Recalibrated Legacy 40Ar/39Ar Ages for the Upper Jurassic Morrison Formation, Western Interior, U.S.A. GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 2 2015 Recalibrated Legacy 40Ar/39Ar Ages for the Upper Jurassic Morrison Formation, Western Interior, U.S.A. Kelli C. Trujillo1, and Bart J. Kowallis2 1Uinta Paleontological Associates, Inc., Laramie, WY 82070; kellitrujillo@icloud.com 2Department of Geological Sciences, Brigham Young University, Provo, UT 84602; bkowallis@byu.edu © 2015 Utah Geological Association. All rights reserved. For permission to copy and distribute, see the following page or visit the UGA website at www.utahgeology.org for information. Email inquiries to GIW@utahgeology.org. www.utahgeology.org Editors Douglas A. Sprinkel Utah Geological Survey 801.391.1977 dsprinkel@gmail.com Bart J. Kowallis Brigham Young University 801.422.2467 bkowallis@gmail.com Thomas C. Chidsey, Jr. Utah Geological Survey 801.537.3364 tomchidsey@utah.gov Steven Schamel GeoX Consulting, Inc. 801.583.1146 geox-slc@comcast.net Production Cover Design and Desktop Publishing Douglas A. Sprinkel Cover Photograph Morrison Formation near Notom looking towards the east with the Henry Mountains in the far back- ground. The upper Salt Wash and colorful Brushy Basin Members of the Morrison can be seen in the foreground with the Cedar Mountain Formation and Mancos Shale above. The Tidwell and lower Salt Wash Members are not seen in the photo. Photo- graph by Bart J. 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Earthquake Safety Committe Chair Grant Willis gwillis@utah.gov 801.537.3355 GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 2 2015 1 ABSTRACT As a result of recent updating of decay constants and standard ages used for 40Ar/39Ar dating, it is nec- essary to recalibrate legacy ages obtained with older methods. These recalibrations bring legacy 40Ar/39Ar ages into better agreement with ages obtained using 238U/206Pb dating methods. We present nine recalibrat- ed 40Ar/39Ar ages for the Upper Jurassic Morrison Formation of the Western Interior, U.S.A., along with the individual geographic and stratigraphic locations for each sample. These recalibrated ages will be useful for researchers looking to place better age constraints on the flora and fauna of the Morrison Formation, as well as for those working to understand stratigraphic relationships across the formation. The recalibrated ages also can now be used reliably for comparisons with newer 238U/206Pb ages obtained for the Morrison Formation. Recalibrated Legacy 40Ar/39Ar Ages for the Upper Jurassic Morrison Formation, Western Interior, U.S.A. Kelli C. Trujillo1 and Bart J. Kowallis2 1Uinta Paleontological Associates, Inc., Laramie, WY 82070; kellitrujillo@icloud.com 2Department of Geological Sciences, Brigham Young University, Provo, UT 84602; bkowallis@byu.edu Trujillo, K.C, and Kowallis, B.J., 2015, Recalibrated legacy 40Ar/39Ar ages for the Upper Jurassic Morrison Formation, Western Interior, U.S.A.: Geology of the Intermountain West, v. 2, p. 1-8. © 2015 Utah Geological Association. All rights reserved. For permission to use, copy, or distribute see the preceeding page or the UGA website, www.utahgeology.org, for information. Email inquiries to GIW@utahgeology.org. INTRODUCTION The Upper Jurassic Morrison Formation is one of the most-studied rock units in North America. Its ver- tebrate fauna includes fish, amphibians, reptiles, and mammals. Dinosaurs from the Morrison Formation are exhibited in many museums worldwide and are among the best-known and loved dinosaurs in the public imag- ination. The Morrison Formation was deposited under ter- restrial conditions, mainly on floodplains, in river chan- nels, and in small lakes as well as small dune fields in some areas. The formation is exposed across the West- ern Interior of North America, and it is generally recog- nizable across this depositional area. In the areas of the northern Colorado Plateau where this study is focused, three formal members are recognized: the lower Tid- well Member, the middle Salt Wash Member, and the upper Brushy Basin Member. On the southern part of the Colorado Plateau other members are recognized in the lower parts of the formation, whereas in the north- ern and eastern parts of the depositional area no formal members are recognized (Turner and Peterson, 1999). Correlations across this large area can be problematic due to discontinuous outcrops and the variable nature of the strata. As a result, radiometric ages are the best method for comparing the ages of disparate fossil local- ities (Trujillo, 2006). As part of a long-term, multi-faceted study of the Morrison Formation, Kowallis and others (1998) pub- lished nine 40Ar/39Ar ages from the Morrison Forma- tion, as well as listing older 40Ar/39Ar and 40K/40Ar ages previously obtained from these rocks. The ages reported GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 2 2015 www.utahgeology.org 2 Recalibrated Legacy 40Ar/39Ar Ages for the Upper Jurassic Morrison Formation, Western Interior, U.S.A. Trujillo, K.C., and Kowallis, B.J. Geology of the Intermountain West 2015 Volume 2 in Kowallis and others (1998) were obtained from san- idine crystals collected from presumed ashfall beds. All but one age was obtained from localities on the Colora- do Plateau in eastern Utah; the other age was obtained from a sample collected from south-central Colorado, near Cañon City. As these ages were the only existing radiometric ages with good resolution from the Mor- rison Formation, they have been used extensively in many different papers about various aspects of the for- mation (e.g., Turner and Peterson, 1999; Foster, 2003). Over the last decade, researchers have been working to bring the 40Ar/39Ar and 238U/206Pb dating systems into better agreement with one another, with inter-calibra- tion projects adding new data to our understanding of critical times in Earth history (e.g., Sageman and others, 2014). In addition, the 40Ar/39Ar system has undergone revisions to some of its major components. The age of the Fish Canyon Tuff sanidine (FCs), one of the main fluence monitors (standards) used in 40Ar/39Ar dating, has been modified several times and discussions of its age are ongoing (e.g., Renne, 2014; Sageman and others, 2014). Initially, the FCs was proposed as a standard with an age of 27.79 Ma (millions of years before present) (Cebula and others, 1986). This age was determined rel- ative to the age of another standard, the McClure Moun- tain hornblende (MMhb-1). Later, when the MMhb-1 age was revised upward, the FCs age was increased to 27.84 Ma (Samson and Alexander, 1987). This age for the FCs was used until Renne and others (1998) pub- lished an age of 28.02 Ma for this standard, which they determined by comparison with another standard known as the GA1550 biotite. Kuiper and others (2008) published a new FCs age based on inter-calibration with the astronomical time scale. This new, more precise age of 28.201 ± 0.046 Ma also utilized a new decay constant (i.e., Min and others, 2000), and researchers with the EarthTime project (an international scientific initiative supported by the National Science Foundation; www. earth-time.org) voted to adopt this value for the FCs in future publications. Renne and others (2010) then pub- lished new 40K decay constants that used an approach that was independent of astronomical dating and in- cluded data from both 40Ar/39Ar and 238U/206Pb dating. These new constants resulted in an age for the FCs of 28.305 ± 0.036 Ma (Renne and others, 2010). Not all workers agreed with these methods and results, how- ever, and Schwartz and others (2011) published a com- ment that questioned some of the methods used by Renne and others (2010). As a result, Renne and oth- ers (2011) published a reply in which they agreed with questions raised by Schwartz and others (2011) about one aspect of their methods (the use of data from liquid scintillation counting techniques), and they removed this data from their calculations. This changed their age for the FCs to 28.294 ± 0.036 Ma. Even with this change, however, some workers (e.g., Alexandre, 2011; Meyers and others, 2012) have questioned aspects of Renne and others (2010, 2011). Most recently, Sageman and others (2014) looked at three proposed sets of 40K total decay constants and associated ages for the FCs: from Renne and others (1998), from Kuiper and others (2008), and from Renne and others (2010, 2011). They took seven Cretaceous samples dated with both 238U/206Pb and 40Ar/39Ar meth- ods, and calculated new 40Ar/39Ar ages using the three different sets of decay constants and FCs ages. They concluded that in all seven pairs, using the FCs age of 28.201 ± 0.046 Ma of Kuiper and others (2008) gave the best agreement with the 238U/206Pb ages. Currently, the majority of 40Ar/39Ar researchers have adopted the astronomically calibrated age of the FCs of 28.201 ± 0.046 Ma from Kuiper and others (2008), as well as Min and others (2000) 40K decay constant of 5.463 ± 0.107 x 10-10 (Sageman and others, 2014). These new developments in 40Ar/39Ar dating meth- odology have led to the need to recalibrate the older, “legacy” ages for various samples dated by 40Ar/39Ar methods. Recalibrated ages for samples from the Morri- son Formation are reported here (figure 1; table 1), and it is our hope that researchers will use these ages in lieu of the previously published ages when referring to the age of the Morrison Formation. METHODS Data from the original 40Ar/39Ar dating process (Kowallis and others, 1998) was entered into a recali- bration calculation spreadsheet created by N. McLean (University of Kansas) and available on the EarthTime website (www.earth-time.org). The decay constants, flu- ence monitor ages, and uncertainties used in the cal- 3 Recalibrated Legacy 40Ar/39Ar Ages for the Upper Jurassic Morrison Formation, Western Interior, U.S.A. Trujillo, K.C., and Kowallis, B.J. Geology of the Intermountain West 2015 Volume 2 ? ? ? ? ~1 51 M a S C A LE m et er s fe et 010203040 05 50 1015 C on gl om er at e S an ds to ne S ilt st on e N od ul ar c ar bo na te M ud dy c ar bo na te A lte re d vo lc an ic a sh M ud st on e Li th ol og ie s S w el lin g cl ay s Li ttl e C ed ar M tn (L C M ) Brushy Basin Member C ed ar M ou nt ai n Fo rm at io n 39 ° 12 " 0 2' N , 1 10 ° 29 ' 4 5" W E m er y C ou nt y, U T Salt Wash Member LC M -1 15 2. 14 ±0 .5 1 M a LC M -3 9 15 0. 00 ±0 .5 2 M a R ai nb ow D ra w (R A IN ) U in ta h C ou nt y, U T 40 ° 33 ' 3 0' ' N , 1 09 ° 11 ' 3 6" W Ti dw el l M br .Salt Wash Member R A IN -1 32 5- 4+ 4 15 6. 84 ±0 .5 9 M a Brushy Basin Member C ed ar M ou nt ai n Fo rm at io n N ot om (N TM ) Brushy Basin Member Salt Wash Member Ti dw el l M br . C ed ar M ou nt ai n Fo rm at io n 38 ° 16 ' 4 4" N ,1 11 ° 07 ' 3 5" W W ay ne C ou nt y, U T N TM -1 31 9- 1 15 6. 77 ±0 .5 5 M a N TM -1 7 15 1. 23 ±0 .5 4 M a S um m er vi lle F or m at io n J- 5 un co nf or m ity B ur ro C an yo n Fo rm at io n M on te zu m a C re ek (M C ) S an J ua n C ou nt y, U T 37 ° 19 ' 1 5" N , 1 09 ° 26 ' 2 6" W Salt Wash Member M C -5 2 15 1. 34 ±0 .5 4 M a M C -3 9 14 9. 74 ±0 .6 4 M a Brushy Basin Member D in os au r Q ua rr y W es t ( D Q W ) R ed w at er M em be r S tu m p Fo rm at io n W in dy H ill M em be r of S un da nc e Fo rm at io n J- 5 un co nf or m ity U in ta h C ou nt y, U T 40 ° 26 ' 2 0" N 1 09 ° 17 ' 4 0" W Ti dw el l M br .Salt Wash Member D Q W -2 1 15 0. 91 ±0 .4 3 M a Brushy Basin Member C ed ar M ou nt ai n Fo rm at io n MORRISON FORMATION Ly tle F m . G ar de n Pa rk (G P) Fr em on t C ou nt y, C O 3 8° 3 2' 3 9" N ,1 05 ° 11 ' 5 2" W B el l R an ch Fm . Morrison Fm., undifferentiaed G P- 13 46 -2 8+ 23 15 2. 29 ±0 .2 7 M a ID U T W Y N E K S N M A Z O K D Q W & R A IN LC M N T M M C G P C O T X Fi gu re 1 . R ec al ib ra te d 40 Ar /39 Ar a ge s f ro m th e M or ri so n Fo rm at io n, p la ce d on to lo ca l s tra tig ra ph ic se ct io ns w he re sa m pl es w er e co lle ct ed . St ra tig ra ph ic se ct io ns lo os ely co rr ela te d ba se d on 40 Ar /39 Ar a ge s t ha t a re w ith in a na ly tic al er ro r o f e ac h ot he r ( bl ue d as he d lin es ). O ra ng e d as he d lin e s ho w s a n es tim at e of th e pl ac em en t o f 1 51 M a tim eli ne b as ed o n str at ig ra ph ic po sit io ns o f 40 Ar /39 Ar a ge s. S am pl e ar ea s s ho w n on in se t m ap . N ot om se ct io n fro m K ow al lis an d H ea to n (1 98 7) ; L itt le Ce da r M tn ., D in os au r Q ua rr y W es t, an d M on te zu m a Cr ee k se ct io ns m ea su re d by C .E . T ur ne r a nd F . P et er so n, U .S . G eo lo gi ca l Su rv ey , w rit te n co m m un ica tio n (1 98 8) ; R ai nb ow D ra w se ct io n fro m T ur ne r a nd P et er so n (1 99 9) ; G ar de n Pa rk se ct io n m ea su re d by F . P et er so n, U .S . G eo - lo gi ca l S ur ve y, w rit te n co m m un ica tio n (1 99 1) . 4 Recalibrated Legacy 40Ar/39Ar Ages for the Upper Jurassic Morrison Formation, Western Interior, U.S.A. Trujillo, K.C., and Kowallis, B.J. Geology of the Intermountain West 2015 Volume 2 Sample name Published age (1 error with error in J) Recalibrated age internal 1 internal + std internal + std + Strat. level of sample Sample area County State Lat. LCM-39 148.07±0.51 150.00 ±0.52 ± 0.53 ± 2.99 104.5 m above base of Brushy Basin Mbr. Little Cedar Mtn Emery UT DQW-21 148.97±0.42 150.91 ±0.43 ± 0.44 ± 2.99 55.4 m above base of Brushy Basin Mbr. Dinosaur Quarry West/Douglass Draw Uintah UT MC-52 149.39±0.53 151.34 ±0.54 ± 0.55 ± 3.01 63.5 m above base of Brushy Basin Mbr. Montezuma Creek San Juan UT NTM-17 149.29±0.52 151.23 ±0.54 ± 0.54 ± 3.01 48.5 m above base of Brushy Basin Mbr. Notom Wayne UT MC-39 147.82±0.63 149.74 ±0.64 ± 0.65 ± 3.00 51 m above base of Brushy Basin Mbr. Montezuma Creek San Juan UT LCM-1 150.18±0.5 152.14 ±0.51 ± 0.52 ± 3.02 3.8 m above base of Brushy Basin Mbr. Little Cedar Mtn Emery UT GP-1346-28+23 150.33±0.27 152.29 ±0.27 ± 0.30 ± 3.00 56 m above base of fm. Garden Park Fremont CO NTM-1319-1 154.75±0.54 156.77 ±0.55 ± 0.56 ± 3.12 2.4 m above base of Tidwell Mbr. Notom Wayne UT RAIN-1325-4+4 154.82±0.58 156.84 ±0.59 ± 0.60 ± 3.13 2.7 m above base of fm. Rainbow Draw Uintah UT converted age uncertainties Location of top of section Long. Location of top of section Table 1. Recalibrated ages in Ma for samples from the Upper Jurassic Morrison Formation, dated by single-crystal 40Ar/39Ar laser fusion methods. Geographic and stratigraphic information from Kowallis and others (1998). See table 2 for details on recalibrations. All samples processed at the Berkeley Geochronological Center. culations are listed in table 2, as are the legacy age data used such as the J value (a parameter associated with the irradiation of 39K to create 39Ar) for each sample and their uncertainties. It should be noted that although we chose the age for the FCs of 28.201 ± 0.046 Ma from Kuiper and oth- ers (2008) and the 40K decay constant of 5.463 ± 0.107 x 10-10 from Min and others (2000) as discussed above, because of ongoing discussions and research on these topics other workers may chose to use different values (e.g., Irmis and others, 2013). The practical differences in the recalibrated ages are small (0.03% in the case of Irmis and others, 2013), however, regardless of which values are used. DISCUSSION The recalibrated 40Ar/39Ar ages for nine samples from the Morrison Formation reported here (figure 1; table 1) are useful for researchers looking to place better age constraints on the flora and fauna of the Morrison Formation, as well as for those working to understand the stratigraphic relationships across the formation. They are also now more useful for comparisons with newer 238U/206Pb ages that have recently been published (Kowallis and others, 2007; Bradshaw and Kowallis, 2009; Trujillo and others, 2006, 2008, 2014; Trujillo and Chamberlain, 2013). It should be noted, however, that comparing ages obtained by 40Ar/39Ar and 238U/206Pb methods is not simply a case of looking at the numbers. Two poten- tial issues regarding the uncertainties in ages must be addressed. First, there are differences in the ways that uncertainties are reported between the 40Ar/39Ar and 238U/206Pb systems. With 40Ar/39Ar ages, the convention is to report the uncertainty in the ages as 1-sigma, while 238U/206Pb ages are almost always reported with 2-sigma uncertainties. Workers should be aware of this differ- ence, as it may result in misunderstood comparisons between ages obtained with the two different systems. Second, although recalibrated 40Ar/39Ar ages are 5 Recalibrated Legacy 40Ar/39Ar Ages for the Upper Jurassic Morrison Formation, Western Interior, U.S.A. Trujillo, K.C., and Kowallis, B.J. Geology of the Intermountain West 2015 Volume 2 now in much better agreement with 238U/206Pb ages overall, attention needs to be paid to the uncertainties propagated by recalibration when comparing ages ob- tained by the different methods. For this study, in tables 1 and 2 the first uncertainty given (converted age uncer- tainties, internal 1σ column) is the uncertainty involved in the analysis itself. This is the uncertainty that should be used when comparing recalibrated 40Ar/39Ar ages obtained by the same lab (Berkeley Geochronological Center) using the same methods, standards, and decay constants. In the data reported here, this is the uncer- tainty that should be used when only these reported re- calibrated ages are of interest. The second uncertainty given in tables 1 and 2 (con- verted age uncertainties, internal + standard column) includes the analytical uncertainty as well as the un- certainty in the age of the fluence monitor (standard, the Fish Canyon Tuff sanidine in this case). This is the uncertainty that should be used when comparing reca- librated 40Ar/39Ar ages from different labs or when dif- ferent fluence monitors are used. The final, largest uncertainty given in tables 1 and 2 (converted age uncertainties, internal + standard + λ column) includes the analytical uncertainty, the un- certainty in the age of the fluence monitor, and the un- certainty in the decay constant. This is the uncertainty that should be used when comparing ages obtained by 40Ar/39Ar methods with those obtained by 238U/206Pb methods. The size of this largest uncertainty is disconcerting, as with a range of approximately 6 million years it spans much of the understood depositional time of the entire Morrison Formation. As a result, it would seem that using the new recalibrated legacy 40Ar/39Ar ages along with new 238U/206Pb ages obtained for the Morrison For- mation could be fraught with error. The importance of this uncertainty to the practical use of these ages is unclear at present. Without more data, it is difficult to make this determination; however, two of the localities where legacy 40Ar/39Ar ages were obtained from the Morrison Formation at Notom, Utah, have also been dated using 238U/206Pb methods (Kowal- lis and others, 2007; Bradshaw and Kowallis, 2010). If we look at the recalibrated 40Ar/39Ar ages without tak- ing the uncertainties into account, they are in very close agreement with the 238U/206Pb ages for these same lo- calities (figure 2). These preliminary data suggest that although the uncertainties are large when comparing ages obtained by the two different dating systems, the data themselves may still be useful. old 5.543E-10 ( age of standard, old 27.84 (age of Fish Canyon Tuff sanidine standard from Renne and others, 1998) new 5.463E-10 ± 1.07E-11 /yr, 1 ( = Decay constant, from Min and others, 2000) age of standard, new 28.201 ± 0.023 Ma, 1 (age of Fish Canyon Tuff sanidine standard from Kuiper and others, 2008) INPUT legacy data age: legacy data age sample age, old (Ma) sample age, new (Ma) Relative Change 1 internal J value J 1 unct. internal internal + standard internal + standard + LCM-39 148.07 150.00 1.29% ± 0.51 0.03801 ± 0.00013 ± 0.52 ± 0.53 ± 2.99 DQW-21 148.97 150.91 1.29% ± 0.42 0.01067 ± 0.00003 ± 0.43 ± 0.44 ± 2.99 MC-52 149.39 151.34 1.29% ± 0.53 0.03832 ± 0.00013 ± 0.54 ± 0.55 ± 3.01 NTM-17 149.29 151.23 1.29% ± 0.52 0.03824 ± 0.00013 ± 0.53 ± 0.54 ± 3.01 MC-39 147.82 149.74 1.29% ± 0.63 0.03823 ± 0.00013 ± 0.64 ± 0.65 ± 3.00 LCM-1 150.18 152.14 1.29% ± 0.5 0.03816 ± 0.00013 ± 0.51 ± 0.52 ± 3.02 GP-1346-28+23 150.33 152.29 1.29% ± 0.27 0.01674 ± 0.00001 ± 0.27 ± 0.30 ± 3.00 NTM-1319-1 154.75 156.77 1.29% ± 0.54 0.03793 ± 0.00013 ± 0.55 ± 0.56 ± 3.12 RAIN-1325-4+4 154.82 156.84 1.29% ± 0.58 0.03811 ± 0.00013 ± 0.59 ± 0.60 ± 3.13 Decay constant and standard age used for conversion: /yr Ma Sample Name (in stratigraphic order) OUTPUT Input for Error Propagation: legacy data J Decay constant and standard age used for legacy data: INPUT FOR ALL SAMPLES Output (all 1 absolute) converted age uncertainties:converted age: Table 2. Data and calculations used for recalibrations of legacy 40Ar/39Ar data from the Upper Jurassic Morrison Formation. Sample data from Kowallis and others (1995, 1998). Results calculated using spreadsheet developed by N. McLean (University of Kansas), available at www.earth-time.org. 6 Recalibrated Legacy 40Ar/39Ar Ages for the Upper Jurassic Morrison Formation, Western Interior, U.S.A. Trujillo, K.C., and Kowallis, B.J. Geology of the Intermountain West 2015 Volume 2 SUMMARY The recalibration of these legacy 40Ar/39Ar ages from the Upper Jurassic Morrison Formation adds new use- ful data for researchers interested in this widespread rock unit. Along with these ages, additional radiometric ages from other geographic areas across the deposition- al area of the formation will help in decoding the tem- NTM-17 151.2 ± 1.8 Ma U/ Pb LA-ICP age 157.2 ± 1.9 Ma 151.23 ± 3.01 Ma Ar/ Ar age 156.77 ± 3.12 Ma Notom (NTM) B ru sh y B as in M em be r S al t W as h M em be r Tidwell Member Cedar Mountain Formation 38° 16' 44" N,111° 07' 35" W Wayne County, UT 40 39 206238 U/ Pb LA-ICP ageAr/ Ar age40 39 206238 NTM-1319-1 NTM_2 NTM_17 M O R R IS O N F O R M A TI O N Summerville Formation J-5 unconformity Conglomerate Sandstone Siltstone Nodular carbonate Muddy carbonate Altered volcanic ash Mudstone Lithologies SCALE meters feet0 10 20 30 40 0 5 50 10 15 Figure 2. Comparison of 40Ar/39Ar and 238U/206Pb ages on samples from the same localities at Notom, Utah. 40Ar/39Ar ages shown with analytical uncertainty, uncertainty in fluence monitor, and uncertainty in decay constant included. See figure 1 for location of section. Section from Kowallis and Heaton (1987). 7 Recalibrated Legacy 40Ar/39Ar Ages for the Upper Jurassic Morrison Formation, Western Interior, U.S.A. Trujillo, K.C., and Kowallis, B.J. Geology of the Intermountain West 2015 Volume 2 poral relationships among the floras and faunas. Tech- niques for isolating and analyzing very small crystals continue to improve, and more radiometric ages from the Morrison Formation are forthcoming. In addition, dating of more samples by both 40Ar/39Ar and 238U/206Pb methods would help in determining how much empha- sis to place on the high uncertainties when comparing ages obtained by the two dating methods. ACKNOWLEDGMENTS We thank Noah McLean (University of Kansas) and John Foster (Museum of Moab) for helpful discussions. Reviews from D. Sprinkel and T. Chidsey (Utah Geo- logical Survey) and M. Heizler (New Mexico Tech) im- proved the manuscript. 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