in Jurassic and Cretaceous Stratigraphy

Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321
at Yen Tu is divided into three parts. No fossils have
been found in the lower part, but based on the plant
fossils from the middle part, the middle and lower parts
are placed together to Late Triassic Norian–Rhaetian.
The upper part, composed of coarse grained sediments,
mainly “Chua Dong conglomerate”, which has a thick-
ness of about 300 m, is placed to Early Jurassic (J1).
Thus, the section of the Hon Gai Formation here is
1150-1300 m thick and is aged Late Triassic (Norian) -
Early Jurassic (T3n-J1hg)
Correlation with coal-bearing sediments of
Nong Son Formation
Many geoscientists have proposed the similarities
between the Upper Triassic coal-bearing sediments in
two areas: Yen Tu (in Uong Bi town, Quang Ninh
Province, NE of Bac Bo) and Nong Son (Que Son and
Dai Loc districts, Quang Nam Province, North of Trung
Bo). Both of them are continental sediments, and
contain the same plant fossil assemblage belonging to
the “Hon Gai flora” aged Late Triassic (Norian-
Rhaetian). Both of them contain anthracite.
The section of coal-bearing sediments from Thach
My to Ngoc Kinh coal mine in Nong Son has been
described (Nguyen Chi Huong, 1983; Bui Phu My and
Hoang Dinh Kham, 1999). With the new data and
cognition, the section of the Nong Son Formation can
be divided into three parts.
Lower part can be divided into two members:
Member 1, overlying unconformably the Paleozoic
limestone of Thach My Formation, is basal pebble
conglomerate, with the pebbles mainly composed of
quartz, quartzite, some of granite which is similar to the
granite of Dai Loc complex, poorly rounded, with
cement of light brown sandstone, with thickness 10-15
m. Further upward are light brown thick-bedded
sandstone and siltstone intercalated with gravelite, with
thickness of 550 m. In some thin intercalated layers of
siltstone are met poorly preserved plant remains which
are hard to do identification. Member 2 composed of
pebble conglomerate, cobble conglomerate intercalated
with some layers of thick and medium-bedded fine-
grained sandstone, siltstone with light brown color,
with thickness of 205 m. In this part no fossils have
been found. The common thickness of the lower part is
about 360 m.
Middle part: It consists of thin to medium bedded,
in some places cross-bedded medium to coarse grained
sandstone of light gray color, intercalated with siltstone,
clay shale of dark gray color, few lenses of pebble con-
glomerate of coarse quartz pebbles. This part contains
3-5 industrially minable coal seams, and in some places
the coal seams are up to 30m thick. The coaly shale
layers near the coal seams contain some plant fossils
belonging to the "Hon Gai flora", including Claphro-
pteris obovata, Dictyophyllum nathorsti, Podozamites
distans, P. lanceolatus, Ptilozamites vennis, Goepper-
tella vietnammica, Cycadocarpidium erdmanni, Clado-
phlebis raciborski aged Late Triassic (Norian–Rhaetian
132
collected and determined by Nguyen Chi Huong, 1983).
At the bank of Khe Tre stream, 150 m from the mouth
of the stream, the clay layers intercalated with
conglomerate and coarse sandstone between thin coal
seams yields many plant fossils: Thaumatopteris re-
mauryi, Neocalamites hoerensis, Clathropteris mongu-
gaica, Cladophlebis (Todites) shensiensis, Equisetites
sarrani, Squamiofolium dictyoneruum, Palissya brun-
nii, Yuccites vietnamensis, Taeniopteris cf. ensis
(collected by Bui Phu My, Hoang Dinh Kham, 1999,
determined by Nguyen Chi Huong).At the same loca-
lity, Nguyen Chi Huong (1983) collected and deter-
mined plant imprints of Neocalamites carrerei,
Clathropteria longilobata, Dictyophyllum, nathorstii,
D. muensteri, Thaumatopteris remauryi, Podozamites
lanceolatus, P. reini, Pterophyllum bavieri, P. sp.,
Hausmania sp., Ferganiella sp and Bui Duc Thang
(1981) found pollens of gymnosperm plant Classopollis
sp. The thickness of the middle part as it is exposed
along the road in the area of Ngoc Kinh coal mine is
350 m and along the Khe Tre stream from its mouth
upstream is 150 m; the common thickness is 500 m.
In summary, the section of the coal-bearing
sediments in Nong Son is also divided into 3 parts as
the section in Yen Tu. But as no fossils have been found
in the lower part, the middle and lower parts are placed
in Late Triassic (Norian–Rhaetian) based on the plant
fossils from the middle part.
Upper part: The upper part changes transitionally
from the middle part, composed of coarser grained sedi-
ments, mainly conglomerate, containing fossils of
Classopollis sp etc., extending from the outcrop on the
bank of Khe Tre stream, 150 m from its mouth to the
water divide, with a thickness of 360 m
Bui Phu My once compared the Khe Tre conglo-
merate with the Bao Dai–Yen Tu conglomerate (which
is now called Chua Dong conglomerate). In this part of
the section there are no fossils, but on the top of Ban Co
mountain, the similar conglomerate yield plant fossils
belong to the “Hon Gai flora”. The new finding of
Coniopteris, Classopollis in the Chua Dong conglo-
merate at Yen Tu, and especially the presence of
Classopollis in the Nong Son coal-bearing sediments
agree with Bui Duc Thang’s remarks (1981) “Although
the number of analyzed samples has been very small (3
samples), the large amount Classopollis pollens allows
to initially think about the Liassic element (J1) in the
uppermost part of the Nong Son coal-bearing sedi-
ments”. Further detailed studies will surely give more
data to clarify this issue.
Correlation with suoi bang coal-bearing sedi-
ments
The Suoi Lao key section (Vu Khuc and Nguyen
Vinh, 1967) can be divided into 3 parts.
Lower part consists of 2 members: Member 1
composed of siltstone, bedded gray shale, with
intercalations of gray marl, shell limestone, shelly
calcareous sandstone, intercalated calcareous sandstone,