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