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will focus on certain aspects of the geology that have been the subject of recent research and present some previously unpublished observations and interpretations including: (1) distinguishing between subduction complexes and deformed assemblages that overlay subduction zones; (2)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 The structure and stratigraphy of the Miocene formations east of San Francisco Bay have been described in multiple studies for over a century. We integrated the results of past investigations and provide new data that improve understanding of formation age, the timing of deformation, and the amount of dextral displacement on selected faults. New geologic mapping and better age control show that formations previously inferred to be separate units of different ages are correlative, and new names are proposed for these units. Miocene structures associated with the development of the San Andreas transform system exerted significant control on Miocene deposition in the East Bay area. The developing structure created five distinct stratigraphic sections that are differentiated on the basis of differences in the stratigraphic sequence, lithology, and age. The stratigraphic changes are attributed to significant dextral displacement, syndepositional faulting, and distal interfingering of sediment from tectonically elevated source areas. New stratigraphic evaluations and age control show that prior to ca. 6 Ma, the developing fault system created local tectonically induced uplift as well as spatially restricted subbasins. Regional folding did not occur until after 6 Ma. Past evaluations have inferred significant dextral displacement on some of the faults in the East Bay. The spatial relationships between unique conglomerate clasts and known source areas, as well as the distribution of well-dated and unique tuffs, suggest that dextral displacement on some faults in the East Bay is less than previously reported.