A High-resolution Record of Deep-water Processes in a Confined Paleofjord, Quebrada de las Lajas, Argentina
Mason Dykstra, Ben Kneller, Juan Pablo Milana, 2008. "A High-resolution Record of Deep-water Processes in a Confined Paleofjord, Quebrada de las Lajas, Argentina", Atlas of Deep-Water Outcrops, Tor H. Nilsen, Roger D. Shew, Gary S. Steffens, Joseph R. J. Studlick
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Fjords can become so overdeepened below sea level during protracted glaciations that they fill with hundreds of meters (>1000 ft) of seawater when glacioeustatic rise occurs during and following deglaciation. Fjords, therefore, can host true deep-water environments, which are commonly laterally confined but longitudinally extensive. Outcrops of ancient paleofjord sediments offer three-dimensional views of the evolution of these deep-water, confined sedimentary environments, where the factors controlling sediment supply are both climatic (deglaciation and eustasy) and tectonic (isostatic rebound). Quebrada de las Lajas, near San Juan, western Argentina, preserves a Pennsylvanian glacial to postglacial succession that was deposited in an over-deepened paleofjord.
The sedimentary succession exposed in the paleofjord is divided into four evolutionary stages: Stage I was an ice-contact delta and proglacial lake, Stage II was a relatively quiet, deep-water marine environment punctuated by turbidity currents, Stage III was an aggradational confined sheet system, and Stage IV was the subaqueous portion of a progradational, coarsening- and shoaling-upward fan delta. The entire sedimentary succession comprises approximately 350 m (1150 ft) of remaining exposed thickness.
Each of the four evolutionary stages has distinct architectural characteristics associated with its depositional environment. Stage I is chatacterized by predominandy lobe-shaped, sheetlike conglomerate and sandstone bodies associated with the ice-contact delta and a subaqueous fan. Stage I also preserves several small, turbidite channel bodies and a small-scale, highly aggradational channel-levee system with a conglomeratic channel axis and thin-bedded sandstone and siltstone levees. Stage II is characterized by thin beds of shale and siltstone punctuated by medium
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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