Sullivan et al., eds., 2011, Record 3. New Mexico Museum of Natural History and Science, Bulletin 53. 602 MAGNETOBIOSTRATIGRAPHY OF THE TROUBLESOME FORMATION, MIDDLE PARK BASIN, CENTRAL COLORADO

DONALD R. PROTHERO

Department of Geology, Occidental College, Los Angeles, CA 90041

Abstract—The Troublesome Formation consists of a discontinuous sequence of tuffaceous floodplain siltstones about 300 m (1000 feet) thick mantled over Laramide paleotopography in the Middle Park Basin, between Kremmling and Granby, Colorado. It first produced Miocene fossil mammals in 1908, but the chronostratigraphy has never been fully resolved. New paleomagnetic studies, along with previous 40Ar/39Ar dates and mammalian biostratigraphy, greatly improve the dating of the deposits. Paleomagnetic sampling was conducted over most of the limited exposures that produce diagnostic mammal assemblage or radiometric dates. Most samples had a slight overprint due to goethite, but yielded directions during the low-temperature thermal demagnetization that seemed to be held in magnetite and passed a reversal test. Petrographic examination of the rocks showed abundant magnetite with no significant hematite. The oldest localities (Moore Reservoir, Granby) contain early-middle mammals and an ash date of 23.5 Ma, and correlate with Chrons C6C-C6r (23-28 Ma). Except for the Highway 40 locality, which spans the Arikareean-Hemingfordian boundary (Chron C5Er, 18.75-19 Ma), the latest Arikareean (19-23 Ma) is virtually missing from the region. The Barger Gulch section appears to span the early Hemingfordian to early (Chrons C5Br-C5Er, 15-18.5 Ma), based on four ash dates and mammals. The Four Dogs-Beached Canoe localities span the early-late Barstovian boundary (Chron C5ADr, 14.5-14.8 Ma). Most of the late Barstovian is missing. The Junction and Gravel Gulch localities are both earliest Clarendonian (Chron C5n-C5r, 9.8-11.0 Ma) based on the ash date of 11.0 Ma and fossil mammals. This new chronostratigraphy shows that although the Troublesome Formation spans the interval between 11 and 25.5 Ma, the formation is very discontinuously deposited, with long intervals of time unrepresented by rocks or .

INTRODUCTION by J.W. Gidley (which is actually a deciduous “Merychippus” tooth, according to Peter Robinson, pers. commun.). Lovering (1930, p. 74) Fossiliferous rocks in the Middle Park Basin of Colorado (Fig. 1) reported additional mammal fossils from the region (the musk deer were first informally called “lake beds” by Marvine (1874, p. 157), but Blastomeryx, the camel Procamelus, the horse Parahippus, and the were named the Troublesome Formation (after the ghost town of Trouble- chalicothere Moropus elatus), identified by Harold Cook as late some and Troublesome Creek, just east of Kremmling) in an unpublished Arikareean in age (then considered , but now is mostly late doctoral dissertation by Richards (1941). They were then formally named – Tedford et al., 2004). Lovering and Goddard (1950) reported and published by Lovering and Goddard (1950, p. 41). The unit consists an Oligocene, Miocene, and age for the formation, based on of deeply weathered and (for the most part) poorly exposed tuffaceous supposed brontothere bones collected by Richards (1941) from near floodplain siltstones (not lake deposits, as Izett, 1968, showed) which Barger Gulch. As Izett (1968, p. 46) pointed out, the material is too mantle Laramide topography over about 20 square miles of land covered fragmentary to confidently identify as brontothere. Izett and Lewis then by grasses and sagebrush. Because they fill local sub-basins and thin made further collections in 1961 and 1962 at several places, including the toward the Laramide uplifts, the thickness of the formation is highly U.S. Highway 40 Roadcut locality (see below), which yielded fossil variable. The few good exposures mean that most outcrops are discon- mammals of latest Arikareean and earliest Hemingfordian age. These tinuous small patches of siltstone in gullies or in roadcuts. Except for collections were discussed by Izett and Lewis (1963) and further collec- Barger Gulch, there is no place where a relatively thick section can be tions were identified by Lewis (1969). These authors concluded that the measured. Consequently, the true thickness of the formation is hard to Troublesome Formation yielded both early Arikareean (Marsland equiva- determine. Izett (1968, p. 40-41) estimated a maximum thickness of 400 lent) and Hemingfordian (Sheep Creek equivalent) faunas. In the 1960s, feet (121 m), but in the same paper (1968, p. 58) he gave a measured the University of Colorado began to make extensive collections in the thickness of 969 feet (295 m) in the Barger Gulch section (which only area, briefly summarized by Robinson (1968). These existing collec- spans the time interval between 18-15 Ma, or early Hemingfordian to tions, along with new collections, became the dissertation project of the early Barstovian). As Kron (1988, p. 22) noted, the numerous beds of late Donald G. Kron, who fully documented dozens of localities with at ash and siltstone are hard to tell apart, let alone trace over long distances least 101 species of fossil mammals, 34 of which were unique to the area. with extensive vegetative cover. A complete composite section is impos- Kron completed his doctorate, but never published it, so his dissertation sible to assemble, because there are multiple eruptions of ashes of similar (Kron, 1988) is still the primary reference for mammalian paleontology appearance and geochemistry. of the formation. Relatively little work has been undertaken on the Starting in 1961 and continuing past his recent retirement, Glen Troublesome fossils since Kron’s death in 2001. This is tragic, since the Izett conducted the most detailed mapping and collection of the forma- collections at the University of Colorado Museum in Boulder, and espe- tion. His work is published in a series of reports and maps (Izett, 1968, cially at the U.S. Geological Survey in Denver, are large and impressive 1974; Izett and Barclay, 1973), and is the foundation for all other re- (many nearly complete skulls and partial skeletons of camels, pronghorns, search on the formation. rhinos, horses, oreodonts, carnivores, and many other taxa) and deserve The first discoveries of fossils mammals, and attempts to date the further study. Troublesome Formation, estimated the age as “Miocene” (Cockerell, According to Kron (1988), these collections now include faunas 1908; Burbank et al., 1935) based on a Parahippus specimen identified from the middle Arikareean (latest Oligocene, about 24 Ma) to the early 603 At each suitable exposure, paleomagnetic sites (3-6 samples per site) were taken as oriented blocks of rock with simple hand tools, and then wrapped and carried back to the laboratory. There they were subsampled into core-sized cylinders using sandpaper, or if the sample was too crumbly, casts into disks of Zircar aluminum ceramic in a mag- netically shielded room. The samples were then analyzed on a 2G Enter- prises cryogenic magnetometer with an automatic sample changer at Occidental College. After measurement of natural remanent magnetiza- tion (NRM), the samples were demagnetized in alternating fields (AF) of 2.5, 5.0, 7.5 and 10.0 mT (millitesla) to prevent the remanence of multi- domain grains from being baked in, and to examine the coercivity behav- ior of each specimen. AF demagnetization was followed by thermal demagnetization of every sample at 50°C steps from 100 to 630°C to get rid of high-coercivity chemical overprints due to iron hydroxides such as goethite, and to determine how much remanence was left after the Curie temperature of magnetite (580°C) was exceeded. Results were plotted on orthogonal demagnetization (“Zijderveld’) plots, and average directions of each sample were determined by the least-squares method of Kirschvink (1980). Mean directions for each sample were then analyzed using Fisher (1953) statistics, and classified according to the scheme of Opdyke et al. (1977). RESULTS Rock magnetic analysis Orthogonal demagnetization (“Zijderveld”) plots of representa- tive samples are shown in Figure 2. In a few samples (Fig. 2A) there was a simple single component of remanance, but most samples (Figs. 2B-G) had an overprinted component that gradually disappeared as the samples were AF demagnetized, then vanished after 200°C, suggesting that it was largely held in chemical remanence of an iron oxide or hydroxide, such as goethite. After this overprint was removed, a stable component was apparent in the first low-temperature demagnetization steps that steadily lost intensity toward the highest temperatures (suggesting that it was held in a low-coercivity mineral), and was usually gone by 580°C, the Curie temperature of magnetite. Both the low coercivity and the lack of remanence above 600°C suggests the primary remanence is largely held FIGURE 1. A, Index map showing distribution of outcrops of the Troublesome in magnetite, with only slight overprints due to goethite and minimal Formation (after Lewis, 1969, fig. 1). B, Detail map showing major localities hematite present. A few samples had very unstable behavior and could and geographic features. Abbreviations: 4D BC = Four Dogs-Beached Canoe; BG = Barger Gulch; GE = Granby East; GG = Gravel Gulch; Gr = not be used, and others crumbled before they could be analyzed. These Granby locality; Hwy 40 = Highway 40 locality; Jct = Junction locality; were eliminated from the calculations, so some sites have fewer than the MR = Moore Reservoir. 3-6 sample directions originally collected and measured. This magnetic behavior and the suggested mineralogy were con- Clarendonian (late middle Miocene, about 10 Ma), or about a 14 m.y. firmed by petrographic examination of the tuffaceous specimens. Izett span of time in a section that is at most just a few hundred meters thick. (1968, p. 47) found abundant magnetite and ilmenite crystals in his Thus, the section is not only patchy and sporadically exposed, but samples but no hematite. clearly must be full of unconformities (most of them cryptic) as well. The mean direction for all normal samples is D = 9.7, I = 67.3 (k

This age span determined by mammalian biostratigraphy is confirmed = 23, 95 = 4.3, n = 50). The reversed mean (D = 186.9, I = -49.6, k = 5.4, 40 39 by Ar/ Ar dates that supplant the older fission-track dates on the 95 = 10.8, n = 60) is antipodal within error limits to the normal mean section. The dated ashes range from 11.0 Ma at the top to 23.5 Ma in the (Fig. 3), so it passes a reversal test for stability. This suggests that most oldest part of the section (Izett and Obradovich, 2001). Thus, the sec- overprinting has been removed. The formational mean (averaging all nor- tion is very long in duration but highly discontinuous, and can only be mal and inverted reversed site mean directions) is D = 11.0, I = 64.4 (k = calibrated on the basis of the detailed biostratigraphy or ash dates on 17.8, 95 = 4.5, n = 110). small local outcrops. Magnetic stratigraphy METHODS Because most of the sites were isolated patches of exposure with Sampling was conducted in the summers of 2007 and 2008 on limited stratigraphic context or thickness of section, we will discuss each some of the most important exposures of the Troublesome Formation of them in turn. Only the Barger Gulch section allowed stratigraphic that had biostratigraphically significant faunas and/or radiometric dates sampling and measurement of a true thickness of a sequence of beds. The (Fig. 1B). These were located using field notes from the University of magnetic data of all the sites and localities are given in Table 1. Precise Colorado Museum, Kron’s (1988) unpublished dissertation, and other locality information in GPS coordinates are archived at the UCM to published information from the USGS. Most samples were taken wher- prevent the information from reaching poachers who might exploit these ever there were decent exposures. Only the Barger Gulch section allowed already sparsely fossiliferous localities. Each site or short section is collection of samples in a long stratigraphic sequence. The Barger Gulch correlated individually with the magnetic polarity time scale of Lourens section was measured with a Jacob’s staff and described and matched et al. (2004), using the mammalian magnetobiostratigraphy of Tedford et with Izett’s (1968, p. 52) published section. al. (2004) and Woodburne (2004), or by the volcanic ash dates of Izett and Obradovich (2001), or both. 604

FIGURE 2. Orthogonal demagnetization (“Zijderveld”) plots of representative samples. Solid squares indicate declination (horizontal component); open squares indicate inclination (vertical component). First step is NRM, followed by AF steps of 2.5, 5.0, 7.5, and 10.0 mT (millitesla), then thermal steps from 100 to 630°C. Each division equals 10-6 emu. 605 TABLE 1. Paleomagnetic data from the Troublesome Formation. N = Barger Gulch: This is the only relatively thick and complete number of usable samples analyzed; DEC = mean declination; INC = mean section through the Troublesome Formation (Fig. 4). Our sampling closely inclination; K = precision paramater; 95 = ellipse of 95% confidence around followed the published section of Izett (1968, p. 52-53) to make sure the mean. Precise location of sites in GPS coordinates are archived at the that we could match his units and locate the key dated volcanic ashes and UCM. mammal localities. However, as Kron (1988, p. 22) noted, tracing ashes across short distances is problematic, even in well-exposed outcrops like those at Barger Gulch. We attempted to trace ashes over long horizontal distances, and found that they varied considerably and could not be traced consistently. Therefore, we focused on a single steep vertical traverse to the top of the lowermost exposures (N40° 02.17’, W106° 18.125’), and a second traverse from the highest exposures down to the bottom of the wash (N40°0.614’ W106°17.303’). This resulted in as complete a section as could be reliably measured, but a large part of the middle of the section (units 14-24 of Izett, 1968, p. 52) was, as Izett noted, very poorly exposed and not amenable to continuous sampling. The lower 75 m of section yields rocks that are of reversed polar- ity (sites 2-5) except the uppermost site (site 6). The section from 240- 600 m (Izett’s units 14-24) is poorly exposed, deeply weathered, or covered, and thus could not be sampled. The upper 100 m of section was completely sampled with the highest site (site 7) of reversed polarity, and the rest of the sites (8-12) of normal polarity. Numerous dated ashes (Izett and Obradovich, 2001) in this sec- tion tightly constrain the dating, along with some diagnostic vertebrate faunas. The ash dates include the “Chalky White” (CW ash) near the base (40Ar/39Ar dated at 18.9 ± 0.22 Ma) in the lowermost reversed magnetozone, just above the Barger Gulch microsite, University of Colo- rado Museum (UCM) locality 77028. This fossil locality produces (Kron, 1988, p. 248) a diagnostic early Hemingfordian fauna most like that of the Runningwater Fauna of Nebraska, including the ochotonid Oreolagus wilsoni, and the rodents Miospermophilus bryanti, Ergodemys bareia, Mesogaulus paniensis, Grangerimus sp., plus bats and a mustelid. Just above the reversed magnetozone at the base of the section and near the base of the covered interval was the “Pure White” (PW) ash (dated at 18.3 ± 0.07 Ma). The LTW ash (17.1 ± 0.06 Ma) falls near the top of the covered interval. The top of the section is dated by the Camel Ash (15.2 ± 0.04 Ma), along with a diagnostic early Barstovian fauna from many different UCM and USGS localities (Kron, 1988, pp. 250- 251). This fauna includes the oreodont Brachycrus buwaldi, a gomphothere mastodont tooth, the pronghorn Cosoryx alticornis, the rodent Peridiomys oregonensis, the carnivore Brachypsaloides modicus, and the camel Protolabis heterodontus, plus abundant material of Mylagaulus, Merychippus, Aepycamelus, Aphelops, and Protolabis that could not be identified to species. Thus, the dates and the faunas suggest that the basal reversed magnetozone and the Site 6 normal zone in the Barger Gulch section is Chron C5E (18.0-19.1 Ma), consistent with the CW ash date of 18.9 ± 0.22 Ma and the PW ash date of 18.3 ± 0.07 Ma just above this part of the section, as well as the Runningwater-correlative mammal fauna, which also correlates with Chron C5D and C5E (MacFadden and Hunt, 1998; Woodburne, 2004). The upper part of the section is constrained by the date of 15.2 ± 0.04 Ma at the top, and the LTW ash date of 17.1 ± 0.06 Ma (Izett and Obradovich, 2001) just below the long normal magnetozone. Such dates constrain the upper normal magnetozone as Chron C5Cn (15.9-16.7 Ma), and the short reversed interval above it (Site 7) as the base of Chron C5Br (Fig. 5). The early Barstovian faunas from the upper part of the section are also consistent with the beginning of the Barstovian, which starts in Chron C5Br (Woodburne, 2004). These dates and biostratigraphic constraints show that the largely covered middle part of the section (Fig. 5) also spans most of the late Hemingfordian (17.0-18.2 Ma). It is also possible that there are unrecog- nized unconformities in this interval, since no late Hemingfordian fossils have ever been found at Barger Gulch (Kron, 1988). Highway 40 Roadcut Locality: One of the first major sites to be discovered by Glen Izett in 1962 was a roadcut on the north side of U.S. Highway 40 (shown in Izett, 1968, p. 42, fig. 16), which was published 606 Spermophilus sp., Peridiomys borealis, Schaubemys sabrae, and large heteromyids like Cupidinimus sp. Kron (1988) interpreted these taxa as a late early Barstovian assemblage, transitional between the classic early Barstovian assemblages (such as Barger Gulch) and late Barstovian as- semblages, with “Beached Canoe” at the base of the sequence, overlain by the Megahippus roadcut locality, and then in turn overlain by “Four Dogs”. There are no volcanic ash dates in these rocks, so the simplest interpretation is that this relatively thin sequence of faunas spans the early-late Barstovian boundary (Chron C5ADr, 14.5-14.9 Ma) (Fig. 5). Alternatively, the “Beached Canoe” beds could be from the early Barstovian Chron C5Br, while the upper “Four Dogs” site is from Chron C5ADr. Moore Reservoir: The section to the southwest of Moore Reser- voir (UCM locality 82097) yields just a few fossil mammal taxa: the giant entelodont “Dinohyus” (now synonymized with Daeodon); the rodents Gregorymys riggsi and Dikkomys; and the mole Proscalops secundus. These are most similar to the fauna of the Harrison Formation Nebraska, according to Kron (1988, p. 242-243), which correlates with the interval between 19 and 25 Ma (MacFadden and Hunt, 1998, fig. 16). Thus, the fauna is not very age diagnostic. However, Izett and Obradovich (2001) obtained a date of 23.5 ± 0.06 Ma in the base of the Moore Ranch section, which is the oldest date so far obtained from the Troublesome Formation. The section strikes 325° and dips 17° to the southwest, so the section was measured perpendicular to strike, and four sites were taken spanning about 100 m (300 feet). The dated tuff (site 10 in Table 1) is reversed in polarity, then the sites at 30 m (90 feet) and 65 m (200 feet) above the tuff are normal (sites 11 and 12). The site at the top of section (site 13) was again reversed. Based on the date (Fig. 5), the section FIGURE 3. Stereonet of means of all sites. Solid circle and dot shows mean probably best correlates with Chron C6Cr-C6Br (22.5-23.5 Ma). and 95% confidence limits of the normal sites. Open circle and dashed line Granby and Granby East localities: The Granby locality (UCM indicate mean of reversed samples (upper hemisphere projection). Solid 77027) is a small area of isolated badlands just south of Tenmile Creek square surrounded by a solid circle shows projection of reversed mean into and due east of the highway south out of Granby (precise locality infor- lower hemisphere. The reversed mean is antipodal to the normal mean mation archived at the UCM). According to Kron (1988, p. 238-241) it within error estimates, so the directions are primary and overprinting has produces a uniquely early Arikareean assemblage including the rodents been removed. Entoptychus near E. minor, Gregorymys and Promylagaulus ovatus, the marsupial Peratherium youngi, the insectivores Parvericius montanus, by Izett and Lewis (1963). It is USGS locality 2 in Izett’s geologic maps Amphechinus horncloudi, and Proscalopus secundus, and the canid (Izett, 1968, fig. 2), and also UCM locality 82068. Izett and Lewis Nothocyon annectens. According to Kron (1988), these fossils are most described distinctive oreodonts from the locality, including the latest similar to those of the Monroe Creek Formation of Nebraska (28-22 Ma, Arikareean oreodont Merycochoerus matthewi from the base of the sec- according to MacFadden and Hunt, 1998) and especially those of Unit K tion, and another oreodont from higher in the section, the early in the John Day Formation, Turtle Cove Member (Deep Creek Tuff to Hemingfordian taxon Merycochoerus proprius, as defined by Schultz Tin Roof Tuff, 27.9-25.9 Ma, Chrons C7r-C8r) (Albright et al., 2008). and Falkenbach (1940). These fossils place the locality near the The two paleomagnetic sites (5 and 6 in Table 1) were both reversed, so Arikareean-Hemingfordian boundary. However, most paleontologists the Granby section probably correlates with Chron C8r. today (Stevens and Stevens, 1996, 2007; Prothero and Sanchez, 2008) The Granby East locality (UCM locality 82094) is an isolated regard Schultz and Falkenbach’s oreodont taxonomy as grossly oversplit, roadcut on the west side of the road just south of Granby (precise based on no statistical data, and full of names that are not based on location archived at the UCM). The locality produces the rodents morphology, but due to different stratigraphic position or sometimes Pseudodikkomys cyanathos, Endemus montanus, along with the absence due to post-depositional deformation. As a result, these taxa may be of the rodent Mucidimus, which is common at the slightly older Granby invalid, and the correct systematic assignment of these oreodont speci- locality. Kron (1988, p. 241-242) estimates the age as a lower Harrison mens is not resolved. The collection also includes the rodent Mesogaulus Formation equivalent. The single site (Site 7 in Table 1) was reversed in and the beardog Amphicyon, along with other less diagnostic taxa. The polarity. Based on correlations with the Nebraska section (MacFadden paleomagnetic site (Site 2) recovered from this deeply weathered and and Hunt, 1998), this suggests a correlation with Chron C6r (19.8-20.0 crumbly siltstone was reversed in polarity (Table 1). Based on its fossils Ma) or C6Ar (20.6-21.1 Ma). found near the Arikareean-Hemingfordian boundary, it best correlates Junction locality: The Junction locality, UCM locality 77035 with Chron C5Er (18.5-18.75 Ma), according to the magnetobio- (along with the nearby Doig Ranch West locality, UCM 84004) are the stratigraphic time scale of Lourens et al. (2004). youngest mammal localities in the Troublesome Formation reported by Beached Canoe and Four Dogs localities: These two UCM Kron (1988, p. 254-255). They are in the northernmost part of the localities (UCM 86108 and 84162, respectively) are recovered from Middle Park Basin Troublesome exposures north of Kremmling. Two roadcuts on the south shore of the Williams Fork Reservoir; their GPS paleomagnetic sites (sites 14-15 in Table 1) were taken in the small coordinates are archived at the UCM, with one site taken at each limited badlands to the north and east of the road north from Kremmling. Both exposure (Table 1). Both sites were reversed in polarity. According to sites were reversed (Table 1). According to Kron (1988, p. 254-255), the Kron (1988, p. 251-252), the composite fauna spans the early-late Junction locality has produced typical early Clarendonian taxa such as Barstovian boundary, with the horses Merychippus isonesus and the rodents Copemys dentalis, Eucastor dividerus, Diprionomys parvus, Megahippus mckennai, the lagomorph Oreolagus wilsoni, the rodents as well as other small mammals. Based on these fossils, the best correla- 607

FIGURE 4. Lithostratigraphy and magnetic stratigraphy of the Troublesome Formation at Barger Gulch. Stratigraphy after Izett (1968, p. 52-53). Declination and inclination of magnetic sites are shown. Solid circles are sites that are statistically removed from a random distribution at the 95% confidence level (Class I sites of Opdyke et al., 1977). Circles with diagonal pattern have fewer than 3 samples, so site statistics could not be calculated (Class II sites of Opdyke et al., 1977). Open circles have two consistent directions, but a third sample which is divergent (Class III sites of Opdyke et al., 1977). 608 the fossils and the dates suggest a time span for the formation of at least 11-28 Ma (about 17 m.y. in duration), the Troublesome Formation is no more than 330 m (about 1000 feet) thick in places like Barger Gulch, and no individual section outside Barger Gulch seems to preserve more than a short interval of geologic time. Long time intervals (especially most of the late Arikareean and late Barstovian) were not represented by samples in this study, and these stratigraphic gaps are also suggested by Kron (1988, p. 239, fig. 10). Only his Beaver locality and possibly Doig Ranch West locality produce latest Barstovian mammals (neither was acces- sible for our paleomagnetic analysis due to difficulties with private land ownership), so most of the late Barstovian is unrepresented in the Trouble- some Formation. Likewise, there appears to be a temporal gap between the Moore Reservoir section (early Arikareean) and the Highway 40 exposures (latest Arikareean), with only the poorly dated Granby East locality spanning the time interval between 19 and 22 Ma. Although the oldest localities in the Troublesome Formation are at the south edge of the outcrop and the youngest at the north, there is no evidence of a continuous sequence dipping to the north. Instead, it ap- pears that these beds were discontinuously deposited as they mantled the underlying Laramide structures at different times and places, so many intervals of time are not represented by fossils or sediments. Such discontinuity is no longer surprising considering the work of Ager (1973), but older notions about the Troublesome Formation suggested that it was once continuously deposited between 28 and 11 Ma. Instead, it is (in Ager’s words), “more gaps than record.” CONCLUSION The Troublesome Formation in the Middle Park Basin of Colo- rado was discontinuously deposited in thin wedges across pre-existing Laramide uplifts, resulting in patchy sequences of rock in which only short slices of time are represented punctuated by large unconformities. There was deposition during the early Arikareean (28-22 Ma), early Hemingfordian to early Barstovian (14.5-19 Ma), and the Barstovian- Clarendonian boundary (13-11 Ma), with significant time gaps through most of the late Barstovian and late Arikareean (somewhat bridged by scattered faunas that could not be sampled in this study). Based on the 40 39 FIGURE 5. Correlation of the Troublesome Formation paleomagnetic Ar/ Ar ash dates, the formation spans the time interval between 11.0 sections, based on the dates and biostratigraphic age constraints discussed in and 23.5 Ma, although the fossils suggest the base of the formation could the text. Time scale after Lourens et al. (2004) and Woodburne (2004). be as old as 28 Ma. Thus it spans the latest Oligocene to the late middle Miocene. tion (Fig. 5) of this reversed section is Chron 5r (11.4-11.9 Ma), based on the magnetobiostratigraphy of Woodburne (2004). ACKNOWLEDGMENTS Gravel Gulch Locality: The Gravel Gulch locality is also in the Prothero dedicates this research to his friend, the late Don Kron, northwestern part of the Troublesome outcrop north of Kremmling (ex- who urged him to collect paleomagnetic samples in the Middle Park in act location archived at the UCM). It yields the youngest ash date in the the 1980s; he is glad to finally fulfill Don’s wish, even if a bit later than formation of 11.0 ± 0.05 Ma (Izett and Obradovich, 2001), but no we had hoped. I thank G. Cromwell for all his help in the field with this fossils. We collected magnetic site 4 (Table 1) from the tuff bed, which project. I thank T. LeVelle for help with sampling. I thank P. Robinson, was normal in polarity, and site 3 about 20 m below site 4; site 3 is T. Culver, and J. Eberle of the University of Colorado and G. Izett reversed in polarity. Based on this ash date on the tuff and the short (USGS, retired) for their help and suggestions. I thank H. Armstrong, F. reversed-normal pattern, we correlate the Gravel Gulch section with Rupp, and D. Stout of the BLM for permission to work on BLM prop- Chron C5n-C5r (11.0-12.0 Ma) (Fig. 5). erties. I thank S. Bogue and J. Kirschvink for help maintaining the Occi- DISCUSSION dental paleomagnetics lab. I thank L.B. Albright, M.O. Woodburne, P. Robinson, and G. Izett for helpful comments on this paper. This re- When all the magnetobiostratigraphic data are correlated to the search was supported by grants to Prothero by the Donors of the Petro- global time scale, a striking pattern emerges (Fig. 5). Even though both leum Research Fund of the American Chemical Society.

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