Regional-scale assessment of a sequence-bounding paleosol on fl uvial fans using ground-penetrating radar, eastern San Joaquin Valley, California George L. Bennett V† Gary S. Weissmann‡ Department of Geological Sciences, Michigan State University, 206 Natural Sciences Building, East Lansing, Michigan 48824-0001, USA Gregory S. Baker§ Department of Earth and Planetary Sciences, 1412 Circle Drive, University of Tennessee, Knoxville, Tennessee 37996-1410, USA David W. Hyndman# Department of Geological Sciences, Michigan State University, 206 Natural Sciences Building, East Lansing, Michigan 48824-0001, USA ABSTRACT nel activity on these fan surfaces during the eastern San Joaquin fans are dominated by fl u- last outwash event. Differences in channel vial processes (Janda, 1966; Huntington, 1971, Recently developed sequence stratigraphic activity between fans indicate that the Kings 1980; Marchand, 1977; Marchand and All- models for fl uvial fans suggest that sequence River migrated across its fan during the last wardt, 1981; Lettis, 1982, 1988; Harden, 1987; boundaries in these deposits are marked by outwash event, as evidenced by the large Weissmann et al., 2002a, 2005), we use the term laterally extensive paleosols; however, these number of areas with increased GPR signal “fl uvial fan” to distinguish these fans from allu- models were based on paleosol correlations penetration and the presence of numerous vial fans dominated by sheet-fl ood or debris- inferred between wells. To test this, we col- channel deposits recorded on the soil surveys, fl ow processes (Weissmann et al., 2005). On the lected ~190 km of ground-penetrating radar while the Tuolumne and Merced Rivers only Kings River fan, episodes of widespread fl uvial (GPR) profi les on three fl uvial fans from the deposited fl oodplain fi nes, with the channels deposition occurred in response to signifi cant eastern San Joaquin Valley, California, to remaining inside a shallow incised valley, as increases in sediment supply during periods of determine the lateral extent and character evidenced by the relatively low number of glacial outwash (Weissmann et al., 2002a). Con- of a buried near-surface sequence-bound- areas with increased GPR signal penetration versely, loss of glacial outwash sediment load ing paleosol. This paleosol, recognized on and the presence of primarily fi ne-grained during interglacial periods led to incision of GPR by rapid shallow signal attenuation, material recorded on the soil surveys. Fac- the fans, leaving the upper fl uvial fan exposed extends across large areas on all three fl uvial tors controlling these differences may include to widespread soil development. Basin subsid- fans. Limited areas of signifi cantly increased variable valley subsidence rates and differ- ence provided accommodation space, such that signal penetration were also identifi ed, and ences in the San Joaquin Basin overall width subsequent aggradational events buried the these zones are interpreted to indicate the at each fan location. interglacial soils, thus forming stratigraphic absence of the paleosol. The zones where the sequences, where a sequence is defi ned as paleosol is missing likely correspond to paleo- Keywords: fl uvial sequence stratigraphy, “…a relatively conformable succession of genet- outwash channel activity on the fan surfaces ground-penetrating radar, fl uvial fans, Qua- ically related strata bounded at its top and base that, when active, was able to partially or ternary, San Joaquin Valley. by unconformities, or their correlative confor- fully scour through the paleosol and deposit mities” (Mitchum, 1977, p. 210). The sequence- coarse-grained channel sediments in place INTRODUCTION bounding unconformities of the Kings River of the sequence boundary. Erosional breaks fan are marked by relatively mature paleosols are most common on the Kings River fan, Fluvial and alluvial fan stratigraphy often in the stratigraphic succession. This sequence while few breaks on the Tuolumne and Mer- appears to be complex and chaotic, thus com- stratigraphic approach provides an important ced River fans may indicate less paleochan- plicating recognition of stratigraphic patterns. framework for groundwater studies (e.g., Weiss- However, Wright and Alonso-Zarza (1990) and mann and Fogg, 1999; Weissmann et al., 2002a, †Present address: U.S. Geological Survey, Califor- Alonso-Zarza et al. (1998) showed that paleo- 2002b, 2004; Burow et al., 2004). nia Water Science Center, 6000 J Street, Placer Hall, sol distributions can help identify stratigraphic Three main issues remain in the sequence Sacramento, California 95819-6129, USA; e-mail: [email protected]. patterns in fan deposits. In recent work, Weiss- stratigraphic model generated in previous stud- ‡Corresponding author present address: Depart- mann et al. (2002a) recognized that the Kings ies. We attempt to address these issues by inte- ment of Earth and Planetary Sciences, MSCO3– River fl uvial fan of the eastern San Joaquin grating ground-penetrating radar (GPR) surveys 2040, 1 University of New Mexico, Albuquerque, Valley in California responded to Quaternary with outcrop and borehole data on three fl uvial New Mexico 87131-0001, USA; e-mail: weissman@ unm.edu. climate change through cycles of deposition fans in the San Joaquin Valley: the Kings River, §E-mail: [email protected]. and erosion, thus producing paleosol-bound the Tuolumne River, and the Merced River fans #E-mail: [email protected]. stratigraphic sequences. Since deposits on the (Fig. 1). The fi rst issue is that in previous work GSA Bulletin; May/June 2006; v. 118; no. 5/6; p. 724–732; doi: 10.1130/B25774.1; 6 fi gures. 724 For permission to copy, contact [email protected] © 2006 Geological Society of America SEQUENCE-BOUNDING PALEOSOLS ON FLUVIAL FANS Tuolumne and Merced River Fluvial Fans Kings River Fluvial Fan CALIFORNIA Figure 1. Maps of study area fans showing surface exposures of stratigraphic units, based on soil surveys by Arkley (1964), Huntington (1971), Arroues and Anderson (1976), McElhiney (1992), Ferrari and McEl- hiney (2002), and USDA/NRCS (2003). The Turlock Lake unit consists of deposits from two major glaciations in the subsurface, e.g., upper and lower Turlock Lake units (Weiss- mann et al., 2002a). The reported stratigra- phy is from Marchand and Allwardt (1981), with unit relationships to soil series based on Janda (1966), Helley (1966), Marchand (1977), Marchand and Allwardt (1981), Lettis (1982, 1988), Harden (1987), and Huntington (1980). Boxes outline the areas covered by maps of Figure 4. Geological Society of America Bulletin, May/June 2006 725 BENNETT et al. (Weissmann and Fogg, 1999; Weissmann et al., nifi cant refl ector encountered by the GPR signal; On the Kings River fan, as well as other fans 2002a), buried paleosols of the Kings River fan thus, in many profi les this shallow refl ector con- in the eastern San Joaquin Valley, depositional have only been correlated between widely spaced structively and destructively interferes with the cyclicity occurred with changes in the sediment wells; thus, uncertainty exists with respect to the direct EM wave, and interpretation is diffi cult. supply to stream discharge ratio through the gla- validity of these correlations and whether the However, due to the highly attenuating behavior cial-interglacial cycles during the Pleistocene paleosols are truly laterally extensive and iden- of the conductive clay-rich unit (where it exists), (Wahrhaftig and Birman, 1965; Janda, 1966; Hun- tifi able on a regional scale. Second, Weissmann the lateral continuity of the uppermost paleosol tington, 1971, 1980; Marchand, 1977; Marchand (1999) and Weissmann et al. (2002a, 2002b, is inferred via depth of penetration through use and Allwardt, 1981; Lettis, 1982, 1988; Harden, 2004) suggested that discrete breaks or “holes” of extensive GPR surveys. 1987; Weissmann et al., 2002a, 2005). Fluvial exist through the sequence-bounding paleosols processes dominated open-fan deposition (depo- in the San Joaquin Valley. They proposed that Study Area sition across the entire fan surface), and aggrada- the breaks were formed by channel erosion that tion occurred during periods of signifi cant glacial occurred when channels reoccupied the fl uvial The Kings, Tuolumne, and Merced Rivers outwash, with channel incision occurring during fan surface during initial aggradation of the form large fl uvial fans where they enter the interglacial periods. Deep (~10–30 m) incised sequence. Evidence for the distribution of ero- San Joaquin Valley from the Sierra Nevada in valleys formed across the length of the fans dur- sional breaks, however, was limited to interpreta- California (Fig. 1). The Kings River enters the ing interglacial incision, leaving widespread tions of paleochannel distribution observed from San Joaquin Valley southeast of Fresno, and the areas on the upper portions of the fans exposed C-horizon textures reported in county soil sur- Tuolumne and Merced Rivers enter the valley east to soil development (Weissmann et al., 2002a, veys (Weissmann, 1999; Weissmann et al., 1999, and southeast of Modesto. Fluvial fans produced 2005). Sequences in the upper and middle por- 2002a, 2005). If such breaks exist, they create by these rivers have low gradients (~0.0013), tions of the fl uvial fans consist of fl uvial open-fan high conductivity pathways that may provide with the drainage trending west to southwest on and coarse-grained incised valley-fi ll deposits, rapid groundwater recharge along with associ- the Kings and Merced River fan,