The Baca Formation of New Mexico and the Eagar

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Formation and Mogollon Rim gravels of Arizona comprise an Eocene-early Oligocene sequence of claystone, mudstone, sandstone, and conglomerate which crops out in discontinuous exposures along an east-west-trending belt from Socorro, New Mexico, to Show Low, Arizona. The maximum exposed thickness is about 1,200 ft (365 m).

The outcrop belt transects the southern part of the east-west-trending Baca-Eagar basin. The basin is bounded on the north by the Defiance and Zuni uplifts, on the south by the Mogollon highland of Arizona and New Mexico, and on the east by the Sierra-Sandia uplift. These uplifts were the primary sources of sediments for the basin. Measurement of maximum clast size, gravel lithology counts, thin-section data, and paleocurrents were used to determine source areas and sediment dispersal patterns. Southward tilting and erosional stripping of the northern part of the basin resulted from uplift of the Colorado Plateau in Miocene-Pliocene time.

A depositional model is presented which consists of a braided alluvial plain-meander-belt-lacustrine facies tract. The meander-belt facies includes both fine- and coarse-grained point-bar deposits. The lacustrine facies contains both the classical Gilbert-type delta and the fine-grained marine-type delta. High concentrations of calcium carbonate in the lacustrine sediments indicate a closed-lake system.

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