QUATERNARY EVOLUTION OF MOHAWK VALLEY, NORTHERN CALIFORNIA. PRELIMINARY RESULTS FROM GEOLOGIC MAPPING AND STRATIGRAPHIC INTERPRETATIONS OF QUATERNARY DEPOSITS

Mohawk Valley is located in northeastern California at the northern end of the Sierra Nevada and at the northern end of the Walker Lane Belt. Here, along the Middle Fork of the Feather River and its tributaries, numerous outcrops of Quaternary sediments containing volcanic ashes spanning the last 750,000 years are exposed. Prior interpretations of these sediments suggest a long-lived period of aggradation of marsh, shallow lacustrine, and braided river deposits dating from ~ 750 ka to 160 ka, followed by incision by glacial outwash deposits of Marine Isotope Stage 2 and older.

The current study has focused on detailed mapping of the distribution of these sediments and interpretation of their depositional environments, particularly in locations with tephra beds. Six new locations were documented where the Rockland tephra bed (~ 573 ka) was deposited within fluvial sediments in a low gradient fluvial system extending more than 15 km along the present day Mohawk Valley. To the north of this low gradient system, however, are fluvial sediments perched up to 60+ m above that reflect a poorly understood larger fluvial system (coarser material) that input into this otherwise low-gradient stream. Tectonic influence on these deposits is not clear though this study has resulted in some first order, general observations regarding the location of faults within the Mohawk Fault Zone and has documented that faults cut the Rockland tephra in multiple locations.

Multiple generations of relatively coarser grained fluvial deposits overlie and cut into finer grained fluvial and marsh deposits that are from ~ 740 ka to 160 ka and crop out at modern river level, which suggests a repeating cycle of filling and cutting. This study provides a second location where the change from a primarily aggradational sequence to a regime of incision occurs after the deposition of the Summer Lake LL tephra bed (~160 ka). What drives the repeated changes in base level is not known, but may have resulted from repeated damming by large-scale landslides and/or tectonics. Alternatively, or in addition to outside base-level controls, these cycles of cutting and filling may represent changes in process driven by climate change, which can affect stream power and/or sediment-water ratios that could change a fluvial system from aggradational to degradational or vice versa.