STRATIGRAPHIC RELATIONS BETWEEN TWO VOLCANIC UNITS AND THE POSSIBLE LACUSTRINE SEDIMENTS BETWEEN THEM: IMPLICATIONS FOR THE PALEOENVIRONMENT IN TIMES GULCH, NW ARIZONA

Times Gulch lies 8 km west of Oatman, AZ in the southern Black Mountains and hosts a volcaniclastic section that resides between two intermediate lava sequences. Using both field and remotely sensing based approaches, we constructed a geologic map of an area south of Times Gulch centered near 35.04° N, 114.44° W to help constrain the extent of this section and unravel the processes it records.

The volcaniclastic section (~50 meters thick) consists (from base to top) of layers of sandstones, possible subaqueous tuffs (Beard et al., 2015), sandstones and, in some places, a capping carbonate (magnesite). The lower contact is defined by sediment interfingering with the lower lava (Alcyone Formation of Ransome, 1923). The contact with the overlying lava (Oatman Andesite of Ransome, 1923) is marked by soft sediment deformation and load-casting in the capping magnesite (where present). The entire section (average attitude ~210° 30° E) is repeated in multiple places by SW-NE striking, N-dipping normal faulting and is cut by multiple intermediate dikes.

Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) 7-3-1 RGB band combination images displayed the most qualitatively noticeable differences between the volcaniclastic section and enveloping lavas. These images, combined with band math techniques performed using ENVI image analysis software to emphasize alteration minerals present within the units, allowed us to trace the section for an additional 2 km south (total map area of ~3 km²); at this point the spectral properties of the volcaniclastics and lavas became too indistinct to confidently discriminate between them. However, reconnaissance mapping and ASTER imagery both suggest the volcaniclastics and capping Oatman Andesite crop out again near 35.03° N, 114.42° W implying the section continues farther beyond our map area.

Our mapping supports the interpretation that between two periods of effusive volcanism, a lacustrine environment existed that records one or more episodes of explosive volcanism (Beard et al., 2015). The nature of the lower and upper contacts of volcaniclastic section suggests that lacustrine and effusive activity occurred in close succession.