Cosmogenic dating of fluvial terraces, Fremont River, Utah
Introduction
Strath terraces represent ancient floodplains of a river that is incising into bedrock. They are formed when, after remaining at the same elevation and widening its floodplain for a period of time, a river begins to incise again, abandoning its old flood plain. Accurate dating of strath terrace deposits can constrain the rate of incision of the stream system as well as the timing of the events that control incision rates. This information can be used to determine the response time of a fluvial system to base level lowering or to rock uplift within the drainage basin. River terraces are excellent chronosequence sites for the study of soil profile development (e.g., [3]), clast weathering, and eolian inflation, the rates of which may be constrained by dating the deposits.
Dating fluvial terraces is often difficult, however. If suitable organic remains can be located, 14C may be employed to date material deposited in the most recent of the late Pleistocene climatic cycles. Some terraces sequences have been dated using tephra 3, 4, or by correlation with moraines 5, 6. U/Th dating of soil carbonate coatings on subsurface clasts has been employed in some settings (e.g., 3, 7), although this technique suffers from an unknown lag between deposition of the clast and accumulation of the innermost carbonate coating.
While in situ produced cosmogenic radionuclides 10Be, 26Al, and 36Cl are now widely used for dating of bedrock surfaces 8, 9, 10, 11, this method has seen limited use in depositional environments. Phillips and co-workers 3, 8have used 36Cl to date large boulders on moraines and associated outwash terraces. We argue that a significant source of uncertainty in this and all other depositional systems arises from the accumulation of nuclides in the sampled clasts prior to deposition (i.e. `inheritance').
Previous studies on depositional surfaces, including our own work, have revealed significant scatter in the effective ages derived from individual clasts sampled from such surfaces. At issue is how to interpret this scatter. One possible source of scatter is post-depositional turbation of clasts within the deposit, resulting in a mean nuclide production rate that is lower than that of the surface. In this case the surface clast with the largest effective age would provide a lower bound on the age of the deposit. If the scatter reflects instead the stochastic nature of pre-depositional nuclide inheritance, the surface clast with the lowest effective age provides an upper bound on the age of the deposit. The resolution requires sampling of the subsurface and averaging over many clasts to deal with the stochastic nature of inheritance. If the deposit has been static since deposition, all clasts will have experienced post-depositional nuclide production rates determined only by their depth within the deposit. The mean concentration profile should be an exponential profile that asymptotically approaches the mean inheritance at depth. If there has been significant post-depositional stirring of clasts (bio-, pedo- or cryoturbation), this expected profile will be disrupted in some way.
We describe here an amalgamation technique which greatly reduces the effort required to determine mean cosmogenic nuclide concentrations of surfaces composed of many clasts with disparate exposure histories. Anderson et al. [15]outlined this method and presented preliminary results. In this paper we further illustrate the inheritance problem and our proposed dating method with a set of numerical simulations, and then employ the method to estimate the ages of the Fremont River terraces.
Section snippets
Description of Fremont River terraces
The Fremont River drains the basalt-capped Aquarius and Fish Lake plateaus and cuts through two monoclines, the Waterpocket Fold and Caineville Reef, before joining the Muddy River near Hanksville to form the Dirty Devil River, a tributary to the Colorado River (Fig. 1). Tills and outwash deposits found in the drainages skirting the Aquarius Plateau indicate that the Fremont has at times been glacier fed. The Blind Lake and Donkey Creek tills have been correlated to the Pinedale glaciation and
Numerical simulation of our technique
We present a numerical model of cosmogenic radionuclide accumulation histories for clasts within a hillslope/fluvial transport system that both illustrates the problem of constraining the terrace ages and provides a theoretical backdrop for our dating strategy.
Our model numerically integrates the differential equation for production and decay through time. Once the clasts are in the fluvial system, a series of three random numbers dictate how long the clast will spend within the system, whether
Assumptions
We assume that the deposition rate of the terrace gravels is sufficiently rapid that there is no age structure within the deposit. There is no stratigraphic evidence for significant time spent during deposition of the terrace gravels, and rapid deposition is consistent with our interpretation of these gravels as braided stream deposits. We assume that the cosmogenic radionuclide inheritance signal is truly random. We assume that the mean inheritance of the clasts arriving on the surface can be
Single clasts
The spread of effective ages derived from single clasts is very large (Table 1; Fig. 5A), demonstrating that the problem of inheritance is significant in this geomorphic system. In all cases, 26Al and 10Be results yield similar ages, implying that processing errors or long burial are not important. Even the cobbles sampled from the modern stream system show significant nuclide concentrations.
Amalgamated surface samples
Results from the surface amalgamated samples (numbers of clasts vary from 25–40; Table 2; Fig. 5B) show
Conclusions
The large spread of single clast ages implies a wide scatter in the inheritance signal, raising a cautionary flag against the use of small numbers of clasts in dating depositional terraces. This scatter can be reduced significantly by using samples consisting of aliquots from large numbers of clasts. The reproducibility of the concentration values from samples amalgamated from 25–40 clasts implies that these are sufficient numbers to constrain the mean concentrations in this particular
Acknowledgements
We gratefully acknowledge the Petroleum Research Fund of the American Chemical Society, a grant from the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory, and a Cole award from the Geological Society of America, for support of this research. We thank the following individuals for their help in the field, the lab, and/or for reviewing early versions of this paper: Christian Brauderick, Alex Densmore, Greg Dick, James Georgis, Joe Koning, Craig Lundstrom, Greg
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