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Geological Society, London, Special Publications; 1982; v. 10; p. 77-94;
DOI: 10.1144/GSL.SP.1982.010.01.05
© 1982 Geological Society of London

Central America

Facies belts of the Middle America Trench and forearc region, southern Mexico: results from Leg 66 DSDP

J. Casey Moore, Joel S. Watkins, Kenneth J. McMillen, Stephen B. Bachman, Jeremy K. Leggett, Neil Lundberg, Thomas H. Shipley, Jean-Francois Stephan, Floyd W. Beghtel, Arif Butt, Borys M. Didyk, Nobuaki Niitsuma, Les E. Shephard & Herbert Stradner

Earth Sciences Board, University of California, Santa Cruz, California 95064, U.S.A.
Gulf Research and Development Company, Pittsburg, Pennsylvania, U.S.A.
Gulf Research and Development Co., P.O. Box 2038, Pittsburgh, Pennsylvania 15230, U.S.A.
Department of Geological Sciences, Cornell University, Ithaca, New York, U.S.A.
Department of Geology, Imperial College of Science and Technology, London, England
Earth Sciences Board, University of California, Santa Cruz, California, U.S.A.
Scripps Institution of Oceanography, La Jolla, California, U.S.A.
Département de Géotectonique, Université Pierre et Marie Curie, Paris, France
Phillips Research Center, Bartlesville, Oklahoma, U.S.A.
Universität Tubingen, Tubingen, Federal Republic of Germany
Empresa Nacional del Petroleo Concon, Chile
Department of Geosciences, Shizuoka University, Shizuoka, Japan
Texas A & M University, College Station, Texas, U.S.A.
Geologische Bundesanstalt, Vienna, Austria

The Middle America Trench SE of Acapulco is flanked by a steep canyon-incised slope and narrow shelf, showing one of a variety of sedimentary facies patterns possible at convergent margins. Piston and drill cores from this region define eight facies belts including: (1) a pelagic facies of brown clay, (2) an outer slope mud facies, (3) a trench sand facies, (4) a foraminiferan-free facies on the lower slope, (5) a foraminiferan-bearing facies on the mid-slope, (6) a laminated mud facies on the upper slope, (7) a shelf facies of sand and mud, and (8) a canyon facies of sand and gravel. The superposition of trench and lower slope sediment during accretion results in a fining upward sequence reflecting a gradual uplift of the seafloor through the trench sediment-plume. The lower limit of the foraminiferan-bearing facies is defined by the absence of in situ calcareous foraminiferans and is controlled by the calcite compensation depth. The upper slope laminated mud facies probably reflects the depth range of the oxygen minimum zone.

In the Leg 66 area sedimentation rates are high in the trench and on the outer and lower slope, decrease on the mid-slope, and increase again on the shelf. On the inner shelf, waves and currents concentrate sand which funnels through a prominent submarine canyon, bypassing the mud-dominated slope and accumulating in the trench. A terrigenous sediment-plume generated by trench turbidity flow causes accelerated sediment accumulation to about 500 m above and 40 km seaward of the trench. The volume of material transported by the trench sediment-plume is five or six times greater than that moved by the shelf sediment-plume which supplies detritus to the shelf, upper slope and mid-slope environments.