MOHAWK LAKE OR MOHAWK MEADOW? (REVISITED). STRATIGRAPHY, GEOMORPHOLOGY, AND SOILS SUPPORT TWO IMPERFECT HYPOTHESES TO EXPLAIN THE QUATERNARY HISTORY OF A NORTHEASTERN CALIFORNIA INTERMONTANE BASIN

Mohawk Valley has long been recognized as an important record of Quaternary history, as incision by the Middle Fork of the Feather River and tributaries exposes ~200 m of basin-fill spanning the last 750 ka. The spatial extent and character of these sediments has been reconstructed using geologic mapping and correlation of 37 tephra beds from 18 locations. Present and prior studies offer two imperfect interpretations from these data, which include long-lived aggradation within a: 1) closed lake basin; and, 2) meadow - shallow lake/ marsh/ low-gradient fluvial system.

The overall character of these deposits suggests a lacustrine/fluvial-deltaic origin. The thick aggradational package is composed of massive diatom-rich silty clay, organic-rich silt, and sands to medium pebbles. Bedforms include plane, wavy, lenticular, tabular and trough cross-beds, shallow channels, wave-formed ripples, and dewatering structures. Glacial deposits grade to near the top of these fine-grained sediments, yet no coarse material lies in the axial valley. Tephras show lake and fluvial deposits of the same age vary in elevation by up to 85 m, perhaps most simply explained by bathymetry in a lake basin. Vertical displacement by the primarily dextral Mohawk fault zone is probably not large enough to account for the elevation differences.

Conversely, evidence for a “meadow/marsh” interpretation includes diatom assemblages indicative of shallow lakes, sediments rich in organic matter and emergent aquatic vegetation, and weak paleosols that suggest periods of subaerial weathering. How do these interpretations co-exist with those of a large, long-lived lake?

An OIS 6-aged tephra caps the fine-grained, basin-fill sediments, providing a maximum age for basin integration. Large landslides of several ages lie in the narrow downstream canyon and probably formed the dam(s). Downstream from the landslides, well developed soils in outwash deposits overlie much stronger soils developed into bedrock, indicating long periods of stability prior to deposition of outwash from Mohawk Valley and apparently agreeing with the upstream history of a confined Mohawk Valley until ~OIS 6 time. One possibility is that both deep lakes and shallow lakes/marshes existed in Mohawk through time depending on the height and dynamics of the landslide dams.