1. ABSTRACT The geology of the northern Sierra Nevada of California records >400 million years of active plate margin tectonic events as a part of the North American Cordilleran orogenic belt. This field-trip guide provides geologic background and description of field-trip stops for a two-day field trip of the 2023 Geological Society of America Cordilleran Section Meeting based in Reno, Nevada. In two days, we cannot sample the complete geologic record of the northern Sierra Nevada, so this guide does not provide an exhaustive review of this geology. We will focus on certain aspects of the geology that have been the subject of recent research and present some previously unpublished observations and interpretations including: (1) distinguishing between subduction complexes and deformed assemblages that overlay subduction zones; (2) evidence for subduction initiation, recorded in high-pressure ( P ), high-temperature ( T ) amphibolites and possibly greenschist facies rocks structurally beneath them; (3) finding of high- P , high- T amphibolite blocks in mélange zones in subduction complex units accreted structurally beneath intact high- P , high- T amphibolite horizons; (4) differences in stream profiles between southern Cascade and northern Sierra drainages, suggesting different forcing mechanisms for stream erosion in those regions; and (5) complex relationships between stream incision, volcanic deposition, and Late Cenozoic faulting. • DEDICATION • This field–trip guide is dedicated to Eldridge Moores (1938–2018) and Jason Saleeby (1948–2023) who were giants in Sierra Nevada geologic research. Eldridge passed away in October 2018 while leading a field trip in this area on which the leaders of the current field trip (JW, DS) were participants. Jason passed away during the writing of this guide. As will be clear from reading this guide, the saying “standing on the shoulders of giants” applies.

Abstract Lead-zinc sulfides were deposited during the Devonian in the locality of the Jason prospect, Yukon Territory. Massive and laminated sulfides are interbedded with clastics of the Canol Formation; galena and sphalerite, among other minerals, precipitated on the sea floor from exhalative fluids along the fault margins of a small graben. The depositional graben center consists of a thick sequence of conglomerates, sandstones, and finer clastics. These were derived from a western source, and were deposited largely by turbidity currents in channel and channel-flanking areas. Conglomerates are massive or graded, and some show faint parallel stratification. Sandstones show Bouma-interval sequences and some grooves and flutes. In contrast, the ore area has a thinner sequence which consists dominantly of sedimentary breccias, pebbly mudstones, and shales. Breccias and pebbly mudstones were deposited from debris flows and show a range of plastic mass-flow structures. Stratigraphic thinning and presence of laterally discontinuous and locally derived mass failure deposits indicate the existence of a penecontemporaneous fault zone. A few conglomerates and sandstones are interbedded with the ore horizon, and these and siltstones and shales were likely deposited from large turbidity currents. Involvement of sediments which had been deposited near the bottom of the graben in later mass failures indicates episodic fault movement during sedimentation. The fault zone appears to have positioned a sub-sea surface, geothermal system which formed metal-depositing fluids. Diamond drilling is underway to determine ore reserves. Core 78-30 shows the range of sedimentary structures and bedding styles of the ore zone.