New evidence for alternating effusive and explosive eruptions from the type section of the Stanislaus Group in the 'Cataract' palaeocanyon, central Sierra Nevada

摘要

High-K volcanic rocks of the Stanislaus Group record magmatic events that occurred when the Sierra Nevada microplate began to calve off from the western edge of the Nevadaplano at 10-9Ma. Despite the fact that these rocks have been studied in the Sierra Nevada for 120 years, their stratigraphy is not well known, because they were largely deposited in a complicated palaeocanyon network that drained the Nevadaplano and are preserved on high peaks in roadless areas. We present new 1:6000 scale mapping and a detailed measured section in the Bald Peak-Red Peak area of the Sonora Pass, in the upper reach of the 'Cataract' palaeocanyon, or palaeo-Stanislaus river. The Bald Peak-Red Peak area was proposed as the type section for the Stanislaus Group in 1953 but had not been mapped or measured in sufficient detail to demonstrate stratigraphic relations. Our work demonstrates the following stratigraphy (from base to top): (1) Table Mountain Latite (TML) trachyandesite (latite) and basaltic trachyandesite (shoshonite) lava flows, with phenocrystic clinopyroxene and skeletal plagioclase and groundmass olivine, and minor olivine basalt and basaltic andesite lava flows; (2) Tollhouse Flat Member of the Eureka Valley Tuff (EVT) welded ignimbrite, with abundant phenocrystic biotite; (3) a single trachydacite lava flow with phenocrystic amphibole, referred to here as the Lava Flow Member of the EVT; (4) By-Day Member of the EVT welded ignimbrite, which lacks phenocrystic biotite; (5) Upper Member of the EVT, consisting of unwelded ignimbrite; and (6) a single, very thick (60 m) aphyric basaltic trachyandesite (shoshonite) lava flow that we assign to the Dardanelles Formation. Our work confirms for the first time that the Dardanelles Formation exists as a unit that overlies all members of the EVT, and that its geochemistry is the same as that of the TML in the type section. Our documentation of a trachydacite lava flow within the EVT in the central Sierra Nevada shows that widespread effusive volcanism alternated with the explosive volcanism, rather than merely preceding and following it, and that magmas of trachydacitic composition were erupted as lavas as well as pyroclastic flows. The preservation of all formations and members of the Stanislaus Group in the 'Cataract' channel suggests that it was an important palaeogeomorphic feature.