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NEW BIO- AND MAGNETOSTRATIGRAPHIC DATA AT THE JURASSIC-CRETACEOUS BOUNDARY OF THE CHIGAN СAPE (VLADIVOSTOK REGION, RUSSIA)

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NEW BIO- AND MAGNETOSTRATIGRAPHIC DATA
AT THE JURASSIC-CRETACEOUS BOUNDARY
OF THE CHIGAN ?APE
(VLADIVOSTOK REGION, RUSSIA)
Guzhikov A.Yu.1,
Arkadiev V.V.2,
Baraboshkin E.Yu.3,
Feodorova A.A.4,
Shurekova O.V.4,
Baraboshkin E.E.3,
Manikin A.G.1,
Surinsky A.M.1,
Golozubov V.V.5,
Kasatkin S.A.5,
Nechaev V.P.5
1Chernyshevsky
Saratov State
University, Saratov, Russia,
2Saint
Petersburg State University,
St. Petersburg, Russia,
3Lomonosov
Moscow State
University, Moscow, Russia,
4Research
and Production
Enterprise Geologorazvedka, St.
Petersburg, Russia,
5Far
East Geological Institute FEB
RAS, Vladivostok, Russia,
Vladivostok
42�'37.8''N
123�'47.0''E
The section was supposed to comprise the level corresponding to the Jurassic-Cretaceous boundary in the basement
of the Berriasellajacobi zone (according to the data from Konovalov and Konovalova, 1997; Sej and Kalacheva, 1999).
B. - Buchia, Ph. - Phylloceras, Ps. - Pseudosubplanites, Ber.- Berriasella
We were involved in searching for macrofauna within the section, making geologic
descriptions, selecting paleomagnetic samples and samples for microfaunistic and palynologic
identifications and for thin-section analyses.
There has been no data so far on the Chigan formation acceptability for paleomagnetic or
micropaleontologic determinations. The investigation results were meantto substantiate
conclusions on the prospects of the section further detailed complex examination.
The apparent thickness of
the section makes 33 m; it
extends 500 m northwards
in a series of coastal cliffs.
Paleomagnetic
samples
have been selected from
33 levels of the section.
The
rocks
lie
subhorizontally and are
transected by a series of
Late Cretaceous gabbroids.
Paleomagnetic
samples
were selected from the
gabbro, as well, to use the
back contact test for
grounding the ancient
magnetization nature.
The section is represented by five members
of gray, fine-grained, poorly sorted
sandstones, clayey to various degrees,
intensely burrowed. Ichnofossils and
lithology are typical for the lower shoreface
zone environments (members 1-3 and 5).
5
Polymictic sandstones of lower shoreface
with Schaubcylindrichnus4coronus.
Member 2. Thin-section 1314-1-11, nicols +.
3
2
1
5
4
3
Recrystallised rudstones with quartz
grains and bioclasts of bivalves (Bi) and
crinoids (Cr) in calcite cement.
Member 5. Thin-section 1314-3, nicols +.
2
1
The middle part of the section comprises a 1.5-meterthick interval of massive sandstones of the middle ?
upper shoreface zone; this is limited with erosion
surfaces from above and from below (member 4). The
bottom of this very interval has been conventionally
adopted as the level of the Jurassic-Cretaceous boundary
by the previous researchers (Konovalov, Konovalova,
Sej, Kalacheva).
The sandstones are transgressively overlain with a 10centimeter-thick layer of cross-bedded sandstones from
the base of the member 5. Those are substitutedlaterally
by bioclasticrudstones.
Selected ichnofossils
from Chigan section
1. Schaubcylindrichnus coronus Frey
et Howard, 1981
2. Asterichnus lawrencensis Bandel,
1967
3. Ophiomorpha irregulaire Frey,
Howard et Pryor, 1978
4. Rhizocorallium commune
irregulare (Mayer, 1954)
5. Phycosiphon incertum FischerOoster, 1858
6. Neonereites uniserialis Seilacher,
1960
7. Bichordites monastiriensis Plaziat
et Mahmoudi, 1988
Paleomagnetic samples were also
taken from the exposure 3050, about
1.1 km southwards from Cape Chigan.
This outcrop is represented by rocks of similar
compositions. Konovalov and Konovalova
believe these rocks to be of the same age as
the deposits from Cape Chigan, while Sej and
Kalacheva judge them to be older.
Berriasella cf. jacobi Mazenot, cast.
Pseudosubplanites cf. combesi Le H間arat.
Collection OAO ?Primorgeologiya?, sample 15-1547.
Collection of I.V. Konovalova, 1993. Scale bar = 10 mm.
Collection of V.P. Nechaev, 2014.
Some ammonites were found in the talus in the base of the section: Pseudosubplanites, Berriasella and others.
All those finds characterize the Berriasian Jacobi zone, but don?t answer the question on the position of the Berriasian
bottom level (the Jacobi zone bottom) within the section.
Palynology. Only one of 14
samples examined proved to
comprise
vegetable
detritus,
pollen and dinocysts, with the
common age interval from the
Oxfordian to the Early Valanginian.
The results show that the
possibility
of
acquiring
palynological data from the Chigan
formation still exists.
Generic rate of foraminifers from Chigan formaition
3050
uppermost
3046
3047
3050
lowermost
14 samples were examined for microfauna presence. Those contained assemblages of agglutinated beach/bar
type foraminifers; some taxa were determined down to the species level.
Due to different sedimentation environments, however, it is difficult to correlate the Far East associations, with complexes of zones and intercalations, and
foraminifers from the Spitsbergen, Pechora and Siberia having some species in common.
The Far East association comprises certain foraminifers of the same species as the Siberian ones, characteristic of both, theTaimyrensis zone of the Upper Volgian
substage (16 common species), and of the Buchiavolgensis zone, corresponding to a substantial part of the boreal Berriasian(13 common species).
Thus, the acquired microfaunistic materials are in accord with the ammonite data in terms of assigning the study section interval to the Jacobi zone, but they don?t
specify the deposit age or clarify the position of the Jurassic-Cretaceous boundary within the section.
Paleomagnetic investigations.
In the Cape Chigan section, samples
from 34 stratigraphic levels have been
examined; in exposure 3050 ? from
six ones. Gabbro samples from dikes
have also been studied.
sandstone
gabbro
Thermomagnetic analysis
Thermomagnetic analysis
K (To)
Kt
[E-6]
-100
3048g
1.20E-05
-110
3048g
Js (To)
1.00E-05
-120
8.00E-06
cooling
-130
6.00E-06
-140
-150
2.00E-06
heating
-170
second heating
4.00E-06
pyrrhotine
-160
first heating
0.00E+00
-180
0
-190
0
100
200
300
400
500
600
700
800 T [癈]
200
400
600
800
Magnetic saturation
Magnetic saturation
oxidized pyrrhotine
and (or) magnetite
magnetite
Magnetic saturation
In sandstones, oxidized pyrrhotine made the principal magnetization carrier, in
gabbro - magnetite. Hematite was recorded in the sedimentation-gap zone, peculiar
for intense oxidation of magnetite particles. Pyrrhotineis registered from its phase
transition to magnetite at the temperature of 300?C in the thermomagnetic analysis
curves. Hematite is registered from the lack of magnetic saturation in the fields of
above 700 mTand from high values of remanent coercive force.
hematite
Magnetic cleanings with temperature and alternating field have shown a presence of reverse and normal components. Gabbro samples are practically singlecomponent.
The paleomagnetic column from the Chigan cape section has been formed by the dominantly reverse-polarity zone, complicated with two contiguous narrow
normal-polarity intervals in its middle part. Reverse polarity domination in the section is suggestive of rock remagnetization.
Results of the back contact test
NRM projection on
the lower hemisphere
NRM projection on
the upper hemisphere
The back contact test,
however, is
positive:sandstone layers
are reversely magnetized,
while the transecting
gabbros and sandstones
from the contact zone are
normally magnetized.
Moreover, opposite sign
polarity is reliably registered
in outcrop 3050. These
circumstances provide
solidevidence in favor of
magnetization primariness.
gabbro
contact zone
sandstone
Statistic paleomagnetic characteristics of the Chigan cape section
n
Dav (o)
Iav (o)
?
?95 (o)
(R-) sandstones
38
253.3
-82.1
9.15
8.1
(N+) gabbros and the contact zone
16
297.3
83.8
49.09
5.3
CRITICAL ANGLE 9.5o
The angle between the mean of the N-polarity set and the mean of the R-polarity set is 13.3o
THE REVERSAL TEST IS NEGATIVE
sandstones
Average paleomagnetic
directions in dikesand host
sandstones are close to being
antiparallel, but the test of
reversal is, nevertheless,
negative.
This, however, does not provide
the basis for rejecting the
hypothesis of magnetization
primariness, since the Late
Cretaceous gabbros are tens of
millions years younger than the
gabbros and
the contact zone
Normal polarity has been registered in
section 3050; this confirms Sej and
Kalacheva?s viewpoint on different ages
of deposits from outcrop 3050 and from
Cape Chigan.
On the assumption that all the ammonite finds testify to the Jacobi zone presence in the section, one may suggest that the R-zone
corresponds either to the M18r or to M17r chron. It is not improbable that it should be identified with M18 + M17r.
Stratigraphic correlation from foraminifers theoretically admits the version of R-zone identification with the M17 + M16r chrons. But this should be excluded,
because the previous ammonite finds describing the Jacobi zone (according to the data from Konovalov, Konovalova) come from the uppermost of the section.
All the variants rule out the version of the M18r chron bottom presence in the study section.
All the variants rule out the version of the M18r chron bottom presence in the study section. Thus, it has been determined from paleomagnetic data, that in Cape
Chigan the level corresponding to the Jurassic-Cretaceous boundary lies below the sea level, whether it is determined from the base of the Jacobi zone or from
the bottom of the M18r chron, according to Gradstein et. al., 2012, etc. According to paleomagnetic data, the N-zone that lies lower in the section of the Chigan
formation, most likely corresponds to the M19n chron in the outcrop 3050.
The major result of the above research involves the very
possibility
of
acquiring
paleomagnetic
and
micropaleontologic data from the section under
consideration.
Detecting probable analogues of the M18r and M19n chrons
in the Chigan formation coupled with paleontologic data
holds out a hope of solving the problem of grounding the
Jurassic-Cretaceous boundary in Primorye upon more
complete and thorough examination of the Chigan formation
section.
In view of rare macrofaunistic finds and difficult correlations
from foraminifers, the paleomagnetic criterion ? the base of
the M18n chron ? might constitute the most preferable
marker of the Jurassic-Cretaceous boundary in Primorye.
Thank you for attantion!
Автор
Nastya
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