Article Middle-Late Triassic radiolarian assemblages from chert clasts of the Excelsior Gulch conglomerate (Wallowa terrane, Oregon, U.S.A.) KHALIL, Hany, et al. Abstract New radiolarian data from the polymict Excelsior Gulch conglomerate (Wallowa terrane, Blue Mountains Province, Oregon, USA)allow narrowing the age of the chert components. Chert clasts sampled at three distinct localities yielded radiolarians that have Middle to Late Triassic ranges. Individual taxon ranges allow for the distinction of four continuous substages: early Ladinian (presence of Triassocampe nishimurae and T. postdeweveri), middle Ladinian (with Pseudotriassocampe hungarica), late Ladinian (presence of Muelleritortis cochleata and Tritortis kretaensis), and early Carnian (with the co-occurrence of species such as Muelleritortis spp. ex. gr. M. cochleata, Tritortis dispiralis, ?Corum delgado and Triassocingula perornatum). The reworked chert clasts represent Middle to Late Triassic Panthalassan open ocean sediments that have not been described in the Blue Mountains Province. Reference KHALIL, Hany, et al. Middle-Late Triassic radiolarian assemblages from chert clasts of the Excelsior Gulch conglomerate (Wallowa terrane, Oregon, U.S.A.). Revue de Paléobiologie, 2020, vol. 39, no. 2, p. 565-579 DOI : 10.5281/zenodo.4465568 Available at: http://archive-ouverte.unige.ch/unige:148434 Disclaimer: layout of this document may differ from the published version. 1 / 1 REVUE DE VOLUME 39 (2 ) – 2020 PALÉOBIOLOGIE Une institution Ville de Genève www.museum-geneve.ch Revue de Paléobiologie, Genève (décembre 2020) 39 (2): 565-579 ISSN 0253-6730 Middle-Late Triassic radiolarian assemblages from chert clasts of the Excelsior Gulch conglomerate (Wallowa terrane, Oregon, U.S.A.) Hany KHALIL1,2, Peter O. BAUMGARTNER3, Tetsuji ONOUE4, Nicolò DEL PIERO2, George STANLEY Jr.5, Sylvain RIGAUD6 & Rossana MARTINI2 1 Department of Geology, Alexandria University, Moharam Bey, 21511 Alexandria, Egypt. E-mail: geo.hmk@gmail. com, [email protected] 2 Department of Earth Sciences, University of Geneva, 13 rue des Maraîchers, CH-1205 Genève, Switzerland. E-mail: [email protected], [email protected] 3 Institute of Earth Sciences, University of Lausanne, Bâtiment Géopolis, CH-1015 Lausanne, Switzerland. E-mail: [email protected] 4 Department of Earth and Environmental Sciences, Kumamoto University, Kumamoto, 860-8555 Japan. E-mail: [email protected] 5 Department of Geosciences CHCB 302, 32 Campus Drive #1296, Missoula, MT 59812 USA. E-mail: george.stanley@ umontana.edu 6 Asian School of the Environment, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459. E-mail: [email protected] Abstract New radiolarian data from the polymict Excelsior Gulch conglomerate (Wallowa terrane, Blue Mountains Province, Oregon, USA) allow narrowing the age of the chert components. Chert clasts sampled at three distinct localities yielded radiolarians that have Middle to Late Triassic ranges. Individual taxon ranges allow for the distinction of four continuous substages: early Ladinian (presence of Triassocampe nishimurae and T. postdeweveri), middle Ladinian (with Pseudotriassocampe hungarica), late Ladinian (presence of Muelleritortis cochleata and Tritortis kretaensis), and early Carnian (with the co-occurrence of species such as Muelleritortis spp. ex. gr. M. cochleata, Tritortis dispiralis, ?Corum delgado and Triassocingula perornatum). The reworked chert clasts represent Middle to Late Triassic Panthalassan open ocean sediments that have not been described in the Blue Mountains Province. Keywords Excelsior Gulch conglomerate, Wallowa terrane, Blue Mountains Province, reworked Middle-Late Triassic Radiolaria. 1. INTRODUCTION important part of the tectonic and stratigraphic history of the Wallowa terrane. It is notably suspected to be a The Excelsior Gulch conglomerate is the youngest dated key unit to link terranes of the Blue Mountains Province sedimentary unit of the southern Wallowa Mountains, and to better constrain their geodynamic evolution Oregon (Follo, 1986, 1992, 1994). The unit records an (e.g., Follo, 1992). Many aspects of this conglomerate, unusual composition of polymictic conglomerate mainly however, remain a matter of conjecture since both its age consisting of limestone, volcanic and volcaniclastic and source, or sources, are uncertain. rocks, and radiolarian cherts. All observed rock types The conglomerate of the Excelsior Gulch unit, in the commonly form rounded clasts, evidencing that a fluvial southern Wallowa Mountains, were first investigated at transport took place prior to their deposition on the top the top of a ridge, east of Excelsior Gulch, where they of deep-basin argillites of the Hurwal Formation (Follo, directly overlie deep-water basinal sediments of the 1986, 1992). As radiolarian cherts are not found in Hurwal Formation (Follo, 1986). Hurwal sedimentary other parts of the Wallowa terrane, this conglomeratic rocks, with a total thickness of 400-500 m in the southern unit is regarded allochthonous and thought to record an Wallowa (Follo, 1986), are mainly formed by dark grey Submitted October 2017, accepted May 2020 Editorial handling: L. Cavin DOI: 10.5281/zenodo.4465568 566 H. KHALIL et al. to brown argillites, possibly intercalated with calcareous zone) through the middle Norian (M. columbianus zone) siltstone and limestone along with volcanic tuffs (Follo, in North America and north-eastern Asia (McRoberts, 1986). They contain abundant fossils and display sedi- 1993, 1997, 2011). A middle Norian age range would mentary structures like cross bedding, convolute bedding, be in accordance with new data from U-Pb analyses of channel scours and fills with some rip-up clasts. detrital zircons of lower middle Norian age (ca. 215 Ma) Follo (1986, 1992) designated the Excelsior Gulch (Massoll & LaMaskin, 2016). conglomerate as a separate stratigraphic unit and regarded This paper provides new radiolarian data from chert the depositional conditions as deep. Absence of visible clasts of the Excelsior Gulch conglomerate. signs of reworking or unconformity led Follo to describe these (elsewhere erosive) levels as conformable, on the top of deep basinal sedimentary rocks of the Hurwal 2. GEOLOGICAL SETTINGS Formation. Follo reported a well-preserved Halobia sp. cf. H. cordillerana and the belemnoid Aulacoceras sp. The Blue Mountains Province (Fig. 1) extends from from beds above the first chert-rich pebbly sandstones. He central Oregon north-eastward into Idaho as a part used these occurrences to date the base of the Excelsior of the Columbia embayment. It is composed of four Gulch conglomerate as early Norian. However, this age tectonostratigraphic terranes (Brooks & Vallier, 1978; assignment is no longer valid, because H. cordillerana Dickinson & Thayer, 1978). From the most outboard is now known to range from the lower Norian (S. kerri position with regard to the North American Craton, these Fig. 1: Fig. 1.1: Location of the Blue Mountains Province in NW USA. WA, Washington State; OR, Oregon State; ID, Idaho State. Fig. 1.2: Regional geologic map of the Blue Mountains Province. JM, Juniper Mountain; H, Huntington; BC, Baker City; JD, John Day; IM, Ironside Mountain; CMC, Canyon Mountain Complex; DB, Dixie Butte; MI, Mitchell Inlier; BMQ, Black Marble Quarry; BMB, Bald Mountain Batholith; WB, Wallowa Batholith; CH, Coon Hollow; CM, Cuddy Mountains; IB, Idaho Batholith; SC, Sparta Complex; O, Oxbow; SDM, Seven Devils Mountains; PL, Pittsburg Landing; MH, Mountain Home; MCQ, Mission Creek Quarry. Modified from LaMaskin et al. (2011). Rectangle indicates location of Fig. 2. Radiolarian assemblages from chert clasts of the Excelsior Gulch conglomerate 567 terranes are: (i) the Wallowa intraoceanic arc terrane Formation (Follo, 1992). They were informally named the (e.g., Vallier, 1977; Walker, 1986); (ii) the Baker terrane, Excelsior Gulch conglomerate in the southern Wallowa a subduction-accretion complex (Brooks & Vallier, Mountains (Follo, 1986) and the Deadman Lake breccia 1978; Coward, 1983); (iii) the Triassic to Jurassic clastic in the northern Wallowa Mountains (Nolf, 1966). sedimentary succession of the Izee terrane (Silberling The Excelsior Gulch conglomerate has been regarded as et al., 1984), which originally rested depositionally a member of the Hurwal Formation (Follo, 1986). It is an on the Baker terrane (Brooks, 1979; Dickinson, 1979; 80-100 m thick sequence of polymict pebbly sandstones Silberling et al., 1984; Tumpane, 2010); and (iv) the to clast-supported, poorly sorted conglomerate, with Olds Ferry pericratonic arc terrane, which represents the abundant rounded reef-type limestone, argillites and most inboard position of the province (Brooks & Vallier, rounded to subrounded volcanic clasts, along with less 1978; Brooks, 1979). These terranes are discontinuously abundant clasts of coarse-grained intrusive rocks (e.g., exposed, as erosional inliers and intruded by a Late quartzite), metamorphic rocks (e.g., greenstones) and Mesozoic plutonic complex (e.g., Walker, 1977; Orr et radiolarian-bearing cherts (Flügel et al., 1989; Follo, al., 1992). Numerous studies consider that terranes of the 1992). This conglomerate were mapped in isolated Blue Mountains were first located outboard of the area outcrops (e.g., Excelsior Gulch, Red Gulch, Sanger that forms today NW Nevada (Dorsey & LaMaskin, 2007; Gulch, O’Brien and East Eagle Creek) throughout the LaMaskin et al., 2011) with a N-S orientation during the