Project Title: Climatic Effects on Terrace Formation in the Ventura Area, California
Mendenhall Fellow: Richard V. Heermance, (520) 670-5508, firstname.lastname@example.org
Start Date: March 19, 2007
Education: Ph.D., University of California, Santa Barbara, 2007
Research Advisors: Robert Powell, (520) 670-5505, email@example.com; Jonathan Matti, (520) 670-5577, firstname.lastname@example.org; Chris Menges, (520) 670-5022, email@example.com; John Tinsley, (650) 329-4928, firstname.lastname@example.org; Martha Eppes, University of North Carolina, Charlotte, (704) 687-3498, email@example.com; Gary Landis, (303) 236-5406, firstname.lastname@example.org; John Chesley, University of Arizona, (520) 621-9639, email@example.com; Ari Matmon, The Hebrew University of Jerusalem, 972 2 658 6703, firstname.lastname@example.org
Project Description: Fluvial terraces result from past episodes of active erosion and/or deposition within a landscape, and thus can provide clues to past climatic regimes surrounding the time of terrace formation. Within mountainous regions, the processes that form terraces, such as changes in discharge, sediment supply, or tectonic uplift, are also factors affecting sedimentation rate, grain-size variability, and facies changes within basins. Thus a causative link exists between subareal terrace levels and sub-surface depositional units within adjacent basins. Unfortunately, the relationship between surface geomorphology and subsurface-deposits can rarely be unambiguously defined due both to lack of stratigraphic continuity between a basin and its hinterland and to poor age-resolution from the terrace remnants or the adjacent basin. Further complicating issues is the poorly understood climatic controls on terrace formation, specifically on what causes rivers to stop incising and erode laterally (thus creating terrace straths) and over what time-periods lateral erosion and incision occur.
This project will use cosmogenic isotopes (10Be, 26Al) to determine surface ages on the multiple levels of fluvial strath and fill terrace-surfaces along the Ventura River, Calif. This suite of ages will be temporally correlated with a detailed record of climate change and sedimentation from the adjacent Santa Barbara channel (Kennett, 1995) in order to address questions about the climatic versus tectonic controls on fluvial-terrace formation.
The western Transverse Range of Southern California north of the city of Ventura is an actively uplifting region that discharges sediment into the Santa Barbara channel (Figure 1). Putnam (1942) first recognized the sequence of fluvial and marine terraces along the Ventura River, and named these the Oakview terraces. Rockwell (1984, 1988) assigned ages to five terraces along a region of the lower Ventura River; 92±13 ka(Qt6c), 54±10 ka (Qt6b), 38±2 ka (Qt6a), 30 ka (Qt5b), and 16 ka for the terraces based on 14C dates from two terraces (Qt5b, Qt6a) and inferred incision rates (Figure 2). Rockwell suggested a causative link between sea-level and terrace formation for these terraces, but the evidence for this remains inconclusive.
Figure 1. Shaded relief digital elevation model (30 m) of the proposed study area in southern California Previously mapped fluvial and marine terraces are shown in proximity to ODP drill hole 893A Mapped and unmapped (shown yellow and white, respectively) terraces are indicated along the Ventura and Santa Clara Rivers. Red terraces are dated terraces from Rockwell and others (1988).
Fluvial terraces along the Ventura River and San Antonio Creek, north of Ventura, Calif., will be mapped and surveyed using a differential global positioning system (GPS). The terrace profiles and mapping will allow correlation of terraces between the river valleys and across faults and folds, and permit reconstruction of the initial terrace geometry. The presence of similar terrace sequences across structures implies that climatic perturbations, and not local tectonic influences, are the primary control on terrace formation.
Terrace ages will be determined primarily from in-situ produced cosmogenic isotopes (10Be, 26Al). These isotopes are produced from exposure of Si and O atoms at the Earth’s surface, and their production rate is a function of time, depth from the surface, elevation and latitude (Lal, 1988). By sampling a depth profile of recent (<1 m.y) deposits, in this case fluvial terrace deposits, an inheritance-corrected depositional age of the surface can be determined. In addition to cosmogenic sampling, organic debris will be collected from within the terrace deposits overlying each strath for 14C dating. The combination of 14C and cosmogenic ages will allow us to determine surface abandonment surfaces and depositional age of the alluvium, as well as permit cross-checking of our geochronology. Furthermore, shielded samples with known inheritance collected at depth within terrace deposits will provide regional erosion rates from the time of deposition.
Although a large part of this project involves field work to understand the terrace ages, the second aspect of this research involves coordinating the setup of a 10Be and 26Al isotope lab in collaboration with researchers at the USGS (Robert Webb) and in the Department of Geosciences at the University of Arizona (Jay Quade, Nat Lifton, Timothy Jull). This new lab will have the capability to prepare the quartz separates and targets for analysis on an Acceleration Mass Spectrometery (AMS).
Figure 2. Schematic cross-section through strath-terraces on the lower reaches of the Ventura River after Rockwell (1984). Red stars show potential sample sites for cosmogenic and 14C analysis Notice that the wide Qt6b terrace has 2 samples spaced out perpendicular to the upper terrace riser.
New geochronology of the sequence of terraces from the Ventura River and tributaries will provide a foundation for understanding the links between climate, tectonics, and erosion within the Ventura region of Southern California.
Recent drilling by the Ocean Drilling Program (ODP) in the Santa Barbara Channel of hole 893A (Figure 1,Kennett, 1995) provides a detailed, 14C-dated record of sedimentation (Figure 3A) back to greater than 160,000 years before present (y.b.p.). Studies of the cores also reveal δO18changes (Figure 3B) from benthic foraminifera that record past ocean temperature, and can be used to infer past regional climate conditions. This high-resolution drill-core record provides a unique opportunity to correlate basin sedimentation and local climate with sub-aerial terrace ages from the adjacent mountain range in the Ventura area.
Preliminary comparison of terrace levels with ODP core 893A indicates a general correlation between climate and terrace age, although errors in the terrace ages make the correlation ambiguous (Figure 3A, 3B). Three of the five terraces (Qt6c, Qt6b, Qt 4) were deposited, at least in part, just after the peaks of glaciation at oxygen isotope stages 5b, 4, and 2. These terraces imply that strath formation occurs during or just after times of maximum glaciation, associated with high sediment supply and hydraulics associated with a cooler, wetter climate (e.g. Bull, 1991). But the current resolution of the data is not sufficient to detect any consistent correlation between the drill core and terraces. At present, a detailed comparison between deposition rates and terrace formation is not possible for three reasons: (1) the age uncertainties on the terraces are too large, (2) the chronology of deposition has not been refined through tuning, and (3) only one detailed record exists from the adjacent basin at this time. All of these restrictions will change during this study. First, more absolute ages with improved resolution will define more accurately intervals of terrace formation. Second, the strong correlation between the Santa Barbara basin record and the Greenland ice cores means that the local record can be tuned to the better dated ice cores (annual layers in those cores). Third, a recent cruise in the Santa Barbara basin has yielded several high quality cores that span the interval of interest (Nicholson and others, 2005). Hence, over the next year, dating of these cores will enable a spatial integration of sediment accumulation rates across the Santa Barbara basin, thereby providing a more robust depositional data set against which to compare the terrace history.
Figure 3. A, Plot of age versus depth for ODP core 893A (Kennett, 1995), showing relative rates of sedimentation (flatter = faster sedimentation, steeper = slower sedimentation). Dashed horizontal lines show the age of fluvial terraces (Rockwell et al, 1984, 1988) with the gray regions indicating 1 sigma error around the age. B, δ18O plot for benthic foraminifers for ODP hole 893A from Kennett (1995).
This project proposes to provide new ages for terrace sequences in the Ventura region. Our results will show variation in ages between and within strath terraces, and shed new insight into the relationship between terrace formation, climate, and sedimentation adjacent-to and within the Ventura basin.
Bull, W.B., 1991, Geomorphic responses to climate change: New York, Oxford University Press.
Kennett, J.P. 1995, Latest Quaternary benthic oxygen and carbon isotope stratigraphy: Hole 893A, Santa Barbara, California: Proceedings of the Ocean Drilling Program, Scientific Results, v. 146, pt. 2, p. 3–18.
Lal, D., 1988, In situ-produced cosmogenic isotopes in terrestrial rocks: Annual Review of Earth and Planetary Sciences, v. 16, p. 355–388.
Nicholson, C., Sorlien, C.C., Hopkins, S.E., Kennett, J.P., Normark, W.R., Sliter, R.W., and Kooker, L., 2005, Extending the high-resolution global climate record in Santa Barbara Basin: Developing the predictive 3D model for core site locations, American Geophysical Union Fall Meeting: San Francisco, Calif.
Putnam, W.C., 1942, Geomorphology of the Ventura region, California: Geological Society of America Bulletin, v. 53, p. 691–754.
Rockwell, T.A., Keller, E.A., Clark, M.N., and Johnson, D.L., 1984, Chronology and rates of faulting of Ventura River terraces, California: Geological Society of America Bulletin, v. 95, p. 1466–1474.
Rockwell, T.A., Keller, E.A., and Dembroff, G.R., 1988, Quaternary rate of folding of the Ventura Avenue anticline, western Transverse Ranges, southern California: Geological Society of America Bulletin, v. 100.
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