CALIFORNIA STATE UNIVERSITY, NORTHRIDGE Investigation of The
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CALIFORNIA STATE UNIVERSITY, NORTHRIDGE Investigation of the depositional environment of clustered boulders, Nojoqui Valley, Santa Barbara County, California A thesis submitted in partial fulfillment of the requirements For the degree of Master of Arts in Geography By Danielle D’Alfonso May 2018 The thesis of Danielle D’Alfonso is approved: _________________________________________ ______________ Dr. Antony Orme Date _________________________________________ ______________ Dr. Julie Laity Date _________________________________________ ______________ Dr. Amalie Orme, Chair Date California State University, Northridge ii Acknowledgement I am grateful to Bill Giorgi of Nojoqui Ranch and Art Green of the Alisal Ranch for providing access to their properties, my advisor Dr. Amalie Orme for her guidance and support, and my field assistant Eiko Kitao. iii Dedication This paper is dedicated to the long-time residents of Nojoqui, and to people everywhere who wonder why boulders are in their yard. iv Table of Contents Signature Page ……………………………………………………………………………ii Acknowledgment ………………………………………………………………...………iii Dedication …………………………………………………..……………………………iv List of Tables ……………………………………………………………………………vii List of Figures ……………………………………….…………………………………viii Abstract …………………………………………………………………………..………ix Chapter 1: Introduction …………………………………………………...………………1 1.1 Statement of Purpose …………………………………………………………1 1.2 Location ………………………………………………………………………2 Chapter 2: Scientific Background and Previous Work ……………………...……………5 2.1 Geologic and Tectonic Framework …………………………………………..5 2.2 The Santa Ynez Fault …………………………………………………………6 2.3 Bedrock Geology ………………………………………………………..……8 2.4 Surficial Boulder Deposits ………………………………………………..…10 2.5 Climatic History ……………………….………….…………………………13 Chapter 3: Physical Setting ………………………….……..……………………………14 3.1 Landscape …………………………………………...………………………14 3.2 Anthropomorphic Influence on Landscape ………………………………….16 Chapter 4: Methods ………………………………………………………………...……17 4.1 Developing a Basemap………………….………………………...…………17 4.2 Determining Boulder Locations …………….………………………………17 v 4.3 Identifying a Source for the Boulders……….………………………………18 4.4 Distribution of Boulders……………………………………………………..19 4.5 Geomorphic Analysis………………………………………………………..23 Chapter 5: Results ……………………………………………………….………………24 5.1 Lithologic Description ………………………………………………………24 5.2 Boulder Distribution …………...……………………………………………31 5.3 Geomorphology……. …….…………………………………………………54 Chapter 6: Discussion ……………………………...……………………………………60 6.1 Lithology and Bedrock Source………………………………………………60 6.2 Distribution and Depositional Environment ………...………………………61 6.3 Timing of Events……………………………………………………………..65 6.4 Future Work …………………………………………………………………66 Chapter 7: Conclusions …………………………….……………………………………68 References …………………………………………………………………………….…70 vi List of Tables Table 1: Lithologic Comparison of Bedrock in Nojoqui …………………………………9 Table 2: Section A clast count on Nojoqui Ranch ridge…………………...…………….39 Table 3: Section B clast count .………………………………………….………………43 List of Figures Figure 1: Geographic location of Nojoqui………………………………………...………4 Figure 2: Nojoqui watersheds ……………………………………...……………………15 Figure 3: Boulder distribution …………………………………..………………………21 Figure 4: Grain-size classification ………...…………………………………………….22 Figure 5.1a: Round arkosic sandstone boulder…………………………………………..25 Figure 5.1b: Angular arkosic sandstone boulders…………………………………….….26 Figure 5.1c: Boulder exhibiting both fresh and weathered surfaces……….………...…..27 Figure 5.1d: Varicolored lichens on conglomerate……………………………………....29 Figure 5.1g: Open-framework conglomerate boulder pile…………………………...….29 Figure 5.1e: Large 2 m conglomerate boulder covered with gray and green lichen…….30 Figure 5.1f: Conglomerate boulder with large quartzite and rhyolitic cobbles …...…….31 Figure 6: Boulder Ridge in Section A …………………………………...………………34 Figure 7: Nojoqui Ranch, Section A……………………………………………….…….35 Figure 8a: Section A Group 1……………………………………………………………36 Figure 8b: Section A Group 12……………………………………………..……………37 Figure 8c: Section A Group 13……………………………………………….….………38 Figure 8d: Section A Group 17………………………………………………….…….…38 Figure 9: Section B, Forty Acre Ranch…………………………………………………..41 vii Figure 10: Detailed grid survey……………………………………………..…………...44 Figure 11: Section C, Alisal Ranch ……………………….………………….………….46 Figure 12a: Detailed drawings of boulder deposits…………………………...…………47 Figure 12b: Section C Station 18………………………………………………...………48 Figure 12c: Section C Station 17………………………………………………..….……48 Figure 13: Section D the North Slope………………………………………….……...…50 Figure 14: Section D Conglomerate at Station 5………………………………...………51 Figure 15: Section D Station 6 Arkosic boulders pile. ……………………………….…52 Figure 16: Section D Matilija sandstone ……………………………………………..…53 Figure 17: Section B Unconformity …………………………………………….…....…56 Figure 18: Geologic map………………………………………………………..…….…57 Figure 19a: Section D Unconformity …………………………………………..……....58 Figure 19b: Section D Fanglomerate ……………………………………………………58 Figure 19c: Angular Unconformity……………………………………………...………59 Figure 20a: Very large-sized boulder-levee deposits in Montecito, CA………..…….…63 Figure 20b: Medium-sized boulders, Montecito debris flow……………………………63 viii Abstract Investigation of the depositional environment of clustered boulders, Nojoqui Valley, Santa Barbara County, California By Danielle D’Alfonso Master of Arts in Geography Large boulder deposits of uncertain origin were mapped in Nojoqui Valley, Santa Barbara County, California, raising questions concerning the nature of these deposits and their provenance. Using detailed field mapping and remote sensing of the study area, this investigation seeks to explain the geomorphic history of the region based on the lithology and distribution of the boulders, and present and past tectonic and climatic conditions. Field observation shows a large percentage of boulders composed of arkosic sandstone, likely sourced from exposures of the Eocene Matilija Sandstone in the Santa Ynez Mountains directly adjacent to the study area. A smaller percentage of conglomeratic boulders, originally thought to be derived from the Oligocene Sespe Formation, contain exclusively felsic clasts, indicating the most likely source to be a previously unmapped basal conglomerate member of the Matilija Sandstone. Distribution of boulders expressed as linear and lobate structures across the landscape, suggest deposition by debris flows with the Santa Ynez Mountains as a plausible source for most, but not all of these boulders. ix Though absolute temporal constraints on the timing of debris flow events cannot be derived directly from this study, a sequence of events for deposition and subsequent displacement along the Santa Ynez fault is interpreted based on locations of groups of boulders. There appears to be an initial deposition of boulders from multiple debris flows followed by a second series of debris flow events that today are found on shutter ridges along the fault. Fluvial transport and removal of material is indicated by boulder deposits perched above the modern valley floor. Incision of the valley floor continues today. x Chapter 1: Introduction 1.1 Statement of Purpose The purpose of this study is to investigate the depositional environment of surficial boulders in Nojoqui Valley with implications for revealing a more complete geomorphic history of the region. The Nojoqui Valley in Santa Barbara County, CA is host to anomalous groupings of surficial boulders of unknown origin. Depositional environments for very large (>2 m) to small (20 cm) boulders are typically characterized by high energy and source proximity. The Santa Ynez Mountains provide a plausible source for most, but not all of these boulders. A large percentage of boulders are composed of arkosic sandstone, likely sourced from exposures of the Eocene Matilija Sandstone in the mountains directly adjacent to the study area; however, a source and transport mechanism for emplacing a significant number of conglomeratic boulders at Nojoqui are absent in the present geomorphic setting. The presence of the boulders questions previous workers who have reported on the geology of this area (Dibblee 1950, Byrd 1983). This study aims to fill a gap in the literature on the geomorphic evolution of Nojoqui by examining the source for the boulders and relative timing of boulder deposits based on their lithology and distribution, and the availability of present and past tectonic and climatic conditions. Specific research questions are: 1) What can detailed mapping of the distribution and placement of boulder deposits within the study area reveal about their depositional environment?; 2) What processes are responsible for the unexplained presence of conglomerate boulders?; and 3) What do these observations reveal about the 1 tectonic and fluvial history of Nojoqui? A combination of remote sensing and field observations were performed to answer these questions. 1.2 Location Nojoqui is located within the Transverse Ranges geomorphic province on the northern boundary of the east-west trending Santa Ynez Mountains, approximately 8 km (5 mi) north of the Pacific coastline, and 5 km (3 mi) northeast of Gaviota Pass (Figure 1). The Transverse Ranges are unique for their series of east-west striking topographic and geologic structures and are often divided by these landforms into the Western, Central, and Eastern Transverse Ranges. The study area is located in the western Santa Ynez Mountains, a feature of the Western Transverse Ranges (WTR). The Santa Ynez Fault