2) they must have dateable material; and 3) they must have a high degree of preservation with respect to time frame of the processes that form them. However, if all the criteria are not

met they still can be useful tools for tectonic and climate analysis. For example, if dates

- 6 - cannot be extracted from the terrace material, they can still be useful in deciphering relative displacement but not a displacement rate (Burbank and Anderson 2001).

Figure 3: Schematic configuration of river terraces A. Cross-sectional sketches of aggradational and degradational fluvial terraces. B. Paired and unpaired river terraces. C. Cross-section showing complex sequence of aggradational and degradational surfaces. Multiple cut-and-fill events are outlined in right- hand box. (Modified from Burbank and Anderson, 2001)

- 7 -

Deriving uplift rates can be difficult because a terrace can form by multiple

processes occurring at the same time, such as marine terraces that form as a result of sea

level changes and tectonic uplift. Sea level fluctuations are known from various studies,

which have dated marine terraces using different methods. Sea level curves, showing sea

level fluctuation, have been developed using data derived from the studies and can

facilitate the derivation of uplift rates. Marine terraces have been used to derive late

Quaternary uplift and potential. For example, marine terrace uplift in the San

Joaquin Hills in southern California was determined using Quaternary sea level curves

and 230Th dating of solitary corals found in the terraces (Grant et al., 1999).

The formation of lacustrine terraces can also involve multiple processes. An early study by G. K. Gilbert in 1890 of shorelines along Lake Bonneville in Utah showed that the terraces there formed by shrinkage of the lake and crustal rebound of the Wasatch

Range (Burbank and Anderson, 2001). The rate of uplift due to crustal rebound derived from just dating the terrace material and terrace displacement of the Lake Bonneville terraces would be incorrect because multiple processes led to the formation the terraces.

Other information would be needed to correct for Lake Bonneville water levels in order to derive an accurate uplift rate.

Fluvial terrace also form reliable geomorphic markers. They can form when rivers or streams cut into flood plain deposits, alluvial fan deposits, or bedrock material during a period of relative stability. As the river down-cuts below the original surface it forms a terrace. If episodes of uplift occur, the relative base level changes and the river continue to down-cut to develop a sequence of terraces that record the uplift event. If the

- 8 - terraces are preserved, uplift rates are calculated by dating the terraces and measuring the height differences of the terraces. The terraces may be continuous or isolated surfaces along the path of the river. Isolated surfaces can be correlated to each other if they maintain the same heights relative to one another (Leopold, 1992).

Regional Geology

The area where the Sea of Cortez and Colorado River meet is geologically diverse and is located near major population centers, of San Diego, Los Angeles, and the

Mexicali-Calexico border region. Many processes have shaped and continue to affect this area; two of the most significant of these processes are faulting and the depositional and erosional effects of the Colorado River delta. The San Andreas and Elsinore faults extend through the region and are of great concern to geologists in Southern California.

The Laguna Salada Fault is believed to be the southern extension of the Lake Elsinore fault (Figure 1). Studies on the Laguna Salada fault suggest an earthquake of 7.1 occurred on February 23, 1892 (Mueller and Rockwell, 1995). Additional recent seismicity suggests that the area near and around the Laguna Salada is a seismically active area (Figure 4). Several M4-5 have occurred in the last 70 years in the vicinity of the eastern Sierra El Mayor- Sierra Cucapa region (Figure 4).

Geology of the Sierra Cucapa and Sierra El Mayor

The Sierra Cucapa and Sierra El Mayor are the two most prominent topographic features in the study area. They reach a maximum elevation of just under 1100 m and are bounded by the Laguna Salada and Cañada David detachment faults on the west and by

- 9 -

SITESITE

Explanation Q(al)---Alluvial Deposits Q(B-Bvd)---Basalt/Volcanic Breccia M(Gn)---Mesozoic Gneiss Q(eo)---Eolian Deposits T(cg)---Tertiary Conglomerates P(M)---Paleozoic Marble Q(cg)---Conglomerate Deposits K(Tn)---Cretaceous Tonalite P(Gn)---Paleozoic Gneiss Q(la)---Lacustirne Deposits K(Gd)---Cretaceous Granodiorite Figure 5: Geologic Map of the Sierra Cucapa and Northern Sierra El Mayor (INEGI, 1983).

-11- the Colorado River Delta on the East (Figure 5), (Axen et al., 1998). The bedrock of the

Sierra Cucapa and Sierra El Mayor consists mostly of Paleozoic gneiss of meta-

sedimentary origin that was intruded by continental magmatic arc tonalite and

granodiorite in the Cretaceous (Barnard, 1968; Siem, 1992; INEGI, 1983). Younger

conglomerate deposits (TQcg) and the Pliocene-Pleistocene Palm Spring Formation

(Tcg) and younger conglomerate deposits (Qcg) are exposed northwest of the site in an

uplifted basin, the Lopez Mateos Basin (Figure 5). The younger conglomerate deposits

are traction and debris flow deposited gravels (Axen et al., 1998). The Palm Spring

Formation consists of pebbly quartz sandstone, arkosic pebble conglomerate, and marine

fossil-bearing pebbly quartz sandstone. These deposits are believed to be deposited

synchronous with the detachment faulting in the area (Axen et al., 1998). Holocene

basalt and volcanic breccia (QB-Bvd) are exposed north of the site at Cerro Prieto (Figure

5).

Faults in the Sierra Cucapa and Sierra El Mayor

The Sierra Cucapa is bounded and cut by the dextral northwest striking faults of the Laguna Salada fault system. This faulting began in middle or late Pliocene and has continued to the present (Mueller and Rockwell, 1991; 1995). In 1892, 22 km of the Laguna Salada fault ruptured along its westernmost strand and along a fault farther south, the Cañon Rojo fault, which intersects the Laguna Salada Fault at about 90o

(Mueller and Rockwell, 1999). The total displacement occurring on the fault was measured at 4 m of right-lateral offset and 3.5 m of vertical offset (Mueller and

- 12 - Rockwell, 1999). The Cañon Rojo Fault is a normal fault with an estimated slip rate of

2-4 mm/yr (Dorsey and Martin-Barajas, 1999).

In the past 50,000 yr, the Laguna Salada fault has exhibited a strike-slip to dip- slip ratio of approximately 1:1 with a slip rate of 2-3 mm/yr and a recurrance interval of

1000-2000 yr (Mueller and Rockwell, 1995). Activity on the fault is isolated on the western range-bounding fault and the strands that dissect the Sierra Cucapa are inactive.

The activity along the fault system post-dates the detachment faulting observed along the northern Sierra El Mayor but not in the southern extents of the range where detachment faulting may still continue (Figure 4) (Axen et al., 1998, 1999; Mueller and Rockwell,

1995).

The Sierra El Mayor is bounded in the north by the northeast striking Cañon Rojo fault and divided into three plates by the Cañada David detachment faults (Axen and

Fletcher, 1999). The Cañada David fault consists of two major segments: (1) a north- striking, west-dipping segment that divides the northern ranges’ Miocene(?)-

Pleistocene(?) sedimentary upper plate and metamorphic rocks of the middle plate; (2) a northwest striking segment that offsets the western front of the Sierra El Mayor (Axen et al., 1998). Along the northern section of the Sierra El Mayor, the Cañada David detachment fault has been active since the late Miocene through the Pliocene and maybe into the early Pleistocene (Vazquez-Hernandez et al., 1996). Presently the fault is active along the southwestern end of the range, which shows evidence of Quaternary range- front fault scarps (Axen et al., 1999; Mueller and Rockwell, 1999). A range-front fault striking N65W to N75W and dipping 45 degrees southwest bounds the southern end of the Sierra El Mayor (Carter,1977).

- 13 - Colorado River Delta and Ancient Lake Deposits

The Colorado River delta has been active since the late Miocene or Pliocene

(Carter, 1977). The delta deposits transitioned from marine to terrestrial since the

deposition of the latest marine units of the Pliocene Imperial Formation (Carter, 1977).

The Laguna Salada and -Northern Gulf trough have accommodated these

deposits (Carter, 1977). They cover an area of approximately 7700 km2, but the present

active delta is about 600 km2 (Luecke et al., 1999). The decrease in area is due to the construction of dams and conversion of wetlands to agriculture (Luecke et al., 1999).

Total river discharge has decreased into this area from approximately 27000 x 106 m3 in

1918 to less than 1000 x 106 m3 per year in some more recent years (Figure 6), (Cohen et

al., 2001).

Figure 6: Colorado River discharge at the California-Mexico boundary 1910-1998. (Modified from

Cohen, 2001)

- 14 -

Ancient Lake Cahuilla existed in and around the area that the now occupies.

The lake existed as late as the late 1500’s and encompassed an area of about 5700 km2 with a depth of over 95 m (Walters, 1983). Lake Cahuilla formed due to natural damming of the Colorado River caused by the deposits of the delta, which resulted in the diversion of the river into Lake Cahuilla. The Lake disappeared when the natural dam was breached (Walters, 1983). Its southernmost extents reached into Mexico around the area of Cerro Prieto, which is the divide between the Salton Basin and the Gulf of

California (Figure 7).

Geology of Terraces

The deposits into which the terraces are cut consist of a wedge of interbedded clays, silts, sands, and gravels deposited at the base of the Sierra El Mayor, which slope and thin eastward toward the basin. These wedge deposits are the result of the intermingling of the coarser alluvial deposits derived form the Sierra El Mayor (sands and gravels) and the finer deltaic flood plain deposit (silts and clays) from the Colorado

River (Figure 8a and 8b). Coarser material is found near the range front and becomes finer at the more distal, basinward edge of the wedge. The wedge deposits are believed to be the same as the deltaic deposits described in Carter (1977). Using Uranium Series dating of caliche blebs at two exposures, Carter (1977) established a minimum age of deposition of about 60,000 years before present. The straths that dominate the geomorphic features on the site were carved mostly into the wedge deposits. A veneer of gravel up to 1m thick , containing mostly schistose, gneissic, and plutonic clasts derived from the Sierra El Mayor, cap the wedge deposits.

- 16 - CAPPING GRAVEL

Figure 8a: Picture showing interlayering of silts, clays, sands, and gravels of the wedge deposits exposed along stream channel. Deposits are relatively close to mountain front. Daypack shown on lower center of picture for scale. Note the veneer of gravel capping the section.

CAPPING GRAVEL

Figure 8b: Picture of deposits exposed by stream channel near the distal extents of the deposits. The upper most gravel is about 1.5 meters thick. Sands, Silts, and clays are interbedded below the capping gravel. (Daypack shown on lower left of picture for scale).

- 17 - Ten trenches were dug into the terrace surfaces to a depth 0.4 to 1.8 m. The

trenches were dug using a shovel and pick and measured 0.5 m by 0.5 m. The trench

deposits were broken down into units, which were measured and described. Samples

were collected from each of the units for analysis in the lab (Figure 9a).

Clay Units

The clay units are brown and resistant with abundant gypsum and range from silty

clay to clay. The units range from about 2 to 20 cm in thickness and are more abundant and thicker farther from the range front of the wedge deposits and thin closer to the mountain range.

Silt Units

The silt units are tan to light brown with some interbedded fine sand. The units are relatively less resistant than the clay, and contain abundant biotite grains. The units range from about 20 to 75 cm in thickness and are most abundant and thickest at the distal parts of the wedge deposits and thin closer to the mountain range.

Sand Units

The sand units are light brown, fine- to coarse-grained, well sorted and poorly

graded with minor gravel, silt, and clay components. The sand unit exhibits minor trough

cross bedding near range front, likely due to fluvial interaction. The grains are composed

of quartz, feldspar, and biotite. The units from about 0.5 to 1 m thickness and coarsen

and thicken toward to the mountain range.

- 18 -

Figure 9a: Photo showing trench dug below one of the terrace surfaces to expose finer deposits exposed beneath the coarse capping gravel.

Figure 9b: Picture showing desert pavement on upper surface and desert varnish on quartzite cobble (large rock in middle of photo). Note the litter of angular, weathered granodiorite clasts.

- 19 - Gravel Units

The gravels are poorly graded, poorly sorted sandy gravels consisting of sub angular to sub rounded clasts composed of granodiorite, tonalite, schist and vein quartz.

The gravels are abundant closer to the mountain front and are absent at the farthest extent of the deposits where clays and silt dominate the units. The gravels grade to coarse to medium sand with distance away from mountain front.

Capping Gravels Unit

The terraces are capped by a 0.5 to 1.5 m thick veneer of alluvial gravel that

consist of pinkish to gray, poorly sorted, and poorly graded cobble gravels. The cobbles

are composed of subangular to subrounded granodiorite, tonalite, schist, marble, and

quartzite. The gravels are thickest near the base of the mountain range. The uppermost

capping gravel extends to the base of the Sierra El Mayor where it forms a gradational

contact with talus deposits.

The uppermost pediment surface is the oldest of the surfaces, as indicated by the

well-formed desert pavement (Figure 9b). Desert pavement forms due to the influx of

windblown dust on a surface and its subsequent pedogenic modification. With time the

dust particles are transported below the coarser surface material. The desert pavement is

also evident on the lower surfaces but is less evolved, suggesting that these lower terrace

surfaces are younger than the top pediment surface. The granitic bedrock and clasts

derived from it are easily weathered. This high susceptibility to weathering causes the

granite to break down fairly rapidly, which limits the development of desert varnish

surfaces. Desert varnish is common on the more resistant rocks such as quartzite (Figure

9b).

- 20 - Geomorphic Analysis

The geomorphology of the terraces was extracted by assessing field data that include: 1) elevation data collected using total station and differential GPS; 2) Aerial photography from a blimp. The data collection section describes the equipment used to collect the data and how the data were used. Data analysis was achieved using computer programs such as Vertical Mapper, Excel and Rockworks.

Data Collection

A total 3580 survey points were gathered using the Leica 1050L total station, which has a maximum vertical accuracy of 0.01 m and horizontal accuracy of 0.001 m.

The points were collected on ten different days. Four base stations were permanently set up on the terrace surfaces (Plate 1) and are marked with iron rebar hammered into the ground. All survey points were surveyed using a local grid system, which was based on an arbitrary point (BJ-01) with local coordinates of 10,000 m easting and 10,000 m northing, and an arbitrary elevation of 100 m. The terraces were mapped by surveying points on the outer limits of the terraces surface at intervals of about 5 to 10 m between points. Some points were also surveyed on the inner surfaces but at a relatively larger spacing between points. The main focus was on the terrace surfaces and resulted in fewer survey points in the active floodplain surface and lower channeled surface of the terraces.

A Trimble Pro XR Differential GPS Unit was used to determine absolute locations of the four base stations. The GPS unit was set up for 30 to 60 minutes at each base station to collect data at one second intervals. The data collected by Trimble GPS unit was then phase differentiated using base station data collected at Point Loma which is located approximately 225 km away. A station 60 km away at INEGI in downtown

- 21 - was not used. Using data from INEGI would have resulted in more accurately located base stations but data were not available at the time. Phase differentiation corrects locations using the different signals received by the Trimble receiver in the field and a permanent base station with known coordinates. The differentiated data has a maximum horizontal accuracy of 0.01 m and a vertical accuracy of 0.1 m. The data differentiated with Point Loma data which resulted in an accuracy of approximately .05 m horizontal and a vertical accuracy of 0.4 m. The differentiation calculations were preformed with Trimble Office software. After the data were corrected, the absolute location of the survey points and base stations were back-calculated (Table 1).

TABLE 1: Table Showing Corrected Location of Base Stations (units in meters, coordinate system Universal Transverse Mercator (WGS84))

Base Survey Survey Survey Absolute Absolute Absolute Stations Easting Northing Elevation Easting Northing Elevation BJ-01 10000.0 10000.0 100.0 661105.8 3557462.3 9.6 BJ-02 9902.0 9938.8 101.5 661007.8 3557401.1 11.1 BJ-03 10105.9 9831.8 113.7 661211.7 3557294.1 23.3 BJ-04 9816.7 9791.6 105.9 660922.5 3557253.9 15.5

A 21-foot helium-filled blimp with a 35-mm camera attached at the base was used to take oblique photographs of the thesis site. The photographs were taken at an elevation of 50 to 100 m and were then used in the mapping of the terraces (Figure 10).

Data analysis and Results

The survey data collected by the total station were used to create a Digital

Elevation Model (DEM), that shows terrace surface distribution, and to allow extraction of terrace surface profiles.

- 22 -

Figure 10: Oblique aerial photograph of terraces taken from blimp showing the dissected terraces.

A 1 m grid DEM was created using the survey data with the use of Natural

Neighbor Interpolation (Figure 11a). Since most of the survey points were collected on terrace surfaces, the active flood plain surface and the channels that incise the terrace surfaces have sparse survey coverage. This lack of data on the active flood plain and in the channels results in lower accuracy compared to the terrace surfaces. A topographic map of the terraces was derived from the DEM at a one-meter contour interval (Figure

11b). The DEM shows some distinct areas where the color remains relatively constant, which is produce by of areas with relatively constant elevation. The DEM shows four relatively flat NNW trending arms with slight NNW sloping surface that are punctuated by incised channels. Multiple flat surfaces extend out along the arms. The DEM shows

- 23 - BJ-01BJ-01 --

BJ-02BJ-02 --

BJ-03BJ-03 -- BJ-04BJ-04 --

25 0 25 50 75 Meters BedrockBedrock

Figure 11a: Digital Elevation Model (DEM) showing terrace surfaces. Cooler shades are lower elevation.

BJ-01BJ-01 -- 111 000 000

BJ-02BJ-02 --

111 111 000 000 000 000 111 111 000 111 111000000 000 111111000 111000000 555 111000 555 BJ-03BJ-03 --

111 111111 BJ-04BJ-04 111000 -- 000

000 111000 111111

111 111 25 0 25 50 75 000 555 111 111 Meters BedrockBedrock

Figure 11b: Digital Elevation Model with 1-meter contour interval. Elevations are relative to base station BJ-01 at an arbitrary elevation of 100 m. (Note: base station BJ-01 is at at an absolute elevation of 9.6 +/- 0.4m above MSL)

-24- the larger, higher elevation pediment surface in the southeastern corner, which is shaded

orange, and the lower active flood plain shaded in darker blue.

Evaluation of the DEM, topographic profiles, and field mapping shows that the

area consists of six geomorphic surfaces; the pediment surface (Qp), four terrace straths

(Qt1 thru Qt4 , with Qt1 being the oldest), and flood plain surface (Qf) (Figure 12). Survey

data collected in the field were measured by surveying points along the boundary of the

terrace surface and terrace risers in order to facilitate mapping of the terrace surfaces

edges. The surfaces from the DEM analysis were verified in the field and also checked

against oblique aerial photographs (Figures 13). Some surfaces are exposed along the

side of the drainages as thin strands caused by dissection of the surfaces, while other

terrace sequences were isolated or are not present in some areas due to erosion (Figure

13). Correlations of discontinuous segments were mapped by using relative elevation

and/or correlation to nearby known terrace surfaces.

Five topographic profiles were measured in the field to evaluate the relative

correlation of the terrace surfaces (Figure 14). The cumulative distances between points

were plotted against relative elevation. The profiles are oriented north-south and collected from five distinct areas (Figure 12). Relative elevations were used in the profiles, which were then standardized using the elevation at the intersection of the flood plain and the base of the terrace slope of line 1.

Not all terrace straths are visible in each of the topographic profiles (Figure 14).

Profiles 3 and 5 show six distinct surfaces spaced between 2- and 4-m apart while profiles 1, 2 and 4, show five surfaces. Profiles 1 and 2 do not show all surfaces due to their close proximity to the range front where surfaces are small, steep, and do not show

- 25 - ExplanationExplanation QfQf FloodFlood plainplain depositsdeposits QtpQtp UndiffertiatedUndiffertiated terraceterrace andandand pedimentpedimentpediment surfacessurfacessurfaces BJ-01BJ-01 Qt4Qt4 TerraceTerrace surfacesurface 44 -- Qt3Qt3 TerraceTerrace surfacesurface 33 Qt2Qt2 TerraceTerrace surfacesurface 22 Qt1Qt1 TerraceTerrace surfacesurface 11 QpQp PedimentPediment surfacesurface BedrockBedrock BJ-02BJ-02 SurfaceSurfaceSurface ProfileProfileProfile LineLineLine --

LL LLiiii 25 0 25 50 75 iiiinnn eee 555 LLLL Meters LLLL iiiiiii iiiniiniinn nnnn eeeeeee eeeeeee

3 3 33 3333 LLLL iiiiiii nnnn eeee

4444 BJ-03BJ-03 -- -26- BJ-04BJ-04 --

1111

eeee nnnn iiiiiii LLLL

2222 2 222 eeee nnn iiininn LLL

BEDROCKBEDROCK

Figure 12: Map of terrace surfaces showing profile lines and mapped terrace surfaces. a)a) QpQp

QpQp

G Qt1Qt1 Qt1Qt1 F

D Qt2Qt2 C Qt3Qt3 B Qt4Qt4

A

Qt3Qt3 Qf

E

ExplanationExplanation b)b) QfQf FloodFlood plainplain depositsdeposits Qt4Qt4 TerraceTerrace surfacesurface 44 Qt3Qt3 TerraceTerrace surfacesurface 33 Qt2Qt2 TerraceTerrace surfacesurface 22 Qt1Qt1 TerraceTerrace surfacesurface 11 QpQp PedimentPediment surfacesurface G

F

C B D

A NNN

E

Figure 13: Shows correlation of terraces surfaces on oblique aerial photographs and terrace map. Note: Letters correspond to matching surfaces between a) and b). -27--27--27- 125

120

115

Elevation (m) 110 Line 1 -28- Line 2

105 Line 3

Line 4

Line 5 100 0 +.32 + 1.4 +0.8 +2.4

95 0 100 200 300 400 500 600 700 800 900 Distance (m) Figure 14: Topographic Profiles (see figure 13 for location) with correlation of surfaces (shaded regions), total of six surfaces (5 in shaded regions and at one the an elevation of 100. Number represents elevation offset used to normalize surfaces. up at resolution of profiles. Field observation indicates that terraces near the range front

are more subdued. Profile 4 does not show all terraces because it is located in area that exhibits greater degree of erosion.

The surfaces can be correlated between the profiles. The profiles show that the total relief from the top of the pediment surface to the active flood plain is between 13 and 16 m (Figure 14). The profiles also show the absolute elevation changes between the surfaces of 3 to 4m for the flood plain and the lowest surface and continuing upward at 4 to 6 m, 1 to 3 m, 1 to 3 m, and 3 to 5 m for successive terrace surfaces.

In Figure 15 surface profiles of the terraces were plotted to show continuity of the surfaces. The elevations of the surface were plotted versus distance from an arbitrary line

located in the active flood plain perpendicular to the general slope of the terraces. This

figure shows the general slope of the individual terraces and the pediment surface. The

slopes of the terraces are relatively parallel to one another while the slope of the pediment

surface is slightly steeper. The figure also shows two discontinuous Qt2 terraces located

upslope from the main Qt2 surface. These two discontinuous surfaces can be correlated

to the larger Qt2 surfaces further down slope by extrapolating the slope of the main

terrace.

Figure 16 shows surface profiles for most of the mapable terrace surfaces. The

graph shows five clusters of surface profile. The clustered profiles are indicative of

correlation between surfaces. The surface profiles were measured at different locations

on the site and show the elevation range for surfaces, Qp, Qt1, Qt2, Qt3, and Qt4. With

the exception of only a few profiles, most profiles lie within a small elevation range of

- 29 - 30

28

26 Qp

Absolute Elevation(amsl) 24

22 Qt1 Qt2 20 -30- 18 Qt2 Qt3 16

14

12 Qt4

10 0 50 100 150 200 250 300 350 400 450 500 Distance (m)

Figure 15: Surface Profiles of elevation vs distance to an arbitrary line in the active floodplain parralel to the strike of the terrace slope. 30

25

Elevation (m) 20 -31-

15

10

5 0 50 100 150 200 250 Distance (m) Figure 16: Graph showing distribution of terrace surface profiles . (Profile Colors: Blue=Qt4, Dk Green=Qt3, Lt Green=Qt2, Orange=Qt1, Red=Qp) other surfaces of the same unit. The three profiles that deviate are located at the distal parts of the site. Since the terraces slope away from the range front, the surfaces at the distal end of the area have lower elevations, but still can be correlated to the more proximal surfaces by extrapolating the terraces surfaces (Figure 15).

The survey data were used to derive elevation ranges of each of the terrace surfaces (Table 2). The elevation change for each surface ranges between 4.6 and 7.9 m.

The pediment surface elevation changes up to 12.9 m. The pediment surface has a maximum elevation of 31.8 m, which is found near base of the mountain range. The elevation changes of each surface reflect their general downward sloping toward the north.

TABLE 2: Maximum and minimum elevation of the surfaces (MSL,units in meters) Surface Minimum Maximum Range Qp 18.9 31.8 12.9 Qt1 18.1 22.7 4.6 Qt2 14.1 21.6 7.5 Qt3 11.1 17.5 6.4 Qt4 8.7 16.6 7.9

The survey data were also used to solve three point problems in order to obtain slope angles and orientation for the surfaces, which were then used to construct rose diagrams and stereo plots in order to anlysize the slope direction and slope angle of the terrace surfaces.

The 3-point problems used ten sets of three randomly selected surveyed points

(Table 3). The points ranged in distance form 25 to 300 meter from each other. The terraces slopes range locally from 0.4 to 4.1 degrees, but generally fall within l and 2

- 32 -

Sample Strike Dip Dip Average Dip Standard Average Dip Standard pts Azimuth Devation Azimuth Devation QP-1 105.3 0.8 15.3 Qp-2 105.5 1 15.5 Qp-3 97.3 1.7 7.3 Qp-4 99.4 1.2 9.4 Qp-5 69.1 0.6 339.1 Qp-6 66.2 1 336.2 Qp-7 70.7 1.3 340.7 Qp-8 76.1 1 346.1 Qp-9 80.2 1.2 350.2 Qp-10 149.2 0.4 239.6 1.0 0.4 1.9 25.3 Qt1-1 14.5 2.3 284.5 Qt1-3 39.2 0.9 309.2 Qt1-4 101.4 1.2 11.4 Qt1-5 101.5 1.2 11.5 Qt1-6 2.8 0.9 272.8 Qt1-7 84.2 1.2 354.2 Qt1-8 53.6 0.7 323.6 Qt1-9 4.3 1.2 274.3 Qt1-10 156 2.4 246 1.3 0.6 331.9 52.6 Qt2-1 97 1.9 7 Qt2-2 109.5 3.4 19.5 Qt2-3 31 2.4 301 Qt2-4 32.6 2.3 302.6 Qt2-5 69.8 0.7 339.8 Qt2-6 20.3 1.9 290.3 Qt2-7 34.3 1.1 304.3 Qt2-8 6.4 1.7 276.4 Qt2-9 17.1 1.8 278.1 Qt2-10 35.9 0.9 305.9 1.8 0.8 315.4 34.8 Qt3-1 70.9 3 340.9 Qt3-2 71 3.9 341 Qt3-3 58.9 4.1 328.9 Qt3-4 29.4 1.7 299.4 Qt3-5 29.5 2.1 299.5 Qt3-6 18.7 1.2 288.7 Qt3-7 27.4 1 297.4 Qt3-8 52.2 0.9 322.2 Qt3-9 52.2 2.4 322.2 Qt3-10 60.3 2.5 330.3 2.3 1.1 317.1 19.2 Qt4-1 65.1 1.3 335.1 Qt4-2 60.6 2.3 330.6 Qt4-3 86 1.8 356 Qt4-4 97.3 1.2 7.3 Qt4-5 81.8 1.5 351.8 Qt4-6 75.2 0.9 345.2 Qt4-7 67.4 1.6 337.4 Qt4-8 71.3 1.1 341.4 Qt4-9 68.2 1.1 338.2 Qt4-10 63 1.2 333 1.4 0.4 343.6 11.6

Table 3: Table Showing Slope Analysis Results (all in degrees)

- 33 - degrees. Terrace Qt3 contains the steepest surface slope with and average dip of 2.3. The pediment surface Qp shows the gentlest slope of the five surfaces with an average dip of

1.0 degrees. The 3-piont problems also show that the terraces dip azimuth ranges between 315.4 to 1.9 degrees.

The random 3-point solutions were used to construct rose diagrams in order to evaluate average surfaces orientation. The mean down slope direction for the terrace straths Qt1 thru Qt4 gradually changes direction form northwest to north, respectively,

toward the toes of the straths. The confidence interval or the deviation in slope direction

decreases with increasing elevation. The slope direction of the pediment surface Qp does

not specifically change with the terraces straths.

Discussion

As outlined above, there are several mechanisms that can produce strath terrace.

At this point, I can not unequivocally determine the genesis of the Sierra El Mayor

terraces. However, I can rule out some processes and offer a hypothesis. The terraces

are not of marine origin because the last sea level high stand occurred 125,000 years ago

at 7 m above present sea level (Gallup et. al, 1994) (Figure 17). The maximum elevation

of the upper most terrace surface is 22 m (amsl) (12 m above present flood plain plus 9.6

m absolute elevation of flood plain). This high sea level stand would have not reached

the elevation need to form the terraces. Also the terraces are only found on the eastern

side of the Sierra El Mayor. If they were solely related to sea level high stand, they should be found on the other side of the range along the Laguna Salada and along other range fronts of the Sea of Cortez. No sequences like the ones on the eastern side of the

- 34 - range are found along the Laguna Salada or northern Colorado delta region of Sea of

Cortez.

The elevation needed to form the terraces could have been attained if the terraces had formed as a result of a high sea level stand and tectonic uplift. The elevation difference between the active flood plain and the average elevation of the highest terrace surface is about 15 m (Figure 14). If the surfaces were formed 125,000 years ago at an elevation of 7 m above present sea level and have since been uplifted tectonically, the terraces would have to been uplifted at least 15 m in order to justify the present position of the highest terrace surface. The offset during this time frame suggests an uplift rate of about 0.10 mm/yr, which is inconsistent with that derived for the Laguna Salada Fault and Cañon Rojo fault which is about 2-3 mm/yr and 2-4 mm/yr, respectively (Mueller and Rockwell, 1995; Dorsey and Barajas, 1999).

) msl , m ( Elevation

Figure 17: Graph showing relative sea level changes. Note high sea level stand 125,000 years ago. (Modified from Gallup et al., 1994). Numbers underneath profile are Oxygen Isotope Stages.

- 35 - Another means by which the terraces could form would be by lacustrine wave-cut

action when Lake Cahuilla was present in the area. The lake is thought to have never extended this far south. The southernmost extent of the ancient Lake Cahuilla lies 35 km north of the terraces (see figure 7). In the past, the Colorado River was diverted by natural dams into the present day Salton Sea basin forming vast lakes just like the ancient

Lake Cahuilla. The Shores of Lake Cahuilla are thought to have never reached this far south and probably did not carve the terraces.

Another scenario is that the terrace formed from a combination of the breaching of Lake Cahuilla and uplift of the Sierra El Mayor since 695 AD. In this scenario, the uplift of the range would have been too rapid since the flow would have occurred within

a relatively short time. The terraces could have formed from multiple breaches of Lake

Cahuilla and its predecessors, since there is evidence that 4 different lakes existed to the

north between 695 A.D. and 1589 A.D (Waters, 1981). These four breaches would be

consistent with the terraces, but the uplift of the surfaces also would have been

inconsistent with locally derived uplift rates (Mueller and Rockwell, 1995; Dorsey and

Barajas, 1999). An unreasonable uplift rate of 17 mm/yr during the 900 yr time span

between breaches would be needed to account for the 15 m elevation difference between

the terrace surfaces. Lastly if the terraces had formed from these flows of water, other

terraces would have formed farther north along the Sierra El Mayor and Sierra Cucapa.

There is no evidence of these types of terrace sequences at the base of the Sierra Cucapa

farther north.

The terraces are found only on the eastern side of the Sierra El Mayor, which

coincides with the present-day location of the Colorado River adjacent to the Sierra El

- 36 - Mayor/Cucapa area (Figure 18). At present-day, the waters of the Colorado occasionally reach the active flood plain at the base of the terraces, but pre-1935, water would have probably reached the base of the terraces due to a larger flow rate. Before the dams were built, maximum flow on the river has reached a rate of at least 10 times greater than present day flood stages (Cohen et al., 2001), but may have not had sufficient energy to carve the terraces. Also the terraces dip towards the Colorado river. If the terraces had been carved by the Colorado River the surfaces should slope parallel to the river.

I interpret that the terrace sequences formed as the result of at least 4 seismic events and incision due to the base level lowering. Since the ages of the wedge deposits are estimated to be 120,000 years old (Carter, 1977), and the age of the terraces surfaces are not known at this time, ages of the terrace-forming events must of occurred less than

60,000 years ago. Using the displacement rate derived by Mueller and Rockwell (1995) of 2-3 mm/yr on the Laguna Salada fault and the total elevation change between the present flood surface and the top of Qt1 of 15 m, I estimate that surfaces Qt1 and Qt4 formed within 5,000 to 8,000 year of each other. Rock varnish on the Qt4 terrace surface suggest ages greater than 10,000 years, which is inconsistent with the terraces forming

8,000 years ago. However if the terraces formed in wetter climate such as 20,000 years ago during the last ice age (Gallup et al., 1994), it could account for the age suggested by the rock varnish. The increase flow in the past could also account for the erosion pattern observed in the in the Northern Sierra El Mayor. The LANDSAT7 image shows where erosion due to the Colorado River has cut into the Northern Sierra El Mayor (Figure 18).

If the maximum range of 8000 years for the formations of Qt1 and Qt4 is used, then the

Qt4 surface would have formed 12,000 years ago at the end of the Pleistocene. The

- 37 -

Study Area

Figure 18: LANDSAT7 TM Image of Sierra El Mayor. Note the darker red region which is the current stream channel of the Colorado River. Agriculture (bright red rectangles) areas lie in flood plain of the delta.

climatic change at the end of the Pleistocene would have brought on dry conditions

leading to a decrease in the flow of the Colorado River. This then lead to the deposition

of finer sediments at the base of the terraces and the termination of the terrace-forming

process.

Hypothesis for terrace genesis

Terrace genesis began with the deposition of sediments along the Sierra El Mayor as flood deposits on the Colorado River delta. Coarser alluvial sediments derived from the Sierra El Mayor intermingled with the finer flood plain deposits creating the wedge

- 38 - deposits at the base of the range. Uranium-Series ages of Carter (1977) suggest the deposition of the wedge sediments began no earlier than 60,000 years ago.

Approximately 17,000 to 20,000 years ago vertical displacement occurred on the western side of the Sierra El Mayor, likely attributed to the Laguna Salada Fault or

Cañada David detachment fault, resulting in the lowering of the local base level that consequentially carved the first strath. This change in level also disrupted the intermingling of alluvial and flood plain sediments. The disruption resulted in an accumulation of capping alluvial gravels on the uplifted surface since the finer sediment was no longer able to interact with the coarser deposits.

At least 4 other uplift events are evident from the terraces. The process of uplift, the craving of the straths, and the accumulation of gravel was repeated in the same way in these other 4 uplift events. Since all the terrace surfaces have capping gravels one might interpret that the deposition of the capping gravels occurred after all the straths were carved. However, the desert pavement on the lower surfaces is less evolved than on the upper surfaces, which implies that the gravel on the upper surface formed earlier than the lower ones.

Between each successive uplift event, the straths were incised. Thin terrace strath remnants are observed on either side of these incised gullies. The strath segments can be easily correlated to the larger segments of that surface, which implies that the surfaces were at one time continuous prior to incision after base level change due to uplift.

Conclusions

The terraces are interpreted to have formed by the down cutting of flood plain deposits at the base of the Sierra El Mayor by the Colorado River and the episodic uplift

- 39 - of the mountain range. The terraces are interpreted to record of at least 4 seismic events between about 20,000 and 12,000 year ago. A reoccurrence interval of 1200 to 1800 years per event was calculated by using the displacement rate from Mueller and

Rockwell, (1999), and the total elevation change measured in this study. This region remains active and has the potential of continued uplift but one element that helped to create the terraces is long gone. The area at presents experiences very little moisture thus the terraces have seized to form. Ages obtained in future dating studies of the terrace surfaces will provided more control on the age of the events that lead to the formation of the terraces.

Acknowledgements

I would like to thank the Cal State Fullerton Geological Science department for the use of their equipment and facilities. I also owe a large amount of gratitude to John

Foster, Richard Laton, John Copper, Brady Rhodes, Jerry Brem, and Diane Clemens-

Knott, who through their instruction have greatly influenced this thesis. I would also like thank Jimmy Gonzales and Sean Hunt for all their help out in the field. Without them the surveying of the terraces would have taken a very long time. Phil Armstrong has advised and mentored me throughout this project. His knowledge and experience have been a great benefit to this project.

- 40 - References Cited

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- 42 -

Appendix

Rose Diagrams

- 43 -