HR: 0830h
AN: T31E-0882    [PDF]
TI: A possible widespread Upper Miocene to Lower Pliocene rift-related sequence under the marine shelf from Mazatlan to the Tres Marias islands, southeastern Gulf of California, Gulf of California MARGINS project
AU: * Umhoefer, P J
EM: paul.umhoefer@nau.edu
AF: Northern Arizona University, Dept. of Geology, 4099, Flagstaff, AZ 86011-4099 United States
AU: Paramo, P
EM: paramo@uwyo.edu
AF: University of Wyoming, Dept. of Geology and Geophysics, Laramie, WY 82071-3006 United States
AU: Brown, H E
EM: hbrown@uwyo.edu
AF: University of Wyoming, Dept. of Geology and Geophysics, Laramie, WY 82071-3006 United States
AU: Holbrook, W S
EM: steveh@uwyo.edu
AF: University of Wyoming, Dept. of Geology and Geophysics, Laramie, WY 82071-3006 United States
AU: Sutherland, F
EM: fsutherl@ucsd.edu
AF: Scripps institute of Oceanography, Institute of Geophysics and Planetary Physics, 9500 Gilman Drive, La Jolla, CA 92093-0225 United States
AU: Kent, G M
EM: gkent@igpp.ucsd.edu
AF: Scripps institute of Oceanography, Institute of Geophysics and Planetary Physics, 9500 Gilman Drive, La Jolla, CA 92093-0225 United States
AU: Harding, A J
EM: aharding@ucsd.edu
AF: Scripps institute of Oceanography, Institute of Geophysics and Planetary Physics, 9500 Gilman Drive, La Jolla, CA 92093-0225 United States
AU: Fletcher, J M
EM: jfletche@cicese.mx
AF: CICESE, Km 107 Carretera Tijuana - Ensenada, Codigo Postal 22860, Ensenada, BC 22860 Mexico
AU: Lizarralde, D
EM: danl@eas.gatech.edu
AF: Georgia Institute of Technology, School of Earth adn Atmospheric Sciences, 221 Bobby Dodd Way, Atlanta, GA 30332-0340 United States
AU: Gonz lez-Fern ndez, A
EM: mindundi@cicese.mx
AF: CICESE, Km 107 Carretera Tijuana - Ensenada, Codigo Postal 22860, Ensenada, BC 22860 Mexico
AU: Axen, G
EM: gaxen@ess.ucla.edu
AF: UCLA, Earth & Space Sciences, 595 Young Drive East, Los Angeles, CA 90095-1567 United States
AB: The margins of the plate boundary on either side of the southern Gulf of California differ substantially. The western margin along Baja California is narrower and has mush less sediment than the eastern margin. The eastern margin is dominated by a 30 - 60-km wide coastal plain and a 40 - 100-km wide marine shelf, both of which cover faults and basins. The Tres Marias islands are the only chance for a detailed geologic window into the shelf. Therefore, seismic reflection lines and the Tres Marias islands are critical to understanding the geology of the southeastern margin of the plate boundary. Previous workers described 1100 - 1600 m of strata on Maria Madre Island that is Late Miocene (~8 Ma) to Pleistocene in age. The section changes abruptly from basal marginal(?) marine sandstone into thick mudstone and diatomite formed in a Late Miocene to Early Pliocene slope to deep marine setting. Within the Lower Pliocene section is an angular unconformity. The Pliocene section shallows up to outer shelf to upper slope conditions, possibly across a second major unconformity that separates the lower and upper Pliocene. The Pliocene marine strata are overlain abruptly across a third unconformity by Pleistocene nonmarine to shallow marine sandstone and conglomerate. Two of our seismic reflection lines run onto the modern shelf between the Tres Marias islands and Puerta Vallarta. The lines show an undeformed upper sequence that lies on a major unconformity above a lower sequence. The upper sequence is ~100 m to ~1 km thick and onlaps and fills previous topography. The lower sequence is consistently broken by normal faults and shows common growth sedimentation related to faulting. The lower sequence is most commonly from 0.5 to 1.0 seconds (two-way travel time). The upper and lower sequences are also well defined on a seismic line offshore of Mazatlan; the lower sequence is under the shelf and slope out to ~80 km offshore; the upper sequence is unfaulted, while the lower sequence is commonly faulted. A final seismic line that runs across the shelf between Mazatlan and Puerta Vallarta has hints of the lower sequence. We tentatively correlate the upper seismic sequence with the upper Pliocene to Quaternary shallowing up section on the Tres Marias islands. We correlate our lower syn-rift sequence with the upper Miocene to lower Pliocene deep marine section on the island (~8 - 3.5 Ma). Further seismic processing and geologic work on the Tres Marias islands should clarify the spatial extent and age of the lower sequence.
DE: 3025 Marine seismics (0935)
DE: 8105 Continental margins and sedimentary basins
DE: 8109 Continental tectonics--extensional (0905)
DE: 8150 Plate boundary--general (3040)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting