Wine-Making Conversations about Where Have All the Humongous Deposits of Oceanic Turbidite Systems Gone?
D. W. Scholl1 and W. R. Normark2
1University of Alaska Fairbanks and USGS-Retired, [email protected]
2Nello spirito e nell'anima presente
Oceanic turbidite systems accumulate above igneous oceanic crust and are commonly humongous in areal and volumetric dimensions. For example, the Zodiac fan of the Gulf of Alaska is a ~300,000-km3 mass of mostly Oligocene sediment deposited over ~1,000,000 km2 of abyssal sea floor. Other large oceanic systems construct the Amazon cone and flood the Bay of Bengal abyss. In contrast, deep-water systems that accumulate above continental crust of an ocean margin or borderland tend to be orders of magnitude smaller. Those of the California Borderland, as examples, are typically 1000-5000 km2 in expanse and 1000-3000 km3 in volume.
Despite the disproportionate dimensions of large oceanic and small ocean-margin systems, the rock record of the past 2-3 billion years has disproportionately stored those of continental margin settings. This circumstance does not apply to Cretaceous and Lower Tertiary rock sequences of the north Pacific rim where turbidite deposits exhibiting high P/T mineral phases are widely exposed, e.g., the Catalina-Pelona-Orocopia-Rand schist of California, the Chugach-Kodiak complex of Alaska, and the Shimanto complex of Japan. A Paleozoic example is the Pennsylvanian Western Series of coastal south-central Chile. These accretionary bodies are exhumed crustal underplates of deeply (15-30 km) subducted oceanic systems.
Nonetheless, older high P/T bodies are uncommonly described and virtually unreported from pre-Neoproterozoic (>1 byr) ocean-margin sequences. Oceanic systems deposited in passive-margin oceans, e.g., the Paleozoic Iapetus and Rheric oceans of the Atlantic realm, are also mostly missing from the rock record.
The implication of these observations is that oceanic turbidite systems, even if temporarily stored as a crustal underplate, are ultimately recycled to the mantle by the kindred processes of sediment subduction and subduction erosion. This hypothesized fate is linked to the circumstance that oceanic turbidite systems are mostly deposited on lower plate crust destined for destruction in a subduction zone.
AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009