1267
ISSN 1028-334X, Doklady Earth Sciences, 2006, Vol. 411, No. 8, pp. 1267–1271. © Pleiades Publishing, Inc., 2006.
Original Russian Text © A.V. Maslov, Yu.L. Ronkin, M.V. Isherskaya, O.P. Lepikhina, 2006, published in Doklady Akademii Nauk, 2006, Vol. 410, No. 6, pp. 799–803.
The Shakapovo–Shikhan Depression—one of the
largest in the eastern part of the East European Plat-
form—is filled with Upper Vendian sedimentary rocks
of the Bizhbulyak Complex about 1.6–1.8 km in total
thickness. The depression is framed by the Orenburg,
Tatar, and Komi–Permyak arches in the southwest,
west, and northwest, respectively. These arches are
composed of the Archean (~80% of total area of ancient
drainage basin [2]) and Paleoproterozoic crystalline
rocks. According to the seismostratigraphic data [3],
the Upper Vendian sedimentary rocks give way east-
ward to the coeval Asha Group of the Bashkir Anticli-
norium. The transition occurs via a system of fold–
thrust dislocations.
Different aspects of the formation of the Upper Ven-
dian rocks in the Shkapovo–Shikhan Depression were
considered by E.M. Aksenov, M.M. Aliev, M.M. Balash-
ova, Yu.R. Bekker, T.V. Belokon, I.K. Chepikova,
Z.P. Ivanova, T.V. Ivanova, A.A. Klevtsova, N.S. Laguten-
kova, R.Kh. Masagutov, S.G. Morozov, L.D. Ozhiganova,
I.E. Postnikova, and other researchers. According to their
data, the Archean garnet and biotite–plagioclase gneisses
of the Tatar Arch served as the main source of clastic mate-
rial during the deposition of the Baikibashevo and Staro-
petrovo formations [4, 5, and others]. Furthermore, it is
suggested that the Salikhovo and Karlin times were
marked by the transport of clastic material from the east,
where the Riphean, Lower Vendian, and initial Upper Ven-
dian metasedimentary rocks were eroded [4, 6]. This inter-
pretation was based on mineralogical and petrographic
data.
The systematics of rare earth elements (REE) and
estimation of the Nd model age for the fine-grained ter-
rigenous rocks of the Bizhbulyak Complex allowed us
to get principally new information on the composition
of rocks, their relationships in the paleodrainage basin,
and the contribution of the western and eastern prove-
nances to the formation of the sedimentary sequence of
the Shkapovo–Shikhan Depression in the Late Vendian.
The Bizhbulyak Complex comprises (from bottom
to top) the Baikibashevo, Staropetrovo, Salikhovo, and
Karlin formations [1]. The Baikibashevo Formation
(10–85 m thick) is composed of sandstone, gravelstone,
and fine-pebble conglomerate, as well as silty sand-
stone and silty argillite mainly occurring in its upper
portion. The Staropetrovo Formation (85–320 m) con-
sists largely of siltstone and argillite with a variable
amount of sandy material as separate interbeds and
members or dissemination. The Salikhovo Formation
(100–450 m) comprises sandstone and siltstone with
subordinate argillite interbeds. The Karlin Formation
(0–600 m) is made up of argillite with interbeds and
packets of lighter colored siltstone and sporadic sand-
stone. The K–Ar age of glauconite from silty sandstone
of the Staropetrovo Formation is ~595 Ma. Glauconite
from sandstone and siltstone of the Basa Formation was
dated with the same method at 600–557 Ma [7]. In the
unified stratigraphic chart of Vendian sequences in the
Volga–Ural province, the Baikibashevo Formation is
correlated with the Uryuk Formation; the Staropetrovo
Formation, with the Basa Formation; the Salikhovo
Formation, with the Kukkarauk Formation; and the
Karlin Formation, with the Zigan Formation of the
Bashkir Anticlinorium [1].
More than 40 argillite samples that represent all for-
mations of the Upper Vendian were taken from cores of
boreholes Kipchak-1, Akhmerovo-6, and Severokush-
kul-1 (Fig. 1) for isotopic and geochemical studies.
After macroscopic and microscopic examination of the
samples, we determined the contents of major oxides in
them and omitted the samples for which the data points
did not fall into the shale field in the
iO
2
/Al
2
O
3
)–
e
2
O
3
/K
2
O)
diagram proposed by Herron [8]. In
the remaining 23 samples, the REE contents were
determined with acid decomposition and subsequent
S(log
F(log
REE Systematics and Nd Model Age of Upper Vendian Argillites
of the Shkapovo–Shikhan Depression
Corresponding Member of the RAS
A. V. Maslov
a
, Yu. L. Ronkin
a
,
M. V. Isherskaya
b
, and O. P. Lepikhina
a
Received March 6, 2006
DOI:
10.1134/S1028334X06080241
a
Zavaritsky Institute of Geology and Geochemistry,
Uralian Division, Russian Academy of Sciences,
Pochtovyi per. 7, Yekaterinburg, 620151 Russia;
e-mail: maslov@igg.uran.ru
b
Institute of Geology, Ufa Scientific Center,
Russian Academy of Sciences, ul. Karla Marksa 16/2,
Ufa, 450000 Russia
GEOCHEMISTRY