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ABSTRACT: Sedimentological Study in the Northwestern Gulf of Mexico Using Tephrochronology and Oxygen Isotope Stratigraphy

Cynthia L. Brown, Carol L. Ostergren, Michael T. Ledbetter

Microprobe analyses of tephra have been used in conjunction with oxygen isotope (^dgrO18) and seismic reflection data in a sedimentary study of the 1000-ft-deep deep-sea core E67-13 from the northwestern Gulf of Mexico (latitude 27°28^primeN, longitude 95°54^primeN). No megascopic tephra layers are found in this core, but electron microprobe analysis of glass shards from five intervals of dispersed rhyolitic tephra indicate that the tephra is both heterogeneous and altered. Extensive hydration and mobility of iron and other ions into carbonate material (pyritization) hinders accurate geochemical fingerprinting. The intervals of dispersed tephra coincide with increases in terrigenous material suggesting that the tephra is reworked from land deposits an redeposited via fluvial systems in the gulf.

Volcanic tephra widespread in the region are absent from core E67-113. The oxygen isotope stratigraphy in the core indicates two hiatuses at 0-0.93 k.y. and 0.12-1.2 m.y. A discrete tephra layer found in another core (E67-126A) in the same region is geochemically and biostratigraphically correlated to the Lava Creek rhyolitic eruption from Yellowstone (0.6±0.01-0.02 m.y.). This tephra, as well as many other region-wide tephra, fall into the missing time spans in E67-113. The

absence of discrete tephra layers in core E67-113 is due to deep-sea erosional processes that removed sediment from the area. The seismic reflection data show that core E67-113 is drilled in a channel adjacent to a large salt diapir. This channel cuts into onlapping sediments. As the diapir ascended through the overlying sediments, bottom currents in the gulf were diverted around the diapir eroding significant amounts of sediment and creating a channel or moat.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990