Description
The rock walls of Sumner Branch and the adjacent seacliffs are cut by en echelon N-S extension fractures and by en echelon N30E fractures associated with shearing. Evidence indicates that the Salk, Torrey Pines, and Scripps faults are en echelon, subsurface, left-lateral, oblique-slip faults striking around N30E, with major splays extending to the surface with strikes of N45E to N60E. The character and strikes of these fractures and faults are those predicted by a strain ellipse oriented over a right-lateral wrench trending N40W. This is the average strike of San Diego's Rose Canyon Fault Zone. Since Dike Rock has an age-date of approximately 11 million years, its possible injection into NE-striking tension fractures may be an event predating the initiation of movement along the Rose Canyon fault zone. An abrasion platform on the Dike Rock tectonic block at a depth of about 75 ft bsl appears to correlate with the world-wide Pleistocene 3e sea-level stillstand. It is estimated that 3 decades ago the average annual rate of down-canyon sand loss was about 65,000- 75,000 yd3/yr. During this study, from 1983-1986, the estimated rate was about 32,000 ± 5,000 yd3/yr. This volume is approximately equal to the sum of the estimated average annual rate of southbound longshore sand transport, 14,600 yd3/yr, plus the estimated average annual rate of sand loss from Blacks Beach, 16,500 yd3/yr. Sand transport along Blacks Beach was episodic and partly controlled by the seasonal location of the wave divergence zone and associated rip-currents. Infilling of the South Branch depression occurred only after infilling of the Sumner Branch depression. Once the South Branch depression had filled, net southbound sand transport around Dike Rock was possible. Annual organic deposition into the canyon gorges was estimated at 17,000 ±4,500 yd3/yr. This debris, primarily surfgrass and kelp, forms a mat up to 30 ft thick along the canyon floor. Gases resulting from decomposition may play an important role in periodic massive down-canyon slumping. From 1983-1986, 37 gas samples were collected from bubbling vents at different locations, depths, and at different times during the tidal cycle. The samples averaged 83% methane and 14% carbon dioxide, with minor amount of air, hydrogen sulfide, and C2+ hydrocarbons. δ13C and δD ratios of methane ranged from -62 to -430/00 and -331 to -2800/00 respectively, while δ13C values of carbon dioxide ranged from -17 to 30/00. Massive down-canyon slumping occurred in December, 1984. From 1985-1986, as a new mat was deposited, bubbling rates increased from intermittent to over 1 cu.inch/sec. The most powerful vents were located near the canyon walls or over the intersections of fractures. Due to hydrostatic pressure changes, bubbling did not begin until after the tide began to ebb, then increased rapidly, and finally ceased shortly into the flood tide. Isotopically lighter methane and carbon dioxide tended to be more abundant during the first hours of bubbling. Carbon dioxide concentrations reached their peak during the second half of the ebb tide. Carbon dioxide concentrations also decreased in down-canyon samples. As the new mat aged, δ13C ratios of carbon dioxide increased by 80/00, while δ13C and δD ratios of methane increased by 5.30/00 and 7.40/00 respectively. δ13C values of carbon dioxide averaged -3.60/00, 1.5 times heavier than the values of most marine sedimentary carbon dioxides, and 3.5 times heavier than those of freshwater. These differences are probably due to the unusually heavy average δ13C values of canyon organic debris, estimated at -170/00. δ13C ratios of methane averaged -510/00, significantly heavier than average values of other marine sedimentary methanes, but within the range of freshwater methanes. δD values averaged -3090/00, about 1000/00 lighter than the average of other marine methanes, but nearly identical to freshwater values. In addition, the carbon dioxide/methane production ratio was estimated at 0.17, or 8.5 times the production ratio of most other marine sediments, but only 1.5 times average production ratio found in freshwater sediments. As the mat aged, average carbon dioxide concentrations increased by about 6%. In addition, there were strong positive correlations between percent carbon dioxide and its δ13C values, and between the δ13C values of carbon dioxide and methane. Moreover, there was a positive correlation between increasing concentrations of carbon dioxide lighter than –l0/00 and increasing concentrations of hydrogen sulfide. Methanogens from within the mat's sulfate reduction zone reportedly utilize methanol and trimethylamines (TMA). Moreover, canyon debris may be a significant source of TMA. Acetate and TMA dissimilation are similar in that both reactions generate carbon dioxide. It may be speculated, therefore, that TMA dissimilation in Scripps canyon is more than a minor methanogenic pathway. In addition, because the isotopic values of the canyon gases are similar to those of gases from fresh water sediments, carbon and hydrogen isotopic fractionations occurring during TMA and acetate dissimilation may be similar.
