Relationships Between Bottom Topography and Marine Sediment Properties in an Area of Submarine Gullies

Abstract

ABSTRACT A precise bathymetric survey, including the collection of 21 cores, was accomplished by the submersible DEEP QUEST along a gullied section of the upper San Diego Trough slope off Del Mar, California. The primary purpose of the study was to investigate the relationship between topography and the mass physical properties of marine sediments. A secondary objective was to gain more information on the nature and origin of these submarine gullies. The cores were obtained along precisely controlled transects down the gully axes as well as on transects approximately perpendicular to the axes. Sediment samples from the cores were tested in the laboratory for shear strength, water content, bulk density, grain size distribution, and mineralogic composition. Submarine gullies within the area appear to begin at the edge of the shelf and trend almost directly westward. The lateral extent of these gullies is from 800 to 1200 meters. Width of the larger gullies varies from 203 to 274 meters. The gullies have a relief of 18 to 30 meters. The morphology of the gullies strongly suggests that they were originally formed by subaerial erosion. Within the submarine gullies the only sediment property which appears possibly to be related to the topography is the grain size distribution. The expected progressive decrease in the grain particle size occurs in the seaward direction. A trend was observed in the water content-sediment depth profiles, which is believed to be a result of the grain size distribution and not of the topographic changes. This trend indicates water content values decreasing as the size of the sediment particles increase. Water content values also indicate a non-linear decrease with sediment depth. X-ray diffraction and heavy mineral analysis indicate that the sediments are very homogeneous in mineralogic composition throughout the area studied. Quartz was usually identified as the major constituent of the entire sample. Illite and mixed layer micas were the predominate clay minerals in the clay size fraction. The principal heavy minerals were amphiboles, pyroxenes and opaques.