Regional-scale assessment of a sequence-bounding paleosol on fl uvial fans using ground-penetrating radar, eastern ,

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 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 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 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, and west on the bounded by the paleosols that developed during ated contaminant transport into the San Joaquin Tuolumne River fan (Weissmann et al., 2005). past interglacial periods (Fig. 2) (Weissmann et Valley aquifer system (Weissmann, 1999; Weiss- The Sierran drainage basins upstream of the al., 2002a, 2005). These paleosols consist of red- mann et al., 2004). The third major issue is that if Kings, Tuolumne, and Merced River fans cover dish, thick (~1 m) argillic (clay-rich) horizons the sequence stratigraphic model is reasonable, areas of ~4385 km2, 3980 km2, and 2750 km2, that are readily recognized in cores and well logs sequence-bounding paleosols should be recog- respectively, while the fans cover areas within and appear to be similar to modern soils that have nizable in the shallow subsurface of other fans in the San Joaquin Valley of ~2400 km2, 630 km2, had signifi cant time to develop (e.g., ~100,000 yr) the eastern San Joaquin Valley. and 840 km2, respectively. These fans are sites (Weissmann et al., 2002a). Since its introduction, GPR has allowed for of ongoing hydrostratigraphic characterization Multiple glacial episodes are represented by quick and effective imaging of the stratigraphy, and groundwater modeling studies (e.g., Burow four Pleistocene-age stratigraphic units in the internal structure, and, in some cases, the lithol- et al., 1997, 2004; Gronberg et al., 1998; Weiss- eastern San Joaquin Valley fans (Fig. 1), with ogy of near-surface sands and gravels (e.g., mann and Fogg, 1999; Weissmann et al., 1999, several minor aggradational events recorded Davis and Annan, 1989; Beres and Haeni, 1991; 2002b, 2004), because increased urbanization within each of these units (Marchand and All- Jol and Smith, 1991; Smith and Jol, 1992; Hug- of this predominately agricultural land has led wardt, 1981). The paleosol imaged in this study genberger, 1993; Beres et al., 1995; Jakobsen to stresses on the groundwater resources in the caps the Riverbank unit (Illinoisan-equivalent and Overgaard, 2002). GPR surveys produce area. Therefore, assessment of anthropogenic glacial stage). This paleosol was formed across continuous high-resolution profi les of the sub- contaminant input and withdrawal demands on the entire upper and middle fan surface (except surface that are similar to those produced using the system are required for protection of this for the location where a relatively deep incised seismic-refl ection methods. Relatively short increasingly valuable resource. valley was present) during the last interglacial (~1.5 km) GPR surveys on the Kings River fl u- vial fan by Burow et al. (1997) indicated that the uppermost sequence-bounding paleosol could be tracked using GPR. In the survey by Burow et al. (1997), as well as surveys con- ducted for this study, the electrically conductive clay-rich paleosols rapidly attenuate the GPR signal, thereby forming a coherent unit, below which very little or no signal is detected. This study focuses on imaging the uppermost buried paleosol of the fl uvial fans (approximately mid- dle Quaternary in age). This paleosol is buried under late Quaternary fan deposits in the middle to distal portions of the fan, while toward the fan apex, it is exposed as a terrace above these deposits (Weissmann et al., 2002a). At the electromagnetic (EM) frequencies used for the GPR surveys (50 and 100 MHz), direct imaging of this paleosol was limited due to its shallow Figure 2. Schematic diagram showing the distribution of facies within sequences in the San depth. The paleosol in most cases is the fi rst sig- Joaquin Valley fl uvial fans.

726 Geological Society of America Bulletin, May/June 2006 SEQUENCE-BOUNDING PALEOSOLS ON FLUVIAL FANS period (Huntington, 1980; Weissmann et al., within the time window, thereby improving the GPR profi les are typically displayed as 2002a). It was then buried over most of the signal to noise ratio in the presence of random either two-way traveltime (ns) of the transmit- study area by glacial outwash deposits of noise by a factor of the square root of n, where ted signal or depth (after conversion) versus the Modesto unit (Wisconsin-equivalent gla- n is the number of stacks. GPR soundings along the distance (m) along the transect. In order to cial stage). Because this paleosol (and other the survey lines were collected every 0.5 m (step translate the two-way traveltime of the refl ected sequence-bounding paleosols) was developed size), with each transmitted pulse triggered by signal to depth below the surface (i.e., time-to- across the upper and middle fan surface during an odometer wheel towed behind the cart. The depth conversion) at the study area, an averaged periods of fan incision, we expect the paleosol common-offset distance (antenna separation) near-surface velocity of 0.14 m/ns was used. to be a widespread stratigraphic marker. was 2 m. Profi les were processed and plotted This velocity was determined from ten common with software developed specifi cally for GPR mid-point (CMP) surveys conducted at several METHODS data presentation, WINEKKO version 1.0 and sites on both the Kings and Tuolumne River fans EKKOMAPPER version 2, both of which were (Bennett, 2003). The variance for this average We collected GPR data from the study areas developed for use with the Sensors and Soft- was 1.4 × 10−4 m/ns. The CMP survey sites were using a Sensors and Software Inc. pulseEKKO ware GPR hardware. Only minimal processing selected to capture a variety of stratigraphic set- 100TM system. Common-offset refl ection sur- was required for the type of data interpretation tings, including areas with a shallow Riverbank veys (i.e., the standard collection method that required in this study. First, a low-frequency paleosol, areas with channel fi ll sands within maintains a fi xed distance between the trans- (“dewow”) fi lter was applied to remove any a Riverbank paleosol break, and areas that are mitting and receiving antennas) were collected low-frequency energy generated from electri- representative of the typical background facies using this system in EH mode (q.v., Jordan and cal inductive processes or as a result of possible encountered during the overall GPR survey. Baker, 2003; also known as parallel broadside or dynamic range limitations of the unit. Second, The GPR surveys were designed to constrain TE mode) with a 400 V transmitter pulser volt- automatic gain control (AGC) was uniformly the lateral extent of the paleosol that caps the age and interchangeable sets of 50 and 100 MHz applied to the data with consistent parameters. Riverbank unit (e.g., the uppermost paleosol in center frequency antennas. We opted to use The AGC is designed to recover and reapply the stratigraphic succession on these fans). The 50 MHz antennas for the large-scale surveys. signal amplitude information potentially lost surveys were run along the preexisting grid of This preference resulted in a decrease in near- due to geometrical spreading of the transmitted county roads on the fl uvial fan surfaces; the surface GPR resolution yet added to the maxi- waves or other attenuation factors. The reason roads were chosen on the basis of the following mum depth of signal penetration, which was for using an AGC gain rather than some other criteria: they had (1) the upper fl uvial fan sur- important to the project objective. The system method (e.g., constant gain or spherical diver- face recorded as either Riverbank or Modesto was towed on a fi berglass GPR cart one meter gence correction) is that the preservation of rela- deposits on the soil surveys, thus, we expected behind a car at ~5 km/h (Fig. 3). Each shot loca- tive amplitude relationships among refl ections to observe the Riverbank-age paleosol in the tion in refl ection mode was stacked at least eight was not required. Instead, the objective was to shallow subsurface beneath the Modesto-age times within a time window of 800 ps (1600 in examine depth of penetration as affected by the deposits, (2) a smooth surface, long distance, some cases). Stacking is a summation function conductive paleosol; thus, enhancing all exist- and relatively straight paths, (3) low traffi c vol- that enhances the incoming refl ections collected ing refl ection information at depth was desired. ume, and (4) they did not have sources of sig- nifi cant GPR signal interference (e.g., abundant overhead high-power lines and urban areas). In total, ~120 km of GPR data were obtained on the Kings River fan, ~34 km on the Tuolumne GPR Console, Laptop, River fan, and ~36 km on the Merced River fan and Power Supply in Car (Fig. 4; for a more extensive presentation of the GPR data, see Bennett, 2003). The Tuolumne River fan survey focused on deposits south of the Tuolumne River, while the Merced River fan survey focused on deposits north of the Mer- ced River. Signifi cant urbanization of the area north of the Tuolumne River combined with limited time and funding prevented collection of GPR data across the entire surface of either Transmitting the Tuolumne or Merced River fan. Instead, we 50 MHz Antenna performed an exploratory survey of the interfan Receiving area, which allows us to compare the GPR data 50 MHz Antenna GPR Odometer from three separate eastern San Joaquin Valley Trigger Wheel fl uvial fans and discuss the observed differences between each region. Figure 3. The ground-penetrating radar (GPR) fi eld setup used for continuous-refl ection mode is shown with 50 MHz antenna and transmitters attached to a fi berglass cart. The Data to Ground-Truth the Paleosol odometer wheel trailing behind the cart triggered the system at regular distance intervals. A Location deep-cycle 12 V battery located in the trunk of the car powered the system, while fi ber optic cables connected the antenna to the systems console (in the back seat), which sent the data Detailed lithologic core and well logs (Weiss- to a laptop computer in the passenger seat. mann, 1999; Harter et al., 1999, 2005) were used

Geological Society of America Bulletin, May/June 2006 727 BENNETT et al.

A Kings River Fluvial Fan B Tuolumne and Merced River Fluvial Fans ModestoModesto

SSangeranger Tuolumne River

{ 6A 5C{ { Hwy 5B 99 5A TurlockTurlock

FowlerFowler { 6B

GPR PROFILES Gravel Hardpan Paleosol Hwy Sand Rivers/Lakes Deep Incision Overbank Shallow Incision Merced River 99 Corrupt Profile Questionable Figure 4. Interpreted ground-penetrating radar (GPR) lines on (A) the Kings River fan and (B) the Tuolumne and Merced River fans showing the distribution of the Riverbank paleosol and locations of breaks through this paleosol. The background map shows locations of channel and overbank deposits interpreted from soil textures (Weissmann et al., 2002a). Numbers show locations of GPR profi les in Figures 5 and 6.

to compare interpretations of GPR data to identi- Additionally, a section located along Del and the depth of the paleosol, where it was pres- fi ed presence or absence of the shallow near-sur- Rey Road (Fig. 5B) also shows attenuation of ent, was compared to the depth noted in core face paleosol. Small-scale surveys were collected the GPR signal beneath the shallow paleosol, and well logs. The lines could not be supported at the Kearney Agricultural Center, an extension where lithologic information from nearby well by ground data at all locations, especially on of the University of California, located on the logs and cores were used for ground truth of the Tuolumne and Merced River fans, because Kings River fan (Fig. 5A). This relatively small- the GPR interpretations. Conversely, when the of the cost limitations imposed by the regional scale (~1 ha) study area was formerly a nectarine paleosol is absent, the GPR signal penetrates scale of the study. However, surveys on these orchard plot that has had its trees removed and is to signifi cantly greater depths (Fig. 5C). Thus, fans appear to have similar character to those of now the site of ongoing vadose zone hydrostrati- shallow GPR signal attenuation helps us to read- the Kings River fan; thus, we infer the presence graphic investigations (Harter et al., 1999, 2005). ily identify areas where the paleosol is present. of a shallow paleosol where the signal is attenu- Numerous continuous sediment cores were drilled ated in the near-surface (~1–4 m). Soil surveys down to a depth of 15.8 m along east-west and RESULTS and drillers’ logs from water wells in the area north-south cross sections within the 1 ha orchard support this interpretation (Arkley, 1964; McEl- plot. GPR data were collected on a grid matching Our regional-scale GPR survey effectively hiney, 1992; Burow et al., 2004). the geometry of the former nectarine tree rows, identifi ed the distribution of the paleosol cap- Shallow paleosols are present in all sur- resulting in fourteen profi les that pass as close as ping the Riverbank unit on all three fl uvial fans veys conducted on the fans, indicating that the possible to borehole locations used in the Harter (Figs. 4, 5, and 6) by identifying regions with paleosols are widespread stratigraphic mark- et al. (2005) study. Figure 5A shows the well log rapid attenuation of GPR signal caused by the ers (Fig. 4). Initially, we expected to see the descriptions superimposed on the GPR profi les clay-rich paleosol unit. As previously noted, Modesto unit signifi cantly thicken basinward, produced using the 50 MHz antenna. This GPR the interpretation that the paleosol caused the with the Riverbank paleosol dipping deeper profi le shows that the signal becomes attenuated observed GPR signal attenuation was verifi ed into the subsurface below a thickening Modesto just below the paleosol, supporting the hypothesis by comparing nearby core and driller’s well logs unit. However, these GPR surveys indicate that that the presence of the paleosol rapidly attenu- to the GPR results at several locations on the the Modesto unit is a relatively thin (~1–4-m- ates GPR signal and thus signifi cantly reduces Kings River fan (Fig. 5). Presence or absence of thick) aggradational unit over the surveyed the depth of signal penetration. the paleosol was confi rmed in these well data, areas of the three fans.

728 Geological Society of America Bulletin, May/June 2006 SEQUENCE-BOUNDING PALEOSOLS ON FLUVIAL FANS

A Kings River Fan - Kearney Agricultural Center - Paleosol present Raw GPR Data South North

0

5

Depth (m) 10 Well Log Key 0 10 20 30 40 50 60 70 80 - Topsoil Position (m) Kearney x51 Section - Sand Interpreted GPR Data - Clay South North - Paleosol Figure 5. (A) Raw data and 0 Paleosol interpreted ground-penetrating 5 radar (GPR) profi les (50 MHz antennas) from the Kearney Depth (m) 10 Agricultural Center showing 0 10 20 30 40 50 60 70 80 signal attenuation below the Position (m) Riverbank paleosol (marked by the dashed line). Lithology B Kings River Fan - Smith Road - Paleosol present from core provides ground North Raw GPR Data South truth for paleosol depth and presence. (B) A typical GPR

0 profi le (50 MHz antennas) from 0 5 the Kings River fan showing signal attenuation at the River- 105 bank paleosol (marked by the

Depth (m) 15

Depth_in_meters 10 dashed line on the interpreted 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 20 Position (m) Smith Road Segment 1 profi le). Presence and depth Interpreted GPR Data of the paleosol is supported by lithologic logs from nearby Paleosol Near Surface Paleosol Beneath Modesto Deposits Smith1_Depth_Section wells. The prominent diffrac- 0 tions (hyperbolas) observed at 0 5 ? ? Paleosol 285, 505, 600, 670, 1100, and

10 1190 along the profi le represent 5 refl ections caused by passing 15 Depth (m) 10 under overhead power lines or 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 20 over irrigation canals. (C) GPR Smith Road Segment 1 Position (m) profi le (50 MHz antennas) from the Kings River fan showing an C Kings River Fan - Del Rey Road - Paleosol absent interpreted paleochannel that has eroded through the River- Raw GPR Data West East bank paleosol. A paleochannel

0 is interpreted to exist between 5 110 and 400 m along this tran- 5 10 sect. Absence of the paleosol in

Depth (m) 1015 this area is supported by litho-

Depth_in_meters 20 logic logs from nearby wells. 15 0 100 200 300 400 500600 700 800 900 Position (m) Del Rey Road Segment 18 20 Interpreted GPR Data Shallow Shallow Paleosol Absent Paleosol Deep Channel Incision Shallow Channel Incision _Dr18_Depth_Section

0 5 5 Channel Scour 10 Paleosol

Depth (m) 1015 Depth_in_meters 1520 0 100 200 300 400 500600 700 800 900 Position (m) Del Rey Road Segment 18

Geological Society of America Bulletin, May/June 2006 729 BENNETT et al.

A Tuolumne River Fan North South Paleosol Beneath Modesto Deposits Smith1_Depth_Section Figure 6. (A) A typical ground- 0 0 2 Paleosol penetrating radar (GPR) pro- 4 fi le (50 MHz antennas) from the 6 Tuolumne River fan showing

Depth (m) 8 signal attenuation at the River- 10 0Depth_in_meters 250 500 750 1000 1250 1500 1750 2000 bank paleosol (marked with an Position (m) Mitchell Road Segment 1 arrow). The prominent diffrac- tions (hyperbolas) observed B Merced River Fan along this line represent refl ec- North South tions caused by passing under Deep overhead power lines or over Shallow _Dr18_Depth_SectionPaleosol Present Channel irrigation canals. (B) Typical Incision ?? GPR profi le (50 MHz anten- 0 Paleosol nas) from the Merced River fan 2 showing signal attenuation at 4 6 the Riverbank paleosol. Deeper signal penetration on the south-

Depth (m) 8 10 ern end of this profi le shows a 0 250 500 750 1000 1250 1500 1750 2000 possible deep channel incision. Position (m) Palm Road Segment 1

Each of the three fl uvial fans described is sections of the laterally continuous Riverbank Modesto deposits on both the Tuolumne and unique in its basinal position and geomorphic paleosol. The areas where the GPR surveys Merced River fans is consistent with similar character (Weissmann et al., 2005). The follow- indicate paleosol breaks exist but channel sedi- sequences on the Kings River fan. In the case ing sections describe specifi c results from each ments are not observed on the soil survey have of the Tuolumne River fan, the lack of recorded fl uvial fan, which are then followed by a discus- a number of possible explanations, including channel breaks through the paleosol appears to sion of how these factors may be related to fan (1) a paleochannel may exist but does not “out- indicate that either the Modesto (Wisconsin- development. crop” on the soil surveys, (2) the paleosol was equivalent age) Tuolumne River never migrated removed for agriculture (e.g., ripping to increase across the fan surface or our limited surveys in Kings River Fan Results drainage), or (3) a shallow paleosol was exca- this region did not cross a Modesto-age channel vated for road construction. position. If the Tuolumne River never migrated Paleosols on the Kings River fan appear to across the fan surface, the Modesto incised val- have been dissected by paleochannels, as shown Tuolumne and Merced River Fan Results ley fi ll would lie beneath the present Tuolumne by zones of relatively deep signal penetration on River channel. The depth of the modern incised the GPR profi les (Figs. 4 and 5C). Areas along On the Tuolumne River fan, the Riverbank valley (30 m) and the similar thickness of past the profi les that showed notable increases in the paleosol was again identifi ed as a widespread incised valley fi ll deposits observed on the depth of signal penetration are assumed to be stratigraphic marker, yet it lacks the abundant Tuolumne River fan (Weissmann et al., 2005) areas where the paleosol is missing (i.e., break in paleochannel incisions observed on the Kings indicate that partial fi lling of the Modesto continuity). The GPR refl ections in some of these River fan, thus indicating greater lateral con- incised valley must have occurred. Without par- breaks show refl ectors consistent with channel tinuity of the paleosol (Figs. 4 and 6A). Addi- tial fi lling, fl oods during Modesto deposition scour shapes (Fig. 5C). These apparent channel tionally, overlying Modesto deposits appear to could not have spilled onto the fan surface. incisions varied in character from deep (>10 m), be thinner on most of the Tuolumne River fan Since the Riverbank paleosol is present in the relatively narrow incisions to shallow and wide (~1–2 m) than those observed on the Kings Tuolumne and Merced River fan subsurface, we lateral scour zones that removed the paleosol but River fan (~1–4 m). Finally, soil surveys across expect to fi nd other sequence-bounding paleo- rarely incised deeper than 6 m (Fig. 5C). In every the Tuolumne and Merced fans show that fi ne- sols in the deeper subsurface of these fans (e.g., case, the paleosol was observed to exist between grained facies dominate the Modesto deposits paleosols capping the Turlock Lake and Laguna discrete channel incisions. (e.g., fl oodplain sediments) (Fig. 4). units). Recent drilling in the Modesto area indi- In most instances, paleosol breaks identifi ed Several small breaks were identifi ed in the Mer- cates that these sequences are indeed present in in the GPR profi les on the Kings River fan are ced River fan paleosol based on GPR data (Figs. 4 both fans (Burow et al., 2004). directly correlated to paleochannel locations and 6B); however, more recent eolian deposits observed on county soil surveys, where chan- cover the surface of this fan. Therefore, we could Observed Differences Between Fans nels are shown by sandy C-horizon textures not interpret these breaks as Modesto channel of the soil series (Fig. 4; Weissmann et al., incisions. The overlying Modesto deposits appear Although similar sequences have been 2002a). This observation, along with the chan- to be ~1–2 m thick on the Merced River fan. observed and described on the Tuolumne, Mer- nel shape evaluated from the GPR surveys, sug- The presence of the Riverbank paleosol dip- ced, and Kings River fans, notable differences gests that Modesto-age paleochannels eroded ping basinward at a low angle (<1°) below the exist between them. These include differences

730 Geological Society of America Bulletin, May/June 2006 SEQUENCE-BOUNDING PALEOSOLS ON FLUVIAL FANS in (1) the apparent volume of Modesto-aged fans and do not believe this to be a major control groundwater resources, as low regions in the sediment preserved in each fan, (2) the number on differences observed between fans. paleosol surface may provide recharge points of observed erosional breaks in the underlying Finally, the variable number of breaks through the vadose zone by focusing and forcing Riverbank paleosol, and (3) the overall size observed in this study between these fl uvial water through the paleosol layer. The approach of each of the fans. Thus, these results aid in fans could be related to the amount of data col- presented in this study shows promise for identi- interpreting controls on sequence development lected on each fan. Fewer GPR survey lines on fi cation of high-conductivity pathways through within the San Joaquin Basin. the Tuolumne and Merced River fans may have the paleosol layer(s). Imaging such areas of Variable subsidence rates between the north- resulted in a biased sampling, thus showing potential focused recharge in these fl uvial sys- ern and southern parts of the San Joaquin Valley fewer observed breaks. tems may lead to an improved understanding may have infl uenced the observed morphology of fl uid/contaminant pathways into the regional and size of these fl uvial fans (Weissmann et al., DISCUSSION aquifers of the San Joaquin Valley fl uvial fans. 2005). Lettis and Unruh (1991) calculated the subsidence rate of the San Joaquin Basin using Paleosol Character and Distribution CONCLUSIONS the Corcoran clay, a regionally extensive 615 ka lacustrine unit, and showed that the southern The paleosol that caps the Riverbank sequence Imaging the presence or absence of the near- valley is subsiding faster then the northern val- was constrained using GPR surveys across all surface Riverbank paleosols on the Kings, ley (0.4 m/k.y. and 0.2 m/k.y., respectively). three study area fans; however, numerous and Tuolumne, and Merced River fans is feasible Thus, the Tuolumne and Merced River fans potentially signifi cant breaks were observed on using a towed GPR cart. Identifi cation of the have subsided at roughly half the rate of the the Kings River fan, and possible minor breaks shallow sequence-bounding paleosols on the Kings River fan to the south. The differing were observed on the Merced River fan. Breaks GPR surveys is made possible due to the clay- subsidence rates may therefore be a factor con- in the Kings River fan paleosol identifi ed on rich nature of these sediments. The penetration trolling differences in the observed Modesto GPR surveys generally correlated with loca- depth of the radar signal is limited by rapid unit thickness and lithologies, with the lower tions of Modesto paleochannels observed on signal attenuation associated with the highly subsidence rate in the northern valley provid- county soil surveys (Fig. 4). However, the chan- electrically conductive paleosol, therefore, these ing less accommodation space for sediments on nel widths observed on the GPR profi les tended conductive zones can be “mapped” by correlat- the Tuolumne and Merced River fans (Weiss- to be narrower then those observed on the soil ing signal penetration with the existence of the mann et al., 2005). This is refl ected in Modesto surveys. This difference could be due to the paleosol. We identifi ed discrete erosional breaks deposits on the Tuolumne and Merced Rivers cross-sectional shape of the channels, whereby through the paleosols on the Kings River fan, fans that are approximately half the thickness the paleosol breaks exist at the narrower base of and hypothesize that they are paleochannels that of those on the Kings River fan. Additionally, paleochannels where scouring took place, while incised into, fi lled, and then migrated across the the greater accommodation space on the Kings the soil survey shows the full width of past chan- upper fl uvial fan surface during glacial peri- River fan may have provided room for Modesto nel tracts. This research indicates that geophysi- ods (aggradational periods). Obvious increases channels to reoccupy the Kings River fan sur- cal surveys may provide a better estimation of in the depth of GPR signal penetration effec- face, leading to paleosol breaks on the Kings break geometries than estimates based on sur- tively highlight the location of erosional breaks River fan in contrast to the limited breaks and face mapping. This difference in break width through or incisions into the paleosol. Generally, fi ne-grained nature of Modesto deposits on the may be signifi cant when incorporating facies breaks identifi ed using GPR correlate with loca- Tuolumne and Merced River fans. geometries into groundwater fl ow models that tions of mapped paleochannels observed on the The overall width of the San Joaquin Valley attempt to capture fl uvial fan heterogeneities. county soil surveys (shown by sandy C-horizon at each of the fan locations may also control the Since these breaks through the paleosols likely textures). These erosional breaks in the clay-rich differences in fl uvial fan size (Weissmann et al., provide high-conductivity pathways for ground- paleosols are believed to be hydrostratigraphi- 2005). The Tuolumne and Merced River fans are water fl ow and contaminant transport (Weiss- cally signifi cant, since they may act as fast paths signifi cantly smaller than the Kings River fan, mann, 1999; Weissmann et al., 2002b, 2004), dif- (zones of high conductivity) into the aquifers. yet all the fans have drainage basins that were ferences in large-scale groundwater movement The Tuolumne River fan lacked evidence for glaciated during the Quaternary and probably are expected between the three study area fans. similar breaks, and only limited breaks were experienced similar glacial outwash sediment These paleosol breaks may also have an effect on observed on the Merced River fan. loads. The northern San Joaquin Basin (near the local ecology due to different levels of interaction The differences in both the number of ero- Tuolumne and Merced River fans), however, is between groundwater and surface water in these sional breaks through the paleosol between these signifi cantly narrower than the southern basin three systems. Further studies that image deeper fl uvial fans and in the thickness of Modesto near the Kings River fan (see Fig. 1). Thus, sequence-bounding paleosols are needed in deposits may indicate different historical con- overall accommodation space appears to be lim- order to understand whether similar breaks exist trols on aggradational cycles between the north- ited by basin width. through other sequence-bounding paleosols. ern fl uvial fans (Tuolumne and Merced River It could be argued that sediment supply to In addition to the paleosol breaks, the GPR fans) and the southern fl uvial fans (Kings River discharge ratio differences between these riv- surveys indicate that topography exists on the fan in particular). The Kings River fan exhib- ers could also have caused some differences in paleosol surfaces, where the top surface of the its more erosional breaks and thicker Modesto sequence thickness between the fans. However, paleosol is uneven or slightly undulating rather deposits than the Tuolumne and Merced River all three fans have similar drainage basin charac- than strictly planar (Figs. 5 and 6). This topogra- fans, possibly indicating that the Kings River teristics (size and connection to glaciated portions phy most likely refl ects the interglacial surface was more apt to aggrade and switch its channel of the Sierra Nevada; Weissmann et al., 2005). character of the fan prior to open-fan aggradation location than the Tuolumne and Merced Rivers. Therefore, we expect that similar sediment sup- during the glacial outwash event. This topogra- Factors that controlled channel aggradation (or ply to discharge ratio changes occurred on these phy is important to understand in the context of accommodation space) during glacial periods

Geological Society of America Bulletin, May/June 2006 731 BENNETT et al.

ogy, v. 23, no. 12, p. 1087–1090, doi: 10.1130/0091- Marchand, D.E., 1977, The Cenozoic history of the San Joa- may have included differing valley subsidence 7613(1995)023<1087:MTAOGS>2.3.CO;2. quin Valley and the adjacent Sierra Nevada as inferred rates (more in the south, less in the north) and Burow, K.R., Weissmann, G.S., Miller, R.D., and from the geology and soils of the eastern San Joaquin differences in the San Joaquin Basin’s overall Placzek, G., 1997, Hydrogeologic facies characteriza- Valley, in Singer, M.J., ed., Soil development, geomor- tion of an alluvial fan near Fresno, California, using phology, and Cenozoic history of the northeastern San width (Weissmann et al., 2005). geophysical techniques: U.S. Geological Survey Open- Joaquin Valley and adjacent areas, California: Davis, Identifi cation and characterization of the File Report 97-46, 15 p. University of California Press, Guidebook for Joint Burow, K.R., Shelton, J.L., Hevesi, J.A., and Weissmann, G.S., Field Session, Soil Science Society of America and sequence-bounding paleosols, along with breaks 2004, Hydrogeologic characterization of the Modesto Geological Society of America, p. 39–50. in these paleosols, may provide an important area, San Joaquin Valley, California: U.S. Geological Sur- Marchand, D.E., and Allwardt, A., 1981, Late Cenozoic strati- framework for future groundwater management vey Scientifi c Investigations Report 2004-5232, 54 p. graphic units, northeastern San Joaquin Valley, Califor- Davis, J.L., and Annan, A.P., 1989, Ground-penetrating nia: U.S. Geological Survey Bulletin, v. 1470, 70 p. and modeling in the San Joaquin Valley. Future radar for high resolution mapping of soil and rock stra- McElhiney, M.A., 1992, Soil survey of San Joaquin County, work should build on this framework in order tigraphy: Geophysical Prospecting, v. 37, p. 531–551. California: U.S. Department of Agriculture, Soil Con- to better understand the complex hydrodynamic Ferrari, C.A., and McElhiney, M.A., 2002, Soil survey of servation Service, 480 p. Stanislaus County, California, western part: U.S. Mitchum, R.M., Jr., 1977, Part eleven: Glossary of terms system of these fl uvial fan aquifers. Department of Agriculture, Natural Resources Conser- used in seismic stratigraphy, in Payton, C.E., ed., Seis- Through application of GPR surveys, we show vation Service, 381 p. mic stratigraphy—Applications to hydrocarbon explo- Gronberg, J.M., Dubrovsky, N.M., Kratzer, C.R., Domagal- ration: American Association of Petroleum Geologists that sequence-bounding paleosols form laterally ski, J.I., Brown, L.R., and Burow, K.R., 1998, Envi- Memoir 26, p. 205–212. extensive stratigraphic markers in the fl uvial fans ronmental setting of the San Joaquin–Tulare Basins, Smith, D.G., and Jol, H.M., 1992, Ground-penetrat- of the San Joaquin Basin. We hypothesize that California: U.S. Geological Survey Water-Resources ing radar investigation of a Lake Bonneville delta, Investigations Report 97-4205, 45 p. Provo level, Brigham City, Utah: Geology, v. 20, similar paleosols may be observed in other fl uvial Harden, J.W., 1987, Soils developed in granitic alluvium p. 1083–1086, doi: 10.1130/0091-7613(1992)020<1083: fans, thus providing improved stratigraphic char- near Merced, California: U.S. Geological Survey Bul- GPRIOA>2.3.CO;2. acterization of this complex depositional system letin, v. 1590-A, 65 p. USDA/NRCS (U.S. Department of Agriculture, Natural Harter, T., Heeren, K., Weissmann, G.S., Horwath, W.R., Resources Conservation Service), 2003, Soil survey of type. Additionally, this work indicates that one and Hopmans, J., 1999, Field scale characterization of Fresno County, California, western part: Soil Survey should expect differences in paleosol form and a heterogeneous, moderately deep vadose zone: The Geographic (SSURGO) data set available from www. Kearney Research Site: Proceedings, characterization nrcs.usda.gov. distribution, and thus sequence geometry, related and measurement of the hydraulic properties of unsat- Wahrhaftig, C., and Birman, J.H., 1965, The Quaternary of to fan position within a basin. These geophysi- urated porous media: Riverside, California, United the Pacifi c mountain system in California, in Wright, cal surveys provide a regional-scale approach to States Salinity Laboratory, p. 415–426. H.E., Jr., and Frey, D.G., eds., The Quaternary of the Harter, T., Onsoy, Y.S., Heeren, K., Denton, M., Weissmann, : A review volume for the VII Congress of determining sequence geometry in fl uvial fans, G.S., Hopmans, J.W., and Horwath, W.R., 2005, Deep the International Association for Quaternary Research: thus offering a relatively low-cost tool for strati- vadose zone hydrology demonstrates fate of nitrate Princeton, Princeton University Press, p. 299–340. graphic evaluation of fl uvial fans. in eastern San Joaquin Valley: California Agriculture, Weissmann, G.S., 1999, Toward new models of subsurface v. 59, no. 2, p. 124–132. heterogeneity: An alluvial fan sequence stratigraphic Helley, E.J., 1966, Sediment transport in the Chowchilla framework with transition probability geostatistics ACKNOWLEDGMENTS River basin, Mariposa, Madera, and Merced Counties, [Ph.D. thesis]: Davis, University of California, 279 p. California [Ph.D. thesis]: Berkeley, University of Cali- Weissmann, G.S., and Fogg, G.E., 1999, Multi-scale alluvial Acknowledgment is given to the Donors of the fornia, 153 p. fan heterogeneity modeled with transition probability Huggenberger, P., 1993, Radar facies: Recognition of facies geostatistics in a sequence stratigraphic framework: American Chemical Society Petroleum Research Fund patterns and heterogeneities within Pleistocene Rhine Journal of Hydrology, v. 226, p. 48–65, doi: 10.1016/ for support of this research (PRF#37731-G8). We also gravels, NE Switzerland: Geological Society [London] S0022-1694(99)00160-2. thank Dietz Warnke and the Department of Geological Special Publication 75, p. 163–176. Weissmann, G.S., Carle, S.F., and Fogg, G.E., 1999, Three- Sciences at California State University, Hayward, for Huntington, G.L., 1971, Soil survey, eastern Fresno area, dimensional hydrofacies modeling based on soil sur- the use of their ground-penetrating radar (GPR) equip- California: Washington, D.C., U.S. Government Print- veys and transition probability geostatistics: Water ment (purchased under the National Science Founda- ing Offi ce, U.S. Department of Agriculture, Soil Con- Resources Research, v. 35, no. 6, p. 1761–1770, doi: servation Service, 323 p. 10.1029/1999WR900048. tion grant DUE-9980881). Discussions with and com- Huntington, G.L., 1980, Soil–land form relationships of por- Weissmann, G.S., Mount, J.F., and Fogg, G.E., 2002a, ments provided by the Geophysics graduate group at tions of the and Kings River alluvial Glacially driven cycles in accumulation space and State University of New York Buffalo are sincerely depositional systems in the Great Valley of California sequence stratigraphy of a stream dominated alluvial appreciated. The manuscript also benefi ted greatly [Ph.D. thesis]: Davis, University of California, 147 p. fan, San Joaquin Valley, California, U.S.A.: Journal of from comments by three anonymous reviewers. 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732 Geological Society of America Bulletin, May/June 2006