PRE-, SYN-, AND POST- MIOCENE VOLCANIC SOURCES AND DEPOSITIONAL ENVIRONMENTS IN SEDIMENTARY ROCKS AT CRASH HILL, NELSON LAKE VALLEY, CALIFORNIA

Crash Hill in Nelson Lake valley, California, contains a >230-m thick, conformable, Miocene sedimentary basin sequence deformed into a domed structure along the Nelson Lake fault. Volcanism occurred west of the area 18-12 Ma, and the section is divided into three general groups relative to volcanism: pre-, syn-, and post-volcanic.

Pre-volcanic rocks derived from Cretaceous granitic rocks form >50 m of arkosic sandstone and conglomerate. Beds are 0.05-1.0-m thick, fine-grained sandstone to pebble conglomerate, tabular to lenticular, many with crossbeds. Many conglomerate beds have subrounded clasts up to 30 cm, and a few ~80 cm. Fallout tephra beds, 4-15-cm thick, are in the upper 40 m and increase in number and thickness up section.

Syn-volcanic deposits are ~100 m of medium- to coarse-grained tuffaceous sandstone to pebble conglomerate, interbedded with siltstone to fine-grained tuffaceous sandstone and fine-grained ash. The tuffaceous and volcanic lithic materials were probably derived from the volcanic center to the west. Many 1-2-m thick bedsets contain cycles that coarsen upwards, from fine-grained, structureless sandstone to coarse-grained sandstone or pebble conglomerate. Many tuffaceous beds have grains of glass shards or pumice clasts, many altered to clay. Many beds have calcite cement, especially the well sorted, coarser-grained sandstones. Some siltstone to fine-grained sandstone beds have root casts. Fine-grained, 10-30-cm thick, ash beds are mostly vitric glass and small pumice grains with small fragments of quartz, feldspar(?), and biotite.

Post-volcanic rocks are >80 m of sandstone and conglomerate that include volcanic lithic clasts, minor amounts of tuffaceous material as epiclasts, and rare granitic clasts. Beds are 4-40-cm thick, fine-grained sandstone to cobble conglomerate, and internally they are laminated, trough crossbedded, or have normal size grading. The matrix is mostly fine-grained, epiclastic grains similar to the larger clasts along with a small component of fine-grained clay minerals.

Depositional environments for pre-volcanic deposits are interpreted as basin margin, medial to distal, alluvial fan, syn-volcanic deposits as basin axis, distal fluvial to lacustrine (marsh?), and post-volcanic deposits as basin axis, distal to medial alluvial fan.