“SILCRETES” IN THE PALEOCENE SAN JUAN BASIN AND THEIR PALEOCLIMATOLOGIC AND SEDIMENTOLOGIC SIGNIFICANCE

The fluvial Paleocene Nacimiento Formation in the San Juan Basin (SJB), New Mexico, contains numerous resistant, fine-grained, silica-rich strata. These beds are well-cemented, contain root traces and organic material, and are found overlying varying lithologies including sandstone, siltstone, and argillic paleosols. Bedding features are not present within the strata. While most SJB silica-rich beds are laterally continuous for less than 1 km, the horizons at which beds are found can be correlated over tens of kilometers. Of the dozens of silica-rich beds known in the Nacimiento Formation, only one is related to a volcanic ash. These strata have been called silcretes by previous researchers, but likely share few genetic similarities with the pedogenic silcretes that have lateral continuity over tens of kilometers and 10⁶ year-long periods of pedogensis in warm, variable wetness climate regimes.

The most striking feature of these beds is the microcrystalline silica matrix. In thin section, this matrix resembles impure chert and comprises up to 50% of the rock. Kaolinite is present in strata immediately overlying these beds but is absent in the beds themselves. Fledspars, clays, and rock fragments are nearly absent from the silica-rich beds. Paleoclimate analysis based upon chemical index of alteration of associated paleosols indicates MATs of ~15°C and MAPs of 1100 mm.

Given the presence of root traces in all silica-rich beds and organic matter in most silica-rich beds, and the stratigraphic association with paleosols of some silica-rich beds, pedogenesis seems to play some role in the formation of these beds. The abundance of both oxidized red beds and organic-rich black beds within the Nacimiento Formation suggests that there were periods of wetness and dryness of greater than seasonal duration during Nacimiento Formation deposition. The lack of paleoultisols in the Paleocene SJB suggests that the duration of pedogenesis was shorter than what was required to form ultisols, so the presence of silica-rich pedogenic horizons (typically seen in soils or paleosols with very long periods of duration) is enigmatic. The data we present on SJB silica-rich beds introduces more questions concerning seasonality, climate, sedimentation, and environments during the early Paleocene in the SJB.