A Rare Exposure of an Ancient Submarine Canyon, Black’s Beach, California, USA
Jeffrey A. May, John E. Warme, 2008. "A Rare Exposure of an Ancient Submarine Canyon, Black’s Beach, California, USA", Atlas of Deep-Water Outcrops, Tor H. Nilsen, Roger D. Shew, Gary S. Steffens, Joseph R. J. Studlick
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Beach cliffs north of San Diego, California, provide superb three-dimensional exposures of an exhumed Eocene submarine-canyon complex. This interpretation is based on large-scale erosional and depositional geometries, channel architecture, lithofacies relationships, sedimentology, and micropaleontology. The canyon fill is composed of multiple, cross-cutting channels on a multitude of scales and with widely diverse lithologies. Individual channels range from subtly scoured and only a few meters (feet) deep to more than 1 km (0.6 mi) wide and up to 100 m (328 ft) deep. Photomosaics aid in analyzing bounding surfaces and lithologic patterns. An irregular sequence boundary defines the canyon base. Two stratigraphic sequences are truncated by the canyon floor; two sequences compose the canyon fill. Lagoonal and tidal deposits of the underlying Delmar andTorrey sequences are unconformably separated from bathyal units of the Ardath Sequence, which comprises the lowermost canyon interval. A second submarine-sequence boundary occurs within the canyon succession, eroding into the top of the Ardath Sequence and dividing it from the overlying Scripps Sequence. A fifth (shallow-marine) sequence, which is not described here, truncates the Scripps Sequence to the north and inland. Pleistocene wave-cut terraces plane off the Eocene interval at the top of the cliffs. The lowermost canyon section (Ardath Sequence) comprises amalgamated, pebbly, and diffusely laminated sandstones. These fine upward to convolute-bedded, fine-grained sandstones, which then grade into laminated to bioturbated silty mudstones. The mudstones fill channels that exhibit a sinuous morphology. Multiple erosional episodes scoured each channel; channel fill predominantly occurred during abandonment. Subsequent flows evacuated a multitude of cross-cutting conduits.
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Tor H. Nilsen, a red-haired Scandinavian who stood more than six feet tall, died October 9, 2005, at his San Carlos, California, home. This was after a valiant five-year fight with melanoma cancer. He was 63. His ashes were scattered at his family plot in Norway in 2006.
He was born in New York City on November 29, 1941, to Mollie Abrahamson and Nils Marius Nilsen of Mandal, Norway, and was the first of their children to be born in the United States. After graduating from Brooklyn Tech, he earned his B.S. in geology from City College of New York in 1962. While there, his prowess on the basketball court impressed a scout from the New York Knicks, but Tor went on to graduate school and earned his M.S. and Ph.D. degrees in geology from the University of Wisconsin at Madison in 1964 and 1967, respectively. His M.S. thesis was a study of Precambrian metasedimentary deposits in the Lake Superior area, and his Ph.D. thesis was a study of Devonian alluvial-fan deposits of the Old Red Sandstone in western Norway.
Dr. Nilsen’s principal expertise was in depositional systems analysis, stratigraphic analysis, and the relationships among tectonics, eustasy, and sedimentation. He began his industry career in 1967 as a research geologist with the Shell Development Company in Houston, Texas, and Ventura, California, where he worked on the tectonics and sedimentation of Tertiary shelf systems of coastal California. He subsequently spent two years with the U.S. Army Corps of Engineers as the Military