Depositional ages and characteristics of Middle-Upper Jurassic and Lower Cretaceous lacustrine deposits in southeastern Mongolia
Abstract
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.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.
- Publication:
-
Island Arc
- Pub Date:
- May 2018
- DOI:
- 10.1111/iar.12243
- Bibcode:
- 2018IsArc..27E2243H