Ž . Earth and Planetary Science Letters 152 1997 59–73 Cosmogenic dating of fluvial terraces, Fremont River, Utah James L. Repka a , Robert S. Anderson a, ) , Robert C. Finkel b a Department of Earth Sciences and Institute for Tectonics, UniÕersity of California, Santa Cruz, CA 95064, USA b Geosciences and EnÕironmental Technology DiÕision and Center for Accelerator Mass Spectrometry, Lawrence LiÕermore National Laboratory, LiÕermore, CA 94550, USA Received 19 March 1997; accepted 25 August 1997 Abstract Absolute dating of river terraces can yield long-term incision rates, clarify the role of climate in setting times of aggradation and incision, and establish the rates of pedogenic processes. While surface exposure dating using cosmogenic 10 Be and 26 Al would seem to be an ideal dating method, the surfaces are composed of individual clasts, each with its own complex history of exposure and burial. The stochastic nature of burial depth and hence in nuclide production in these clasts during exhumation and fluvial transport, and during post-depositional stirring, results in great variability in clast nuclide concentrations. We present a method for dealing with the problem of pre-depositional inheritance of cosmogenic nuclides. We generate samples by amalgamating many individual clasts in order to average over their widely different exposure histories. Depth profiles of such amalgamated samples allow us to constrain the mean inheritance, to test for the possible importance of stirring, and to estimate the age of the surface. Working with samples from terraces of the Fremont River, we demonstrate that samples amalgamated from 30 clasts represent well the mean concentration. Depth profiles show the expected shifted exponential concentration profile that we attribute to the sum of uniform mean inheritance and depth-depen- dent post-depositional nuclide production. That the depth-dependent parts of the profiles are exponential argues against significant post-depositional displacement of clasts within the deposit. Our technique yields 10 Be age estimates of 60 "9, 102 "16 and 151 "24 ka for the three highest terraces, corresponding to isotope stages 4, 5d and 6, respectively. The mean inheritance is similar from terrace to terrace and would correspond to an error of ;30–40 ka if not taken into account. The inheritance likely reflects primarily the mean exhumation rates in the headwaters, of order 30 mrMa. q 1997 Elsevier Science B.V. Keywords: cosmogenic elements; terraces; fluvial features; absolute age 1. 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 ) Corresponding author. Fax: q1 408 459 3074. E-mail: jl- [email protected]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 Ž wx. the study of soil profile development e.g., 3 , clast 0012-821Xr97r$17.00 q 1997 Elsevier Science B.V. All rights reserved. Ž . PII S0012-821X 97 00149-0
Transcript
Ž .Earth and Planetary Science Letters 152 1997 59–73
Cosmogenic dating of fluvial terraces, Fremont River, Utah
James L. Repka a, Robert S. Anderson a,), Robert C. Finkel b
a Department of Earth Sciences and Institute for Tectonics, UniÕersity of California, Santa Cruz, CA 95064, USAb Geosciences and EnÕironmental Technology DiÕision and Center for Accelerator Mass Spectrometry, Lawrence LiÕermore National
Laboratory, LiÕermore, CA 94550, USA
Received 19 March 1997; accepted 25 August 1997
Abstract
Absolute dating of river terraces can yield long-term incision rates, clarify the role of climate in setting times ofaggradation and incision, and establish the rates of pedogenic processes. While surface exposure dating using cosmogenic10Be and 26Al would seem to be an ideal dating method, the surfaces are composed of individual clasts, each with its owncomplex history of exposure and burial. The stochastic nature of burial depth and hence in nuclide production in these clastsduring exhumation and fluvial transport, and during post-depositional stirring, results in great variability in clast nuclideconcentrations. We present a method for dealing with the problem of pre-depositional inheritance of cosmogenic nuclides.We generate samples by amalgamating many individual clasts in order to average over their widely different exposurehistories. Depth profiles of such amalgamated samples allow us to constrain the mean inheritance, to test for the possibleimportance of stirring, and to estimate the age of the surface. Working with samples from terraces of the Fremont River, wedemonstrate that samples amalgamated from 30 clasts represent well the mean concentration. Depth profiles show theexpected shifted exponential concentration profile that we attribute to the sum of uniform mean inheritance and depth-depen-dent post-depositional nuclide production. That the depth-dependent parts of the profiles are exponential argues againstsignificant post-depositional displacement of clasts within the deposit. Our technique yields 10Be age estimates of 60"9,102"16 and 151"24 ka for the three highest terraces, corresponding to isotope stages 4, 5d and 6, respectively. The meaninheritance is similar from terrace to terrace and would correspond to an error of ;30–40 ka if not taken into account. Theinheritance likely reflects primarily the mean exhumation rates in the headwaters, of order 30 mrMa. q 1997 ElsevierScience B.V.
Keywords: cosmogenic elements; terraces; fluvial features; absolute age
1. Introduction
Strath terraces represent ancient floodplains of ariver that is incising into bedrock. They are formedwhen, after remaining at the same elevation andwidening its floodplain for a period of time, a river
begins to incise again, abandoning its old floodplain. Accurate dating of strath terrace deposits canconstrain the rate of incision of the stream system aswell as the timing of the events that control incisionrates. This information can be used to determine theresponse time of a fluvial system to base levellowering or to rock uplift within the drainage basin.River terraces are excellent chronosequence sites for
Ž w x.the study of soil profile development e.g., 3 , clast
0012-821Xr97r$17.00 q 1997 Elsevier Science B.V. All rights reserved.Ž .PII S0012-821X 97 00149-0
( )J.L. Repka et al.rEarth and Planetary Science Letters 152 1997 59–7360
weathering, and eolian inflation, the rates of whichmay be constrained by dating the deposits.
Dating fluvial terraces is often difficult, however.If suitable organic remains can be located, 14C maybe employed to date material deposited in the mostrecent of the late Pleistocene climatic cycles. Someterraces sequences have been dated using tephraw x w x3,4 , or by correlation with moraines 5,6 . UrThdating of soil carbonate coatings on subsurface clasts
Ž w x.has been employed in some settings e.g., 3,7 ,although this technique suffers from an unknown lagbetween deposition of the clast and accumulation ofthe innermost carbonate coating.
While in situ produced cosmogenic radionuclides10 Be, 26Al, and 36Cl are now widely used for dating
w xof bedrock surfaces 8–11 , this method has seenlimited use in depositional environments. Phillips
w x 36and co-workers 3,8 have used Cl to date largeboulders on moraines and associated outwash ter-races. We argue that a significant source of uncer-tainty in this and all other depositional systems arisesfrom the accumulation of nuclides in the sampled
Ž .clasts prior to deposition i.e. ‘inheritance’ .Previous studies on depositional surfaces, includ-
ing our own work, have revealed significant scatterin the effective ages derived from individual clastssampled from such surfaces. At issue is how tointerpret this scatter. One possible source of scatter ispost-depositional turbation of clasts within the de-posit, resulting in a mean nuclide production rate thatis lower than that of the surface. In this case thesurface clast with the largest effective age wouldprovide a lower bound on the age of the deposit. Ifthe scatter reflects instead the stochastic nature ofpre-depositional nuclide inheritance, the surface clastwith the lowest effective age provides an upperbound on the age of the deposit. The resolutionrequires sampling of the subsurface and averagingover many clasts to deal with the stochastic nature ofinheritance. If the deposit has been static since depo-sition, all clasts will have experienced post-deposi-tional nuclide production rates determined only bytheir depth within the deposit. The mean concentra-tion profile should be an exponential profile thatasymptotically approaches the mean inheritance atdepth. If there has been significant post-depositional
Ž .stirring of clasts bio-, pedo- or cryoturbation , thisexpected profile will be disrupted in some way.
We describe here an amalgamation techniquewhich greatly reduces the effort required to deter-mine mean cosmogenic nuclide concentrations ofsurfaces composed of many clasts with disparate
w xexposure histories. Anderson et al. 15 outlined thismethod and presented preliminary results. In thispaper we further illustrate the inheritance problemand our proposed dating method with a set of numer-ical simulations, and then employ the method toestimate the ages of the Fremont River terraces.
2. Description of Fremont River terraces
The Fremont River drains the basalt-cappedAquarius and Fish Lake plateaus and cuts throughtwo monoclines, the Waterpocket Fold and CainevilleReef, before joining the Muddy River near Han-ksville to form the Dirty Devil River, a tributary to
Ž .the Colorado River Fig. 1 . Tills and outwash de-posits found in the drainages skirting the AquariusPlateau indicate that the Fremont has at times beenglacier fed. The Blind Lake and Donkey Creek tillshave been correlated to the Pinedale glaciation andthe Carcass Creek to the Bull Lake glaciation, basedon morphology and extent of the deposits and on the
w xdegree of weathering of the clasts 16 . We havefocused on the reach near North and South Caineville
Ž .Mesas Fig. 2 , 15–20 km east of Capitol ReefNational Park.
Four terraces can be traced through the field area,ranging in elevation from 20 to 135 m above the
w xmodern floodplain 5,17 . Each terrace consists of aseveral meter thick gravel veneer capping a strathsurface eroded into shale and sandstone bedrock. Wehave labeled them FR0–FR4 from lowest to highestŽ .Fig. 2 , FR0 referring to the modern floodplain.FR1 and FR4 exist within the field as isolated rem-nants, with maximum widths of a few tens of meters.We have designated sublevels of FR2 as FR2A-C,from lowest to highest. FR2A and FR3 are the mostextensively preserved of the terraces.
No absolute dates for either the moraines or thefluvial terraces of the Fremont River exist. Howardw x5,17 relied upon careful correlation of the terraceswith the morainal sequence on Boulder MountainŽ . Ž .Fig. 1 , correlating terrace FR2A his 4A with the
Ž .Pinedale glaciation, and terrace FR3 his FR3A with
( )J.L. Repka et al.rEarth and Planetary Science Letters 152 1997 59–73 61
the Bull Lake glaciation. Recent dating of glacialdeposits in the type localities adjacent to the WindRiver Range, Wyoming, reveals that each glaciation
w xconsists of numerous advances 3,25 . Clearly, abso-lute dating is needed to unravel the timing withinany particular system.
All of the Fremont terraces have in common asingle layer of varnished clasts, forming an interlock-
Žing desert pavement surface capping a thin -10.cm layer of nearly clast-free silt, probably reflecting
eolian inflation of the surface. The silt layer capsseveral meters of river gravels and sands. Thestratigraphy is consistent with that of a braided chan-nel. The total gravel thickness varies from less than 2to roughly 10 m. Where visible, the base of the
deposit makes a relatively planar contact with under-lying shale and sandstone bedrock.
Clasts range in diameter up to tens of centimeters.The deposits are dominated by locally derived sand-stone and shale, quartzite and chalcedony from the
ŽTriassic Shinarump conglomerate presently exposed.50 km upstream in Capitol Reef National Park , and
basalts from lava flows that mantle Boulder Moun-w xtain and Fish Lake Plateau 18 . In our cosmogenic
radionuclide dating we have used only the wellpreserved and ubiquitous quartzite clasts on theseterraces. The sizes of the quartzite clasts are similarfrom terrace to terrace. Their extreme resistance toweathering minimizes the problem of erosional lossfrom the sampled clasts. Finally, as the age of the
Ž . 2Fig. 1. Map of the Fremont River drainage upstream of the field area. Drainage area dashed line is just over 3700 km . The solid lineŽ .surrounding the high plateaus represents the equilibrium line altitude ELA during the last glacial maximum, estimated to be 3170–3260 m
w x Ž .11 . The Caineville area within which the Fremont terraces are studied Fig. 2 lies in the box east of Capitol Reef National Park.
( )J.L. Repka et al.rEarth and Planetary Science Letters 152 1997 59–7362
Ž .Fig. 2. Detailed topography adjacent to the Fremont River as it passes eastward through the gap between North Cainville Plateau NCP andŽ .South Caineville Plateau SCP . The sampled terraces are depicted. Note the single small scraps of FR1 and FR4 surfaces, and the extensive
preservation of FR2 and FR3 surfaces.
conglomerate from which they are derived is muchgreater than the half-lives of the radionuclides, weignore inheritance from exposure in the ancient geo-morphic system.
3. Numerical simulation of our technique
We present a numerical model of cosmogenicradionuclide accumulation histories for clasts withina hillsloperfluvial transport system that both illus-trates the problem of constraining the terrace agesand provides a theoretical backdrop for our datingstrategy.
Our model numerically integrates the differentialequation for production and decay through time.Once the clasts are in the fluvial system, a series ofthree random numbers dictate how long the clast willspend within the system, whether it is in transport or
storage at any time step, and its depth of burial if itis in storage. We specify that fluvial transport takesplace only in short episodes during which the clast isat the surface; the rest of the time the clast is buriedat a depth that is some fraction of a specified maxi-mum. At the end of the transport cycle, clasts areinstantaneously deposited on a terrace at some speci-fied depth. Clasts deposited on the surface experi-ence steady production, while clasts deposited be-neath the surface experience production that de-creases with time due to eolian inflation of the
w xdeposit 26 . At each time step, the model determinesŽwhere the clast is within the system on the hillslope,.in the river, or on the final terrace , determines the
clast depth and the associated cosmogenic radionu-clide production rate, and updates the concentration.
This model incorporates several assumptions,some of which are applicable to most fluvial sys-tems, while others should be considered specific to
( )J.L. Repka et al.rEarth and Planetary Science Letters 152 1997 59–73 63
the Fremont River: We assume a constant cosmic rayflux and therefore production rate. Cosmic ray irradi-ation is assumed to be negligible before hillslopeexhumation brings the clast near the surface. Weneglect variability in the hillslope exhumation pro-cess, assuming it to be spatially and temporallyuniform within the catchment. We assume that topo-graphic shielding from cosmic rays by snow andvegetation is insignificant.
3.1. Theoretical background
Cosmogenic nuclides are produced in situ frominteractions between secondary cosmic ray particlesand material at the earth’s surface. Production de-creases exponentially with depth:
P z sP eyz r z )
1Ž . Ž .o
where z ) sLrr is the e-folding length, or the ratioŽ .of the absorption mean-free path L of ;145–160
y2 Ž . w xg cm and the material density r 20,21 . Inquartz, which is an ideal mineral for measurementw x 26 Ž .9,10 , Al t s0.705 Ma is produced primarily1r2
from silicon, while oxygen is the primary target for10 Ž .production of Be t s1.5 Ma .1r2
For exposure at a constant depth:P
)0ylt yz r z yltN z,t sN z,0 e q e 1ye 2Ž . Ž . Ž . Ž .l
Here the first term represents the decay of the inher-Ž .ited concentration, N z,0 , while the second term
represents the net increase in concentration due toproduction and subsequent decay.
The mean concentration of a cosmogenic nuclideat any particular depth, N, is approximated by mea-suring ‘amalgamated’ samples consisting of equal
w xmass aliquots of a large number of single clasts 15 .Ž .Eq. 2 may now be rewritten as an equation for the
mean concentration. The difference in the concentra-tion between two amalgamated samples, one fromthe surface and one from a known depth within thedeposit, then reflects the expected difference in thedeterministic post-depositional grow-in of cosmo-genic radionuclides represented by the second term
Ž .in Eq. 2 . Ignoring decay, a full profile of the meanconcentrations should reveal a simple shifted expo-nential:
)yz r zN z sN qN e 3Ž . Ž .in s
where N is the mean pre-depositional inheritancein
and N is the nuclide accumulation at the surfaces
since deposition. This approximation is valid fortimes short relative to the decay time of the nuclidemeasured, and for shallow depths within which pro-duction by muons and by other less important mech-anisms is negligible. Since there are two unknowns,N and N , two measurements suffice to solve thein s
equation. Using this ‘pairs technique’ we can calcu-late the age of the surface:
1 DPts ln 4Ž .ž /l DPylDN
where DP is the difference in production rates be-tween the samples and DN refers to the differencebetween the concentrations of the two samples. Foryoung terraces, where decay may be neglected, thisbecomes tsDNrDP.
In the case of a surface inflating by eolian deposi-tion, the assumption of a constant production ratewithin the subsurface is incorrect. In this case our
Ž .measured depth z overestimates the mean depths
since deposition and thus underestimates the meannuclide production rate for the subsurface sample. Inaddition, the production rate difference, DP, is afunction of time. If we make the simplest assumptionthat silt thickness begins at zero and increases at asteady rate to its measured depth of L at the time of
Ž .sampling, the correction factor RsPrP z for thes
mean production rate for the subsurface sample is:
z)
)L r zw xR L s e y1 5Ž . Ž .L
Ž .Eq. 4 should therefore be calculated with the dif-P z z)Ž .s
ference in production rate of DPsP y =0 Lw L r z ) xe y1 . While this correction can become largeŽ ) .of the order of 30% for a silt thickness Ls0.5 z ,the 10 cm thick silt caps of the Fremont River
Ž ) .terraces and z s80 cm result in only a 6% in-crease of the age estimate.
3.2. Model results
Fig. 3 illustrates the stochastic nature of the inher-itance problem, showing simulated 10 Be concentra-tion histories for a large number of clasts for twoend-member scenarios. The terrace age was pre-scribed to be 100 ka, the surface production rate
( )J.L. Repka et al.rEarth and Planetary Science Letters 152 1997 59–7364
P s6 atomrgmryr. In Fig. 3A, the exhumation0
rate is 500 mmryr, the maximum fluvial transporttime is 200 ka, and the post-depositional eolianinflation rate is 10y3 mrka. In Fig. 3B the exhuma-tion rate varies randomly among the clasts, rangingfrom 17.5 to 42.5 mmryr, while fluvial transport iszero, and the eolian inflation rate is zero. The large
spread in the final concentrations of each groupreflects the stochastic nature of the pre-depositionalgeomorphic processes. In the top run, Fig. 3A, the
Ž .age estimated using Eq. 4 is 113 ka, reasonablyclose to the prescribed 100 ka age of the surface. Theinset in Fig. 3B shows the dependence of the ageestimate on the number of clasts used in the amalga-
Fig. 3. Simulated 10 Be concentration histories of many clasts from hillslope exhumation through fluvial transport to terrace deposition. Asteady sea level surface 10 Be production rate of P s6 atomsrgmryr is assumed. At the final site of deposition, half of the clasts are0
deposited in the subsurface at an original depth of 1.0 m, the other half are deposited on the surface. The terrace age is ascribed to 100 ka.Ž . Ž .A Fluvial inheritance dominates. Twenty-four clast histories are shown. Exhumation of all clasts on hillslopes is rapid 500 mmryr .Clasts spend a random time between 0 and 200 ka in fluvial transport. Eolian silt inflates the terrace surface by 0.1 m over 100 ka. An age
6estimate for the terrace of 113"21 ka is obtained using the mean concentrations of N s0.68"0.10 and N s0.25"0.12=10s s sŽ . Ž . Ž .atomrgm and Eq. 4 illustrated as the intersection of gray lines back-tracking concentration histories from mean concentrations . B
Hillslope exhumation dominates inheritance. Twenty clast histories are shown. Exhumation rate is chosen randomly from between 17.5 andŽ .42.5 mmryr. Transport time within the fluvial system is negligible. The mean concentrations of the 10 surface clasts N and of the 10s
Žsubsurface clasts N are shown, from which DN is calculated. Inset depicts dependence of such age estimates on the number of clasts useds sŽin estimating the surface and subsurface mean concentrations, shown for 5 separate runs of 100 total clasts each 50 each in surface and
.subsurface .
( )J.L. Repka et al.rEarth and Planetary Science Letters 152 1997 59–73 65
mated samples. The age estimate improves as moreand more clasts are amalgamated.
In Fig. 4 we depict similar simulations for fourŽ .hypothetical terraces A–D whose height above the
floodplain is dictated by a linear incision rate of 1.2mrka. We show the effective individual clast agesŽ .NrP , as well as the effective mean age, for all0
Ž .surface clasts on each terrace. We then use Eq. 4 toestimate a true surface age for each terrace, using 30randomly selected clasts both from this surface popu-lation and from the corresponding subsurface popula-tion at 1 m depth. The ages are depicted by thecircles. It should be obvious that the mean concentra-tion obtained from a single amalgamated sample ofsurface clasts would over-estimate the age of thesurface. If all of the assumptions made in our model
Ž .are correct, then Eq. 4 yields a reasonable approxi-mation for the age of the surface. Correction of Eq.Ž . Ž .4 for steady eolian inflation with Ls10 cmincreases the age estimate by roughly 6%.
Fig. 4. Results of numerical simulations, resulting in calculated10 Be ages for 2000 clasts from four hypothetical terraces in a
Ž .system in which a linear incision rate 1.2 mmryr; gray line isimposed. The cloud of points at each elevation represents the widespread of clast concentrations converted to effective ages throught sNrP , reflecting the variable inheritance of cosmogenic ra-0
dionuclides during transport to the depositional site. Note that theyoungest single clast age for each terrace is close to the specified
Ž .age of the terrace intersection of the gray line with that elevation .Boxes represent age estimates based upon samples amalgamatedfrom 30 surface clasts selected at random from each of the four
Žterraces. The large circles represent our model terrace age Eq.Ž ..4 using the pairs technique, based upon amalgamated surfaceand amalgamated subsurface samples. These model ages fallwithin 5% of the specified ages for each of the terraces.
3.3. RelatiÕe importance of exhumation and fluÕialtransport
Nuclide inheritance accrues during hillslope ex-humation and fluvial transport. Ignoring for the mo-ment the decay of the cosmogenic radionuclides, theinheritance during exhumation by steady surfacelowering, e , on hillslopes is the product of the˙surface production rate with the time it takes thesample to traverse the final production length scale
) Ž w x.z to the surface e.g., 9 :
P z )
0N s 6Ž .hill
e
Ž .Variation in N arises from several factors: 1hi l l
differences in surface production rates in the catch-ment due to altitudinal dependence of production
Ž .rate; 2 variation in topographic shielding within thecatchment, locally reducing the cosmic ray flux be-
Ž .low that on an unshielded surface; and 3 variationin the local erosion rate in time and space.
Production within the fluvial system is compli-cated by the stochastic nature of the transport and
Ž .burial processes. Clast depth, z t , falls within thew xrange O,H , where O represents the surface and H is
the depth of the gravel fill. Substituting bsHrz) ,the cosmogenic radionuclide accumulation historycan be rewritten:
N sP THbz )
exp yzrz) F z dz 7Ž . Ž . Ž .fluvial 0 0
where T is the total transport time in the fluvialŽ .system and F z dz is the probability of finding the
w xclast within the interval z, zqdz at any time. Forthe simplest case of uniform probability between thesurface and depth H, the integral yields:
1yeyb
N sP T 8Ž .fluvial 0 ž /b
This reduces to the yet simpler expression NfluvialŽ ) .sP Trb in thick deposits H)2z . As H in-0
creases the clast spends less time near the surfaceand accumulates fewer cosmogenic radionuclides.The simulated accumulation histories shown in Fig.3A suggest that difference in total transport time isthe principal factor in clast-to-clast variation in thefluvial component of nuclide inheritance. In addition,the mean rate of accumulation is significantly lowerthan the surface rate, P . The ratio of these mean0
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production rates, as deduced from the slopes in Fig.3A, is roughly 4.6:1, meaning the effective produc-tion rate P sP rb , with bs4.6. Input parameterseff 0
for the model were Hs5 m and z ) s0.8 m; there-fore the expected value of bsHrz) is 5.3. Thesimulation both illustrates and verifies this simplestatistical treatment.
The relative contributions from hillslope and flu-vial processes is the ratio of the effective times spentin the production zone in each of the geomorphicsettings:
N e T 1yeyb e TŽ .˙ ˙fluvials f 9Ž .
)N bz Hhill
When the exhumation rate is low and the transporttime short, or the thickness of the floodplain storagesystem is great, the contributions from the hillslopesystem will dominate the inheritance signal, and viceversa. In the numerical simulations displayed in Fig.
Ž .3A es0.5 mrka, T s200 ka, and Hs5 m ,˙ m a x
for instance, the expected relative contributions tothe inheritance should therefore be roughly 10:1fluvial:hillslope. A more realistic estimate for thehillslope exhumation rate might be a few tens ofmicrons per year, and the transport times in thefluvial system may not exceed several thousand yearsduring times of high glacial discharge. Hillslopeexhumation may therefore become an equal or domi-nant player in producing inheritance, the spread insingle-clast cosmogenic radionuclide values reflect-ing nonuniformity in the erosion rates within thecatchment.
4. Application to dating of the Fremont Riverterraces
4.1. Assumptions
We assume that the deposition rate of the terracegravels is sufficiently rapid that there is no agestructure within the deposit. There is no stratigraphicevidence for significant time spent during depositionof the terrace gravels, and rapid deposition is consis-tent with our interpretation of these gravels as braidedstream deposits. We assume that the cosmogenicradionuclide inheritance signal is truly random. Weassume that the mean inheritance of the clasts arriv-
ing on the surface can be characterized well byaveraging the concentrations of a relatively small
Ž .number of clasts 25–40 . The number of clastsnecessary to constrain the mean concentration wellenough to extract the inheritance signal depends onthe width of the distribution of clast inheritance. Inaddition, inheritance becomes a smaller portion ofthe total nuclide concentration on older terraces,making them more immune to the problems we areattempting to solve. We assume that there is nopost-depositional ‘stirring’ of the terrace deposit.Finally, we assume that there has been no erosionalloss from the horizontal portions of the terrace sur-faces. This is consistent with the inflationary model
w xof desert pavement formation 26 .
4.2. Field methods
Samples were selected from the flattest surfaces,well away from terrace edges, in order to avoid areassubject to gravitational creep. Where possible, sub-surface samples were obtained from pits dug in theinterior of the terrace; otherwise they were fromareas exposed by roadcuts. The location and eleva-tion of each sample was noted. Topographic shield-ing was determined by measuring the vertical angleto the horizon measured in eight directions. Wecollected 30–70 clasts for amalgamation of surfaceand subsurface samples. On FR2 and FR3 we col-lected additional samples every 0.4–0.5 m, to adepth of 1.5–2.0 m, to obtain the concentrationprofiles. Clast sizes ranged from 5 cm to 20 cm.
4.3. Estimation of production rates
We have used the long-term average high latitudesea level production rate of 5.8 atomrgmryr sug-gested in the most recent work of Nishiizumi et al.w x24 . This is approximately 13% higher than that of
w xClark et al. 23 and 4% lower than the that ofw xNishiizumi 22 . The surface production rate for the
given latitude and altitude is then adjusted usingw xcoefficients reported in Lal 9 . Variations in mag-
netic field strength introduce uncertainties into pro-duction rate estimates. Our calculations include aproduction rate uncertainty of 10%. The topographic
w xshielding factor 22 was very near 1.0 in all cases.The soil density was estimated using a measured
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Table 110 Be and 26Al data: single clasts
10 26 10 26 10 26w x w xSample Terrace No. of Latitude Altitude Depth P Be P Al Be Al Be concentration Al concentration Be model age Al model ageŽ . Ž . Ž . Ž . Ž . Ž . Ž . Ž .clasts km cm ppm ppm atomrmgm atomrmgm ka ka
Altitudes and latitudes for samples were determined using: hand held GPS; laser EDM; triangulation with compass and topographic maps; and a combination of the three.w x w xProduction as a function of latitude and altitude is calculated from equations given by Lal 9 and calibrations of Nishiizumi et al. 24 . The surface production rate is then
adjusted to account for the depth to the subsurface samples: we estimate the terrace gravels to contain 30% large clasts, and the porosity of the matrix to be 35%, to arrive at a3 Ž . Ž . w x 2 Ž . 2 Ž .density of 2.1"0.1 grcm ; we then calculate P z using Eq. 1 and Brown et al.’s 20 values of L s145 grcm "5% and L s156 grcm "8% . Errors wereBe Al
propagated using the uncertainties given, with a 10% uncertainty assigned to the surface production rate and 7% to the sample depth. All samples were measured at the CAMSŽ .facility at Lawrence Livermore National Laboratory. Aluminum samples were normalized to standards prepared by Nishiizumi, KNSTD9860 26Al:27Als9.86Ey12 and
Ž . Ž .KNSTD9919 26Al:27Als9.92Ey12 . Aluminum blank ratios were uniformly less than 1.0Ey14. Beryllium samples were normalized to LLNL STD10000 10Be:9Bes1E11 .Beryllium blank corrections take into account interference by the isobar 10 B and were uniformly less than 6.0Ey14.
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30% clasts in the terrace deposit, and an estimatedporosity of 0.35 within the fines, yielding a densityof 2100 kgrm3. We assume no vertical densitystructure, an assumption that results in negligibleerror.
4.4. Laboratory methods
Clasts were individually crushed and sieved to auniform size of 200–500 mm, after which amalga-mated samples, each representing a single depth, areconstructed from equal mass aliquots of 25–40 indi-vidual clasts. Clean quartz separates were isolated by
w xchemical leaching 28 . We added 0.5 mg of Becarrier to a 20 g sample of quartz and dissolved thesample in 3:1 HF:HNO . Aluminum concentrations3
were determined by atomic absorption spectroscopyon an aliquot of the dissolved sample. Al and Bewere separated by ion chromatography and precipi-tated as metal hydroxides which were then oxidizedto Al O or BeO. The ratio of the radionuclide to the2 3
stable isotope was determined by accelerator massspectrometry at Lawrence Livermore National Labo-
w x 10ratory 29 . Be concentrations were determinedrelative to an ICN standard and 26Al to an NBSstandard, both prepared by K. Nishiizumi. 10 BerBeratios were corrected for interference from 10 B. Themeasured ratios of both 10 Be and 26Al were correctedfor process blanks treated in the same way as thesamples. The uncertainties quoted are the 1s AMSerrors only.
5. Results and discussion
5.1. Single clasts
The spread of effective ages derived from singleŽ .clasts is very large Table 1; Fig. 5A , demonstrating
that the problem of inheritance is significant in thisgeomorphic system. In all cases, 26Al and 10 Be re-sults yield similar ages, implying that processingerrors or long burial are not important. Even thecobbles sampled from the modern stream systemshow significant nuclide concentrations.
5.2. Amalgamated surface samples
Results from the surface amalgamated samplesŽnumbers of clasts vary from 25–40; Table 2; Fig.
.5B show significantly less scatter than those for
Fig. 5. Summary of results from 10 Be and 26Al analyses ofŽFremont River terrace quartzite clasts data shown in Table 1Ta-
. 26 10ble 2 . `s Al results; ls Be results. Ages were calculatedusing production rates determined using nuclear disintegration
w xrates reported in Lal 9 , and scaled to fit the production calibra-w x Ž .tions reported in Nishiizumi et al. 24 . A Results for whole
single rock samples. Note the wide spread in effective ages. Theseare all maximum age estimates if the chief error is associated with
Ž .inheritance. B Results for amalgamated surface samples repre-senting at least 25 clasts. Note considerable reduction in age
Ž .scatter from results from single clasts shown in A . These agescorrespond to the boxes in Fig. 4, and should be consideredmaximum ages. The length of the arrows show downward correc-tion of mean ages from these amalgamated surface samples when
Ž . Ž .inheritance is taken into account see C . C Results of the pairstechnique employing two amalgamated samples. Triangles repre-sent the 26Al and 10 Be model ages determined using the profile
Ž .technique see Fig. 4 .
( )J.L. Repka et al.rEarth and Planetary Science Letters 152 1997 59–73 69
single clasts. Most repeat samples using differentsets of clasts show some overlap, both in 26Al and in10 Be. If the spread in effective ages from singleclasts represents variable inheritance, then age esti-mates from these surface amalgamated samples areto be considered maxima.
5.3. Paired–amalgamated sample technique
In Fig. 5C we plot age estimates of the terracesŽ Ž ..adjusted for inheritance Eq. 4 . The model ages
for the higher terraces increase with elevation, al-Ž .though the lowest terrace FR1 appears to be slightly
older than the next highest terrace. The arrows onFig. 5B depict the downward shift in the age esti-mate when inheritance is taken into account.
5.4. Profile technique
Fig. 6 shows vertical concentration profiles offour terraces. Two of the terraces, FR2C and FR3,were sampled at several depths. The profile of FR2Cconsists of two amalgamated samples collected fromthe surface, one from 0.4 m, two from 0.6 m, andone from 1.0 m depth. The FR3 profile consists offour surface samples, one from 0.5 m, and two eachfrom 1.0 and 1.5 m. The best-fitting shifted exponen-
Ž Ž ..tials Eq. 3 fit the shapes of the full profiles quitewell. For the higher surfaces, the shift representingpre-depositional inheritance is similar, about 24"8ka. The inheritance shift for FR1 is minimal. The ageestimates based upon these profiles are shown astriangles on Fig. 5C.
At this location there is no well expressed terraceŽ .that corresponds to the last glacial maximum LGM .
The surface in the proper elevational range is FR1.The pairs technique suggests that FR1 is slightlyolder than FR2C, which is geomorphically impossi-ble. It is likely that the gravel deposits on this smallterrace remnant consist entirely of material quarriedfrom the adjacent older terrace either at the end ofthe depositional phase, or during the post-deposi-tional evolution of the scarp separating the two. Thiswould result in an age estimate for FR1 similar tothat determined for FR2C, which indeed is what weobserve. An alternative explanation is that the LGM
Ž .is represented here by the current floodplain FR0 .In either case, given the minor, narrow exposures inthis reach of the Fremont River, we are not confidentin our dating of FR1.
Fig. 6. Full cosmogenic radionuclide concentration profiles forFR2C and FR3, and the amalgamated sample results used in the
26 Ž . 10pairs techniques on all terraces. Note Al top axis and BeŽ .bottom axis are plotted on scales that differ by a factor of 6.0.
Ž . ) ŽCurves are best fits to Eq. 3 , with z fixed at 0.8 m solid line10 26 .Be; dashed Al . The gray vertical lines represent best fits for
Žthe inheritance, N , width indicates error in estimate of inheri-i n.tance for FR2C, FR3 and FR4. Inheritance on FR1 appears to be
Ž .minimal see text for discussion . The best fit for the post-deposi-tional component of the radionuclide concentration at the surface,N , is then used to estimate the age of the surface.s
()
J.L.R
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Table 210 Be and 26Al data: amalgamated samples
10 26 10 26 10 26w x w xSample Terrace No. of Latitude Altitude Depth P Be P Al Be Al Be concentration Al concentration Be model age Al model ageŽ . Ž . Ž . Ž . Ž . Ž . Ž . Ž .clasts km cm ppm ppm atomrmgm atomrmgm ka ka
Altitudes and latitudes for samples were determined using: handheld GPS; laser EDM; triangulation with compass and topographic maps; and a combination of the three.w x w xProduction as a function of latitude and altitude is calculated from equations given by Lal 9 and calibrations of Nishiizumi et al. 24 . The surface production rate is then
adjusted to account for the depth to the subsurface samples: we estimate the terrace gravels to contain 30% large clasts, and the porosity of the matrix to be 35%, to arrive at a3 Ž . Ž . w x 2 Ž . 2 Ž .density of 2.1"0.1 grcm ; we then calculate P z using Eq. 1 and Brown et al.’s 20 values of L s145 grcm "5% and L s156 grcm "8% . Errors wereBe Al
propagated using the uncertainties given, with a 10% uncertainty assigned to the surface production rate and 7% to the sample depth. The model ages given for subsurfaceŽ ) . Ž .samples labeled with a are calculated with Eq. 4 , using the mean surface production rate and concentration for that terrace. All samples were measured at the CAMS facility
Ž .at Lawrence Livermore National Laboratory. Aluminum samples were normalized to standards prepared by Nishiizumi, KNSTD9860 26Al:27Als9.86Ey12 andŽ . Ž .KNSTD9919 26Al:27Als9.92Ey12 . Aluminum blank ratios were uniformly less than 1.0Ey14. Beryllium samples were normalized to LLNL STD10000 10Be:9Bes1E11 .
Beryllium blank corrections take into account interference by the isobar 10 B and were uniformly less than 6.0Ey14.
( )J.L. Repka et al.rEarth and Planetary Science Letters 152 1997 59–73 71
The profiles and amalgamated sample pairs sug-gest 10 Be-based age estimates of 60"9 ka for FR2C,102"16 ka for FR3, and 151"24 ka for FR4.While the uncertainties are large, comparison withthe global ice volume record depicted by the d 18 O
w xcurve 30 supports the idea that the Fremont terraceswere emplaced in late glacial or glacial maximum
Ž .times Fig. 7 . The amalgamated surface sampleŽ .results Fig. 5B from the FR2A and FR2B surfaces
imply that they may be roughly coeval with theFR2C surface. In the absence of samples from depthon these two extensive lower surfaces, we cannotfurther constrain the time span that these three sur-faces represent. If they are all associated with isotopestage 4, then the vertical range of depositional sur-faces produced during this glacial stage was roughly30 m.
We note that the 102 ka age for the FR3 terracecorresponds well with recently reported ages for theWR3 terrace along the Wind River. The Wind River
w xhas 15 well preserved terraces 14 and displays arich response to small fluctuations in the global ice
ŽFig. 7. Age estimates of the three best dated terraces FR2C, FR3,.and FR4 shown against the last 250 ka of the normalized global
18 Žd O record the scale on the left axis is standard deviations about. w xthe mean 27 ; oxygen-isotope stages are numbered. The terrace
Ž .ages gray bars were determined using production rates scaled byw xthe Nishiizumi et al. 24 calibrations, and post-depositional accu-
mulations of cosmogenic radionuclides constrained by fitting theconcentration profiles shown in Fig. 6. Also shown are thesummer and winter insolation calculations of Berger and Loutrew x31 for 308N. The terrace ages correspond roughly to global icevolume maxima in stages 4, 5d and 6, and to maxima in thesummer insolation and minima in the winter insolation histories.
volume record. The Pinedale moraine and associatedterrace sequence includes last glacial maximum datesranging from 17 to 21 ka. The Bull Lake glaciationincludes a wide temporal range of morainal andterrace features. The associated WR3 terrace has
36 w xbeen dated, using Cl, at 126–103 ka 19 . Resultsof work using 10 Be methods parallel to our own
w ximplies an age of roughly 100 ka for this terrace 25 .While the age the FR2C terrace does not correlatewith any dated surface in the Wind River sequence,the Sierran system does record such an eventŽYounger Tahoe in the Bloody Canyon moraines:
w x.40–55 ka 13 .Knowledge of the ages of these terraces allows us
to estimate various geomorphic process rates withinthe Fremont River system. While the incision rate of
Žthe river varies with time as the existence of ter-.races attests , even the mean incision rate between
times of terrace abandonment appears to vary widely.The terrace ages suggest variation by roughly afactor of 3, from 0.30 to 0.85 mrka around a meanof 0.6 mrka. We therefore caution against placingmuch faith in linear interpolation or extrapolation ofterrace ages from any pair of known ages, as iscommonly done.
The inheritance, N , is of the same order for allinŽ 6 10three terraces 0.42x10 atomrg Be for FR2C,
0.48x106 atomrg for FR3, and 0.44x106 atomrg for.FR4 . This corresponds to the arrival of clasts at the
terrace with mean effective exposure ages of 30–40ka. The inheritance can be used to constrain hillslopeexhumation rates and fluvial transport times. If we
Ž .assume rapid fluvial transport N sN , we canin hillŽ .solve Eq. 6 for the mean hillslope erosion rate to
yield es30 mrMa. If we assume exhumation to be˙Žrapid i. e., N sN , then N corresponds toin fluvial fluvial
Ž Ž .a mean transport time of 166 ka using Eq. 8 with.Hs5 m . Because inheritance is the sum of these
two processes, the exhumation determined here is aminimum and the fluvial transport time is a maxi-mum. Given the 40–50 km distance from our sourcearea to our sampled terraces transport times of theorder 105 yr seem unreasonable. The 30 mrMaerosion rate, on the other hand, is similar to bedrockerosion rates measured elsewhere in the arid Ameri-
w xcan west 12,1 . This argues that the primary sourceof inheritance in the Fremont system is obtainedduring exhumation on hillslopes within the headwa-
( )J.L. Repka et al.rEarth and Planetary Science Letters 152 1997 59–7372
ters, and that clast-to-clast variability in inheritanceis due to high variability in local rates of exhuma-tion. That the mean inheritance is roughly consistentfrom terrace to terrace could be interpreted as signi-fying that the mean weathering rate within the basinover this interval of time has been surprisingly con-stant.
6. Conclusions
The large spread of single clast ages implies awide scatter in the inheritance signal, raising a cau-tionary flag against the use of small numbers ofclasts in dating depositional terraces. This scatter canbe reduced significantly by using samples consistingof aliquots from large numbers of clasts. The repro-ducibility of the concentration values from samplesamalgamated from 25–40 clasts implies that theseare sufficient numbers to constrain the mean concen-trations in this particular geomorphic system.
Using the latest Nishiizumi et al. production rates,we estimate 10 Be ages of roughly 60"9, 102"16and 151"24 ka for three of the terraces along theFremont River. These ages are consistent with aterrace genesis model wherein terrace gravels arelaid down in braided outwash plains associated withhigh water and sediment flux during glacial maxima,and are abandoned as the Fremont River begins toincise due to high water and low sediment flux upon
w xretreat of the glaciers 2,6 . Our dating implies thatclimate swings that generate only small and short-lived variations in global ice volume are sufficient togenerate significant glacial systems in small moun-tain chains. The incision rate between terrace-for-ming events appears to vary by a factor of three,implying that the method of dating strath terraces byfitting them to a single, long-term mean incision rateyields at best a crude approximation.
At this point, one must still exercise considerablegeological experience in the interpretation of cosmo-genic radionuclide dates from these depositional ter-races. By developing a strategy to account for thegeomorphic delivery system of clasts to the final siteof deposition, we have dealt with a large portion ofthe noise in the system. The profile technique, em-ployed here on two of the terraces, generates theexpected inheritance-shifted exponential profile, sup-
porting the use of the pairs technique in this systemfor determining both the age and inheritance. Wesuggest that this is probably not a general result, andthat full concentration profiles should be used to testfor problems related to post-depositional turbation ofclasts and for inflation or deflation of the surfacewithin any system being dated. The inheritance ofcosmogenic radionuclides is significant and must betaken into account in estimating ages of depositionalsurfaces. Studies of the rate of pedogenesis, and ofthe rate of weathering of clasts on these terraces cannow proceed with the knowledge that the timing hasbeen established to within roughly 10–20%, most ofthis error being in choice of cosmogenic productionrate.
Acknowledgements
We gratefully acknowledge the Petroleum Re-search Fund of the American Chemical Society, agrant from the Center for Accelerator Mass Spec-trometry at Lawrence Livermore National Labora-tory, and a Cole award from the Geological Societyof America, for support of this research. We thankthe following individuals for their help in the field,the lab, andror for reviewing early versions of thispaper: Christian Brauderick, Alex Densmore, GregDick, James Georgis, Joe Koning, Craig Lundstrom,Greg Pratt, Eric Small, and Melissa Swartz. In addi-tion, thorough and constructive reviews of an earlierversion of the manuscript by K. Nishiizumi, E. Brownand an anonymous reviewer aided significantly in itscondensation. This work was partially supported un-der the auspices of the DOE by LLNL under contract
[ ]W-7405-Eng-48. MK
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