TWICE-BAKED AMMONOIDS: FAMENNIAN (AND LATER) EVENTS AT DUGAN POND, SIERRA COUNTY, CALIFORNIA

The Elwell Formation (EF) is a thin but very distinctive unit within Paleozoic island-arc deposits in the northern Sierra Nevada. In most places it consisted (before metamorphism) of phosphatic, carbonaceous, radiolarian chert interbedded with sediment-gravity-flow deposits rich in dacitic debris derived from the underlying Sierra Buttes Formation (SBF), the whole intruded by penecontemporaneous andesitic sills and related peperite. At Dugan Pond, a few km north of the Sierra Buttes massif, these rocks have been metamorphosed twice, since they were intruded by a gabbro-quartz diorite stock prior to eventual Jurassic low-greenschist metamorphism (except locally, the grade of contact metamorphism probably did not exceed that associated with the albite-epidote-hornfels facies). The metachert has yielded three forms of goniatites, tentatively identified as a very evolute platyclymenid, Platyclymenia sp. or Clymenia sp. (the dominant species); Platyclymenia sp.; and Pernoceras crebriseptum, to date the only megafossils reported from the EF. These fossils provide a firm Famennian age for the EF, in accord with the mid-Famennian age of conodont-bearing chert in Member C of the SBF (=EF) near Bowman Lake (Hanson and Schweickert, 1986). The best preservation is in the metachert adjoining the stock at Dugan Pond. The chert, much altered to thinly laminated to structureless quartzite having coarse granoblastic fabric, nevertheless retains much evidence of its original character. Relict phosphate nodules contain idioblastic apatite prisms as long as 0.15 mm. Graphite, nearly always present, takes several forms, from “dust” to hexagonal plates often up to 0.02 mm across; it defines stylolites in most of the laminated samples. Relict radiolarians rarely are found, as spheroidal quartz aggregates outlined by graphite or apatite. Locally high temperatures of contact metamorphism are indicated by relict bipyramidal crystals of high-quartz, usually in metachert xenoliths.