名古屋大学博物館報告
Bull. Nagoya Univ. Museum
No. 28, 1–18, 2012

Stratigraphy and geological structure of the Paleozoic system
around Ulaanbaatar, Mongolia
Makoto TAKEUCHI1)*, Kazuhiro TSUKADA2), Toshiya SUZUKI1),
Yuki NAKANE1), Sersmaa GONCHIGDORJ3), Manchuk NURAMKHAAN3),
Tomoyuki KONDO1), Nozomi MATSUZAWA1), Bakhat NURAMKHAAN3),
Khishigsuren SODNOM3), Onon GANTUMUR1), Yusuke KATSURADA2),
Kentaro KAWAHARA1), Masami HASHIMOTO1), Shoko YAMASAKI1),
Asuka MATSUMOTO4), Oyu-Erdene BORKHUU3),
Bulgantsengel MAGNAIBAYAR3), Kundyz SYERYEKHAAN3),
Enkhchimeg LUVSANDORJ3), Ganzorig RANJIN3),
Myagmarsuren GERELBAATAR3),Jamiyandagva OTGONBAATAR3), Molomjamts MUNKHJARGAL3)
1) Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Chikusa-ku, Nagoya, 464-8601, Japan
2) Nagoya University Museum, Chikusa-ku, Nagoya, 464-8601, Japan
3) School of Geology and Petroleum Engineering, Mongolian University of Science and Technology, Ulaanbaatar,
Mongolia4) Department of Earth Science, Graduate School of Science and Technology, Niigata University, Igarashi 2-nocho, Nishi-ku, Niigata, 950-2181, Japan*Corresponding author

Abstract
Recent mapping study in the Ulaanbaatar area has revealed that the geology around Ulaanbaatar previously reported should be revised in regard to stratigraphy and geological structure. Here, the new division ofstrata and geological structure are presented.
Detailed field mapping with careful observation of sedimentary structures such as grading, loadcast, crosslamination, channel structure, and so on which indicate stratigraphic top have been carried out. The results
show that many parts of strata were overturned and the geological structure is characterized with overturnedfolds with south vergence. In particular, the Paleozoic system is newly divided into two accretionary complexes (Altan-Ovoo & Orgioch-Uul Formation and Gorkhi Formation) and a shallow marine formation. Mostof the previous shallow marine Carboniferous system is replaced by the two accretionary complexes and theshallow marine Carboniferous system which occurs with fossils indicating shallow sea environment only
distributed in narrow area between two faults, although the latter has been regarded as extensively distributed
around Ulaanbaatar. The geology suggests that the shallow marine sediments overlie the accretionary complex(Altan-Ovoo & Orgioch-Uul Formation), unconformably.
The accretionary complexes are classified with quite different from the stratigraphy observed in clastic
rocks. The Gorkhi Formation is composed of chert, siliceous shale and clastic rocks dominating massive andthick-bedded sandstone in ascending order. The Altan-Ovoo & Orgioch-Uul Formation consists of basalt,
chert, siliceous shale and clastic rocks dominating thick- to thin-bedded sandstone and mudstone in ascendingorder.
Furthermore, it is found out that the previous Gorkhi Formation in the northwest area and the previous
Carboniferous system in the central south area are regarded as weakly metamorphosed and strongly deformed
equivalents to the Altan-Ovoo & Orgioch-Uul Formation. Geological structure of these strata represents southvergence such as north-dipping thrust and fold-axial plane. The fact strongly suggests that north-directing
subduction formed these geological structures.
—1—

Introduction
96°E
106°E
114°E
90°E
102°E
The Central Asian Orogenic Belt (CAOB) is
Adaatsag Terranean orogenic belt that developed between the
48°N
Haraa Terrane
Eastern European, Siberian, Tarim and North
China blocks associated with subduction,
Ereendavaa Terrane
44°N
magmatism, accretion and collision. The tectonic
Hangay-Hentey basin B
interpretation for constituents in the CAOB can be
improved by recent detailed studies of stratigraphy,
0500 kmpetrology, structural geology and geochronology.
B
Badarch et al. (2002) summarized geology ofwhole Mongolia and described geotectonic
division. Within the unit of the geotectonic division,
Batsumberthe Hangay-Hentey basin is an important key unit
with regard to the end stage of geological evolution
Fig. 2of the CAOB. The tectonic interpretation of the
Hangay-Hentey basin is however still controversial.
Ulaanbaatar
Several major hypotheses have been proposed,
including an oroclinal rotation model (Sengör etNalaikhZuunmodal., 1993), a model involving collision followedby oroclinal bending (Zorin et al., 1993), a backSergelenarc basin model (Badarch et al., 2002), a twosubductions model between island-arc system in0
20 km
the south and continental-margin arc in the northMesozoic terrigenous sedimentary rocks(Kelty et al., 2008; Bussien et al., 2011).
Mesozoic plutonic rocks
Geology around Ulaanbaatar is divided into the
Carboniferous plutonic rocks
Haraa Terrane which consists of Cambrian to
Hangay-Hentey basin
Lower Ordovician metasedimentary rocks
Carboniferous shallow marine sedimentary rocks
Devonian to Carboniferous accretionary complex (Gorkhi F.)
originated from back-arc or fore-arc basin, theDevonian to Carboniferous accretionary complex (Sergelen F.)
Hangay-Hentey basin and the Adaatsag TerraneSilurian (?) accretionary complex (Mandal Group)
which consists of metasedimentary rocks withHaraa Terrane
ophiolite (Badarch et al., 2002). The HangayOrdovician plutonic rocks
Hentey basin is subdivided into the Silurian
Cambrian to Ordovician metasedimentary rocks (Haraa F.)
Adaatsag
TerraneMandal Group (Tomurtogoo et al., 1998), theOrdovician (?) ophiolite and metamorphic rocks
Lower Devonian Sergelen Formation, the MiddleUpper Devonian Gorkhi Formation, the Lower
Fig. 1 (A) Distribution of the Hangay-Hentey basin based
Carboniferous Altan-Ovoo Formation and theon Badarch et al. (2002).
Lower-Upper Carboniferous Orgioch-Uul(B) Simplified geological map around Ulaanbaatar
area (modified from Tomurtogoo et al., 1998).
Formation (Minjin et al., 2006) (Fig. 1). Kurihara
et al. (2009) reported Upper Silurian to Upper
Devonian radiolarian fossils from chert and tectonic stacking structure basalt-chert and chert-turbidite
successions. They interpreted the Sergelen and Gorkhi Formations as a Early Carboniferous accretionary
complex comprising of Upper Silurian to Upper Devonian pelagic rocks and Early Carboniferous turbidite.
On the other hand, they proposed the Orgioch-Uul and Altan-Ovoo Formations as Early Carboniferous(Serpukhovian in Visean) shallow marine strata.
However, the stratigraphic relationship between the accretionary complex and the shallow marine
strata, stratigraphy of the shallow marine strata and their geological structures are obscured. It is therefore

A

B

—2—

Fig. 1

important to understand these facts on discussion of tectonic evolution of the CAOB. Especially, geologicalstructure can make a direction of subduction clear, and which has formed the accretionary complex of
the Hangay-Hentey basin. We also examined stratigraphy and geological structure of the Gorkhi, OrgiochUul and Altan-Ovoo Formations.
Geological outline
The study area is located around Ulaanbaatar, extends 40 km east-west and 25 km north-south andbelongs the Hangay-Hentey basin (Badarch et al., 2002) (Fig. 1). This study reports stratigraphy, geological structure and distribution of the shallow marine strata and accretionary complex, and which arequite different from the geology previous reported.
Basement units in this area are composed of two formations with different stratigraphy (Figs. 2–5).
One is the Gorkhi Formation (Kurihara et al., 2009) abundant in sandstone, and the other is the AltanOvoo & Orgioch-Uul Formation abundant in sandstone and mudstone. Here, “the Altan-Ovoo & OrgiochUul Formation” is used because this study clarified that the geological structure of the previous Altan-Ovoo
and Orgioch-Uul Formations in the study area was quite different from those in previous studies and thestrata could not be stratigraphically divided into the two formations, Altan-Ovoo and Orgioch-Uul
Formations.
The Gorkhi Formation yields Upper Silurian to Upper Devonian radiolarian fossils from chert (Kurihara
et al., 2009). The sandstones of the Gorkhi and Altan-Ovoo & Orgioch-Uul Formations yield detrital
zircons with peak U-Pb age distributions of 339 Ma, 340 Ma, 349 Ma (Kelty et al., 2008), 394 Ma and404 Ma (Bussien et al., 2011), and the clastic rocks are considered as the Devonian and partly Lower
Carboniferous.
The shallow marine Carboniferous system is composed of conglomerate and sandstone in the lower
part and mudstone with thin sandstone beds. The conglomerate in the lower part yields some kind offossils such as brachiopods. The strata could not stratigraphically divided because the distribution islimited narrowly along the fault in the northwest area.
Mesozoic Bogd Uul Granite dated as 181±7.7 to 212±10.8 Ma (Tomurtogoo et al., 1998, Khishigsuren,
et al., 2013) intrudes into the Altan-Ovoo & Orgioch Formation in the southeast area, and rhyolite, andesite
and granite dikes intrude into the Altan-Ovoo & Orgioch-Uul Formation in the northwest area. Granitic
dikes with ages 223.5±30.2 and 294.2±51.4 Ma were reported by Khishigsuren et al. (2009).
Cover sediments on the basement rocks only outcrop in a small part of the study area. The Cretaceous
system (Tomurtogoo et al., 1998) composed of white claystone and conglomerate is exposed in thenorthwest of central Ulaanbaatar.
Talus deposits mainly consisting of gravel bed are distributed along rivers and in the valley with slightly
higher surface than recent river bed.
Fault with large displacement cuts these strata except talus deposits from northwest to southeast. There
are many folds with northeast-directing fold axis in the both accretionary complexes. The folds represent
south vergence (Fig. 3).
Altan-Ovoo & Orgioch-Uul Formation
Difinition: Minjin et al. (2006) divided the strata in this area into the Lower Carboniferous Altan-Ovoo
Formation and the Lower-Upper Carboniferous Orgioch-Uul Formation. However, “the Altan-Ovoo &Orgioch-Uul Formation” is used because it was found out that the strata could not divided into the twoformations in lithostratigraphy. The Altan-Ovoo & Orgioch-Uul Formation includes basalt and chert inaddition with sandstone and mudstone in the north and west of Ulaanbaatar.
Type locality: Mountain range with Ondor tolgoi (section C-D in Fig. 2).
—3—

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Fig. 2 Geological map around Ulaanbaatar. See Fig. 1 for the location of the mapped area. The base map is complied from Google (2012).

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Fig. 3 Geological cross section.
The routes of cross sections are shown in Fig. 2.

Fig. 4 Index map showing the route of columnar section.

—5—

Altan-Ovoo & Orgioch-Uul Formation

15

16

Fault

Gorkhi Formation

Fault
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Conglomerate
Sandstone
Alternating beds of ss. & ms.
Mudstone
Felsic tuff
Chert

28
500 m

Fault

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Fig. 5 Columnar sections measured within the Altan-Ovoo & Orgioch-Uul and the Gorkhi Formations.
Fig. lithostrati5
See Fig. 4 for location details. Correlations indicated by broken lines are based on the correlation of key
graphic boundaries.

Stratigraphy and lithology: The basal bed of the AltanRed chert
Ovoo & Orgioch-Uul Formation consists of pillow and
Mudstonemassive lavas of basalt. It is overlaid by alternating bedsof reddish purple mafic tuff, green basalt and mudstone,
alternating beds of red chert and mudstone, siliceous
mudstone and clastic rocks in ascending order (Fig. 6).
The pillow basalt (Fig. 7A) contains white limestone
Thinnly alternating beds
(Fig. 7B), dolostone (Fig. 7C) and red jasper amongof mudstone &fine-grained sandstone
pillows. Some basalt shows amygdule. White chert
Massive
Purple mafic tuff
basalt lava
lamination in red chert is characteristic (Fig. 7D), and rare
Mudstonebedded chert intercalates with siliceous mudstone. Black
Basalt dike
MnO2 is observed in red chert (Fig. 7E). There is no
outcrop where upper contact with chert is recognized, but
50m
mudstone overlies on siliceous mudstone in the
5m
northwesternmost area.
Lithology of clastic rocks highly varies laterally andBasalt pillow lava
upward (Fig. 5). Sandstone-dominant turbidites are Fig. 6 Columnar section of pelagic sequence (left)intercalated with mudstone (Fig. 8). The turbidites are
and detail lithology (right).
See Fig. 4 for the location.
composed of sandstone beds of several millimeters to
Fig. 6several meters thick, and show grading and load cast (Fig.
9A-C). Parallel and cross laminations are characteristic in some medium- to thick-bedded sandstones.
Some massive sandstone and conglomerate scour underlying strata as channel (Fig. 10). The bottom ofchannel-fill sandstone contains many mudstone intraclasts. Loadcast is rarely observed at the base ofsandstone bed.
—6—

Fig. 7 Lithologies of basalt and chert of the Altan-Ovoo & Orgioch-Uul Formation. See Fig. 4 for the location of following outcrops.
(A) Pillow lava of basalt. (B) Limestone lens (LS) within pillow basalt. (C) Red dolostone lens (Ds) within pillow
basalt. (D) Red bedded chert. (E) Black MnO2 (Mn) with red chert (Ch). (F) Red tuff (upper) and white sandstone(lower).

Conglomerate is matrix-supported and varies in grainsize from granule to boulder. Clasts larger than
granule are well-rounded (Fig. 9C, D). The clasts consist of granite, quartzite, crystalline schist, acidic
volcanic rocks, sandstone and mudstone (Minjin et al., 2006). Adachi et al. (2007) reported granophyre
cobble also.
Felsic tuff yields in mudstone-dominant horizon. It yields as alternating beds of medium-grained whitetuff and fine-grained pale green tuff (Fig. 9E). These beds are several-centimeters thick. Red or green
tuff is rarely intercalated with sandstone (Figs. 7E, 11). The tuff is partly siliceous like chert.
The Altan-Ovoo & Orgioch-Uul Formation in the north of fault which is situated in the northwest to
southeast area is suffered from higher metamorphism and deformation toward the northwest. Slate cleavage(Fig. 9F) is characteristic in mudstone of the northwest area. Outlines of clastic particles in sandstone of—7—North limb

South limb

Fig. 1050 m

Conglomerate
Massive sandstone
Bedded sandstoneThin bedded sandstone & mudstone
Mudstone with sandstone lamination
Massive mudstone
Intraclast of mudstoneFig. 10

Fig. 850 m

Fig. 8 Detail columnar section of the conglomeratic facies in the Altan-Ovoo & Orgioch-Uul Formation.
The two columnar sections were measured from the north limb (left, upper part of columnar section 7 in Fig. 5)
and the south limb
(right, columnar section 8 in Fig. 5) of fold. Lithology largely varies laterally and vertically
Conglomeratebecause of channel facies.
Massive sandstone

the northwest areaBedded
are not
distinct because of recrystallization with metamorphism. Basalt has beensandstone
metamorphosed intogreenschist
in thenorthwest area, and which is composed of metamorphic minerals
Thin bedded sandstone& mudstonesuch as actinolite, epidote, titanite and chlorite (Fig. 12). Chert has also been recrystallized into quartz
Mudstone with sandstone lamination
schist yielding elongated quartz and stilpnomelane. The Altan-Ovoo & Orgioch-Uul Formation in the
mudstonesouth of the fault isMassive
also sufferedfrom higher deformation toward northwest and in Zaisan tolgoi south
Intraclastof mudstoneof central Ulaanbaatar
whereslate cleavage is developed typically (Fig. 2).
Thickness of strata: 4500 m in clastic rocks and 500 m in chert and basalt.
Age: No fossil has been reported. 404 Ma of peak U-Pb age from detrital zircons in sandstones of theFig. 8in the western area and its extension (Bussien et
Altan-Ovoo & Orgioch-Uul Formation were reportedal., 2011). Then this formation is the Devonian or the younger at least. Kelty et al. (2008) reported 349
Ma of peak U-Pb age from detrital zircon in the extension in the western area. It indicates that the clastic
rocks are Lower Carboniferous or the younger.
Gorkhi Formation
Definition: This strata has not been defined formally, but Minjin et al. (2006) assigned the Gorkhi
Formation an accretionary complex composed of sandstone, mudstone, siliceous shale, radiolarian chert,
basalt, dolerite, mafic tuff, gabbro and crystalline limestone around Ulaanbaatar.
Stratigraphy and lithology: In the Zuunmond ca. 30 km south of Ulaanbaatar, the Gorkhi Formation
is typically distributed (Kurihara et al., 2009). The oceanic plate stratigraphy which consists of ca. 30 m
—8—

Fig. 9 Lithology and sedimentary structures in the Altan-Ovoo & Orgioch-Uul Formation. See Fig. 4 for the location.
(A) Very thin-bedded turbidite. (B) Thin turbidite with grading. Stratigraphic top is upper on the picture. (C) Loadcast at the base of sandstone bed (upper). (D) Cobble conglomerate. (E) Felsic tuff. Fine-grained part is pale green
and medium-grained part is white. (F) Mudstone with strong slate cleavage.

Fig. 10 Channel-filled sediment in the Altan-Ovoo & Orgioch-Uul Formation.
See Fig. 8 for the location.
—9—

A

B
slate with thin ss.
green tuff
red tuff
green ss.
pale green chert
ms. with ch.
green slate

black ms.

green slate

red tuffF
F

white tuffaceous ss.
green ss.
red tuffF

white tuffaceous ss.
red tuff

20 m

5m
white chert

Fig. 11 Columnar sections showing occurrence of tuffaceous beds in the
Altan-Ovoo & Orgioch-Uul Formation. See Fig. 4 for the location.

Fig. 11

Fig. 12 Photomicrographs of thin sections of mafic rocks.
(A) Glassy basalt of the Altan-Ovoo & Orgioch-Uul Formation (Fig. 7A). Pyroxene pseudomorph (Psm) has
been replaced by epidote. Plane-polarized light. (B) Same as (A). Cross-polarized light. (C) Weakly metamorphosed basalt of the Altan-Ovoo & Orgioch-Uul Formation (Fig. 7C), which is composed of epidote (Ep), actinolite (Act), chlorite (Chl), titanite (Ttn) and calcite (Cal). Plane-polarized light. (D) Same as (C). Cross-polarized
light.

— 10 —

thick of basalt lava, hyaloclastite and tuff, ca. 5 m thick of alternating beds of chert and dark red tuff, ca.
100 m thick of chert, 10 to 20 m thick of siliceous shale and clastic rocks showing upward-thickening
and coarsening was reported by Kurihara et al. (2009). The thickness of the clastic rocks is highly
thicker than pelagic rocks judging from their geological map.
In this study area, there are abundant clastic rocks and a few chert and siliceous shale. Total thickness
is unknown because the Gorkhi Formation cut by several faults. In Gachuurt area, the Gorkhi Formaiton
consists only of clastic rocks, massive and thick-bedded sandstones in the upper and lower parts andalternating beds of sandstone and mudstone and thick-bedded sandstone in the middle part (Fig. 5). In
the east of Ochirvaani Uul, siliceous shale overlies chert (Fig. 13A) and underlies sandstone (columnersection 27 in Fig. 5). The siliceous shale contains sand-sized clastic grains. The Gorkhi Formation in theeasternmost area is dominant in mudstone and thin-bedded turbidite (Fig. 5).
Massive sandstone is medium- to coarse-grained and poor sedimentary structure. Thick-bedded
sandstone is also medium- to coarse-grained, and shows parallel and cross laminations and grading (Fig.
13B). Thin-bedded turbidite is ca. 10 cm thick of a bed, shows grading from medium- or fine-grainedsandstone to mudstone and parallel and cross laminations. In the route of columnar section 28 (Figs. 4,5), the upper sandstone includes red chert block (Fig. 13C), ca. 100 cm × 30 cm, and medium-grained
clastic chert fragments. There is conglomerate dominated with sandstone clasts in the easternmost unit
(Fig. 13D).
Fossil and age: Upper Silurian to Upper Devonian radiolarian and conodont fossils were reported
from chert in the Zuunmod area (Kurihara et al., 2009). Nakane et al. (2013) reported Devonian radiolarian fossils from chert in this formation of the east of Ochirvaani uul. The deposition of the sandstone isFig. 13 Lithology of the Gorkhi Formation. See Fig. 4 for the location.
(A) Siliceous shale overlying red bedded chert. (B) Thick bedded sandstone with parallel and cross laminations.
Stratigraphic top is left on the picture. (C) Sandstone including exotic red chert blocks. The chert block is sedimentary but not tectonic, because the sandstone contains also chert fragments. (D) Conglomerate containing
many sandstone cobbles.
— 11 —

after Mississippian of Early Carboniferous because 394 Ma of peak U-Pb age from detrital zircon in thesandstones in the southwest of Gachuurt (Bussien et al., 2011) and 339 Ma and 340 Ma near Nalaikh(Kelty et al., 2008) were reported.
Shallow marine Carboniferous system (undivided)
This formation is composed of sandstone and conglomerate
in the lower part and mainly of mudstone with conglomerate
Thinly alternating beds ofand sandstone in the upper part which is distributed along thefine-grained sandstone & mudstonefault in the northwest area (Figs. 2, 14, 15).
Parallel lamination
Type locality: Mountain range east of Mt. Tamir, 25 km
northwest of central Ulaanbaatar.
Stratigraphy and lithology: We found new locality ofsandstone and conglomerate containing brachiopods fossils(mostly impression fossil) in mountain range at 2.5 km northof Otogon Hustin Tolgoi which is 8 km southeast of the type
locality. The conglomerate is considered to overlie basalt and
Massive mudstonechert of the Altan-Ovoo & Orgioch-Uul Formation
unconformably, although outcrop of unconformity is notobserved. The upper part of this formation in the area is
Fine-grained sandstonecomposed of mudstone with thin sandstone beds.
In the type locality, this formation is composed of bedded
Pebble to granule conglomeratewith fossils
fine- to medium-grained sandstone containing brachiopods,
gastropods and bryozoan fossils (Fig. 15D-G). Twenty-fivemeters thick of mudstone overlying the sandstone intercalates
sandstone beds with ill-sorted granule to pebble conglomerate(Fig. 14). The conglomerate bed yields fossils above mentioned.
Massive mudstone
It is inferred that the fossils were transported and deposited
together with gravels, because the fossils occur randomly
oriented in a bed. The upper part of the strata consists mainlyof mudstone with medium- to thin-bedded and fine- to mediumgrained sandstone beds (Fig. 15A).
5m
Fossil and age: Minjin et al. (2006) described that the aleuroBedded medium-grained sandstone
with fossils
sandstone horizon of the Altan-Ovoo Formation contains Lower
Carboniferous, Visean bryozoans: Fenestella invulgata Shishiva, Fig. 14 Columnar section of the shallow maLyrocladia mariae Shishova, Sulcoretepora mergensis
rine Carboniferous system. See Fig.
4 for the location.
Nekhoroshev (Donakova, 1961, not listed within Minjin et al’s
Fig. 14references). Furthermore, they also reported that Popeko (1961) found brachiopods: Spirifer dublicicostus
Mart, and Radchenko (1961) collected ferns: Asterocalamites sp. and Zalesskyodendron sp. (both literatures
do not listed within Minjin et al’s references), although fossil localities are unknown. We also found somefossils at two localities mentioned above, with the details to be reported in a separate paper.
Geological structure
We have carried out detailed field mapping with careful observation of sedimentary structures suchas grading, loadcast, cross lamination, channel structure, and so on, which indicate stratigraphic top. It
was recognized that many parts of the strata in this area were overturned (Fig. 17A) and the geologicalstructure is characterized with overturned folds with south vergence (Figs. 2, 3). The axial planes of folds— 12 —

Fig. 15 Lithology of the shallow marine Carboniferous system. See Fig. 4 for the location.
(A) Alternating beds of sandstone and mudstone of the upper part. (B) Ill-sorted conglomerate. (C) Mudstone.
(D) Impression fossil of brachiopod. (E) Impression fossil of brachiopod. (F) Impression fossils of bryozoan.
(G) Impression fossil of brachiopod.
— 13 —

B

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Dacite dike

Cover gravel

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Mudstone70

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3m

Fig. 16 Occurrence of fault cutting dacite dike (A) and its sketch. See Fig. 4 for the location.

strike northeast and dip 20 to 80° to the north, and they are overturned or recumbent fold. The wave lengthis ca. 5 km.
Fig. 16
The Gorkhi Formation is in fault contact with the Altan-Ovoo & Orgioch-Uul Foramtion. The fault
trending to the northeast cuts the fold structures within the both formations. This type of fault is developedin the Gorkhi Formation and active before the formation of northeast-trending fault.
There is a major fault distributed from the northwest to the southeast in this area. This fault diverges
in the northwest, and exotic blocks of massive sandstone, chert, basalt and limestone occur between thediverging faults. The fault seems to have moved after Cretaceous time, because the Cretaceous systemin the northwest of central Ulaanbaatar which is bounded by the fault dips 20°to the south.
There are east- and north-ternding faults around the Uliastain gol, which cut fold structure. The northtrending fault is associated with 80 cm wide of cataclasite zone and cut dacite dike (Fig. 16). It is obliquefault because slicken line plunges at 25°to the north. Fault plane of east-trending fault dips to the north,on which vertical slicken lines are recognized (Fig. 17A, B). Fault gouge and breccia with the fault havebeen consolidated. Direction of displacement of the fault is unknown. There several quartz veins several
meters long and 10 cm wide around the faults (Fig. 17C, D).
North-trending fault in the west area is associated with several meters wide of fracture zone and there
are small faults in 20 m wide. The fracture part was weathered into white material (Fig. 17E).
Discussion
Shallow marine Carboniferous system has been previously divided into the sandstone-dominant AltanOvoo Formation and the mudstone and sandstone-dominant Orgioch-Uul Formation only with lithology.
Bedding plane of stara in the northern-half of this area dips to the north. Then the Carboniferous system
has been considered as monocline and that the Orgioch-Uul Formation overlies the Altan-Ovoo Formation(Magic project, 1998, Tomurtogoo et al., 1998) (Fig. 18).
We carefully observed sedimentary structures and judged stratigraphic top on outcrops to understandstratigraphy and geological structure. Consequently, it is found out that many parts of strata were overturned— 14 —

Fig. 17 Deformation structures. See Fig. 4 for the location.
(A) Overturned sandstone and mudstone (slate) with vertical slate cleavage. Stratigraphic top is right on the
picture. The sandstone bed grades into mudstone (slate) to right. (B) Outcrop of dip-slip fault. Fault plane isvisible. (C) Close up of (B). Vertical slicken line on the fault plane. (D) Quartz vein in sandstone. (E) Close upof (D). (F) Outcrop of fault with wide fracture zone.

and that the geological structure is occupied by overturned folds which are quite different structure.
Although the younger Altan-Ovoo Formation is distributed in the north and the older Orgioch-Uul
Formation is distributed in the south in the geological map (Magic project, 1998; Tomurtogoo et al.,1998), previously, the stratigraphy itself should be reexamined because of the new facts by our observation.
Distribution of chert seems to have been regarded as important to decide the previous boundary between
the Gorkhi Formation and the shallow marine Carboniferous system (Magic project, 1998; Tomurtogoo
et al., 1998). This study made this problem clear and found two obviously different stratigraphies (Fig.
5). Consequently, two formations are newly defined as the Gorkhi and the Altan-Ovoo & Orgioch-Uul
Formations. The Gorkhi Formation in this area is regarded as an accretionary complex because theformation is characterized by an oceanic plate stratigraphy from chert, through siliceous shale to clastic— 15 —

０５km

Fig. 18 Comparing of distribution of the geological boundaries between previous map (Tomurtogoo et al., 1998) andthis study. C1–2, C2–3 and D1–2 represent the early to middle Carboniferous Altan-Ovoo Formation, the middle
to late Carboniferous Orgioch-Uul Formation and the Early to Middle Devonian Gorkhi Formation, respectively. Thick and thin broken lines represent fault and formation boundary, respectively, in the previous map.

rocks in ascending order (Nakane et al., 2013) and the Gorkhi Formation can be correlated with the
Gorkhi Formation in the Zuunmod area which is regarded as an accretionary complex (Kurihara et al.,2009). Such strata are distributed extensively to the northwest of Gachuurt. Then the boundary northernend of the Gorkhi Formation should be revised (Fig. 18).
On the other hand, the Altan-Ovoo & Orgioch-Uul Formation shows different stratigraphy from the
Gorkhi Formation (Fig. 5), and is characteristic with variable lithological changes towards horizontallyand vertically by sandstone and mudstone. It is impossible to divide simply into the Altan-Ovoo and the
Orgioch-Uul Formations in lithofacies. The formation is associated with chert and siliceous shale in thenorthwest area. Furthermore, the Altan-Ovoo & Orgioch-Uul Formation never yields fossils indicating
shallow marine environment, even any fossils. So the Altan-Ovoo & Orgioch-Uul Formation is regarded
as an accretionary complex. However, it is necessary that the Altan-Ovoo & Orgioch-Uul Formation
should be correlated with the Altan-Ovoo and Orgioch-Uul Formations in the type localities because
these formations are regarded as shallow marine sediments (Minjin et al., 2006).
The stratigraphy of the Altan-Ovoo & Orgioch-Uul Formation is quite different from one of the Gorkhi
Formation. This fact indicates that these formation are deposited originally apart far each other.
In the northwest area of this study, the Devonian system in the northwest and the Carboniferous systemin the southeast are illustrated in the geological map (Magic project, 1998; Tomurtogoo et al., 1998) (Fig.
18). However, the previous Devonian system is regarded as the lower part of the Altan-Ovoo & OrgiochUul Formation which has been strongly deformed and weakly metamorphosed.
This study revised that the shallow marine Carboniferous system is distributed in narrow area only
along the faults in the northwest area, although it has been regarded to be distributed in wide area around
Ulaanbaatar, previously (Magic project, 1998; Tomurtogoo et al., 1998; Minjin et al., 2006). It is largely
possible to overlie unconformably the Altan-Ovoo & Orgioch-Uul Formation. This suggests that the— 16 —

Altan-Ovoo & Orgioch-Uul Formation was accreted in the Early Carboniferous time and the shallow
marine Carboniferous system unconformably overlaid the Altan-Ovoo & Orgioch-Uul Formation after
the Altan-Ovoo & Orgioch-Uul Formation was uplifted to shallow sea environment, and that the shallow
marine Carboniferous system fell down between faults with younger faulting. Most of the shallow marine
Carboniferous systen might be eroded with uplifting the accretionary complex.
The Altan-Ovoo & Orgioch-Uul Formation and the Gorkhi Formation are occupied with south vergence(Fig. 19). The Gorkhi Formation in the Zuunmod area also shows stacking thrust sheets indicating southvergence (Kurihara et al., 2009). On the other hand, in the upper reaches of the Haraa Gol River northnorthwest of Ulaanbaatar, the Haraa and the Mandal Groups also show stacking thrust sheets dipping to
the north, i.e., south vergence (Gordienko et al., 2012). Metamorphic and deformation grades in the
Altan-Ovoo & Orgioch-Uul Formation increase toward northwest and it is considered that the Altan-Ovoo& Orgioch-Uul Formation is continuous with the neighbor Mandal Group which is composed of similarlithology with the Altan-Ovoo & Orgioch-Uul Formation, i.e., quartz schist with mafic and intermediate
metavolcanics, polymictic sandstone and flyschoid sand-siltstone (Gordienko et al., 2012). This meansthat whole strata in the Hangay-Hentey basin show south vergence and strongly suggests that north-directing subduction formed these geological structures.

Mesozoic plutonic rocks

Zuunharaa

Carboniferous plutonic rocks
Rocks of the Adaatsag Terrane
Gorkhi Formation
Altan-Ovoo & Orgioch-Uul Formation
Metamorphosed Altan-Ovoo &Orgioch-Uul Formation (Mandal Group)
Devonian plutonic rocks

So

BatsumberhutHaraa Group

ce

en

rg

ve

ThrustSyncline
Anticline
Overturned syncline
Overturned anticline

Ulaanbaatar

NalaikhZuunmod

20 km
Fig. 19 Map showing geological structure of the Haraa Terrane, the Hangay-Hentey basin and the Adaatsag Terrane.
Complied with Tomurtogoo et al. (1998), Kurihara et al. (2009), Gordienko et al. (2012) and this study.

Fig. 19
Acknowledgements
We are grateful to Professor Ch. Minjin, of the Mongolian University of Science and Technology andProfessor H. Yoshida of the Nagoya University Museum for very usefull suggestions and comments.
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This work was supported by Grant in Aid for scientific research by the Ito Kagaku Shinkokai, theJapan Society for the Promotion of Science (No. 20403013), and the Socho-Sairyo-Keihi of Nagoya
University. A part of this work was carried out as a project on “A Synthetic study on geological structure;mineral deposits; natural environment analysis; nurture of human resources; improvement of geologicalinformation in Mongolia” by Ministry of Economy, Trade and Industry and Japan Oil, Gas and Metals
National Corporation (JOGMEC).
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（2012 年 10 月 15 日受付，2012 年 12 月 20 日受理）

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