Integrative Analysis of the Fish Creek - Vallecito Basin, Western Salton Trough

Participants in this NSF-funded study include Tom Peryam (Ph.D. student here at the UO), my colleague Bernie Housen at Western Washington University, Bernie's student Chris DeBoer, colleague Mike Oskin at U.C. Davis, and Mike's students Nicole Longinotti and Kim Le. The central goal of this project is to develop a continuous record of 10Be-derived paleo-erosion rates from the Plio-Pleistocene Fish Creek - Vallecito basin (FCVB) of southern California. This research is both a proof-of-concept that a robust inherited 10Be paleo-erosion-rate signal can be obtained from such a setting, and a study to test a hypothesis for enhanced Pleistocene erosion due to more frequent, larger-magnitude climate fluctuation. The Fish Creek-Vallecito basin is particularly well suited to address this problem because it accumulated sediment from a slowly eroding upland (= high 10Be signal) into a rapidly subsiding depocenter that has been subsequently rapidly exhumed (= low added 10Be noise). The continuous, high-fidelity stratigraphic record permits comparison of the paleo-erosion rate data to other indicators such as paleosols and sedimentary facies. Because the observed stratigraphic signals could have been produced either by changes in climatic drivers (rainfall, sediment influx rate) or tectonic drivers (uplift and erosion rate in source, subsidence rate in the basin), we need multiple tools to answer questions that have long challenged basin analysts. Our approach benefits from being highly interdisciplinary, as it integrates data from field based stratigraphy and sedimentology, paleomagnetism, geomorphology, and studies of stable and cosmogenic isotopes.

Geologic map of the Fish Creek - Vallecito basin, western Salton Trough

Upper ~ 2km of the FCVB section

Above figures modified from a recent paper by Dorsey et al. (2011).

Preliminary results of this study show an overall increase in 10Be concentration (decrease in erosion rate) over time from about 4 Ma to the present, with a lot of scatter that we are presently working to reduce through improved sampling methods. Our new data set includes samples from below the base of the Hueso Formation (ca. 2.8 Ma), which records abrupt progradation of locally-derived sediment into the basin. We find similar paleo-erosion rates on both sides of this stratigraphic datum, which suggests that progradation was not caused by an erosion-rate driven increase in sediment supply from the upland source area to the west. We suspect that another factor, such as a decrease in basin subsidence rate, more likely caused this progradation.

To further test this idea, we are using paleosol descriptions and isotopic analysis of pedogenic carbonate to document changes in paleoclimate between about 4.0 and 0.75 Ma, and test hypotheses for the timing of creation of the modern rain shadow by uplift of the Peninsular Ranges. High-resolution magnetostratigraphy allows us to determine the age of paleosol horizons with average uncertainties of +/- 0.06 m.y. Analysis of 82 paleosol horizons ranging in age from 3.9 to 0.7 Ma reveals two primary populations: vertic paleosols with abundant mud cracks and calcic paleosols with pronounced calcite accumulation zones. Vertic paleosols are dominant in older deposits but disappear from the stratigraphic record by 2.8 Ma, after which point calcic paleosols are nearly the only paleosol type present. Pedogenic carbonate nodules from 47 horizons were analyzed for oxygen and carbon isotopic compositions. In late Pliocene time carbonate del 13C values become depleted, from an average of -5.1‰ between 3.9 and 3.5 Ma to -7.3‰ (VPDB) between 2.75 and 2.25 Ma, recording an increase in C3 plants at the expense of C4 grasses. This finding suggests that summer precipitation decreased in the study area from 3.8 to 2.5 Ma, possibly due to a weakening of the summer monsoon. The absence of vertic paleosols in post-2.8 Ma deposits supports this hypothesis. The increase in abundance of calcic paleosols through time suggests a long-term increase in aridity. A gradual enrichment of about 2‰ in del 18O values from 3.9 to 0.7 Ma also likely resulted from increased aridification and evaporative enrichment of soil water. Our findings suggest that global cooling in late Pliocene time resulted in increased aridity in southern California due to decreased monsoonal activity and the end of persistent El Niño conditions in the Pacific Ocean. The observed enrichment in del 18O in the Fish Creek–Vallecito basin is opposite the change that would be produced by the onset of a rain shadow, suggesting that significant uplift of the Peninsular Ranges occurred before 3.8 Ma. (modified from Peryam et al. 2011).

The combined 10Be and paleosol record together suggest a scenario of decreasing erosion rate as aridity increased. There is no increase in erosion rate associated with the global increase climate variability after 3 Ma. To confirm that these are regional signals and not associated with a particular sediment source area, we are now working to replicate the paleosol and 10Be record from a new stratigraphic section in the southern Fish Creek-Vallecito basin. Sediments in that area (Canyon Sin Nombre) are derived from a completely separate, southern source area. Thus if climate (aridification and decreasing erosion rate) was the main cause of the patterns we have documented thus far, we should see the same signal in this southern section.

The above material is based upon work supported by the National Science Foundation (Grant No. EAR-0838119). Any opinions, findings or conclusions expressed herein are those of the authors and do not necessarily reflect the views of the National Science Foundation (NSF). We thank George Jefferson and the volunteers in the Stout Paleontology Lab, Anza-Borrego Desert State Park, for their generous help and ongoing collaboration.

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This page was last updated August, 2011, by Becky Dorsey.