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Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM

Abstract: An Example of the Combined Influences of Tectonics and Volcanism on Fluvial Aggradation: The Abiquiu Formation (Oligocene-Lower Miocene), North-Central New Mexico

In many tectonically-active settings, volcanism and basin subsidence may occur simultaneously, making a strong combined contribution to aggradation. While differences in the relative importance of these factors on fluvial deposition can be difficult to resolve, consideration of stratigraphic relationships can lead to a qualitative distinction of their influences. The Oligocene-lower Miocene Abiquiu Formation, deposited in northern New Mexico under the combined influence of volcanism and extensional tectonism, offers a laboratory to investigate these factors. These strata lie across the boundary between the Rio Grande rift and Colorado Plateau. The upper member, a ~250-350 m thick volcaniclastic sandstone, was at least in part deposited synchronous with Rio Grande rift subsidence. The volcanic sources of upper Abiquiu sediment are the Latir volcanic field, on the opposite side of the rift basin 60 km northeast of the Abiquiu exposures, and the San Juan volcanic field, 70 km to the north.

Although the pieces of data that indicate a dominance of one control on aggradation over another seem entangled, the controls can be teased apart. Stratal thickening between key horizons of the upper Abiquiu Formation into the rift implicates the role of active rift subsidence in causing deposition, while the aggradation of Latir-derived material beyond the structural limits of the rift basin suggests an abundance of reworked pyroclastic material overwhelming streams, which in consequence regraded their profiles across the developing margin of the rift.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana