TY - JOUR

T1 - Depositional ages and characteristics of Middle–Upper Jurassic and Lower Cretaceous lacustrine deposits in southeastern Mongolia

AU - Hasegawa, Hitoshi

AU - Ando, Hisao

AU - Hasebe, Noriko

AU - Ichinnorov, Niiden

AU - Ohta, Tohru

AU - Hasegawa, Takashi

AU - Yamamoto, Masanobu

AU - Li, Gang

AU - Erdenetsogt, Bat Orshikh

AU - Heimhofer, Ulrich

AU - Murata, Takayuki

AU - Shinya, Hironori

AU - Enerel, G.

AU - Oyunjargal, G.

AU - Munkhtsetseg, O.

AU - Suzuki, Noriyuki

AU - Irino, Tomohisa

AU - Yamamoto, Koshi

N1 - Funding information: Grant-in-Aid for Scientific Research (B), Grant/ Award number: 21403008 and 25302011; Fujiwara Natural History Foundation; Grant for Environmental Research Projects from the Sumitomo Foundation, Grant/Award number: 143485; Young Scientific Research (B), Grant/ Award number: 25870309 We are grateful to B. Gantsolmon, S. Kito, S. Yogo, M. Nozaki and A. Matsumoto for their analytical and technical assistance. We are also 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, and AE S. Bajpai. This paper is a contribution to UNESCO-IUGS IGCP608 “Cretaceous ecosystems and their responses to paleoenvironmental changes in Asia and the Western Pacific”. This study was supported financially by Grant-in-Aid for Young Scientific Research (B) (No. 25870309), Grant for Environmental Research Projects from the Sumitomo Foundation (143485), and Fujiwara Natural History Foundation provided to H. Hasegawa, and Grant-in-Aid for Scientific Research (B) (No. 21403008 and 25302011) provided to H. Ando.

PY - 2018/5

Y1 - 2018/5

N2 - Lower Cretaceous lacustrine oil shales are widely distributed in southeastern Mongolia. Due to the high organic carbon content of oil shale, many geochemical studies and petroleum exploration 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 is about 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 were formed by lake level changes, reflecting precipitation changes. Shales were deposited in the center 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 was deposited at around 165–158 Ma of Callovian–Oxfordian. The calculated sedimentation rate of the Shinekhudag Formation is between 4.7 ±2.6 cm/ky and 10.0 ±7.6 cm/ky. Shales in the Shinekhudag Formation show micrometer-scale lamination, consisting of algal organic 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–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.

AB - Lower Cretaceous lacustrine oil shales are widely distributed in southeastern Mongolia. Due to the high organic carbon content of oil shale, many geochemical studies and petroleum exploration 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 is about 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 were formed by lake level changes, reflecting precipitation changes. Shales were deposited in the center 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 was deposited at around 165–158 Ma of Callovian–Oxfordian. The calculated sedimentation rate of the Shinekhudag Formation is between 4.7 ±2.6 cm/ky and 10.0 ±7.6 cm/ky. Shales in the Shinekhudag Formation show micrometer-scale lamination, consisting of algal organic 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–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.

KW - Cretaceous

KW - humid climate

KW - Jurassic

KW - lake

KW - Mongolia

KW - oil shale

KW - U–Pb age

KW - varve

UR - http://www.scopus.com/inward/record.url?scp=85042543180&partnerID=8YFLogxK

U2 - 10.1111/iar.12243

DO - 10.1111/iar.12243

M3 - Article

AN - SCOPUS:85042543180

VL - 27

JO - Island Arc

JF - Island Arc

SN - 1038-4871

IS - 3

M1 - e12243

ER -