in Jurassic and Cretaceous Stratigraphy

Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321
ribbon. The centre of the depression is covered by
Quaternary deposits and contemporaneous sea water. In
the northwest limb and on the Cai Chien, Vinh Thuc
Islands, the Ha Coi Formation covers with angular
unconformity the Tan Mai Formation (O2-S1tm), while
in the southwest and south it rests unconformably upon
or has tectonic contact with Upper Triassic coal-bearing
formation.
The Ha Coi Formation (Zhamoida A.I., 1962) was
named in accordance to the geographic name of the
centre of the area and the Ha Coi Bay in Mong Cai.
Based on Liassic fossils collected in its lower part it
was firstly dated as Undifferentiated Jurassic. In the
geological mapping at medium scale (1: 200,000) and
in thematic studies, it has been subdivided into two
(Nguyen Cong Luong et al., 2001; Fir.1) or even three
(Nguyen Xuan Khien et al., 2003) subformations. New
collections of fossils have allowed geologists to date
the formation more precisely as Early-Middle Jurassic.
Geological and stratigraphic characteristics of
these continental red beds have been described by some
works (Zhamoida A.I., 1962; Vu Khuc and Bui Phu
My,1988; Bui Cong Luong et al., 2000; Nguyen Xuan
Khien et al., 2003; etc.), but there are still lots of unclear
problems related to characteristics of lithological facies,
forming conditions and sedimentary composition. Our
study is aimed to contribute in the clarification of these
problems, then to establish paleogeographic chara-
cteristics of the basin, where there are the famous
World Natural Heritage Ha Long Bay.
II. Stratigraphic characteristics of the Ha Coi
Formation
The most sufficient section of the Ha Coi
Formation has been observed on the Ban Chat - Ban
Thin Road and on the talus of the Tan Mai - Ha Coi Car
Road. It is composed of 3 following parts (Nguyen
Xuan Khien et al., 2003, with supplementation and
adjustment of the authors):
(1) Lower part: including mainly coarse-grained
sediments, which were formerly described by French
geologists as “Continental Sandstone” or “Upper Sand-
stone”. It is composed of 4 members:
Member 1: Thick-bedded quartz sandstone with
some interbeds of white-pink sandstone and violetish
siltstone; 50 m thick.
Member 2: White-grey medium- to coarse-
grained quartz sandstone with some interbeds of fine
sandstone, red-brown siltstone and claystone; 200 m
thick.
Member 3: Red-brown siltstone with some dark
grey beds due to the content of black or black-brown
coaly shale, lenses of coaly shale, marl and calcareous
siltstone; 30 m thick. Yielding leaf imprints of Anomo-
zamites sp., Equisetites sp., and Neocalamites sp.
Member 4: White-grey quartz sandstone con-
taining poorly preserved gastropod fossils with some
interbeds of red-brown siltstone and calcareous
siltstone ; 110 m thick.
The total thickness of the lower part is about 390
m.
(2) Middle part: Mainly of the intercalation
fine-grained sediments (siltstone, claystone, calcareous
clayey siltstone) with some interbeds of sandstone with
4 members:
Member 1: Red-brown siltstone interbedded with
fine- to medium-grained sandstone and some lenses of
yellow-greenish marl; 10 m thick.
Member 2: Red-brown siltstone and calcareous
siltstone with some interbeds of cross-bedded fine
sandy siltstone; 82 m thick.
Member 3: Brown-violetish, cross-bedded siltstone
and calcareous silty claystone interbedded with
lighter coloured fine-grained sandstone; 120 m thick.
Member 4: Brown-violetish siltstone interbedded
with reddish sandstone; 160 m thick.
The total thickness of the middle part is 372 m.
Near the Cai Lam Village, leaf imprints have been collected
in this part, such as: Coniopteris sp., Equisetum
sp., Podozamites sp. together with the phyllopods
Amussia sp., Baidestheria sp. and spores and pollens
Ginkgo sp., Benettites sp., Protoconiferus sp., Chomotriletes
sp., and Laevigatosporites sp.. Among these
fossils, Coniopteris sp. is usually considered as characterizing
Liassic.
(3) Upper part: consisting of an intercalation of
fine- and coarse-grained sediments with 2 members:
Member 1: Quartz sandstone containing small
calcareous concretions, light violet-brown calcareous
sandstone interbedded with thin-banded silty sandstone
of the same colour, 116 m thick.
Member 2: Light red-brown calcareous siltstone
with some interbeds of thin-banded fine-grained sandstone,
190 m thick.
The total thickness of the upper part is 306m, and
the total thickness of the Ha Coi Formation in this
section reaches 1068 m.
In the foot of the Trang Vinh Dam (Mong Cai,
Quang Ninh Province) and in the Ban Chat - Po Hang
earth road (Dinh Lap, Lang Son Province), Nguyen
Xuan Khien et al. (2004) have dicovered in violetishbrown
calcareous gritty gravelstone of the lower part of
the formation an abundant collection of bivalves with
Cuneopsis sp., Eocuneopsis dehuaensis Guo (sample
MD.140/2). Similar fossils have been found in the
Zhangkezhai Formation (J1-2) in Yunnan (China). Apart
from these, there still is Pseudocardinia sp. (samples
MD.140, MD.140/1). Based on collected fossils and the
unconformable relation of the Ha Coi Formation upon
older sediments, this formation have been dated as
Early-Middle Jurassic.
III. Material composition of sediments of the
Ha Coi Formation
(1) Composition of sediments
a. Conglomerate: Not widespread. It occurs only at
the base of the formation, distributed mainly in the
northwest of the basin. Pebbles have the dimension
from small to medium; coarse pebbles are rare. They
have the composition of monomineral, oligomictic and
319