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