Plate 1. Conodonts
fossils
1-4, 6. Ozarkodina
(Spathognathodus) sp.
5. Palmatolepis sp., a- upper side views; b- lateral side views
The line fragments are equal to 0.1 mm
ON THE AGE AND GE
OF THIÊN NHẪN FORMATION IN
SHIGEKI HADA1, TRẦN VĂN TRỊ2,
ĐOÀN NHẬT TRƯỞNG3
1Research Institute for Higher Education,
Kobe University, Kobe, Japan
2Việt Nam Union of Geological Sciences, 6 Phạm Ngũ Lão, Hà Nội
3Research Institute of Geology and Mineral Resources, Thanh Xuân, Hà
Nội
Abstract: The Thiên Nhẫn Formation occuring in
During recent years, the collaboration between the
Department of Geology and Minerals of Việt
I. LECTOSTRATOTYPE
The Thiên Nhẫn Formation was established by Mareitchev A.
and Zhamoida A. in 1965 [1], named by the name of Thiên Nhẫn Mt situated in
northeast of the Hương Sơn District, Hà Tĩnh Province, and supposedly dated as
Early-Middle Triassic, but its stratotype was not indicated. This formation
occurs in a narrow band of 3-6 km in width, extending in NW-SE direction in the
west of the Thanh Chương and
During recent years, the authors of this paper had
supplementary surveys and established the lectostratotype for the formation
along the road from Yên Vực (18°34’30” N; 105°33’12” E) situated in the west of
the former Road No.15A, extending through the Thiên Nhẫn Mt to Thịnh Hương
(18°33:15” N; 105°31’45” E), situated in the left side of the Ngàn Phố River,
Hương Sơn District, Hà Tĩnh Province. The formation in this section was crosscut
by many fault, and complicatedly folded and deformed, therefore, it is difficult
to recognize the lowermost and uppermost parts of the section. However,
according to the attitude and lithological composition of the beds one can
distinguish 3 parts of the formation as follows.
Lower part: light-grey, dull-white but
violetish-brown, dark-yellow by weathering argillaceous-cherty shale, cherty
shale and cherty mudstone rhythmically interbedded with each interbed of some
to some tens cm in thickness. In this part, locally there still are interbeds
(4-7 cm) of siltstone and fine-grained sandstone. Thickness: over 200 m.
Plate 1. Conodonts
fossils
1-4, 6. Ozarkodina
(Spathognathodus) sp.
5. Palmatolepis sp., a- upper side views; b- lateral side views
The line fragments are equal to 0.1 mm
Middle part: mainly of dark-grey, light-grey,
thin- and parallel-bedded cherty shale, chert, rhythmically interbedded with
argillaceous-cherty shale, usually broken into angular fragments or small
plates. Particularly, in many places there still are interbeds of cherty,
argillaceous-cherty breccia or manifestations of manganese hydroxide and
iron-manganese oxide. Thickness: about 350 m.
Upper part: greenish-grey
argillaceous-cherty shale rhythmically interbedded with dull-white, pinkish,
parallel-banded cherty mudstone; besides there still are interbeds of dark-grey
cherty limestone (30-40 cm), cherty breccia and manganese manifestations.
Thickness: over 250 m.
The total thickness of the formation in this section is
about 800 m.
II. PALEONTOLOGICAL
Fossils in the Thiên Nhẫn Formation were found only in
recent years. They are almost deformed, recrystallized and broken into small
fragments, among them radiolarians are widespread together with some conodonts
that are difficult for specific determination, however, among them some can be
determined.
Plate 2. 1-4. Radiolarian
fossils
The
line fragments are equal to 0.05 mm
The sample 020818 (18°30’50” N; 105°34’10” E by GPS)
consists of cherty shale collected from exposures situated in the southern talus
of the Road No. 8, extending from the Linh Cảm Bridge to Châu Sơn, Châu Lĩnh
villages, Tùng Ảnh Commune, Đức Thọ District, Hà Tĩnh Province, containing the
conodont Ozarkodina
(Spathognathodus) sp. (Photo 1-4, 6, Plate 1) of from Late Silurian to
Carboniferous age; especially the specimen 1 is similar to Ozarkodina semialternans of
Middle-Late Devonian age. The Photo 5 is Palmatolepis sp.
having a large, asymmetrical platform with the curved, but still preserved main
tooth, and morphologically similar to Palmatolepis glabra lepta of Late Devonian
age (Plate 1). Besides, there still are many radiolarians, but of bad
preservation.
Figure 1. Hill scarp situated in the south of Road
No.8, in 3 km east of the
Photo:
Figure 2. Hill scarp situated in 200 m southeast of
the
Photo:
The sample 020818-8 (18°30’50” N; 105°34’25” E by GPS)
situated in 500 m east of the exposure 020818-4 and in 300 m south of the Road
No.8, collected from a cherty shale scarp in the hill slope, contains also many
conodont fossils and, especially, very many radiolarian remains, but also of
bad preservation, that does not allow to determine to genus and species (Plate
2).
Also in this road section, the American CONOCO Inc. in
collaboration with the Department of Geology and Minerals of Việt Nam has been
collecting the chert sample 12/15/1 in the road talus (18°31’ N; 105°35’ E)
containing in abundance radiolarian remains of very bad preservation, but
according to Stanley A. Kling, California, USA some specimens can be determined
as of the Entactiniidae family of from Devonian to Permian age, and some
specimens can be determined as of the Albaillidae family of from Mississippi to
Permian age, and the common age of host rocks can be Middle-Late Paleozoic.
III. GEOLOGICAL RELATIONS
The lower and upper boundaries of the Thiên Nhẫn
Formation have not been observed in any locality yet, because its contact with
other formations was destroyed by faults. According to Zhamoida and Mareitchev
[1] in the west of the Vũ Liệt Village one can suppose that there is normal
attitude of argillaceous shale of the Ladinian Quy Lăng Formation upon cherty
shale of the Thiên Nhẫn Formation. Besides, in the north and east of the Height
497 the Thiên Nhẫn Formation rests unconformably upon sandstone and shale of the
Sông Cả Formation (O-S sc); as
for in the south and northwest of
IV. GEOLOGICAL SETTINGS
The lithological composition of the studied formation
consists mainly of thin-bedded cherty shale rhythmically interbedded with
parallel-banded argillaceous-cherty shale, cherty mudstone, and in some places
there are the beds containing manifestations of manganese hydroxide or
manganese nodules. Besides, there still are cherty breccias of sedimentary
melange type and thin interbeds or lenses of calcareous-cherty shale.
These sediments form a complex of turbidite flysch
containing radiolarians and conodonts of the pelagic environment of the
Việt-Laos branch basin of the PaleoTethys lying between the
V. AGE AND GEOLOGICAL CORRELATION
The Thiên Nhẫn Formation long ago has been considered as
a dumb formation and referred by the authors to different age, such as
Silurian-Devonian [2, 8], Triassic [1, 7] and Carboniferous [5, 9].
The recent fossil finding includes abundant collection
of radiolarians and conodonts. Although the fossils are in deformed situation,
they still allow to determine the conodonts Ozarkodina (Spathgnathodus) sp., O. semialternans. And
especially Palmatolepis sp.
morphologically close to Palmatolepis glabra lepta of Late Devonian
age, possibly Frasnian.
In the southwest of the studied area, on the territory of
the Tuyên Hóa District, Quảng Bình Province there is the Ngọc Lâm Formation
including also thin-bedded, banded cherty shale, argillaceous-cherty shale with
interbeds of manganese hydroxide containing the conodonts Palmatolepis
subrecta, Calvinaria cracowiensis, C. cf. megistanus of Frasnian age,
lying unconformably upon the Động Thờ Formation (D2gv-D3fr
dt) and
under the Xóm Nha Formation (D3 xn) [6]. As for in
the north, in the area of Cốc, Phùng Giao and Lương Ngọc villages, Thanh Hóa
Province, occurs also cherty shale interbedded with argillaceous shale, chert
containing manganese supposedly referred to Upper Permian - Lower Triassic or Upper Devonian [4]. Based on
lithological and paleontological characteristics the Ngọc Lâm Formation and
cherty shale at
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