ABRUPT CHANGES IN DEPOSITION RESULTING FROM HYDROLOGIC AND TECTONIC EVENTS, PLIO-PLEISTOCENE HUECO BASIN, RIO GRANDE RIFT

A study of logs from 27 wells and cuttings from five wells is used to describe the Neogene and early Pleistocene history of the Hueco Basin in the Rio Grande Rift. The wells reveal that the fill is punctuated by dramatic changes in geometry that relate to the changes in sediment sources, hydrology and the relative effects of extrabasinal vs. intrabasinal sources. This reaffirms observations made by several authors that such changes are common during rift development and reflect stages in the integration of rift basins. The oldest sediments pierced by the wells are likely Pliocene or late Miocene in age, although no dates are available. During this time, this part of the basin was a broad, flat surface, whose morphology was dominated by an expansive playa that extended to nearly the mountain front. This geometry and deposition indicate that a broad area contributed ground or surface water to the basin, resulting in rapid basin floor aggradation that buried the basin margin fans. The first abrupt change was to a segmented, basin floor with smaller and isolated playas. This interval was succeeded by progradation of distal alluvial fans and basin floor channels that reflected a transition to deposition from bounding alluvial fans. Finally, a mixture of sandy alluvium from the Rio Grande and basin floor sediment reflected a re-leveling of the basin floor through fluvial input during the middle Pleistocene. The abrupt changes in deposition within the basin are interpreted to reflect changes in basin hydrology, that, in turn resulted from an interplay between tectonism and deposition. When the basin was a regional topographic low, large lakes formed during rapid basin-floor deposition. When the basin was topographically high, alluvial fans prograded into the basin.