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project area. We associate a normal-to-reversed transition in magnetic polarity in the upper member of the Laguna We describe a sequence of upper Tertiary and Quawith the boundary between the Gauss Normaland ternary alluvial deposits for the area of the Feather and Matuyama Reversed-Polarity Chrons dated elsewhere at Yuba Rivers in the Sacramento Valley, California, and we 2.48 Ma. correlate these deposits with existing stratigraphic units to A lahar and volcanic sediments that contain dacitic the south in the northeastern San Joaquin Valley. We also clasts underlie the Laguna Formation and have only limdescribe our studies of relict and exhumed soils formed ited surface exposure. Previous workers interpreted this on six of these deposits at Honcut Creek in the center of volcanic unit to lie stratigraphically below the Miocene the project area. The soils were examined in order to Lovejoy Basalt (basalt of Table Mountain) and attributed a establish a soil chronosequence to evaluate the systematic Miocene age to it. Blocks of dacite from the lahar have a changes in soil profile properties with increasing age of mean whole-rock K-Ar age of 3.2 Ma. New stratigraphic the deposits, and to evaluate the use of soils as correlation and geomorphic data plus the radiometric dating suggest and dating tools for deposits of the upper Cenozoic. a late Pliocene age and further constrain the time of Methods employed in developing the local stratigrainception of alluvial valley filling. phy and in correlating deposits to the San Joaquin Valley The volcanic unit and the Laguna Formation are moninduded: (1) stratigraphic superposition of deposits, sevoclinally folded or faulted along the edge of the foothills. eral of which are separated by buried soils, (2) relative Deformation occurred between about 3.0 and 1.5 Ma and elevation and degree of dissection of original surfaces of is perhaps a southerly extension of the Chico monocline, deposits, (3) contrasting development of soils on relatively which terminates 20 km to the north of the Feather River. uneroded surfaces of the deposits, (4) tephra correlation, Aligned north-south drainages in the Riverbank Formation (5) magnetostratigraphy, and (6) radiometric dating. suggest more recent minor deformation. The alluvial deposits form a poorly expressed set of Distinctive soils are associated with relict surfaces of inset terraces along the edge of the valley t.hat open to the each formation and member. The most recognizable field south and west to form a fan flood plain in which succesproperties that develop systematically with increasing soil sive deposits are buried in normal stratigraphic superpoage are soil color, soil depth, and argillic B horizons. Soils sition. Some of the deposits therefore can be observed in formed in Holocene alluvium have 10 YR brown color both relict-surface and buried positions. Buried soils mark hues and A horizons that are less than 0.5 m thick. In substantial periods of stability between episodes of aggracontrast, soils formed on the lower member of the Laguna dation. We recognize seven principal alluvial units and we Formation have 10 R red color hues and strong argillic B correlate these with members and informal units of the horizons that extend more than 5 m deep. We analyzed Modesto, Riverbank, and Laguna Formations, plus Hothe soils to determine their morphologic, physical, chemlocene alluvium. We did not recognize an alluvial deposit ical and mineralogic properties. Properties that develop equivalent to the middle Pleistocene Turlock Lake Formamost systematically with soil age are total profile develoption. We suggest that a major hiatus in deposition ocment, as quantified from field properties by the Harden's curred between the upper Pliocene and lower Pleistocene soil development index, and depletion of major elements Laguna Formation and the middle Pleistocene Riverbank from silt-size fractions as determined by X-ray fluoresFormation. cence spectroscopy. Changes in clay mineralogy with soil Tephra correlation and magnetostratigraphy provide age also are distinctive but are qualitative data. Nonconnew information on the age of the Laguna Formation. stancy of other soil-forming factors, principally parent Glass from fluvially laid tephra at or near the base of the material composition and topographic position, intralower member is correlated with the Nomlaki Tuff Memduces important variation in the age trends that we obber of the Tehama and Tuscan Formations and provides an served and limits the certainty with which soils can be age of 3.4 Ma for inception of alluvial deposition in the used to correlate and date alluvial deposits. Nevertheless, Late Cenozoic Stratigraphy of the Feather and Yuba Rivers Area, California G1 systematic change in soil properties over the 1.6 million year estimated age span of the chronosequence provides insight into near-surface weathering processes and provides a means to use soils as a tool in geologic studies.

Late Cenozoic stratigraphy of the Feather and Yuba rivers area, California, with a section on soil development in mixed alluvium at Honcut Creek