CARSON PASS-KIRKWOOD PALEOCANYON SYSTEM: IMPLICATIONS FOR THE TERTIARY EVOLUTION OF THE SIERRA NEVADA, CALIFORNIA

Mapping of previously undifferentiated Tertiary strata reveals six unconformity-bounded sequences preserved within an up to 540 m deep, roughly NE-SW trending paleocanyon cut into Mesozoic granitic basement, with paleo-transport direction roughly parallel to the modern Mokelmune River drainage (toward the SW).

Sequence 1 consists of Oligocene silicic ignimbrites sourced from central Nevada. Sequences 2 through 6 consist of dominantly andesitic rocks of the Ancestral Cascades arc; eruptive centers were sited along the modern range crest in the Carson Pass-Kirkwood area, and built lava domes that collapsed to generate block-and-ash-flow tuffs, which in turn were remobilized down-canyon to produce volcanic mudflow and minor dilute flow (stream) deposits. Sequence 2 trachyandesite block-and-ash flow tuff yielded a biotite age of 14.70 ± 0.10 Ma and a plagioclase age of 14.10 ± 0.70 Ma. A local eruptive hiatus is recorded by sequence 3, which consists of stream deposits sourced from andesitic centers that lay to the east of the present-day range crest. Sequence 5 is dated by an andesitic dike with peperitic margins that gives a plagioclase age of 10.50 ± 0.12 Ma and a whole rock age of 10.6 ± 0.20 Ma. The highest sequence (6) forms the highest peaks in the area, with sequence 6A on Red Lake Peak at Carson Pass, and sequence 6B on The Sentinels at Kirkwood. Sequence 6A contains basalt lava flows with a whole rock age of 6.80 ± 0.20 Ma; upsection from this lies a distinctive pyroxene-bearing andesitic block-and-ash-flow tuff (all others in the area bear hornblende) that is correlated with the sequence 6B Sentinels block-and-ash-flow tuff, with a plagioclase age of 6.05 ± 0.12 Ma.

Vertical relief on the unconformities within the paleocanyon ranges from 12 to 303 meters, with paleoslope gradients ranging from 3 to 48 degrees. The deepest uncomformities within the paleocanyon reincised to granitic basement between 14 and 10 Ma (uncomformity 5) and between 10 and 6 Ma (unconformity 6).

In summary, arc axis volcanism occurred in the Carson Pass / Kirkwood Valley area from about 14 to 6 Ma. The paleo-canyon was apparently cut off from rivers draining the Basin and Range province to the east sometime between about 14 and 10 Ma, and after that the paleocanyon fill was reincised to basement levels twice, suggesting the onset of range-front faulting by about 10 Ma.