Received: 25 December 2015

Revised: 2 October 2017

Accepted: 10 October 2017

DOI: 10.1111/iar.12243

RESEARCH ARTICLE

Depositional ages and characteristics of Middle–Upper
Jurassic and Lower Cretaceous lacustrine deposits insoutheastern Mongolia
Hitoshi Hasegawa1,2| Hisao Ando3 | Noriko Hasebe4 | Niiden Ichinnorov5 |Tohru Ohta6 | Takashi Hasegawa7 | Masanobu Yamamoto8 | Gang Li9 |Bat-Orshikh Erdenetsogt10 | Ulrich Heimhofer11 | Takayuki Murata3 | Hironori Shinya6 |G. Enerel12 | G. Oyunjargal12 | O. Munkhtsetseg12 | Noriyuki Suzuki13 | Tomohisa Irino8 |Koshi Yamamoto14
1
Department of Global Environment andDisaster Prevention, Faculty of Science andTechnology, Kochi University, Kochi
780-8520, Japan

Abstract
Lower Cretaceous lacustrine oil shales are widely distributed in southeastern Mongolia. Due tothe high organic carbon content of oil shale, many geochemical studies and petroleum explora-

2

University Museum, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya 464-8601,
Japan
3
Department of Earth Sciences, Faculty of
Science, Ibaraki University, Bunkyo, Mito
310-8512, Japan
4

Institute of Nature and Environmental
Technology, Kanazawa University, Kakuma,
Kanazawa 920-1192, Japan
5

Institute of Paleontology and Geology,
Mongolian Academy of Sciences, P.O.B. 260,
Ulaanbaatar 210351, Mongolia
6tion have been conducted. Although most of the oil shales are considered to be Early Cretaceous in age, a recent study reveals that some were deposited in the Middle Jurassic. The
present study aims at establishing depositional ages and characteristics of the Jurassic and Cretaceous lacustrine deposits in Mongolia. The Lower Cretaceous Shinekhudag Formation isabout 250 m thick and composed of alternating beds of shale and dolomite. The Middle Jurassic Eedemt Formation is about 150 m thick and composed of alternating beds of shale, dolomitic marl, and siltstone. The alternations of shale and dolomite in both formations wereformed by lake level changes, reflecting precipitation changes. Shales were deposited in thecenter of a deep lake during highstand, while dolomites were formed by primary precipitation
during lowstand. Based on the radiometric age dating, the Shinekhudag Formation was deposited between (123.8 2.0) Ma and (118.5 0.9) Ma of the early Aptian. The Eedemt Formation

Department of Earth Sciences, Faculty of
Education and Integrated Arts and Sciences,
Waseda University, Nishiwaseda, Shinjuku,
Tokyo 169-8050, Japanwas deposited at around 165–158 Ma of Callovian–Oxfordian. The calculated sedimentation

7

in the Shinekhudag Formation show micrometer-scale lamination, consisting of algal organic

Department of Earth Sciences, Faculty of
Science, Kanazawa University, Kakuma,
Kanazawa 920-1192, Japanrate of the Shinekhudag Formation is between (4.7 2.6) cm/ky and (10.0 7.6) cm/ky. Shales
matter and detrital clay mineral couplets. Given the average thickness of micro-laminae and calculated sedimentation rate, the micro-lamination is most likely of varve origin. Both Middle–

8

Faculty of Environmental Earth Science,
Hokkaido University, Kita 8, Nishi 5, Kita-ku,
Sapporo 060-0810, Japan
9
Nanjing Institute of Geology andPalaeontology, Chinese Academy of Science,
39 East Beijing Road, Nanjing 210008, China

Upper Jurassic and Lower Cretaceous lacustrine oil shales were deposited in intracontinental
basins in the paleo-Asian continent. Tectonic processes and basin evolution basically controlled
the deposition of these oil shales. In addition, enhanced precipitation under humid climate during the early Aptian and the Callovian–Oxfordian was another key factor inducing the widespread oil shale deposition in Mongolia.

10Department of Geology and Geophysics,
National University of Mongolia, KH
Surguuliin Gudamj-1, Ulaanbaatar 14200,
Mongolia

KEYWORDS

Cretaceous, humid climate, Jurassic, lake, Mongolia, oil shale, U–Pb age, varve

11

Institute for Geology, Leibniz University
Hannover, Welfengarten 1, HannoverD-30167, Germany
12
School of Geology and Mining, Mongolian
University of Science and Technology, Baga

Island Arc. 2018;e12243.
https://doi.org/10.1111/iar.12243

wileyonlinelibrary.com/journal/iar© 2018 John Wiley & Sons Australia, Ltd

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2 of 17

HASEGAWA ET AL.

Toiruu34, Sukhbaatar, Ulaanbaatar 210646,
Mongolia
13

Department of Natural History Science,
Hokkaido University, Kita 8, Nishi 5, Kita-ku,
Sapporo 060-0810, Japan
14
Graduate School of Environmental Studies,
Nagoya University, Furo-cho, Chikusa-ku,
Nagoya 464-8601, Japan

Correspondence
Hitoshi Hasegawa, Faculty of Science andTechnology, Kochi University, Akebono-cho
2-5-1, Kochi 780-8520, Japan.
Email: hito_hase@kochi-u.ac.jp
Funding information
Grant-in-Aid for Scientific Research (B), Grant/Award number: 21403008 and 25302011;
Fujiwara Natural History Foundation; Grant forEnvironmental Research Projects from theSumitomo Foundation, Grant/Award number:
143485; Young Scientific Research (B), Grant/Award number: 25870309

1 | I N T RO D UC T I O N(Johnson et al., 2003, Yamamoto et al., 1993, 1998). However, theage of the oil-prone lacustrine deposits in Mongolia is poorly

Jurassic–Cretaceous sedimentary basins are widely distributed in theconstrained.

southern and eastern parts of Mongolia (Graham et al., 2001; Jerzy-

Recent paleontological study suggests that the oil-prone lacus-

kiewicz & Russell, 1991; Khand et al., 2000; Figure 1). Most of thesetrine deposits in Mongolia are not only of Early Cretaceous age, butbasins are characterized by extensional tectonics of Late Jurassic toalso of the late Middle Jurassic (Li et al., 2014). Some studies also

Early Cretaceous time formed by rifting and filled by syn-rift deposi-suggest that the Lower to Middle Jurassic sedimentary succession

tion. These well-developed rift basins in southeast Mongolia

containing organic-rich lacustrine shales and coals has generated a(e.g. Choibalsan, Tamsag, Choir-Nyalga, East Gobi and South Gobi

significant hydrocarbon charge in Mongolia (Johnson et al., 2003, Sla-basins; Figure 1) include lacustrine sedimentary succession containing

den & Traynor, 2000). These lines of evidence indicate that potential

thick oil shale and coal, which resemble the Daqing oil field of theoil-prone source rocks in Mongolia are not restricted to the Lower

Songliao basin of northeast China (Graham et al., 2001; Powell, 1986;

Cretaceous lacustrine strata but also included in Early–Middle Juras-

Sladen & Traynor, 2000). Previous studies evaluated the potential of

sic lacustrine strata, similar to the adjacent Chinese Jurassic basinscoal and petroleum resources in Jurassic–Cretaceous oil shale-bearing(e.g. Tarim, Junggar and Ordos basins; Powell, 1986; Watson

deposits in southeast Mongolia (Bat-Erdene, 2009; Bat-Erdene &et al., 1987).

Enkhtugs, 1987; Erdentsogt et al., 2009; Johnson et al., 2003; John-

In this paper, we first review the general tectonic and strati-

son & Graham, 2004; Sladen & Traynor, 2000; Yamamoto et al.,graphic framework of the Jurassic and Cretaceous lacustrine deposits

1993, 1998). They are generally characterized by high content ofin Mongolia. We then describe the lithostratigraphy and sedimento-

total organic carbon (TOC), and thus the Lower Cretaceous organic-

logical characteristics of the type locality of both Jurassic and Creta-rich sediments are thought to be the petroleum source rocks

ceous lacustrine oil shale deposits. We also provide new radiometric

Map illustrating the distributionof Jurassic–Cretaceous lacustrine oil shales
in Mongolia. Key localities for Cretaceous(red) and Jurassic (purple) sites are shown.
Modified after Bat-Erdene (2009)
FIGURE 1

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HASEGAWA ET AL.

age data of intercalated tuff and basalt in order to establish detailed

occurrence of the Lower Triassic Lystrosaurus hedini near the south-

depositional ages for the Jurassic and Cretaceous lacustrine oil shale

ern border of Mongolia (Noyon Uul area) and a lystrosaurid in thedeposits in Mongolia. Based on the U–Pb and fission track (FT) ages

Lower Triassic of the Ordos basin in northern China, southern Mon-of zircons in tuff layers, and on the K–Ar age of the basalt, lategolia likely had been connected to northern China as early as in the

Middle–early Late Jurassic and late Early Cretaceous ages of the

Early Triassic (Gubin & Sinitza, 1993; Hendrix et al., 1996).

Mongolian lacustrine oil shales is presented.

One of the largest tectonic events affecting Mongolia and adjacent regions during the Mesozoic is attributed to the closure of theMongol-Okhotsk Ocean from central Mongolia to the Okhotsk Sea

2 | T E C T O N I C S E T T I N G OF J U R A S S I C–
C RE T A C E O U S B A S I N S I N M O NG O L I A

with a scissor-like motion toward the northeast (Cogne et al., 2005;
Donskaya et al., 2013; Metelkin et al., 2010; Yang et al., 2015; Zorin,
1999). Although the precise closure age of the Mongol-Okhotsk

Mongolia is situated in the heart of the Central Asian Orogenic Belt,

Ocean remains controversial, the closure of the ocean in the north-which is fringed by the Siberian Craton in the north, essentially aneastern Mongolia region is thought to have occurred during theamalgamation of lower Paleozoic terranes, and by the Northern

Middle–Late Jurassic. Yang et al. (2015) suggested that development

China and Tarim Blocks in the south, an area of complex middleof back-arc extensional basins with northeast–southwest orientation

Paleozoic–Cenozoic terranes and suture zones (e.g. Badarch, Cun-in southeast Mongolia and northeast China during the early Lateningham, & Windley, 2002; Lamb et al., 2008). By the Late Permian aJurassic, prior to the final closure of the Mongol-Okhotsk Ocean.

major fold belt had been formed by the collision of the North China

Such Middle–Upper Jurassic back-arc extensional basins then experi-

and Central Mongolian plates. The Upper Permian sedimentary strata

enced strong compression, inversion and erosion during the latest

include alluvial and fluvial deposits which incorporated thick bitumi-

Jurassic–earliest Cretaceous period (Graham et al., 2001; Meng,

nous coals and lacustrine shales. Thereafter, throughout the Meso-

2003; Yang et al., 2015). The timing of the compressional tectonics inzoic and Cenozoic, sedimentary deposits are distinctly non-marine,

south and southeast Mongolia is considered as late Late Jurassic

except for the Lower Triassic marine deposits of the northeastern(Kimmeridgian–Tithonian), which is constrained by radiometric dating

margin of Mongolia (Ehiro, Zakharov, & Minjin, 2006). Based on theof intercalated tuffaceous sandstone in the Sharilyn Formation

Comparison ofchronostratigraphic subdivisions ofMongolian Jurassic–Cretaceous
sedimentary successions and their inferred
tectonic stages. Age assigned formations
in each study are shown. Blank parts
represent hiatus and/or formations withunknown ages

FIGURE 2

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HASEGAWA ET AL.

(155 Ma) in the Eastern Gobi basin (Graham et al., 2001: Figure 2).

2000), and by 40Ar/39Ar age of 155 Ma from a tuffaceous sand-After the final closure of the Mongol-Okhotsk Ocean, numerous

stone sample (Graham et al., 2001).

extensional basins with northeast–southwest orientation were devel-

The overlying Lower Cretaceous succession is composed of allu-

oped in southeast Mongolia and northeast China during the Earlyvial fan to fluvio-lacustrine deposits of the Tsagantsav Formation,

Cretaceous.

lacustrine deposits of the Shinekhudag Formation, and coal-bearing

Lower Cretaceous extensional sedimentary basins are widely

fluvial deposits of the Khukhteeg Formation, in ascending order. The

developed in southeast Mongolia and northern China (Graham et al.,Tsagantsav Formation reaches up to 1 km in thickness and the sedi-

2001; Meng et al., 2003). Sedimentary successions of the basins dis-ments are regarded as the main part of the syn-rift sequences by Gra-

play several depositional cycles, starting with alluvial and fluvial con-

ham et al. (2001). The formation consists mainly of fining upwards

glomerates and then passing upward into lacustrine fine-gained

succession from the basal conglomerate to trough cross-bedded,

sedimentary rocks. Strike-slip faulting during rifting and/or inversion

coarse- to medium-grained sandstone, reddish or greenish shale, andalso occurred, as indicated by the occurrence of flower structures

calcretes. Although the overall climate during the period of the Tsa-recorded on reflection seismic data (Graham et al., 2001; Johnson

gantsav Formation is thought to be dry due to the existence of red-2004). The basins were inverted at the end of the Early Cretaceous,

dish beds with calcretes, intermittently humid climate phases possibly

and then overlain by relatively thin Upper Cretaceous sedimentary

existed during this period due to existence of some perennial lacus-

rocks (Graham et al., 2001; Meng et al., 2003).

trine bodies. Graham et al. reported 40Ar/39Ar age of (131 1) Ma(Khara Khutul section) and (126 1) Ma (Tsagan Tsav section) of theintercalated basalts in the middle parts of the formation, suggesting a

3 | G E N E R A L ST R A T I G R A P H Y OFJ U R A S S I C– C RE T A C E O U S F O R M A T I O N S I N
MONGOLIA

Hauterivian–Barremian age. Plant fossil evidence also indicates a
Valanginian to Barremian age (Krassilov, 1982).
The Shinekhudag Formation is composed mainly of welllaminated shale (paper shale), dolomitic marls, dolomite, and siltstone,

Jurassic–Cretaceous sedimentary successions in Mongolia are charac-

which represent characteristics of offshore lacustrine facies in large,

terized by three stages of tectonic regime; (i) pre-rift stage

perennial lakes. Detailed descriptions of this formation are provided(Khamarkhovoor Formation) of the Early–Middle Jurassic age; (ii) syn-in the next section. The Khukhteeg Formation, about 150–300 m

rift extensional stage of the Upper Jurassic to Lower Cretaceous suc-thick, is composed mainly of dark greyish coaly mudstone, light grey-

cession (Sharilyn, Tsagantsav, Shinekhudag, Khukhteeg Formations);

ish sandstones, and conglomerates. It is widespread in the east andand (iii) mid-Cretaceous inversion and Upper Cretaceous post-rift

center of Mongolia with the characteristic feature of abundant coal

succession

Barungoyot,mine (e.g. Shivee Ovoo, Tevshin Gobi, Bayan Erkhet, Khuren Dukh,

Nemegt Formations) (Graham et al., 2001; Hendrix et al., 1996; John-

Nalaikh, Baga Nuur localities). Abundant partial tree trunks and otherson & Graham, 2004; Traynor & Sladen, 1995; Figure 2).
The Lower–Middle Jurassic Khamarkhovoor Formation, exposedwater-borne organic materials are intercalated in this formationin the East Gobi, South Gobi, Gobi Altai, Mongol Altai, Khangai, andyields aquatic champsosaurs (Shuvalov, 2000) and trionychid turtles(Barunbayan,

Bayanshiree,

Djadokhta,(Shuvalov, 2000; Sochava, 1977). The Khukhteeg Formation also

Lang Shan (Inner Mongolia) areas, consists of acidic–intermediate vol-(Nessov, 1985; Vitek & Danilov, 2014). Spores and gymnosperm pol-canics and coarse-grained sediments formed in alluvial fan environ-len are common in all areas, but pollens of ginkos are much less abun-ments with bituminous coals and floodplain lacustrine mudstones

dant (Ichinnorov 1998, 2005; Ichinnorov & Hofman, 2012).

(Jerzykiewicz & Russell, 1991; Sladen & Traynor, 2000). Due to theAngiosperms are also observed but are subordinate. The Khukhteeg

compressional tectonics, the Early–Middle Jurassic was generally a

Formation is dated as Albian or Aptian–Albian based on the basis oftime of widespread erosion resulting in the limited distribution of thestratigraphic occurrences of molluscs, turtles and mammals (Shuvalov,

Khamarkhovoor Formation. At the type section (Khamarkhovoor

1975), and pollen-spores (Ichinnorov, 1998, 2005; Ichinnorov &area), the formation is well exposed along the margins of basement

Hofman, 2012; Nichols, Matsukawa, & Ito, 2006).

uplifts of the Unegt subbasin in East Gobi, which is interpreted to be

A period of structural inversion occurred during the mid-Creta-coeval with sedimentary deposits exposed at the Noyon Uul locality

ceous, which resulted in tilting and erosion of underlying syn-rift strata.

of South Gobi (Graham et al., 2001; Hendrix et al., 1996) and at theThese sediments are overlain by flat-lying Upper Cretaceous strata

Dariv locality of southwestern Mongolia (Graham et al., 1997)(Graham et al., 2001; Jerzykiewicz & Russell, 1991; Johnson, 2004;(Figure 1).

Figure 2). Flat-lying Upper Cretaceous strata of Mongolia, divided into

The initiation of the syn-rift stage is marked by a distinctive

the Barunbayan, Bayanshiree, Djadokhta, Barungoyot, and Nemegt

approximately 250 m thick alluvial to fluvial conglomerate (Upper

Formations, in stratigraphically ascending order, are mostly reddish col-Jurassic Sharilyn Formation). The Sharilyn Formation consists ofored and preserve a near-continuous succession from the Cenomanian

reddish colored breccias, conglomerates and fining-upward succes-

to Maastrichtian (Gradzinski, Kielan-Jaworowska, & Maryanska, 1977;

sion composed of red and green mudstone with some calcrete

Hasegawa et al., 2009; Jerzykiewicz & Russell, 1991; Khand et al.,horizons (Graham et al., 2001). The Sharilyn Formation is assigned

2000; Martinson, 1982; Shuvalov, 2000; Sochava, 1975; Van Itter-to the Late Jurassic (Kimmeridgian–Tithonian) based on freshwater

beeck et al., 2005). The erosional unconformity between the Khukh-assemblages

charophytes

teeg and Barunbayan Formations, which is not identified in outcrop(Jerzykiewicz & Russell, 1991; Keller & Hendrix, 1997; Shuvalov,but penetrated in drilled cores at Zuun Bayan oil field, is correlated to

such

as

molluscs,

ostracodesand5 of 17

HASEGAWA ET AL.

the lower boundary of the Upper Cretaceous succession in the South

clear signs of epigenesis, replacement or secondary infill of large

Gobi Basin (Graham et al., 2001). The upper contact of the Barunbayan

dolomite crystals. Both, shale and dolomite layers occasionally con-

Formation with the overlying Bayanshiree Formation is gradational

tain abundant pyrite framboids, indicating microbial sulfate reduction(Martinson, 1982). Although the age of the Barunbayan Formation iswithin anoxic pore- or bottom-waters. Thin layers of tuff are partly

controversial, similar assemblages of ostracods, molluscans, and charo-intercalated, and analcime is common in the deposits, indicating alka-

phytes occur from Albian to Cenomanian strata in eastern Asialine and saline lake water environments with a certain volcanic input.

(Barsbold, 1972; Makulbekov & Kurzanov, 1986). Thus, rift deforma-

The overlying Khukhteeg Formation is exposed mainly in thetion and inversion are thought to be completed at around the transi-

Khukh Teeg hills, about 3 km northwest of the main section of thetion from the Albian to the Cenomanian.

Shine Khudag locality (N44 440 3000 , E107 530 1800 ; Figures 3a and 4f).
Here, the Khukhteeg Formation is composed of greyish to darkbrownish coaly mudstone, greyish sandstones and conglomerates.

4 | L I T H O S T R A T I G R A P H Y OF J U R A S S I C
A N D CR E T A C E O U S LA C U S T R I N E D E P O S I T S

Abundant remains of woody trunks and stromatolites are also found.
The underlying Tsagantsav Formation, exposed in the southeasternpart of the area, is composed of trough cross-bedded, medium- to

Since 2009, we have investigated the oil-prone lacustrine deposits incoarse-grained sandstones and conglomerates of fluvial channel ori-

the Shine Khudag locality (type section of the Lower Cretaceous Shine-

gin. Dark grey paper shales and greyish dolomitic marls are also partly

khudag Formation) in the Shaazangiin Gobi area, and the Eedemt local-

intercalated within the channel fill deposits in the upper part of the

ity in the Khootiin Khotgor area (Ando et al., 2011). The latter deposits,

Tsagantsav Formation (Figure 3a).

the Eedemt Formation in the Eedemt locality, is newly assigned to the

In 2013 and 2014, two scientific research cores (CSH01, CSH02)

late Middle Jurassic age on the basis of conchostracan assemblages

were drilled in this area (Figure 3a,b). CSH01 core is 150 m long, and(Li et al., 2014). In this section, we describe in detail the geology andcovers the lower to upper parts of the Shinekhudag Formation

lithostratigraphy of the Shinekhudag Formation in the Shine Khudag(Figure 4a). CSH02 core is 192 m long, and covers the middle part oflocality, and the Eedemt Formation in the Eedemt locality.

the Shinekhudag Formation to the lower part of the Khukhteeg Formation beneath the top of the Khukh Teeg hills (Figure 4f). CSH01
and CSH02 cores are correlated to the outcrop succession on the

4.1 | Shine Khudag locality

basis of a marker bed represented by a remarkably thick dolomite

The Shinekhudag Formation, well exposed in the type section of thelayer (Figure 3c). The boundary between the Shinekhudag andShine Khudag locality in the Shaazangiin Gobi area in the Dornogobi

Khukhteeg Formations is not well exposed in outcrop, but is well

Province, is composed of alternating beds of dark grey paper shale

observed in the CSH02 core (Figure 3a). We set the boundary by the(oil shale), light grey dolomitic marl, yellowish to brownish dolomite,first appearance of coaly mudstone in the Khukhteeg Formation.

and rare occurrence of silty mudstone layers (Figures 1, 3, and 4).

The Shinekhudag Formation contains abundant fossil remains,

Here, the Shinekhudag Formation is about 250 m thick and overlies

such as dense occurrence of conchostracans (Figure 4e), ostracods,

the fluvio-lacustrine Tsagantsav Formation. It is overlain by the coal-plant remains, bivalves, and gastropods. Yuan & Chen (2005) exam-

bearing fluvial deposits of the Khukhteeg Formation. Strata mainly

ined ostracode and conchostracan assemblages in the Shine Khudag

dip toward the north to northwest (dip angle: approximately 10 –locality; and correlated the Shinekhudag Formation to the Jiufotang

20 ), and they are cut by northwest–southeast directed strike-slip

Formation (Early Aptian; Sha, 2007) of Liaoning Province, northeast-

faults (Figure 3b). The middle to upper parts of the Shinekhudag For-

ern China, based on the occurrence of Yanjiestheria gobiensis and Neo-



0

00



0

00

mation are well exposed in this area (N44 43 00 , E107 55 45 ; Fig-

diestheria mongolensis. Although some studies assigned the age of thisures 3c and 4a–c). The alternations of shale, dolomitic marl andformation to the Hauterivian–Barremian based on ostracode assem-

dolomite are rhythmically bedded in decimeter-, meter-, tens of

blages (e.g. Khand et al., 2000), floral and molluscan evidence sup-meter-scale (mainly meter-scale; Figures 3c and 4b,c). The lowermost

ports an Aptian or Barremian–Aptian age (Jerzykiewicz & Russell,part of the formation is exposed along the banks of a dry creek

1991; Krassilov, 1982). The 40Ar/39Ar age of intercalated basalts in



0

00



0

00

(N44 42 45 , E107 55 15 ), and composed mainly of light grey dolo-

the upper part of the underlying Tsagantsav Formation (ca 126 Ma;

mitic marl and grey calcareous shales with minor amounts of dark

Graham et al., 2001) is also consistent with an Aptian age for the Shi-

grey shales (Figure 4d). Dark grey shale is well-laminated. Microscopic

nekhudag Formation.

inspection reveals that laminae in shale is composed of micrometerscale lamination, consisting of couplets of organic matter and detrital
clay minerals (Figure 4g,h). Micritic texture of calcite crystals is also

4.2 | Eedemt locality

occasionally present just above the organic matter laminae

The Eedemt Formation, exposed in the Khootiin Khotogor area in(Figure 4h). In contrast, dolomitic marls and dolomites show a micritic

Dundgobi Province is composed of brownish-grey paper shale (oiltexture with only weak lamination or even no lamination (Figure 4i).

shale), light grey dolomitic marl, greyish siltstone, and a small amount

Scanning electron microscope (SEM) images show that dolomiteof pebbly sandstone (Figures 1, 5, and 6a–c). Here, the Eedemt For-

layers consist mainly of aggregates of micrometer-scale crystals, exhi-

mation is about 150 m thick, and overlies the coal-bearing fluvial

biting sub-rounded morphology (Figure 4j). Some dolomite beds show

deposits of the Khootiin Khotogor Formation (Figure 6d). The stratapeloidal textures, but lack a distinct oncolitic texture. There are noin the Eedemt locality mainly dip toward southeast (dip angle: ~15 ).

6 of 17

HASEGAWA ET AL.

Geology (Lithostratigraphy) of the Shine Khudag locality. (a) Composite stratigraphic column of the Tsagantsav, Shinekhudag andKhukhteeg Formations at the Shine Khudag locality. Lithologic columns of two scientific research drill-cores (CSH01, CSH02) are also shown. Fm,
Formation. (b) Geologic map of the Shine Khudag area. (c) Detailed stratigraphic columns of the middle to upper parts of the Shinekhudag Formation

FIGURE 3

The Eedemt Formation is well exposed in two sections named

The Eedemt Formation contains abundant fossil remains, such

Eedemt A (ED-A: N45 410 1000 , E107 430 2000 ) and Eedemt B (ED-B:

as conchostracans (Figure 6e), ostracods, plant remains, bivalves,

N45 400 2700 , E107 430 0000 ) (Figure 5a,b). We have also investigated aand fish remains. Li et al. (2014) examined a conchostracan assem-

drill core (KH0818) (Figure 5c), which was drilled by the NEDO-MIT

blage from the Eedemt locality, and described two new species,

Joint Coal Expedition Project in 2008. The upper part of core

Triglypta eedemtensis Li sp. nov. and Dundgobiestheria mandalgo-

KH0818 consists of the Eedemt Formation, while the basal part cor-biensis Li gen. et sp. nov. Li et al. then demonstrated that Triglypta

responds to the uppermost Khootiin Khotogor Formation. Although aand Dundgobiestheria are common components of the Middle

clear lithological boundary between the Eedemt Formation and theJurassic Euestheria ziliujingensis and Sinokontikia faunas, and repre-

underlying Khootiin Khotogor Formation is not exposed in outcrop,

sents a typical taxon in the Middle Jurassic lacustrine sequences ofinspection of core KH0818 shows that the boundary is gradual.

northern Hebei, Junggar, and Turpan basins in northwest China. In

7 of 17

HASEGAWA ET AL.

Outcrop photographs and photomicrograph of the Shinekudag Formation. (a) Overview of the Shine Khudag hill, which comprises
the middle to upper parts of the Shinekhudag Formation. (b) Outcrop of meter-scale alternating beds of dark grey shale, grey shale, andyellowish to brownish dolomite (C10–C30 horizon of Figure 3b). (c) Outcrop of centimeter-scale alternating beds of dark grey shale and grey
calcareous shale. (d) Outcrop photograph of meter-scale alternating beds of grey dolomitic marl, light grey calcareous shale, and grey shale along
the banks of a dry creek, lowermost part of the Shinekhudag Formation. (e) Gregarious fossil occurrence of conchostracan (Neodiestheria
mongolensis) on a bedding surface of the Shinekhudag Formation. (f) Overview of Khukh Teeg hill, the lower part of the Khukhteeg Formation inthis area. (g) Photomicrograph of a dark grey paper shale layer showing micrometer-scale laminations. Dark brown coloured laminae is composedof algal organic matter, while light grey coloured laminae consists of detrital clay mineral. (h) Photomicrograph of reflected fluorescent image of
micrometer-scale laminae in shale layer. Laminae couplets are mainly composed of highly fluorescent, light yellow-green coloured algal organic
matter, and less fluorescent, dark-green coloured detrital clay mineral. Light blue coloured, calcite crystals of micritic texture commonly present
just above algal organic matter laminae. Yellow coloured, cross section of conchostracans is also present within the clay mineral laminae.
Average thickness of the laminae couplet in this photomicrograph is 60 μm. (i) Photomicrograph of a dolomite layer showing dense aggregatesof micrometer-scale dolomite crystals. Weak lamination, consisting of brown coloured organic matter and dolomite crystals, are also observed. (j)Photomicrograph of scanning-electron micrometer (SEM) image of dolomite crystals showing dense aggregates of micrometer-scale crystals of
sub-rounded morphology

FIGURE 4

particular, the Dundgobiestheria assemblage is correlated with theJurassic, and thought to be age equivalent to the Khamarkhovoor

Sinokontikia fauna of the late Middle Jurassic (Callovian) age of theFormation (Figure 2).

Qiketai Formation in the Turpan Basin, northwest China (Li et al.,2014). Therefore, the age of the Eedemt Formation is considered
as late Middle Jurassic (Li et al., 2014). This interpretation is con-

5 | RA D I O M E T R I C A G E C O N S T R A I N T S

sistent with previous paleontological age constraints of the underlying coal-bearing strata of the Khootiin Khotgor Formation. BasedTo establish the reliable chronology for the Shinekhudag and Eedemton the floral evidence (Sodov, 1980, 1985, 1990) and recent paly-

Formations, U–Pb and FT age dating of intercalated tuff samples andnological work by Ichinnorov, Purevsuren, and Buyantegsh (2008),

K–Ar dating of intercalated basalt were conducted (Figures 7 and 8;

the Khootiin Khotgor Formation is assigned to the Early–Middle

Tables 1–3). U–Pb dating is applied to zircons from two tuff layers in8 of 17

HASEGAWA ET AL.

Geology (Lithostratigraphy) of
the Eedemt locality. (a) Geologic map ofEedemt locality. (b) Stratigraphic column of
main exposures of the Eedemt Formation(ED-A and ED-B) and the underlying
Khootin Khotgor Formation (ED-C).
(c) Lithologic column of drilled core
(KH0818) showing the Eedemt Formationand the upper part of the Khootin Khotgor
Formation. Correlated stratigraphic horizonof exposures of the ED-A and ED-B areshown

FIGURE 5

the lower part of the Khukhteeg Formation in the CSH02 core fromareas. Two samples (CSH-2-3-1, CMK-104-9) were taken from thethe Shine Khudag area (sample CSH-2-3-1) and CMK-104 core fromlower part of the Khukhteeg Formation, two samples (CMK-104-

the Matad area, Dornod Province in eastern Mongolia (sample CMK-

44, CMK-107-50) were taken from the middle and upper parts of

104-9), two tuff layers in the middle and upper parts of the Shinekhu-

the Shinekhudag Formation, and one sample (CMK-107-118) was

dag Formation in the CMK-104 and CMK-107 cores from the Matadtaken from the uppermost part of the Tsagantsav Formationarea, (samples CMK-104-44 and CMK-107-50), a tuff layer in the(Figures 7a–e and 8; Table 1). One sample (ED-C-18-14) from theuppermost part of the Tsagantsav Formation in the CMK-107 core

uppermost part of the Khootin Khotgor Formation was analyzed

from the Matad area (sample CMK-107-118), and from a tuff layer inby both FT method and the U–Pb dating at Kanazawa University

the uppermost part of the Khootin Khotgor Formation in core

(Figure 7f; Tables 1 and 2). Zircons were extracted from tuff sam-

KH0818 from the Eedemt area (sample ED-C-18-14) (Figures 3, 5,

ples, and euhedral zircon crystals were mounted and polished.

and 8). FT dating is also applied to zircons from a tuff layer in the

Extracted and polished zircons were first analyzed by using catho-

uppermost part of the Khootin Khotgor Formation in core KH0818

deluminescence (CL) at Nagoya University to check potential dif-from the Eedemt area (sample ED-C-18-14). K-Ar dating is applied toferences in the internal structure.

thick (150–200 m) basalt beds in the middle part of the Tsagantsav

The LA–ICP–MS analyses of extracted zircons were performed

Formation from cores CMK-57 and CMK-58 drilled in the Suman Uul

on an Agilent 7700× ICP–MS and Electro Scientific Industries

area, Sukhbaatar Province in eastern Mongolia (Figure 8).

NWR213 at Nagoya University. Zircon 91500 was used as an external standard for age calibration and the NIST SRM610 synthetic glass

5.1 | U–Pb and FT age

was applied for instrument optimization. In order to check the accuracy and reproducibility of the zircon ages, another zircon standard

U–Pb dating of zircons from five tuff samples was conducted by

OD-3 (Iwano et al., 2013) was also analyzed. Several zircon crystals

Laser ablation–inductively coupled plasma–mass spectrometry (LA–were measured from each sample and grains with negligible common

ICP–MS) at Nagoya University. Numerous tuff and tuffaceous

lead (204Pb is under the detection limit) were further considered.

sandstone layers are intercalated in the lower part of the Khukh-

They were displayed in a concordia diagram (Figure 7) using Isoplot

teeg Formation and the uppermost part of the Tsagantsav Forma-

3.75 (Ludwig, 2012), and grains plotted on the concordia within 2tion in core samples derived from the Shine Khudag and Matadsigma error range were accepted to calculate weighted mean

9 of 17

HASEGAWA ET AL.

Outcrop photographs of the
Eedemt Formation. (a) Overview ofEedemt hill (ED-A section), the lower partof the Eedemt Formation. (b) Outcrop ofmeter-scale alternating beds of brownishgrey paper shale, grey shale, and lightgreyish dolomitic marl in the ED-A section.
(c) Outcrop of intercalated pebbly
sandstone layer in the ED-B section.
(d) Outcrop of coal seam, coaly mudstone,
whitish sandstone of the upper part of the
Khootin Khotgor Formation in the ED-C
section at an open-pit coal mine.
(e) Gregarious fossil occurrence ofconchostracan (Dundgobiestheria
mandalgobiensis) on a bedding surface of
the Eedemt Formation

FIGURE 6238

U–206Pb and 235U–207Pb ages (Table 1). Because 238U–206Pb ages

are more precise than235

U–207Pb ages,

238

U–206Pb ages arezircons from the analyzed tuff layer were crystallized not immediately
after the time of eruption.

accepted to discuss the depositional age of each formation. Experimental and instrumental details for U–Pb age dating at Nagoya Uni-

5.2 | K–Ar isotopic age

versity, and FT and U–Pb age dating at Kanazawa University aredescribed in Kouchi et al. (2015) and Hasebe, Tamura, and Arai

K–Ar dating has been applied to two samples (CMK-57-72, CMK-(2013), respectively, and references therein.

58-17) taken from two basalt layers in the middle part of the Tsa-The U–Pb dating results (238U–206Pb ages) are shown in Figures 7

gantsav Formation in the Suman Uul area in eastern Mongolia

and 8. Two samples (CSH-2-3-1, CMK-104-9), taken from the lower(Figure 8). The basalt layers show baked basal contacts and weath-part of the Khukhteeg Formation show the youngest ages of

ered and vesicular tops. The basalts are generally fine grained, and(118.5 0.9) Ma and (119.7 1.6) Ma. Two other samples (CMK-

include calcite veins. Thus, fresh samples lacking calcite veins were104-44, CMK-107-50) from the middle and upper parts of the Shine-

selected. K–Ar analysis and extraction of plagioclase from basalt

khudag Formation show older ages of (121.0 0.9) Ma andsamples were carried out at Hiruzen Institute for Geology & Chro-(121.9 2.3) Ma. A single sample (CMK-107-118) taken from thenology, Japan. Plagioclase crystals were extracted from crushed

uppermostTsagantsav

Formation

shows

the

oldestageof(123.8 2.0) Ma. Thus, the depositional age of the Shinekhudag Formation can be constrained to range between (123.8 2.0) Ma and(118.5 0.9) Ma. The older age is consistent with the 40Ar/39Ar ages
for tuff layers corresponding to the uppermost Tsagantsav Formation((125 1) Ma) and the basal part of the Shinekhudag Formationbasalt samples by conventional mineral separation (50–75 μm). To
remove alteration influences, extracted plagioclase samples weretreated with HCl solution. Detailed analytical procedures of K–Ar
method in Hiruzen Institute for Geology & Chronology are described
in Itaya et al., (1991). Resultant ages are (131.1 2.8) Ma for CMK-((121 1) Ma) as reported by Johnson and Graham (2004) (Figure 8).

57-72 and (129.0 2.8) Ma for CMK-58-17 (Table 3; Error estimate

The FT and U–Pb ages of a tuff sample (ED-C-18-14) from theis σ throughout). These values are consistent with the existing

uppermost part of underlying Khootin Khotgor Formation analyzed at

40

Ar/39Ar ages ((126 1) Ma and (131 1) Ma) reported by Graham

Kanazawa University are consistent within two sigma error range

et al. (2001) for corresponding basalt layers in the middle part of the((164.8 2.0) Ma and (158 7) Ma; Tables 1 and 2), assuming that

Tsagantsav Formation.

10 of 17

HASEGAWA ET AL.

U–Pb concordia diagrams fortuff samples. (a,b) The lower part of
Khukhteeg Formation in Shine Khudag andMatad sections. (c,d) The upper part of the
Shinekhudag Formation in Matad section.
(e) The uppermost Tsagantsav Formationin Matad section. (f ) The basal part of the
Eedemt Formation in Eedemt section

FIGURE 7

6 | DISCUSSION

Kusumi) from the lacustrine shale of the lower part of the Zuunbayan
Formation (correlative strata of the Shinekhudag Formation) of East

6.1 | Age of the Shinekhudag and Eedemt
Formations

Gobi. Some of the lacustrine shales within the Tsagantsav Formation

Paleontological age constraints of the Shinekhudag Formation have

(1982) defined the age of the Shinekhudag Formation as Aptian, on

been discussed controversially. Although abundant molluscan andthe basis of two floral assemblage zones including Baierella hastata–possibly were mistakenly correlated to the Shinekhudag Formation,
as pointed out by Graham et al. (2001). On the other hand, Krassilov

ostracod fossils have been cited as evidence of a Barremian–

Araucaria mongolica, and Limnothetis gobiensis–Limnoniobe insignis,

Hauterivian age (Khand et al., 2000; Martinson & Shuvalov, 1973), an

which also occur in dated strata in eastern Siberia. Based on the con-

Aptian age of the formation was suggested by floral, conchostracans,chostracan assemblage from the Shine Khudag locality, Yuan andand palynological evidence (Graham et al., 2001; Ichinnorov 1998,

Chen (2005) concluded that the Shinekhudag Formation can be cor-

2005; Ichinnorov & Hofman, 2012; Jerzykiewicz & Russell, 1991;

related to the Jiufotang Formation of the Liaoning Province, north-

Krassilov, 1982; Yuan & Chen, 2005). Martinson and Shuvalov (1973)

eastern China, given its similar faunal composition, whose age waspreviously considered a Hauterivian–Barremian age for the formation

assigned to the Early Aptian (Sha, 2007). In addition, Ichinnorov

based on a conchostracans assemblage containing Bairdestheria sinen-(1998, 2005) and Ichinnorov and Hofman (2012) report pollen andsis (Chi), B. mattoxi Krassilov, and Pseudograpta asanoi (Kobayashi etspore assemblages from the Shinekhudag Formation from BayanHASEGAWA ET AL.

11 of 17

Litho- and chronostratigraphic correlation of the Tsagantsav, Shinekhudag, and Khukhteeg Formations in several sections in southernto eastern Mongolia. 40Ar/39Ar age* of intercalated basalt in Khara Khutul and Tsagan Tsav localities are from Graham et al. (2001). 40Ar/39Ar
age# of intercalated tuffs in core sample of Zuun Bayan oil field are from Johnson and Graham (2004). K/Ar age of intercalated basalt in Matad
locality, and U–Pb age of intercalated tuff in Shine Khudag and Matad localities are from this study. Fm, Formation

FIGURE 8

Erkhet, Matad, and Shine Khudag localities, which support an Aptian

deposited between (123.8 2.0) Ma and (118.5 0.9) Ma of theage for the formation.

Early Aptian (Figures 7 and 8). 40Ar/39Ar age data of intercalated tuff

On the basis of our U–Pb age dating of zircons in intercalated

samples from a drillcore of the Zuun Bayan oil field (Johnson & Gra-

tuff samples, the Shinekhudag Formation is considered to beham, 2004), which yielded (121 1) Ma in the basal part of the12 of 17

HASEGAWA ET AL.

Zircon U–Pb dating results of the lower part of the Khukhteeg Formation, the upper part of the Shinekhudag Formation, theuppermost Tsagantsav Formation in Shine Khudag and Matad sections, and the basal part of the Eedemt Formation in Eedemt section

TABLE 1

206

U/238U207

U/235U

Sample name

Number ofgrains forconcordia
diagramNumber ofconcordant
grainsAverageratioStandard
deviation

Weightedmean age
(Ma)

Error
(2σ) (Ma)

AverageratioStandard
deviation

Weightedmean age
(Ma)

Error
(2σ) (Ma)

CSH-2-3-1

37

25

0.018 60.000 8

118.5

0.9

0.126

0.014

120.3

3.4

CMK-104-91080.018 70.000 7

119.7

1.6

0.125

0.022

122.2

6.3

CMK-107-50

6449

0.019 00.000 8

121.0

0.9

0.139

0.025

136.7

4.2

CMK-104-441290.019 10.001 0

121.9

2.3

0.121

0.014

121.6

10.5

CMK-107-11810

70.018 90.001 4

123.8

2.0

0.122

0.018

122.4

8.0

ED-C-18-1464

480.024 70.001 1

164.8

2.0

0.201

0.026

184.2

5.7

TABLE 2

Zircon fission track (FT) dating result of the basal part of the Eedemt Formation in Eedemt section
LA–ICP–MS data
Zr

FT datingsamplename

Number Number FT density Averageof grains of FT(106/cm2) 238U/29Si

ED-C-18-14 61

TABLE 3

3 475

5.747

0.079 3

NIST610
Standard
deviation(%)
40

Average
U/29SiStandard
deviation(%)

Pooled mean
Zr/NIST610(238U/29Si)

Error
(2σ)
(%)

Pooledmean U(mg/g)

Pooledmean Error
age
(2σ)
(Ma)
(Ma)

0.230

7

0.324

0.64

70.3

158

238

7

Result of K–Ar dating of basalt samples in the middle part of the Tsagantsav Formation in Suman Uul section

Sample name

K content (wt%)

Average K (wt%)

Radiogenic 40Ar (10−8 cc STP/g)

K–Ar age (Ma)

Atomospheric 40Ar (%)

CMK-57-72

1.414

1.418 0.510

748.0 7.5

131.1 2.8

2.4

1.275 1.110

661.7 6.4

129.0 2.8

1.7

1.422
CMK-58-17

1.268
1.282

Decay constant: λe = 0.581 × 10−10/year, λβ = 4.962 × 10−10/year, 40K/K = 1.167 × 10−4 (Steiger & Jager, 1977).

Shinekhudag Formation and (125 1) Ma age in the uppermost part

strata in China (Tiaojishan Formation in northern Hebei basin) areof the underlying Tsagantsav Formation, and the K/Ar age

also supported by radiometric age data (Li et al., 2014; Zhang,((129.0 2.8) Ma, (131.1 2.8) Ma; Table 3) and 40Ar/39Ar age

Wang, & Liu, 2008). Based on the U–Pb dating of intercalated tuff((126 1) Ma, (131 1) Ma; Graham et al., 2001) of the intercalated

samples, the age of the Tiaojishan Formation is assigned as frombasalt in the upper Tsagantsav Formation are also consistent with our

165 Ma to 156–153 Ma of late Middle–early Late Jurassic (Zhang

chronology for the Shinekhudag Formation (Figure 8). However, theet al., 2008).

error range of our U–Pb age and reported 40Ar/39Ar age of theOn the basis of our U–Pb and FT age of a tuff sample (ED-C-18-

boundary between the Shinekhudag Formation and the underlying14) in the uppermost part of the underlying Khootin Khotgor Forma-

Tsagantsav Formation overlap with each other. Thus, the precise agetion, the Eedemt Formation is considered to be deposited at aroundof the initiation of the Shinekhudag Formation needs further investi-

165–158 Ma of Callovian–Oxfordian (Figures 7 and 8; Gradstein

gation. The total thickness of the Shinekhudag Formation is about

et al., 2012). U–Pb and FT ages ((164.8 2.0) Ma and (158 7) Ma,

250 m in the type section of Shine Khudag and in most of the otherrespectively) are within two sigma error range, assuming that zircons

sections (Figure 8). Given that the deposition of the Shinekhudag For-

from the analyzed tuff layer were crystallized not very early from themation was initiated at (123.8 2.0) Ma (our U–Pb age data) ortime of eruption. Given the age of the overlying Sharilyn Formation is(121 1) Ma (40Ar/39Ar age of Johnson & Graham, 2004), and ended

assigned to the late Late Jurassic (Kimmeridgian–Tithonian) based on

before (118.5 0.9) Ma (our U–Pb age data), the calculated sedimen-abundant freshwater assemblages such as molluscs, ostracodes andtation rate of the formation is in range between (4.7 2.6) cm/ky andcharophytes (Jerzykiewicz & Russell, 1991; Keller & Hendrix, 1997;(10.0 7.6) cm/ky.

Shuvalov, 2000), and 40Ar/39Ar age ((155 1) Ma; Graham et al.,Although the age of the lacustrine deposits in the Khootin2001) of the intercalated tuff in the middle Sharilyn Formation, theKhotgor area (Eedemt Formation) was also controversial, Li

age of the Eedemt Formation likely includes Oxfordian. But the possi-

et al. (2014) demonstrated the existed problems, and assigned its

bility of Callovian age is also not negligible due to the age constraints

age as late Middle Jurassic. Li et al. demonstrated that the Dundgo-

by conchostracans assemblages (Li et al., 2014). Conclusively, the age

biestheria assemblage is correlated with the Sinokontikia fauna ofof the Eedemt Formation is considered to be deposited at aroundthe late Middle Jurassic (Callovian) age of the Qiketai Formation in

165–158 Ma

the Turpan Basin, northwest China. The age of the correlative

Jurassic).

of

Callovian–Oxfordian(late

Middle–earlyLate13 of 17

HASEGAWA ET AL.

6.2 | Depositional environments of shale–dolomite
coupletsof the forest–prairie border setting should be a sensitive indicator ofsalinity variations in the lake, in response to climatically inducedchanges in the balance between precipitation and evaporation

Both, the Shinekhudag and Eedemt Formations are composed mainlyof meter-scale alternating beds of dark grey shale, light grey dolomiticmarl and dolomite, with minor amounts of siltstone and sandstone.
Shale beds are generally well-laminated with no significant bioturbation (Figure 4g,h), indicating low-oxygen or anoxic bottom-water conditions. Permanent or at least seasonal stratification promotes the(Dean & Megard, 1993).
Given that the dolomite beds in the Shinekhudag Formation
show no clear evidence of lacustrine littoral facies such as evaporative facies or desiccation structures (Johnson & Graham, 2004), thedolomite beds are likely to be formed by primary precipitation fromthe lake water in the lake center. Dry climatic periods would alterdevelopment of anoxic bottom waters in deep lakes with several tens

lake chemistry such that magnesium ions could be concentratedof meters depth (O'Sullivan, 1983; Zolitschka et al., 2015). Tropical(e.g. Dean & Megard, 1993). Abundant green algae (e.g. Botryococcus)

lakes in Africa (e.g. Lake Malawi and Lake Tanganyika) are examples

possibly have partly contributed magnesium ions as well. By analogyof modern freshwater lakes where permanent stratification promoteswith prior studies of dolomite formation in lakes, the dolomite beds

stagnant anoxic bottom water and TOC-rich sediments (Brown, Cal-in the Shinekhudag and Eedemt Formations are likely to be formed

lonnec, & German, 2000; Cohen et al., 2006). Palynological evidenceby autochthonous and primary precipitation during phases of low lake

for abundant green algae (incl. Botryococcus), and the presence oflevel with possible influence of microbial mediation. Presence of cal-

β-carotane in oil shale deposits of both formations indicate consider-cite crystals within shale layers (Figure 4h) and predominance of dolo-

able algal export production in slightly saline to freshwater lakes withmite crystals in dolomite layer are also consistent with the afore-

anoxic bottom waters (Johnson et al., 2003; Sladen & Traynor, 2000).

mentioned observations by Dean and Megard (1993).

Yamamoto et al. (1998) also reports geochemical evidence for bloom-

Conclusively, the meter-scale alternations of shale and dolomite

ing of autotrophic prokaryotes and thermal stratification with anoxic

were primarily controlled by lake level changes, which could havelake-bottom conditions in the East Gobi basin.

reflected changes in precipitation. Shale beds formed in the deep lake

In contrast, dolomitic marl and dolomite beds show weakly lami-

during lake level highstand of freshwater to slightly saline lake envi-

nated or no lamination, and are composed of aggregates ofronment, while dolomite beds formed by autochthonous and primary

micrometer-scale sub-rounded dolomite crystals. Similar textures ofprecipitation during lowstand of saline lake environment with possi-

the lacustrine dolomite with small, 1–2 μm crystal aggregates exhibit-

ble influence of microbial mediation. The common presence of anal-

ing sub-rounded morphology are reported from Pliocene deposits incime in tuff and shale layers is also consistent with alkaline and saline

Spain and Holocene deposits in France, which are interpreted to belake waters with a certain volcanic input (Remy & Ferrell, 1989).

formed by bacterially-induced dolomite precipitation in shallow salinelake environments (Breheret et al., 2008; Del Cura et al., 2001).
Because of the lack of replacement textures, micro-crystalline dolo-

6.3 | Origin of micrometer-scale lamination

mite is thought to represent autochthonous and primary precipitation

As described above, shale beds in the Shinekhudag Formation include

from the lake water or early diagenetically from pore water. In addi-

micrometer-scale laminations, which are composed of couplets oftion, almost all modern lacustrine dolomite is formed primary withindark brownish colored organic matter-rich laminae and light greyish

saline lake waters with a high Mg/Ca ratio, in association with biolog-

colored laminae composed of detrital clay minerals (Figure 4g ).

ical activity (Last, 1991).

Reflected fluorescent microscopic inspection confirms that the dark

An observational study of modern lakes by Dean and Megard

brownish colored organic matter-rich laminae consist of highly fluo-(1993) provides significant analogy for dolomite deposition and depo-

rescent algal organic matter, while the light greyish colored laminae

sitional environmental change of shale–dolomite couplets in the Shi-

consist of less fluorescent detrital clay mineral (Figure 4h). Micritic

nekhudag Formation. The sediments of saline prairie lakes in westerntexture of calcite crystals is also occasionally present just above theMinnesota, North Dakota, and South Dakota commonly contain high-algal organic matter laminae (Figure 4h). These characters and order-

Mg calcite and dolomite especially during the mid-Holocene dry cli-

ings of the composition of lamina couplets are similar to the organic-matic period. In contrast, low-Mg calcite is dominant in today’s

clastic varve in lakes of temperate climatic setting as discussed below

forest-dominant lake sediments under more humid climatic setting.

(e.g. Luder et al., 2006; Zolitschka et al., 2015).

Dean and Megard also demonstrated that changes in relative propor-

In addition, the occurrence of calcite crystal above the organic-

tions of dissolved calcium and magnesium in lakes across therich laminae is significantly consistent with observation of modern

precipitation-evaporation gradient setting of 219 lakes from Wiscon-lake sediments by Dean and Megard (1993). Seasonal changes in pri-sin, North Dakota, and South Dakota (data from Gorham, Dean, &mary productivity, carbonate saturation, water chemistry, andSanger, 1982). In response to increasing salinity (decreasing lake-

sediment-trap samples reveal that calcite precipitates during latelevel) and precipitation of calcium carbonate, the relative proportion

summer, triggered by algal photosynthesis. The pH of the lake epilim-of calcium decreases and the relative proportion of magnesium

nion increases to almost 9.0 in late summer in response to photosyn-

increases. Hence, the precipitation of calcite results in an increase in

thetic removal of CO2 during the summer months (Dean & Megard,

the Mg/Ca ratio in lake water. Then, high Mg/Ca ratio in the arid

1993). Thus, by analogy with existing studies of varve sediments insaline lakes results in deposition of dolomite and high-Mg calcite intemperate lakes, algal organic matter laminae and micritic calcite crys-

their sediments. Therefore, carbonate composition in lake sediments

tals in shale layer of the Shinekhudag Formation are likely to be14 of 17

HASEGAWA ET AL.

formed during the late spring to late summer, while detrital clay min-Kravchinsky, 2005; Kravchinsky et al., 2002; Metelkin et al., 2010;eral laminae are dominant during autumn–winter.

Pruner, 1992; Yang et al., 2015; Zhao et al., 1990; Zorin, 1999).

Notably, our interpretation of the micrometer-scale laminations

Based on the reconstructed paleogeographic map of the paleo-Asianas annually-laminated varves can be strongly supported by the sedi-

continent, the Gobi basin was located at approximately 41 –44 N

mentation rates. Given that the average thickness of the laminae cou-

(Pruner, 1992) or approximately 45 –50 N (Cogne et al., 2005) during

plets is about 50–70 μm in shale layers (Figure 4g,h) compared to

Early Cretaceous times, whereas it was located at 42 –55 N during

about 100–120 μm thick, less pronounced lamination preserved in

the Middle–Late Jurassic (Cogne et al., 2005; Kravchinsky et al.,dolomitic marl facies (Figure 4i). If the laminae couplets in the Shine-

2002). Despite considerable uncertainties regarding the exact paleo-khudag Formation are seasonal varves as discussed above, the aver-latitudinal position of the Gobi basin during the Middle Jurassic, aage sedimentation rate is estimated to be approximately 50–120 μm/

mid-latitudinal position at 40 –50 can be inferred for both theyear, which corresponds to approximately 5–12 cm/ky. This estimate

Middle–Late Jurassic and Early Cretaceous times.

is consistent with the chronostratigraphically calculated sedimentation rate of (4.7 2.6) cm/ky to (10.0 7.6) cm/ky described above.

Due to the different paleogeographic settings caused by the closure of the Mongol-Okhotsk Ocean (Kravchinsky et al., 2002;

Varve couplets consisting of algal organic matter and detrital clay

Metelkin et al., 2010; Yang et al., 2015), distance of the Gobi basinminerals in shale layers of the Shinekhudag Formation resemble afrom coastline could be largely different between the Middle–Latetypical feature of modern and Quaternary lake sediments in the tro-

Jurassic and the Early Cretaceous. In addition, the tectonic settings ofpics and temperate latitudes, and they are categorized as organic-

the Jurassic and Cretaceous sedimentary basins are also different.

clastic varves (e.g. Brauer et al., 1999; Brown et al., 2000; Cohen

Nevertheless, both of the lacustrine oil shales were thought to be

et al., 2006; Enters et al., 2010; Luder et al., 2006; Ojala et al., 2012;

deposited in intracontinental extensional basins in mid-latitudes of

Shanahan et al., 2008; Zolitschka et al., 2015). The dark brown col-paleo-Asian continent during the late Middle–early Late Jurassic andored organic-rich laminae were probably formed by phytoplankton

the late Early Cretaceous. Early Cretaceous extension was possibly

remains produced as a result of late spring to summer blooms, while

resulted from strike-slip faults that bounded the plate fragments inthe light grey colored detritus clay-rich laminae represent allochtho-

China and Mongolia. Although cause of the Middle–Late Jurassic

nous terrestrial input deposited during the autumn–winter season

extension is currently unclear, combination of causes was possibly(Figure 4g). Since sedimentary grading structures are absent in the

responsible for extensional basin development, i.e. both subduction-

detrital laminae, formation of clay-rich laminae during rainy summer

related and strike-slip-related extension led to conditions ideal forseasons is unlikely. Micritic calcite is occasionally preserved in justlake basins (Sladen & Traynor, 2000). Tectonic processes and basin

above the organic-rich part of individual laminae (Figure 4h), which

evolution basically controlled the deposition of these lacustrine oil

possibly indicate formation via biochemically-induced precipitation

shales. In addition, the common occurrence of stratigraphically con-during late summer season (Dean & Megard, 1993). This type oftinuous lacustrine deep water deposits associated with coal-bearing

varve couplet composition indicates increased lake surface tempera-

fluvial strata in both, the Middle–Late Jurassic and the late Early Cre-

ture and primary production during summer season as reported fromtaceous indicate enhanced precipitation within a humid climatic set-

modern mid-latitude lakes (e.g. Sacrower See Lake in Germany;ting during the early Aptian and the Callovian–Oxfordian times isEnters et al., 2010; Luder et al., 2006). Given that the lacustrine oilmore likely to be a key factor of the widespread occurrence of lacus-

shales of the Shinekhudag Formation are also deposited in the mid-trine

latitude, the distinct varve composition of this formation suggests

et al., 2012).

existence of seasonality during the mid-Cretaceous “greenhouse”
climate.

environmentsinsoutheast

Mongolia(e.g.

Hasegawa

It is noteworthy that the deposition of lacustrine oil shales in the
Shinekhudag Formation occurred between (123.8 2.0) Ma andShales in the Eedemt Formation also exhibit micrometer-scale(118.5 0.9) Ma of early Aptian, which is nearly contemporary to thelamination, suggesting the possibility of seasonal varve origin. How-

one of the largest Ocean Anoxic Events (OAE) of OAE1a interval

ever, the lack of precise absolute age control of the Eedemt Forma-(Emeis & Weissert, 2009; Erba et al., 2015; Follmi, 2012). In addition,

tion does not allow for reliable estimates of sedimentation rates.

the depositional interval of the Eedemt Formation at around

Further studies are needed to verify the origin of the micrometer-

Callovian–Oxfordian is also nearly accordant to the recently recog-

scale lamination pattern characterizing the Eedemt Formation.

nized OAE event during the Callovian–Oxfordian (e.g. Hautevelle
et al., 2007; Louis-Schmid et al., 2007; Nozaki, Kato, & Suzuki, 2013;

6.4 | Tectonic-climatic setting of the Jurassic–
Cretaceous lacustrine deposits
The paleo-latitudinal position of the Jurassic–Cretaceous sedimentary

Rais et al., 2007; Weissert, 2011).

7 | SUMMARY

basins in Mongolia represents an important aspect for understanding
potential trigger mechanisms for the widespread deposition of lacus-

The present study explored the spatial and temporal distribution oftrine oil shales at that time. The configuration of the paleo-Asian con-the Mongolian oil shales, and established detailed ages for the Juras-

tinent and the closure of Mongol-Okhotsk Ocean have been

sic and Cretaceous lacustrine deposits in Mongolia. The Lower Creta-

reconstructed based on paleomagnetic studies (Chen et al., 1993;

ceous Shinekhudag Formation, which is widely distributed in

Cogne et al., 2005; Halim et al., 1998; Hankard, Cogne, &southeastern Mongolia, is composed of meter-scale alternating beds

15 of 17

HASEGAWA ET AL.

of shale and dolomite. At the type locality of the Shine Khudag area,changes in Asia and the Western Pacific”. This study was supported

the formation is about 250 m thick and overlies the fluvio-lacustrine

financially

Tsagantsav Formation, and is overlain by the coal-bearing fluvial(B) (No. 25870309), Grant for Environmental Research Projects from

deposits of the Khukhteeg Formation. The Middle–Late Jurassic

the Sumitomo Foundation (143485), and Fujiwara Natural History

Eedemt Formation is composed of alternations of shale, dolomitic

Foundation provided to H. Hasegawa, and Grant-in-Aid for Scientificmarl, silt- and sandstone. In the type locality of the Khootin Khotogor

Research (B) (No. 21403008 and 25302011) provided to H. Ando.

byGrant-in-Aid

for

Young

Scientific

Researcharea, the formation is 150 m thick, and overlies the coal-bearing fluvial deposits of the Khootin Khotogor Formation. Meter-scale alter-

ORCID

nating beds of shale and dolomite are thought to record fluctuations
in lake level, which was primarily controlled by precipitation changes.

Hitoshi Hasegawa

http://orcid.org/0000-0003-0925-2717

Namely, shale beds were deposited in the center of a deep lake during a highstand phase of freshwater to slightly saline environment,
while dolomite beds were formed by autochthonous and primary precipitation during lowstand phase of saline lake environment with possible influence of microbial mediation.
To establish a detailed chronology of these lacustrine deposits,
FT and U–Pb age dating of zircons derived from intercalated tuffs
and K–Ar age dating of intercalated basalts were carried out. Basedon the new radiometric age dating, the Shinekhudag Formation was
deposited between (123.8 2.0) Ma and (118.5 0.9) Ma of early
Aptian times, while the Eedemt Formation was deposited at around
165–158 Ma in the Callovian–Oxfordian, although precise ages ofthe both formations need further investigation. The calculated sedimentation

rate

of

the

Shinekhudag

Formationisbetween(4.7 2.6) cm/ky and (10.0 7.6) cm/ky. Shales in the Shinekhudag
Formation exhibit micrometer-scale laminations, which are composedof couplets of algal organic matter and detrital clay minerals. Givenan average thickness of the micro-lamination of about 50–70 μm inshale facies compared to about 100–120 μm in dolomitic marl facies,
the micro-laminations in the Shinekhudag Formation are most likelyof varve origin reflecting seasonal cyclicity.
Lacustrine oil shales of both the Middle–Late Jurassic and thelate Early Cretaceous ages were deposited in intracontinental extensional basins in the mid-latitudes of the paleo-Asian continent.
Although basin tectonics may have affected the deposition of theselacustrine oil shales, enhanced precipitation within a humid climatic
setting during the early Aptian and the Callovian–Oxfordian times ismore likely to be a key factor of the widespread occurrence of lacustrine environments in southeast Mongolia. The depositions of thelacustrine oil shales, both in the Shinekhudag Formation at around
123–119 Ma and in the Eedemt Formation at around Callovian–
Oxfordian, are nearly contemporary to reported OAE intervals.

ACKNOWLEDGEMEN T
We are grateful to B. Gantsolmon, S. Kito, S. Yogo, M. Nozaki and
A. Matsumoto for their analytical and technical assistance. We arealso grateful to Enkhtogs and Orgil of Cameco Mongolia LLC for
donation of chronological samples. We also appreciate to Y. Kouchi
for comments and corrections for the U–Pb age data. We are also
grateful to R. Barsbold, K. Tsogtbaatar, Y. Khand, N. Katsuta and
F. Hirsch for fruitful comments of the manuscript. We are also grateful to detail and constructive reviews by Y. I. Lee, H. Weissert, andAE S. Bajpai. This paper is a contribution to UNESCO-IUGS IGCP608“Cretaceous ecosystems and their responses to paleoenvironmental

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How to cite this article: Hasegawa H, Ando H, Hasebe N,
et al. Depositional ages and characteristics of Middle–Upper
Jurassic and Lower Cretaceous lacustrine deposits in southeastern Mongolia. Island Arc. 2018;e12243. https://doi.org/
10.1111/iar.12243

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Graphical abstract
Depositional ages and characteristics of Middle–Upper Jurassic and Lower Cretaceous
lacustrine deposits in southeastern Mongolia
Hitoshi Hasegawa, Hisao Ando, Noriko Hasebe, Niiden Ichinnorov, Tohru Ohta, Takashi Hasegawa,
Masanobu Yamamoto, Gang Li, Bat-Orshikh Erdenetsogt, Ulrich Heimhofer, Takayuki Murata, Hironori Shinya,
G. Enerel, G. Oyunjargal, O. Munkhtsetseg, Noriyuki Suzuki, Tomohisa Irino, and Koshi Yamamoto

The present study aims at establishing depositional ages and characteristics of the Jurassic and Cretaceous lacustrine deposits in Mongolia.
Based on radiometric age dating, the Shinekhudag Formation was deposited between (123.8 2.0) Ma and (118.5 0.9) Ma of early Aptian, andthe Eedemt Formation was deposited at around 165–158 Ma of Callovian–Oxfordian. Although basin tectonics may have affected the deposition of these lacustrine oil shales, enhanced precipitation within a humid climatic setting during the early Aptian and the Callovian–Oxfordiantimes is more likely to be a key factor of the widespread occurrence of lacustrine environments in southeast Mongolia.

HASEGAWA ET AL.

RESEARCH ARTICLE
Depositional ages and characteristics of Middle–Upper Jurassic andLower Cretaceous lacustrine deposits in southeastern Mongolia
HITOSHI HASEGAWA, HISAO ANDO, NORIKO HASEBE, NIIDEN ICHINNOROV, TOHRU OHTA,TAKASHI HASEGAWA, MASANOBU YAMAMOTO, GANG LI, BAT-ORSHIKH ERDENETSOGT,ULRICH HEIMHOFER, TAKAYUKI MURATA, HIRONORI SHINYA, G. ENEREL, G. OYUNJARGAL,
O. MUNKHTSETSEG, NORIYUKI SUZUKI, TOMOHISA IRINO, KOSHI YAMAMOTO

モンゴル南東部には下部白亜系湖成オイルシェールが広く分布する. 最近の研究により, その年
代は白亜紀前期だけではなく, ジュラ紀中期も含む可能性が示されている. 本研究は, モンゴルのジュラ系および白亜系湖成層の堆積年代と特徴を解明することを目的とする. 下部白亜系シネ
フダグ層および中部ジュラ系エーデムト層は, 共に頁岩と苦灰岩の互層を主体とする. 頁岩は高水位期に, 苦灰岩は低水位期に堆積し, その互層は降水量変動に伴う湖水位変動を反映している. 介在する凝灰岩の U-Pb 年代により, シネフダグ層は123.8 2.0 Ma から118.5 0.9 Ma のア
プチアン期前期に, エーデムト層は約165-158 Maのカロビアン–オックスフォーディアンに堆積した. シネフダグ層の頁岩中には藻類起源有機物と粘土鉱物の互層からなるマイクロラミナが見られ, ラミナの平均層厚と平均堆積速度(4.7 2.6 cm/ky10.0 7.6 cm/ky)から, ラミナは年縞であると解釈された. モンゴル南部の湖成層の堆積は, 内陸盆地のテクトニクス活動に加え, アプ
チアン期前期とカロビアン–オックスフォーディアンにおいて湿潤気候が卓越したことが関係していたと考えられる.