New Mexico Geological Society Annual Spring Meeting & Ft. Stanton Cave Conference — Abstracts

Strata of the Baca Formation in west-central and south-central New Mexico are Eocene siliciclastic strata deposited late in the Laramide orogeny. Although initially described as fluvial deposits in the 1970s, in the 1980s Baca deposition was reinterpreted as having taken place in a closed lake basin, a conclusion not questioned since. A re-evaluation of Baca sedimentation, however, indicates no evidence for deposition in a lake basin but instead in a fluvial system that flowed to the east.

Previous interpretations of Baca deposition as lacustrine identified Gilbert deltas, lacustrine mudrock, extremely thick Baca strata near the lake basin center (Gallinas-Bear mountains) with paleoflow from the east draining off of a Laramide Sierra uplift. None of these conclusions stands up to a critical re-evaluation.

So-called Gilbert delta deposits in the Baca Formation are alluvial deposits consisting of fluvial channel and floodplain deposits. Fluvial channel geometry consists of single- to multi-storey channels with flat to heavily scoured bases, locally with abundant mudstone rip-up lag deposits. Individual storeys display multiple beds of planar to trough cross-bedded and plane-bedded sandstone.

Baca mudstones mostly display evidence of pedogenesis to varying degrees. Where the mudstones are well exposed, they are gray to red, commonly mottled, with wedge to blocky peds with well-developed clay cutans on the ped surfaces, or crumb fabric in places. Root traces are common, sometimes but not always drab. Good mudstone exposures at Dog Springs Canyon in the Gallinas Mountains include paleosol profiles with clear A and B horizons and silty mudstone near the top with prominent drab root traces and blocky fabric. Exposures in the Bear Mountains, supposedly at/near the lake center, display mature calcretes with partially to fully coalesced nodular ledges (Bk horizons) with flat upper surfaces and gradational lower boundaries in which nodule size and frequency decrease downward. Clearly, the bulk of the mudstones record deposition on an alluvial plain subject to vegetation and pedogenesis. Deposits of laminated and organic-rich mudstones occur locally, but these have limited vertical and lateral distribution. Hence, we interpret these as evidence of local ponding on the alluvial plain.

Maximum Baca thickness on outcrop is about 354 m, though average thickness is 150-200 m, much less than the 520 m thickness claimed near the basin center. Paleoflow of Baca rivers from the east was based on a small set of crossbed measurements in the Bear Mountains that, when combined with additional paleoflow measurements (e.g., gravel imbrication), do not indicate westward paleoflow.

As concluded in the 1970s, Baca deposition took place in a fluvial system that flowed east, not in a closed basin with flow into a lake near the basin center. This re-evaluation of Baca sedimentation also calls into question the existence of a Laramide Sierra uplift and of a Laramide Carthage-La Joya basin.