OSTRACODS FROM THE LOWER MEMBER OF THE DEVILS GATE

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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 Sciences University of
Brussels - Polska Akademia Nauk FRFC
2.4501.02 of FNRS and 3P04D 040 22 of CSR
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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.
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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.
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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).
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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.
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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.
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• 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.

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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.
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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).
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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.
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Author addresses Casier, Jean-Georges, Dept.
Paleontology, Belgian Royal Institute of natural
Sciences, Vautier street, 29, B-1000, Brussels,
Belgium. casier_at_naturalsciences.be Olempska,
Ewa, Instytut Paleobiologii PAN, ul. Twarda
51/55, PL-00-818 Warszawa, Poland.
olempska_at_twarda.pan.pl Berra, Ivan, Dept. Earth
Sciences and Environment, University of Brussels,
F.D. Roosevelt Av., 50, B-1050, Brussels,
Belgium. iberra_at_ulb.ac.be Préat, Alain, Dept.
Earth Sciences and Environment, University of
Brussels, F.D. Roosevelt Av., 50, B-1050,
Brussels, Belgium. apreat_at_ulb.ac.be