GENESIS OF THE QUATERNARY TERRACES OF THE EASTERN SIERRA EL MAYOR, NORTHERN BAJA CALIFORNIA, MEXICO 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 Baja California, 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