Refinements of the Late Miocene Fort Rock Formation in South-Central

Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) 165

REFINEMENTS OF THE LATE MIOCENE FORT ROCK FORMATION IN SOUTH-CENTRAL OREGON, THE McKAY FORMATION IN NORTHERN OREGON, AND THE TIMING OF THE PROSOMYS INTERCONTINENTAL DISPERSAL EVENT

James E. Martin*, Jennifer E. Hargrave, and Kristin L. Ball University of Louisiana Geology Museum School of Geosciences Lafayette, LA 70504 *Corresponding author email: [email protected]

ABSTRACT

The Fort Rock Formation has been known for over 50 years, but relatively little has been published concerning the lithostratigraphy, fossil content, and age of the formation. New vertebrate paleontological discoveries in the Fort Rock/ Christmas Valley Basin of the Oregon High Desert provided impetus for lithological and stratigraphical details. Our investigations have provided additional environmental and temporal information concerning the Fort Rock Formation. The formation is exposed in northern Lake County, Oregon, and consists of tuff, diatomite, extrusives, and agglomerates. Some of the original reference areas and lithologies are herein discarded from the definition of the formation, and the unit is extended 50 km farther south than was illustrated in the original description. Lithological evidence indicates the Fort Rock Formation is principally the result of fluvial deposition with extensive volcanic contributions. Paleontological evidence indicates the formation was deposited during the late Hemphillian North American Land Mammal Age (NALMA). That time of deposition is reinforced by 40Ar/39Ar radiometric dates derived from interbedded tephras in the Fort Rock Formation that range from 5.7 to 6.2 Ma. In northern Oregon, the McKay Formation was deposited under fluvial conditions and contains late Hemphillian mammals; the 40Ar/39Ar radiometric dates from tephras in this formation range from 5.5 to 6 Ma. Therefore, vertebrate fossils from both areas are faunally and temporally similar and were deposited under riparian conditions between 5.5 and 6.2 Ma. The biostratigraphic correlation and radiometric dates of these two localities also provide direct evidence for the timing of the dispersal of the arvicoline rodent, Prosomys (=Promimomys), from Eurasia occurring approximately 5.5-6 Ma.

Keywords

Oregon, Miocene, Hemphillian, Fort Rock Fm., McKay Fm., radiometric dates 166 Proceedings of the South Dakota Academy of Science, Vol. 97 (2018)

INTRODUCTION

Our recent discoveries in the Fort Rock Formation of giant camels (Figure 1), bovids, and other vertebrate fossils from northern Lake County in southern Oregon (Figure 2) prompted concurrent geological studies. In addition, the Christmas Valley Locality of Repenning (1968) was relocated, and numerous new fossiliferous localities in the Fort Rock Formation produced significant assemblages that contribute to our knowledge of past depositional environments and their timing. Following the Cascadian Orogeny, fluvial sedimentation during Miocene time carried debris from the Cascade Mountains throughout the Pacific Northwest. Many of these sediments are now exposed across eastern Oregon, from north (represented by the McKay Formation, among others) to south (represented by the Fort Rock Formation, among others). Both formations have produced vertebrate fossils that indicate a late Miocene time of deposition during the Hemphillian NALMA. At the current state of knowledge, radiometric date correlations of the Hemphillian NALMA ranged from 10.3 Ma to 4.9 Ma, and the late Hemphillian NALMA ranged from approximately 6 to 7 Ma until 4.9 Ma (Tedford et al. 2004). Of course, the reliability of these dates and their correlations must stand the test of time and refined technologies, particularly in the Pacific Northwest where faunal provincialism may have occurred.

Figure 1. Megatylopus, a giant camel partial skeleton (University of Louisiana, Geology Museum number V6272) collected from the Fort Rock Formation.

The Fort Rock Formation was designated by Hampton (1964) for sedimentary deposits and extrusives in the Fort Rock/Christmas Valley Basin of northern Lake County. However, few refinements to the lithologies, biostratigraphy, or geochronology of the formation have been contributed subsequently. Hampton (1964) Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) 167 named the Fort Rock Formation for a geographic landmark, a geomorphic tuff ring termed Fort Rock that lies just north of the town of Fort Rock in the Fort Rock/Christmas Valley Basin.

Figure 2. Study areas in Umatilla and Lake counties, Oregon. 168 Proceedings of the South Dakota Academy of Science, Vol. 97 (2018)

He thought that Fort Rock itself may have been an eruptive center for the Fort Rock Formation, but Fort Rock is now considered to have been a much later occurring eruptive formed during the Late Pleistocene when pluvial lakes occurred in the basin (Allison 1979). Also, one of Hampton’s reference areas for the formation, Table Rock Butte, is also likely a Late Pleistocene volcanic struc- ture (Heiken et al. 1981; Orr and Orr 2012). This revision has yet to be applied to the Fort Rock definition. Fortunately, Hampton (1964) designated a type section on Seven Mile Ridge composed of sedimentary rocks (Figure 3) and reference areas to characterize the formation, such as the Fandango Canyon-St. Patrick Butte areas (Figure 4). Although much of the type section now lies beneath a reservoir, adequate exposures occur along the shoreline to provide a lithological basis for the formation. The reference area in the Fandango Canyon-St. Patrick Butte region includes extrusives as well as sedimentary units and lies just to the north and west of the area where vertebrate fossils were recently discovered by University of Louisiana Geology Museum field expeditions. Therefore, our prin- cipal concerns herein include the ages and lithologies of the sedimentary rocks that comprise the Fort Rock Formation. The Fort Rock Formation was originally thought to have been deposited during the Pliocene Epoch (Hampton 1964). However, the understanding of the European type areas of the Lyellian epochs has changed greatly over the last 50 years, and radiometric dating has refined boundaries of the Paleogene-Neogene epochs. What was considered Pliocene in Hampton’s time is now mostly contained within the Miocene Epoch in the Clarendonian and Hemphillian NALMAs (See Tedford et al. 2004). Now, the former latest Pliocene (Blancan NALMA) comprises essentially the revised Pliocene Epoch. Repenning (1968) described arvicoline rodents that were considered Hemphillian NALMA (late Miocene) from the Christmas Valley Locality mapped by Hampton (1964) as the Fort Rock Formation. Additional fossil specimens, correlations, and radiometric dates resulting from our research provide a refined lithostratigraphic and temporal framework for the Fort Rock Formation. Another formation that produced Hemphillian vertebrate fossils is the McKay Formation (Farooqui et al. 1981) in Umatilla County in northern Oregon (Figure 2). Hemphillian vertebrates are well known from the McKay Reservoir Locality (Shotwell 1955, 1956; Martin 1984, 2008). Based upon detailed biostratigraphic correlations, Martin (2008) indicated the McKay Reservoir assemblage was accumulated during the late Hemphillian NALMA. Radiometric dates for the assemblage have been wanting, but we have recovered dateable samples from the McKay Reservoir Locality. The resulting40 Ar/39Ar dates are reported herein. Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) 169

Figure 3. Stratigraphic section at the type locality of the Fort Rock Formation (Hampton 1964). Inset photograph illustrates that the section is now partially covered by a reservoir. 170 Proceedings of the South Dakota Academy of Science, Vol. 97 (2018)

Figure 4. Significant locales in the Fort Rock/Christmas Valley area.

The dates from these two formations have a direct bearing on the timing of the dispersal of arvicoline rodents from Eurasia into North America. The immigration of Prosomys (=Promimomys), the probable ancestor of the Pliophenacomyinae (R.A. Martin 2010), is represented in the Fort Rock and McKay formations. Prosomys mimus was designated by Shotwell (1956) for specimens from the McKay Reservoir Locality, and Repenning (1968) noted similar specimens from the Christmas Valley Locality, which he termed as Promimomys mimus, based on their similarity to the Eurasian genus, but Voohries (1990) resurrected Prosomys. Ultimately, a taxonomic review is required to determine which generic name is appropriate. No matter which name is utilized, the dates recorded here provide a temporal framework for this dispersal event, which heretofore had been approxi- mated at 6.7 Ma (Repenning 1987), 5.5-4.8 Ma (Repenning et al. 1990), and 5.5-5.0 Ma (R.A. Martin 2010).

FORT ROCK FORMATION

Lithostratigraphy. Although the Fort Rock Formation was originally described as a combination of sedimentary and extrusive units, our concentration in this contribution concerns the former. The third author will discuss associated basalts and other extrusives in her M.S. thesis. Our focus on detrital sedimentary units is the result of the fossil vertebrates derived therefrom that can provide evidence for temporal correlations. The majority of the sedimentary units are interbedded volcaniclastics, diatomites, and fluvial/lacustrine sediments now expressed as mudstones, sandstones, and basaltic agglomerates. Diatomites are relatively common and interbedded within the detrital sediments. Diatomite prospect pits occur through the area, and mammalian fossils have been found in interbedded detrital fluvial sediments. Channel and floodplain deposits are common, as well Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) 171 as indurated waterlain tuffaceous sandstones and basalt agglomerates. Fossils are found in channel, floodplain, and indurated tuffaceous sandstones, some with suspended basalt cobbles. As mentioned above, Hampton (1964) included the Fort Rock and Table Rock Butte areas as part of the Fort Rock Formation. The rocks include orangish brown basaltic lapilli tuffs, associated basalts, and hyaloclastic ash and debris. However, these tuff ring deposits are not interbedded with the Miocene deposits and were formed much later during the Pleistocene when large pluvial lakes covered the area (Allison 1979; Orr and Orr 2012). Therefore, lithologies associated with the tuff rings in these two areas should be eliminated from the definition of the Fort Rock Formation. Hampton’s geologicial map (1964) ended just south of the T28S-T29S line in Lake County. However, Walker et al. (1967) mapped similar deposits to the south as Tst, lacustrine tuffaceous sandstone and siltstone; welded and nonwelded vitric-crystal and vitric-lithic tuff; ash and ashy diatomite; tuffaceous sandstone and breccia; conglomerate; and pumice lapilli tuff and tuff breccia. These lithologies are typical of the Fort Rock Formation and suggest that the formation should be extended southeast to Alkali Lake and south to at least Lake Abert (Figure 2). Moreover, representatives of the fossil assemblage described below have been found in this region during our field investigations.

Paleontology. A great number of fossil invertebrates and vertebrates have been recovered from the volcaniclastic sedimentary rocks within the Fort Rock Formation, particularly from the Christmas Valley Locality, BPA Pole Locality 183/4, and BPA Pole Locality 183/5, among others (Figure 4). Typical late Hemphillian taxa were collected from the formation, including Mustela, Pliotaxidea, Dipoides, Parapliosaccomys, Prosomys (regarded as Cosomys in Martin 2017, but should more properly be considered Prosomys (=Promimomys) as in Martin 2008), Teleoceras, Megatylopus, and Neotragocerus. A water mole, Gaillardia, was discovered at BPA Pole Locality 183/5. This rare taxon is an immigrant from Eurasia and has been found principally in late Hemphillian localities as discussed by Martin (2017).

Geochronology. Field investigations resulted in tephras being identified at the Christmas Valley Locality and the BPA Pole Locality 183/5. An 40Ar/39Ar date of 5.74±0.01 Ma derived from anorthoclase from Unit 13 of the latter locality was reported by Martin (2017). Additional dates from BPA Pole 183/5 were derived from tephras interbedded within the most productive fossil horizon (Figures 5, 6). From a lower tephra, a white volcanic ash (Unit 11) from BPA Pole Locality 183/5 was calculated at 5.84±0.08 Ma, based on plagioclase and anorthoclase, whereas that from the upper tephra, a similar white waterlain volcanic ash (Unit 14) was calculated at 5.83±0.06 Ma, based principally on anorthoclase. At the Christmas Valley Locality, a tephra was found just below the fossiliferous horizon (Figure 7), and an 40Ar/39Ar date for this waterlain tephra (Unit 1) derived from feldspar crystals was calculated at 6.19±0.18 Ma. 172 Proceedings of the South Dakota Academy of Science, Vol. 97 (2018)

Figure 5. Stratigraphic section at the BPA Pole Locality 183/5. Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) 173

Figure 6. Upper portion of the stratigraphic section at the BPA Pole Locality 183/5 illustrating levels of dated tephras.

McKAY FORMATION

Lithostratigraphy. The McKay Formation in Umatilla County, Oregon, is comprised of fluvial sediments with interbedded tephras. Both channel and floodplain deposits occur and are well expressed at the McKay Reservoir fossil locality (Figures 8, 9). Similar to those of the Fort Rock Formation, the channels are composed of rounded basalt clasts, but relatively well-weathered red clay- stones appear more abundantly at the northern Oregon locality.

Paleontology. The taxa derived from the McKay Reservoir Locality clearly indicate a late Hemphillian NALMA for the assemblage. Typical Hemphillian taxa such as Gaillardia, Pliotaxidea, Dipoides, Oregonomys, Parapliosaccomys, Prosomys (=Promimomys), Teleoceras, and Megatylopus occur in the McKay Formation (Shotwell 1955, 1956; Martin 1984, 2008).

Geochronology. Two tephras were found interbedded within the fossiliferous units of the McKay Formation at the McKay Reservoir Locality (Figures 8, 9). The lower tephra (Unit 4) is a gray, waterlain ash from which an40 Ar/39Ar date of 5.95±0.30 Ma was obtained from anorthoclase. Fossils are rare below this unit but do include specimens of Megatylopus. The upper tephra (Unit 10) lies within the midst of the most fossiliferous units, and above Unit 8, the white claystone 174 Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) that has produced the most abundant and diverse assemblage from the McKay Formation, including Gaillardia and Prosomys. An 40Ar/39Ar date of 5.51±1.22 Ma was derived from anorthoclase crystals from the waterlain volcanic ash of Unit 10 (Figures 8, 9). Although a relatively large margin of error was calculated for this latter date, its weighted mean, which lies relatively close to the date of Unit 4, appears reasonable, although perhaps somewhat young.

Figure 7. Stratigraphic section at the Christmas Valley Locality. Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) 175

Figure 8. Stratigraphic section at the McKay Reservoir Locality. 176 Proceedings of the South Dakota Academy of Science, Vol. 97 (2018)

Figure 9. Stratigraphic section at the McKay Reservoir Locality. Arrows designate loca- tions of dated tephras.

CONCLUSIONS

The results of our investigations refine the definition, extend the geographic range, and provide a chronological framework for the Fort Rock Formation in northern Lake County and a temporal framework for the McKay Formation in Umatilla County, Oregon. The Fort Rock Formation is dominated by fluvial (tuffaceous and basaltic mudstone, sandstone, and breccia/conglomerate) and lacustrine deposits (ashy diatomites) accompanied by significant volcanic input. These volcaniclastics include welded and nonwelded vitric-crystal and vitric- lithic tuff; pumice lapilli tuffs and breccia. The lithologies associated with the Pleistocene tuff rings that comprise Table Rock Butte and Fort Rock itself (basaltic lapilli tuffs, basalts, and hyaloclastic ash and debris) should be excluded from the definition of the Miocene Fort Rock Formation. The vertebrate fossils recovered from the Fort Rock Formation indicate the formation was deposited during the late Hemphillian NALMA (late Miocene), whose duration appears to have been from 6 or 7 Ma to 4.9 Ma (Tedford et al. 2004). This contention is confirmed by 40Ar/39Ar dates of 5.84±0.08 Ma, 5.83±0.06 Ma, and 5.74±0.01 Ma from the BPA Pole Locality 183/5 (Figure 10) and 6.19±0.18 Ma from the Christmas Valley Locality (Figure 10). These localities are nearly six km apart and separated by significant faulting. The geographic range of the Fort Rock Formation extends south beyond the limit of the geological map of Hampton (1964). We found similar lithologies and fossil vertebrates southeast to Alkali Lake and south to Lake Abert, a southerly extension of at least 50 km. Biostratigraphic correlation of the Fort Rock assemblage has been made with the McKay assemblage from northern Oregon (Martin 2008), based principally Proceedings of the South Dakota Academy of Science, Vol. 97 (2018) 177 on the concurrent ranges of Parapliosaccomys and Prosomys in association with Gaillardia, Pliotaxidea, and Dipoides. These taxa suggest a late Hemphillian NALMA, and this correlation is substantiated by the 40Ar/39Ar dates of 5.95±0.30 Ma and 5.51±1.22 Ma from the McKay Formation (Figure 10). These dates also provide direct radiometric evidence (Figure 10) for the important dispersal event from Eurasia to North America of Prosomys (=Promimomys) noted by Repenning (1968, 1987) and Repenning et al. (1990). This event heralds the ancestry of the subsequent North American clade of the Pliophenacomyinae, a significant event in the history of hypsodont cricetid rodents. Assuming the 40Ar/39Ar dates are reliable, these dates suggest the event was in the 5.5-6 Ma range, slightly older than that predicted by R.A. Martin (2010).

Figure 10. Chronological correlation of Hemphillian localities and the interval produc- ing occurrences of Prosomys. 178 Proceedings of the South Dakota Academy of Science, Vol. 97 (2018)

ACKNOWLEDGMENTS

The field investigations on BLM holdings that culminated in this contribution were supported by the Bonneville Power Administration as part of their Pacific Direct Current Intertie Project. Sincere appreciation goes to Mr. John B. Wiley, who guided the geological-paleontological portion of the project; his direction and vision are greatly appreciated. The support of other BPA personnel is greatly appreciated, especially that of Mr. Eric Orth, Mr. Jeff Lansberg and Mr. Darren Bowman. Cooperation with the Bureau of Land Management is sincerely acknowledged, particularly the efforts of Mr. William Cannon, Lakeview District Archaeologist, for his support and expertise. Also from the Lakeview District, we benefitted from the oversight of Mr. Todd Forbes, Ms. Jami Ludwig, and Ms. Kathy Stewardson. Fossil specimens were collected under BLM Paleontological Resource permits OR-50882 and OR-50889, awarded to the first author. Dr. Randolph Moses, Absaroka Energy and Environmental Solutions, contributed his energy and expertise to this study. Personnel, students, and volunteers at the University of Louisiana Geology Museum were instrumental in providing for the collection, preparation, and reposition of scientifically important specimens. Ms. Cathy Bishop, Ms. Elisabeth Boudreaux, Mr. Terry Paddock, Mr. Brian Quebedeaux, and Mr. Gage Seaux were extremely helpful in the field and in the laboratory. Ms. Lisa Peters of the New Mexico Geochronology Research Laboratory, Socorro, undertook the radiometric dating, and Mr. Blake Lagneaux, Lafayette Science Museum, kindly undertook the locality figures. Reviews by Theodore Fremd, University of Oregon; Dr. Nicholas Famoso, John Day Fossil Beds National Monument; David C. Parris, New Jersey State Museum; and edi- torial suggestions by Dr. Robert Tatina greatly enhanced this contribution.

LITERATURE CITED

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