排序方式： 共有3条查询结果，搜索用时 7 毫秒
1
1.
Eolian sand dune deposits of the Upper Cretaceous Djadokhta Formation at Tugrikiin Shiree, southern Mongolia, yield not only dinosaur skeletal remains but also numerous trace fossils produced by invertebrates. This paper describes the trace fossil Entradichnus meniscus, a long unlined and unbranched trail that is filled with meniscate laminae and occurs characteristically in positive epirelief. The trail is straight to gently meandering, parallel to the foreset laminae of the eolian dunes, and exhibits a significant preferred orientation parallel to the depositional dip of the cross‐stratification laminae. In addition, almost all the crescentic internal laminae of the trail show concave down‐dips. These features indicate that the trails were produced beneath the slipface of eolian dunes by the downward burrowing of the trace‐makers. This occurrence mode of E. meniscus of the Mongolian Cretaceous is very similar to that described from the Jurassic eolian dune deposits in North America. Hence, the downward burrowing of the E. meniscus animal might be a common feature in arid eolian dune deposits at least during the Jurassic and Cretaceous, and possibly reflecting a behavioral response to the morphology of large sand dunes under an arid climate. 相似文献
2.
Depositional ages and characteristics of Middle–Upper Jurassic and Lower Cretaceous lacustrine deposits in southeastern Mongolia 
下载免费PDF全文
下载免费PDF全文 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 《Island Arc》2018,27(3)
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. 相似文献
3.
1