MIOCENE STRATIGRAPHY AND ANALYSIS OF THE ROCKS ASSIGNED TO THE “INFORMAL” ARGOS STATION UNIT, SOUTHERN CADY MOUNTAINS, CALIFORNIA

Miocene rocks of the Mojave Desert’s Cady Mountains in southern California are significant components for tectonic and environmental reconstruction of the Basin and Range. The Cady Mountains are generally composed of Jurassic granite/quartz monzonite overlain by Miocene intrusive, volcaniclastic and tuffaceous sedimentary units. Those sedimentary units and associated volcanic flows provide a window into the initiation and restoration of sedimentation after underlying nonconformal intrusions of the Nevadan Orogeny.

A distinct red-orange ashfall tuff arenite and associated clastics are assigned to the ~19.9 Ma formation of Argos Station i.e., fAS (Glazner, 1988). The tuffaceous arenite is the lowermost unit of the fAS, and contains distinctive stratigraphically alternating petrologic facies. Those facies include 1) beds of large and poorly sorted lithic tuff clasts; and 2) well sorted, tightly compacted beds that lack visible tuff clasts. During field-mapping, eight stratigraphic sections were measured along outcrop strike. Those arenite sections varied from two to 40 meters in thickness. Petrologic examination further indicates the presence of at least three distinct and two intermediate facies.

The fAS unit appears to be part of local Miocene pyroclastic eruptions that predate the super-caldera eruption of the Peach Springs Tuff (~18.8 Ma). Based on the array of sedimentary structures found in the sandstone (e.g., mudcracks, ripples, flute marks, potential trackways), pyroclastic eruptions provided the lithic tuff sedimentation into an ephemeral wetland, or marshy area. The fAS does not appear to be stratigraphically distinguishable as either post- or pre- Hector Formation (Woodburne et al., 1974), nor have biostratigraphic fossils been retrieved from the fAS. The tuffaceous arenite of the Argos Station may be a unique clastic facies correlatable with, or possibly assignable, as a new member of the Hector Formation.