PYROCLASTIC DEPOSITS IN THE PALEOCENE SAN JUAN BASIN: INVESTIGATING THE RELATIONSHIP BETWEEN VOLCANISM, SEDIMENTATION, AND PRESERVATION DURING THE LARAMIDE OROGENY

Silcretes in the Nacimiento Formation of the San Juan Basin, New Mexico, have been interpreted previously as groundwater diagenetic or pedogenic features. New petrographic and geochemical investigation shows that many of the coarse silt to sand-sized clasts in these strata are in fact tephra shards. The presence of numerous tephra-rich silcretes through the Early and Middle Paleocene Nacimiento Formation section indicates frequent episodic deposition and preservation of volcanic ash in the Paleocene San Juan Basin during the Laramide Orogeny. Magma/lava sources are not yet known, but likely potential volcanic sources are in the western Colorado Mineral Belt or south-central Arizona. Here we present data and interpretations showing that the San Juan Basin was frequently affected by explosive volcanism during the Laramide Orogeny, that the environment of deposition in the San Juan Basin was favorable for the preservation of fine-grained volcaniclastic deposits, and that paleoenvironmental conditions allowed for the rapid chemical alteration of these deposits with minimal effects on the encapsulating strata.