OSTRACODS FROM THE LOWER MEMBER OF THE DEVILS GATE FORMATION (FRASNIAN) IN THE TYPE LOCALITY (NEVADA)

RELATION WITH THE ALAMO EVENT

by

Jean-Georges Casier, Ewa Olempska, Ivan Berra, & Alain Préat

Royal Belgian Institute of natural SciencesUniversity of Brussels - Polska Akademia Nauk

FRFC 2.4501.02 of FNRS and 3P04D 040 22 of CSR

Introduction.

The Alamo Event was responsible for the deposition of a huge carbonate mega-breccia, in the eastern part of the Great Basin (Fig. 1). This mega-breccia dated early Frasnian, and more precisely punctata conodont Zone (Sandberg & Warme, 1993), is distributed in a semi-circular pattern, with an average thickness of 10 m, covers an area of over about 100,000 km², and contains a rock volume that may be as great as 1,000 km³ (Morrow et al., 1998; Warme, 2004). The presence of a weak iridium anomaly (Warme & Sandberg, 1995), of shocked quartz (Leroux et al., 1995), of carbonate spherules and tsunami deposits (Warme & Kuehner, 1998) associated to this mega-breccia have more and more supported the idea that the Alamo Event was the result of an extraterrestrial impact. More recently, and because of the number of other Late Devonian impacts, Sandberg et al. (2002) have suggested that the Alamo impact may have been part of a comet shower.

Fig. 1 - Paleogeographic map showing the distribution of the Alamo mega-breccia on the ramp (Zone 1), on the outer carbonate platform (Zone 2) and on the intertidal carbonate platform (Zone 3) (from Sandberg et al., 2003). Location of the Devils Gate section.

The presence of entomozoacean ostracods in the Alamo mega-breccia on the west slope of the Tempiute Mountains (Casier in Sandberg et al., 1997), in the punctata Zone in Belgium (Bultynck et al., 2001) and probably at the same level in Algeria (Casier, 1983), had suggested that the Alamo Event could be related to an hypoxic event. In reality, entomozoacean and (or) cypridinacean ostracods belong to the Myodocopid Mega-Assemblage (Fig. 2), a proxy for hypoxic events (Casier, 2004). These events are frequently related to period of extinction, e.g. close to the Frasnian/Famennian boundary (Fig. 3).

Fig. 2 – Ostracod Mega-Assemblages recognized in the Upper Paleozoic. The Myodocopid Mega-Assemblage indicative of hypoxic water conditions is characterized by entomozoid (eg Fig. 3, n° 12-15) and (or) cypridinoid (eg Fig. 3, n° 11) ostracods.

Fig. 3 - Generalized section of the Frasnian - Famennian boundary beds and their ostracod faunas in the type area (Dinant Basin, Belgium). Most representative ostracod species recovered are shown in successive faunas. The succession of assemblages around the Frasnian - Famennian boundary (= FFB) suggests that the Late Devonian mass extinction was principally linked to a regressive-transgressive cycle, and to an anoxic event. The transgression was progressive and culminated below the FFB as suspected by JOHNSON et al. (1985); the regression was more rapid and important. O.M.Z. = oxygen minimum zone. The survival of several species and among them of Ovatoquassilites avesnellensis (LETHIERS, 1973) (5), and of the cypridinacean Palaeophilomedes neuvillensis CASIER, 1988 (11) shows that very shallow marine environments and surficial waters were not touched during the Late Devonian mass extinction. Stacked valves (8) are indicative of very shallow water conditions (CASIER, 2004).

The Devils Gate section.

In order to contribute to the knowledge of the Alamo Event, we have undertook the study of ostracods present in the Lower Member of the Devils Gate Fm in the type locality. The Devils Gate section is located on US highway 50, and 13 km west of the historic mining town Eureka, in the Eureka County (Fig. 1).

For Sandberg et al. (1997) the Alamo Event is probably related in the lower member of the Devils Gate Fm, to a 29-cm thick bed of slightly sandy, graded, calcareous diamictite with granule-size carbonate clasts and in the top 8-10 cm, parallel laminations and tepee structures (Fig. 4).

Fig. 4 – The Devils Gate section. Location of the bed related to the Alamo Event.

Ostracod study.

Almost 2,000 ostracods have been extracted from 36 samples collected in the lower member of the Devils Gate Fm (Fig. 5), and 26 taxa have been identified (Figs. 6-8). They belong exclusively to the Eifelian Mega-Assemblage indicative of shallow environments. No ostracod indicative of hypoxic water conditions (Myodocopid Mega-Assemblage) or of deep and maybe colder environments (Thuringian Mega Assemblage) has been found.

Fig. 5 - Lithology of the Lower Member of the Devils Gate Fm, and positions of ostracod samples. Other columns indicate the conodont zones of Sandberg et al. (2003), the paleoenvironmental interpretation, and the microfacies evolution.

DEVILS GATE 4 6 A3 A12 A15 A17 21b 21a 21 302 20 19c 19b 19a 19 102 18 17 105 106 15

Voronina? eureka n. sp. • • Knoxiella? sp. A, aff. rauseri • • ? • • • Bairdia sp. A • Westmontia? sp. ? Acratia sp. D, aff. paraschelonica • Rozhdestvenskayites sp. A • Microcheilinella sp. A • ? Cytherellina sp . A • • • • Acratia sp. indet ? • • • • Platycopina indet. • Acratia sp. B • • • • • • • • ? • Pachydomellidae? indet. • Plagionephrodes cf. medius • • Knoxiella sp. C, aff. ornata • • • Knoxiella? rauseri • • • • • • • • Westmontia devilensis ? • • • • • ? • Acratia sp. A • • • • • • Knoxiella sp. B, aff. ornata • Youngiella cf. mica • • • • • • ? Coelonella sp. A • Bairdiacypris cf. martinae • Acratia sp. C • • Bairdia sp. • • Cryptophyllus sp. A • Elliptocyprites lorangeri • • Bairdia sp. B • Serenida dorsoplicata n. sp. • • Amphissites sp. A •

Below the 29-cm thick bed related to the Alamo Event ostracods are present only in 4 out of 17 samples (Fig. 6). Their rarity, the relative abundance of platycopids, and the monospecificity are indicative of semi-restricted water-conditions. The absence of ostracods in the majority of samples could indicate real stressful lagoonal conditions.

Above the bed related to the Alamo Event, ostracods are present in every samples with only one exception. Platycopids indicative of shallow environments are relatively abundant and diversified, however the greater abundance of podocopids is indicative of stronger marine influence. The environment was always very shallow becoming progressively more marine.

Fig. 7- Ostracods present in the lower member of the Devils Gate Fm. A. Amphissites sp A; B,C. Youngiella cf. mica ROZHDESTVENSKAJA, 1972; D. Rozhdestvenskayites sp. A; E. Coelonella sp. A; F,G. Knoxiella? rauseri (TSCHIGOVA, 1977); H. Knoxiella? sp. A, aff. rauseri (TSCHIGOVA, 1977); I. Knoxiella? sp. B, aff. ornata LETHIERS, 1981; J. Knoxiella? sp. C, aff. ornata LETHIERS, 1981; K,L,M. Serenida dorsoplicata n. sp.; O,P. Voronina? eureka n. sp.; Q. Westmontia devilensis CASIER & LETHIERS, 1997.

Fig. 8 - Ostracods present in the lower member of the Devils Gate Fm. A,B. Westmontia devillensis CASIER & LETHIERS, 1997; C. Platycopina indet.; D. Plagionephrodes cf. medius (LORANGER, 1963); E. Microcheilinella sp.; F Pachydomellidae? indet.; G. Elliptocyprites lorangeri LETHIERS, 1981; H. Cytherellina sp. A; I. Bairdiacypris cf. martinae CASIER & LETHIERS, 1997; J. Bairdia sp. A; K. Bairdia sp. B; L. Acratia sp. A, aff. sp. 42 sensu BRAUN (1967); M,N. Acratia sp. B, aff. sp. 271 sensu BRAUN (1967); O. Acratia sp. C, aff. sp. 274 sensu BRAUN (1967); P. Cryptophyllus sp. A, aff. sp. 11 sensu BRAUN (1967).

Conclusions.

The rarity and poor diversification of ostracods in samples collected below the bed related to the Alamo Event are not favorable to display prominently any extinction close to this event. Nevertheless their greater abundance and diversity above this bed seems to indicate the absence of extinction in this shallow setting.

The ostracod fauna displays affinities with fauna described from Western Canada and, however in a smaller extent, from the Russian Platform. The exchange with western Europa, North Africa and the eastern part of the US were very reduced or non-existent.

Bibliography:

Bultynck, P., Casier, J-G., Coen-Aubert, M. & Godefroid, J., 2001. Pre-conference field trip (V1): Couvin-Philippeville-Wellin area, Ardenne (May 11-12, 2001). Field trips guide book (V1) of the 15th International Senckenberg Conference, Joint meeting IGCP/SDS: 1-44.

Casier, J-G., 1983. Les Ostracodes du Frasnien et de la base du Famennien de la coupe du Km 30 (Saoura, Sahara algérien). Bulletin de la Société belge de Géologie, 91 (1982) (4): 195-207.

Casier, J-G., 2004. The mode of life of Devonian entomozoacean ostracods and the Myodocopid Mega-Assemblage proxy for hypoxic events. Bulletin de l'Institut royal des Sciences naturelles de Belgique, Sciences de la Terre, 74 suppl.: 73-80.

Leroux, H., Warme, J. & Doukhan, J-C., 1995. Shocked quartz in the Alamo breccia, southern Nevada: Evidence for a Devonian impact. Geology, 23: 1003-1006.

Morrow, J., Sandberg, C., Warme, J. & Kuehner, H-C., 1998. Regional and possible global effects of sub-critical late Devonian Alamo impact Event, Southern Nevada, USA. Journal of the British Interplanetary Society, 51: 451-460.

Sandberg, C., Morrow, J., Poole, F. & Ziegler, W., 2003. Middle Devonian to Early Carboniferous event stratigraphy of Devils Gate and Northern Antelope Range sections, Nevada, U.S.A. Courier Forschunginstitut Senckenberg, 242: 187-207.

Sandberg, C., Morrow, J. & Warme, J., 1997. Late Devonian Alamo impact Event, global Kellwasser Events, and major eustatic events, eastern Great Basin, Nevada and Utah. Byu geology studies, 42 (1): 129-160.

Sandberg, C., Morrow, J. & Ziegler, W., 2002. Late Devonian sea-level changes, catastrophic events, and mass extinction. Special paper Geological Society of America, 356: 473-487. Sandberg, C. & Warme, J., 1993. Conodont dating, biofacies, and catastrophic origin of Late Devonian (early Frasnian) Alamo Breccia, southern Nevada. Abstracts with Programs Geological Society of America, 25 (3): 77.

Warme, J., 2004. The many faces of the Alamo impact breccia. Geotimes, 49 (1): 26-30.

Warme, J. & Kuehner, H.-C., 1998. Anatomy of an anomaly: The Devonian catastrophic Alamo impact breccia of southern Nevada. Contribution Lunar and Planetary Institute, 40: 189-216.

Warme, J. & Sandberg, C., 1995. The catastrophic Alamo breccia of southern Nevada: Record of a Late Devonian extraterrestrial impact. Courier Forschunginstitut Senckenberg, 188: 31-57.

Author addresses:

Casier, Jean-Georges, Dept. Paleontology, Belgian Royal Institute of natural Sciences, Vautier street, 29, B-1000, Brussels, Belgium. casier@naturalsciences.be;

Olempska, Ewa, Instytut Paleobiologii PAN, ul. Twarda 51/55, PL-00-818 Warszawa, Poland. olempska@twarda.pan.pl;

Berra, Ivan, Dept. Earth Sciences and Environment, University of Brussels, F.D. Roosevelt Av., 50, B-1050, Brussels, Belgium. iberra@ulb.ac.be

Préat, Alain, Dept. Earth Sciences and Environment, University of Brussels, F.D. Roosevelt Av., 50, B-1050, Brussels, Belgium. apreat@ulb.ac.be