REVISITING THE ORIGIN OF SILCRETES IN THE PALEOCENE NACIMIENTO FORMATION, SAN JUAN BASIN, NORTHWESTERN NEW MEXICO

One distinctive feature of the Paleocene Nacimiento Formation in the San Juan Basin, NM, is the occurrence of numerous thin, resistant ledges that are siliceous (Rains, 1981; Williamson et al., 1992; Hobbs, 2016). Individual ledges tend to be laterally extensive, although variable in thickness, and only rarely exhibit intersections and bifurcations. Upper and lower contacts are sharp, but the upper contacts are typically smooth, while the lower contacts are commonly irregular. These beds are termed silcretes in a broad sense by most authors. Rains (1981) interpreted these beds as silcretes in the pedogenic sense, implying very extensive surface exposure times in a warm, moist climate, drawing an analogy to similar features in Australia. Hobbs (2016) rejected this interpretation, noting a lack of petrographic and macromorphological features that would be associated with pedogenic processes and interpreted the origin of the silcretes as devitrification of volcanic ash fall and reworked ash deposits near the surface. Our field observations, particularly the lateral facies relationships and the nature of the upper and lower contacts, suggests that most are channel fill deposits. Although we note the presence of root casts in these beds, evidence of extensive pedogenesis is lacking. Petrographic observations reveal a largely matrix-supported fabric, the presence of abundant quartz tephra shards, but also rounded (detrital) quartz and detrital mudstone clasts. The matrix consists of both silica and clays. We interpret these observations as indicating a volcanic provenance for most but not all of the sediment, with reworking and deposition by fluvial processes. Pedogenesis was very limited, and most cementation was the result of devitrification of glassy components of the original ash components in near-surface conditions, as suggested by Hobbs (2016).

Hobbs, K, 2016. Sedimentation, pedogenesis, and paleoclimate conditions in the Paleocene San Juan Basin, New Mexico, U.S.A. unpubl. PhD dissertation, Univ New Mexico.

Rains, GE, 1981. Paleocene silcrete beds in the San Juan Basin. unpublished MS thesis, Univ Arizona.

Williamson, TE, Crossey, LJ, Lucas, SG, 1992. Silcretes of the Paleocene Nacimiento Formation. New Mexico Geol Soc Guidebook. 43rd Field Conference. New Mexico Geol Soc, San Juan Basin IV, pp. 38–42.