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Recent Sediment Accumulation Rates on The RECENT SEDIMENT ACCUMULATION RATES ON THE MONTEREY FAN A Thesis Presented to The Faculty of the Department of Geology San Jose State University In Partial Fulfillment of the Requirements for the Degree Master of Science By William L. Hughes December, 1988 ACKNOWLEDGMENTS I would like to acknowledge the help and support of many individuals who made this thesis possible. Dr. Alan Shiller provided assistance as the thesis advisor and patiently instructed me in laboratory procedures and geochemistry. Dr. David Andersen has provided suggestions and encouragement during the project. Dr. Michael Ledbetter suggested the subject area and assisted in the sampling of cores. Tom Walsh of Scripps Institution of Oceanography provided samples from cores. Dr. William Normark and Christina Gutmacher of the Marine Geology Branch of the u.s. Geological Survey have been generous with their time and provided radiographs, photos and cores for analysis. Donna Beale of u.c. Santa Cruz provided assistance in calibrating the 210Po tracer. Figures for this research were drafted by Lynn McMasters. I would lilce to thank Applied Earth Technology, Inc. for the use of a computer to translate my thoughts to words on paper. Financial support for this research was provided in part by the Packard Foundation and Moss Landing Marine Laboratories. Finally, I thank rny wife Joanne, who provided continuous support during the duration of this project. iii TABLE OF CONTENTS page ABSTRACT ~ ~ . .. .. .. .. .. .. .. .. ... .. .. • .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. viii INTRODUCTION ......................................................................... 1 Purpose of Study . • . • • • • . • . • • • • • • • • • • . 3 Lead-210 Geochronology . • . • • • • • • • . 8 Initial Assumptions .....••••••••••••••........... 11 METHODS .......... ,. ............................................................ .. l3 RESULTS 19 Radiochemical Data ••........•••••.....•••••••.... 19 Suprafan ..................................................................... 19 Levee ............................................................................. 29 Fan-Valley .................... .. 32 Sur Slide .••.••••••..... 38 DISCUSSION •••••....••••••..•...•.••••..•..•...•••.•••• 43 Supra fan. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 4 3 Levee ••••••••......•••••.....•.•••••••........... 45 Fan-Valley . • • • . • . • • • • . • • • • • . • • 4 7 Sur Slide 48 Effect of Moss Landing Harbor Dredge Spoil ..••••• 49 Comparison with Other Deep-Sea Canyons 50 Sediment Accumulation and Depositional Processes . 51 Implications Regarding Sea-Level ....•.••••••••... 52 CONCLUSION ........................................................ ., .......... 54 REFERENCES CITED . • . • . • • • • • . • • • • . • . • • 57 JV APPENDIX: Total Lead-210 Activity Data ................ 62 v LIST OF ILLUSTRATIONS Figure page 1. Location of study Area 2 2. Core Locations . 5 3. 238u Decay Series 9 4. I d ea1 1zed. Pro f'11 eo f 210 Pb Ac t'lVl 't y .............. 16 5 . E xcess 210 P b At'c lVl 't y P ro f'11 es ................... 20 a. Core 98G .................................... 20 b. Core 100G ................................... 21 c . Core 14G . 21 Core 15G e. Core 16G . 2 2 Core 69Bx Core 49Bx h. Core B3Bx • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 2 4 Core B4Bx j . Core B7Bx . 2 5 k. Core 90Bx • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 2 5 1 • Core 43Bx • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 2 6 m. Core 1PG . • . • • • • • • • • • • . • • • 2 6 n. Core 2PG . 2 7 0 • Core 4PG • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 2 7 6. Photograph of core 14G ........................... 30 7. Radiograph of core 14G . ... 31 8. Photograph of core 15G ........................... 33 vi 9. Radiograph of core 15G ................................. 34 10. Photograph of core 16G ................................. 35 11. Radiograph of core 16G ............................... 36 12. Radiograph of core 1PG .............................. 40 13. Radiograph of core 2PG ............................... 41 14. Photograph of core 2PG ............................. 42 15. Distribution of Sediment Accumulation Rates ...... 44 Tables 1. Cores Used in the Study .........................• 6 2. Sediment Accumulation and Sedimentation Rates ...• 28 vii ABSTRACT Lead-210 geochronology was used to determine sediment accumulation rates between the water depths of 2550 and 4135 m on the Monterey Fan. Fifteen box and gravity cores were studied from the fan-valley, levee, suprafan, and Sur slide. Using excess 210Pb activity profiles, the sediment accumulation rates were calculated. Slumping from the fan-valley walls and changes in the gradient result in varying rates of sediment accumulation along the channel. Upslope of the oxbow meander, sediment accumulation rates increase (0.09 to 0.61 gjcm2;yr) with increasing water depths. Within the meander the rates of sediment accumulation range from 0.17 to 0.51 gjcm2;yr. On the suprafan, sediment accumulation rates range from 0,05 to 0.56 gjcm2jyr. The differences in the rates on the suprafan indicate that hemipelagic sedimentation is not the only active depositional process in this area. Sediment accumulation rates range between 0.12 and 2.04 gjcm2;yr on the northern levee. This would indicate that the crest of the levee is a zone of nondeposition or erosion and that the sediment is deposited farther down the flank. Sediment accumulation rates on the Sur slide are 0.48 to 1.35 gjcm2jyr. Using 210Pb geochronology, the age of the slide was estimated to be between 140 and 600 yr B.P. viii The dumping of dredge spoil from Moss Landing Harbor at the head of the canyon is not reflected in an increase in sediment accumulation rates at the depths studied. Dredge spoil is either trapped further up the canyon or is too widely dispersed to be detected. The recent sedimentation rates on the Monterey Fan were compared with the Astoria Fan off the Oregon continental shelf. It was found that the modern sedimentation rates on the Monterey Fan are greater than both Late Pleistocene and Holocene rates on the Astoria Fan. Variations in sea-level probably affect the sedimentation rates on the fan; however, the tectonic setting is the primary control of the sedimentation rate on the Monterey Fan. ix INTRODUCTION The Monterey Fan is the dominant physiographic feature along the central California coast. The fan is fed by the Ascension and Monterey Canyon systems. The Monterey Canyon contributes more sediment to the fan (Wilde, 1965) because it cuts across the shelf and heads at Elkhorn Slough, about 120 km south of San Francisco (fig. 1). The Monterey Fan has a radius of 300 km and covers approximately 100,000 km2 of the sea floor (Hess and Normark, 1976). The Monterey Fan is thought by some workers to be relatively inactive due to the present high stand of sea-level and the lack of a large river transporting sediment to the canyon (Shanmugam and Moiola, 1988). Evidence for this is a blanket of olive green mud containing Holocene pelagic foraminifera and radiolaria (Komar, 1969). Other authors also have reported finding mostly marine mud in the canyon (Shepard, 1948; Martin and Emery, 1967; Yancy, 1968; Wolf, 1970; Oliver and Slattery, 1973; Greene, 1977); however, thin sand layers (Komar, 1969) and Holocene sand deposits on the suprafan and in Monterey East Valley (Hess and Normark, 1976) indicate infrequent turbidite deposition. The primary modes of sediment transport onto the fan today are hemipelagic sedimentation and turbidity currents initiated by the slumping of material from the walls of the canyon. Greene (1970) was the first to suggest active 2 Northern California Point Reyes Study Area Figure 1. Location of study area. 3 slumping of the canyon walls and later mapped slump features in detail (Greene, 1977). Chinburg (1985) estimated that 150,000,000 m3 of sediment was involved in slumping in the upper 25 km of the canyon. The slumping in the canyon may be caused by the oversteepening of the canyon walls through sedimentation and undercutting of the canyon walls by bottom currents (Chinburg, 1985). A potential recent source of sediment to the Monterey Fan is the dumping at the head of the Monterey Canyon of dredge spoil from the dredging of Moss Landing Harbor. No sediment accumulation rates have been established on the fan using any radioisotope, although Wilde (1965) calculated an average sedimentation rate of 1 cm/1000 yr using the total sediment volume of the fan and its estimated age (Oligocene). Hein and Griggs (1972) estimated a sedimentation rate of 27 cm/1000 yr on the northwest levee of the Monterey-Ascension Fan Valley using the depth of an isochronous biostratigraphic marker. This marker was dated at 12,500 yr B.P. and denotes a change from a dominance of planktonic foraminiferans below to a dominance of radiolarians above (Duncan et al., 1970). Purpose of Study The purpose of this study is to examine sediment accumulation rates using the radioisotope 210Pb at selected locations on the Monterey Fan and to determine if the 4 Monterey Fan can be considered active during this high stand of sea-level. Box, piston, and gravity cores curated at the U.S.G.S. and Scripps Institution of Oceanography were sampled for this study. These cores were taken at locations along the Monterey fan-valley axis, across the northern levee of the fan-valley, on the suprafan, and within
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