Sedimentary Facies, Depositional Environments, and Paleosols of the Upper Tertiary Fort Hancock Formation and the Tertiary Quaternary Camp Rice Formation, Hueco Bolson, West Texas



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The Hueco Bolson is a segment of the Rio Grande Rift, which formed as a result of late Tertiary Basin and Range deformation. The upper Tertiary Fort Hancock Formation and the upper Tertiary-Quaternary Camp Rice Formation compose the basin fill except in the deepest (western) parts of the bolson.

Five lithofacies make up the Fort Hancock Formation: (I) gravel; (II) sand, sandy mud, or sandy silt and gravel; (III) sand, sandy mud, and sandy silt; (IV) clay and sandy clay; and (V) clay, mud, sandy mud, and gypsum. These lithofacies represent the textural gradation from basin margin to basin center of proximal to transitional to distal alluvial fans (lithofacies I through III) to ephemeral lakes (IV) to saline playas (V). In cores from beneath the study area, these same lithofacies are present in a 230-meter-thick (700-ft) upward-fining sequence. The sequence records the lacustrine expansion that occurred over basin-margin alluvial fans as the basin filled.

The Fort Hancock Formation is separated from the overlying Camp Rice Formation by a regional unconformity. The unconformity records a period of extensive erosion that marks the integration of the ancestral southern and northern segments of the Rio Grande approximately 2.25 million years ago.

Fluvial, lacustrine, and eolian sediments accumulated above the unconformity as the Camp Rice Formation. Five lithofacies also make up the Camp Rice Formation: (1) sand and locally derived gravel, which was deposited by tributaries to the Rio Grande; (2) sand and exotic gravel (derived from north of the study area), which was deposited by a through-flowing stream, the Rio Grande; (3) sand, which was deposited as a dune complex; (4) coarse silt and very fine sand, which was deposited as loess; and (5) clay, sandy clay, and gypsum, which was deposited in ephemeral lakes with central playas.


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