Abstract: At least 5 units of Cretaceous clastic rocks have been discovered to widely outcrop on the central-northern Lhasa terrane, Tibet. These sedimentary rocks accurately encode information on a series of major geological events such as the demise of Bangong–Nujiang Ocean, the Lhasa–Qiangtang collision, and the early uplift of the Tibetan Plateau. As the Tibetan Plateau has been understudied in geological research, basic petrological data of the Plateau are scarce and not always publicly accessible, resulting in repetitive petrological work. In this study, we collected polarized light microscopy of 402 pieces of Cretaceous rock samples from the five widely outcropping siliciclastic units—Duoni Formation, Duba Formation, Jingzhushan Formation, Daxiong Formation, and Kcv Unit—to better share geological data. The rock samples include 23 siltstones, 235 sandstones, 15 conglomerates, 98 gravels, 2 mixed siliciclastic and carbonate rocks, 6 calcareous nodules, 14 volcanic clastic rocks, 6 igneous rocks, and 3 metamorphic rocks. This dataset is supplemented by detailed information on 22 stratigraphic sections, including sampling location, stratigraphic age, and rock characteristics. This dataset can be used in both basic geological and interdisciplinary research, e.g., those linked to computer sciences. Moreover, it provides references for mineral, oil, and gas prospecting as well as teaching or general popularization of sciences.
Keywords: thin section; polarized photomicrograph; siliciclastic rocks; central-northern Lhasa Terrane; cretaceous; Tibetan Plateau
|Title||Photomicrograph dataset of Cretaceous siliciclastic rocks from central-northern Lhasa terrane, Tibet|
|Data corresponding author||Hu Xiumian (email@example.com)|
|Data authors||Lai Wen，Zhang Yiqiu，Hu Xiumian，Sun Gaoyuan|
|Time range||The rock samples were collected between 2012 and 2017 from a strata whose stratigraphic age was attributed to the Cretaceous (about 130 to 80 Ma). The polarized photomicrographs were taken in 2019.|
|Geographical scope||Specific areas include central Tibetan Plateau as well as Nagqu and Ali areas in Tibet (longitude and latitude scope: 84°35′20.0″E–90°8′54.42″′E and 30°36′20.53″N–32°08′05.30″N, respectively)|
|Spatial resolution||4908*3264 pixels|
|Data volume||3.03 GB|
|Data format||*.png; *.jpg; *.xls|
|Data service system||<https://dx.doi.org/10.11922/sciencedb.j00001.00013>|
|Source of funding||National Science Fund for Distinguished Young Scholars (41525007); National Key Research and Development Program of China (2018YFE0204201).|
|Dataset composition||The dataset includes four data files, namely, “Photomicrographs.zip,” “Field photos.zip,” “Stratigraphic columns.zip,” and “Information table of database.xls.” (1) “Photomicrographs.zip” stores 876 polarized photomicrographs (*.jpg) of rock thin sections, with a data volume of 2.91 GB; (2) “Stratigraphic columns.zip” consists of 8 pictures showing the rock types, stratigraphic thickness and age of 22 measured sections, and all sampling locations in the stratigraphic columns, with a data volume of 5.68 MB; (3) “Field photos.zip” is a collection of 21 images showing the field macrocharacteristics of the 22 measured sections, with a data volume of 115 MB; (4) “Information table of database.xls” is a data sheet containing the identification information of the rock thin sections, with a data volume of 73.1 KB.|
The Lhasa terrane, which runs from east to west across the central part of the Qinghai–Tibet Plateau, underwent subduction of the Bangong–Nujiang Ocean and Neo–Tethys Ocean, with thick late Mesozoic strata[1-2] (Figure 1). Based on the results of paleocurrents, sedimentary facies, and provenance in the Lhasa terrane, the Cretaceous source-sink system, palaeodrainage patterns, and paleogeography of the northern Lhasa terrane are much clearer.
The study of the Cretaceous strata in the central-northern Lhasa terrane revealed that the clastic sediments on the northern Lhasa terrane showed a different provenance evolution from north to south with Langshan Formation marine limestone deposited at the same time in the Middle Cretaceous (136–94 Ma)[3-4]. That is, the Duoni Formation on the south side received the debris from the highlands of the central Lhasa terrane, whereas the Duba Formation (also known as Tangza Formation) in the northern side accepted sediments from the Bangong–Nujiang suture zone (BNSZ) and Qiangtang terrane[3,5–7]. In the Turonian period of the Late Cretaceous (92–88 Ma), with the uplift of the northern Lhasa Plateau in the central-northern Lhasa terrane, the source-sink system on the Lhasa terrane changed. The Late Cretaceous Daxiong Formation was directly deposited on the Zenong Group or Paleozoic strata in the southern part of the central Lhasa terrane[8-9]. At the same time, the mid-Cretaceous Kcv unit, Duba Formation, or Tangza Formation in the northern margin of the northern Lhasa terrane are replaced by the Late Cretaceous Jingzhushan Formation. Among them, the main source of the sedimentary area in northern Lhasa terrane had undergone a 180° major transformation after 92 Ma, and it had been transformed from the original northern source area to the southern area.
Recent studies have shown that the Cretaceous basins on the northern Lhasa terrane recorded a large amount of key information about the demise of Bangong–Nujiang Ocean, Lhasa–Qiangtang collision, and early uplift of the Tibetan Plateau. Therefore, the Cretaceous sediments in the central and northern Lhasa terrane were one of the best research objects for geological studies.Considering the Lhasa terrane is part of the Tethyan oil and gas belts, the Gangdese metallogenic belt, and the Bangong–Nujiang metallogenic belt[10-11], the sedimentary data can also provide important geological information for the national economy and metallogenic research.
In view of the important scientific and social values, the Cretaceous sediments of the northern Lhasa terrane have received increasing attention, with many geologists participating in the study of Cretaceous strata. However, the basic petrography research data and related microscopic image data of the northern Lhasa terrane are still very scarce. The relevant published papers so far only show a small number of typical thin section photos and sampling sites related to the discussion of the paper[3–5,7–9,12–14] since the basic information, such as the thin sections, is only a small part of the data generated in the papers, which makes the sharing ratio of basic data very low, leading to the need to spend a lot of human resources to repeat the related research in other earth science studies. The repeated research on the same area or section is common but is an unnecessary waste of scientific research in the study of geosciences.
Based on the abovementioned status and existing problems, in this study, the microscopic image data of 22 measured geological sections from five Cretaceous clastic stratigraphic units of the central-northern Lhasa terrane were collected. A total of 402 rock thin sections dominated by clastic rocks were collected to share with colleagues who are interested in these regions and samples, the systematically studied sections, and the basic information of the samples, including a large amount of unpublished data generated during the studies. The basic information, such as global positioning system (GPS) coordinates, stratigraphic units, and the number of thin sections of the 22 geological sections collected by this dataset, are shown in Table 1; the geographic location of each section is shown in Figure 2.
|Period||Unit||Section ID||Name of section||Latitude||Longitude||No. of thin sections|
|Late Cretaceous||Daxiong Formation||S22||West Coqen Section, Coqen||31°08′09.5″N||84°48′48.8″E||24|
|S21||Daxiong Section, Coqen||31°11′33.5″N||84°50′37.3″E||5|
|S20||Dawa Co Section, Coqen||31°10′59.0″N||84°52′15.8″E||2|
|S19||Nam Co Section, Baingoin||30°36′20.53″N||90°11′26.12″E||15|
|S18||Daguo Section, Nima||30°56′48.34″N||86°38′57.15″E||15|
|Jingzhushan Formation||S17||Qiri Section, Nima||32°08′05.30″N||85°26′57.40″E||8|
|S16||North Qiagui Co Section, Nima||31°50′6.46″N||88°10′13.92″E||17|
|S15||Daze Co Section III, Niam||31°41′43.30″N||87°32′32.38″E||2|
|S14||Daze Co Section II, Niam||31°42′31.75″N||87°31′53.24″E||37|
|S13||Mentang Section, Xainza||31°37′9.61″N||88°6′27.08″E||20|
|S12||Jingzhushan Section, Baingoin||31°26′00.6″N||89°42′34.6″E||33|
|Kcv Unit||S11||Daze Co Section I, Niam||31°42′40.57″N||87°32′6.40″E||7|
|Early Cretaceous||Duoni Formation||S10||Guolong Section, Coqen||31°26′23.6″N||85°24′46.6″E||21|
|S9||Xialong Section, Coqen||31°47′55.7″N||84°39′24.5″E||8|
|S8||Zhukang Section, Coqen||31°55′53.3″N||84°35′20.0″E||38|
|S7||Baoji Section, Baingoin||31°3′44.23″N||90°8′54.42″E||11|
|S6||Baoji Ercun Section, Baingoin||30°57′43.94″N||90°18′39.06″E||10|
|S5||Shen Co Section, Baingoin||31°0′56.98″N||90°23′2.68″E||29|
|Duba Formation||S4||Eshaebu Section, Xainza||31°18′14.42″N||88°10′54.37″E||55|
|S3||Zhirong Section, Baingoin||31°22′58.91″N||89°31′58.54″E||13|
|S2||South Langshan Section, Baingoin||31°24′54.88″N||89°32′41.11″E||17|
|S1||North Langshan Section, Baingoin||31°24′24.83″N||89°42′1.25″E||16|
Figure 2 Sketch geological map of the Lhasa block, modified from 
BNSZ: Bangong–Nujiang Suture Zone；SNMZ: Shiquanhe–Nam Co Melange Zone；LMZ: Luobadui–Milashan Fault；IYZSZ: Indus–Yarlung Zangbo Suture Zone; SGAT: Shiquan–Gaize–Amdo Thrust; GST:Gaize–Selin Co Thrust; ET: Emei La Thrust; GLT: Gugu La Thrust; GJT, Geren Co-Jiali Thrust.
Based on scientific issues and assumptions, after literature reviews and field surveys, representative stratigraphic sections were selected for measurement and description, and then rock samples were collected systematically. The original data and information related to the sections were obtained through field observations.
The rock samples collected in the field were sent to the Langfang Chengxin Geological Service Co., Ltd. (Hebei, China) for standard thin sections with 0.03-mm thickness. The methods for photographing and information collection of thin sections, as well as naming of sedimentary rocks, are on the basis of rules of “Rock Microscopic Image Topics”.
This dataset mainly consisted of “Photomicrographs.zip,” “Stratigraphic columns.zip,” “Field photos.zip,” and “Information table of database.xls.” These data subsets can be combined to form a whole database by the identification of measured sections (Table 1) or sample identification.
The “Photomicrographs.zip” consisted of 876 polarized light micrographs of 402 thin sections. Each thin section contained at least one cross-polarized photomicrograph and a single-polarized photomicrograph on the same part. The color on the photomicrograph was consistent with the naked eye observation under the microscope. A red line scale in a white background was placed in the lower right corner. Then, the microscopic images were numbered uniformly according to the rules of “section number” + “m” + “digital serial number of the camera field of view” + “orthogonal light symbol or single polarized light symbol” . For example, the single-polarized and cross-light photos taken by the thin section numbered 13MD06 are marked as 13MD06m1−, 13MD06m1+, respectively (Figure 3). The resolution of the photomicrographs in this dataset was unified to 4908×3264 pixels and saved in JPG format.
3.2 Stratigraphic columns.zip
All the pictures in the “Stratigraphic columns.zip” were in PNG format, showing the formation thickness, name, lithological features, sampling interval, and position of the sample in the measured sections (Figure 4), some of which also included sedimentary age information, such as the youngest age of detrital zircons (as marked DZ on the left in Figure 4).
3.3 Field photos.zip
A total of 21 field photos in JPG format from “Field photos.zip” included 22 measured sections. The field photos were named uniformly according to the rule of “section ID, stratigraphic unit name, and section name” to assist in finding the position of the sections accurately on the basis of the field photos. An example is shown in Figure 5.
3.4 Information table of database.xls
The “Information table of database.xls” consisted of a clastic rock (including other noncarbonate rocks) identification table and a carbonate rock (including mixed siliciclastic and carbonate rocks) identification table. The “Information table of database.xls” mainly contained siltstones, sandstones, conglomerates, gravels in the conglomerates, mixed siliciclastic and carbonate rocks, calcareous nodules, pyroclastic interbeds, igneous rocks, and thermal metamorphic rocks from the above 22 measurement sections, including composition, lithological feature information, classification, and names .
The 23 siltstones (Table 2) mainly consist of polymictic siltstones with some calcareous siltstones and quartzose siltstones. The 235 sandstones mainly include quartzose sandstone, quartzo-lithic, lithic sandstones, litho-quartzose, feldspatho-quartzose, and feldspatho-lithic sandstones, with a few quartzo-lithic and quartzo-feldspathic wacke rocks. The 15 fine-grained conglomerates include polymictic conglomerates, calcareous conglomerates, and volcanic conglomerates. The 98 gravels from conglomerate beds include limestone, sandstone, tuffs, siltstone, metamorphic rock, intermediate-acid volcanic rock, granite, and some other rocks. In addition, there are 2 mixed siliciclastic and carbonate rocks as well as 6 calcareous nodules found in the Cretaceous clastic sections on the central-northern Lhasa terrane. Magmatism in the Cretaceous is frequent on the Lhasa terrane, so 14 layers of pyroclastic rocks, including tuffs, volcanic ash, and volcanic rocks were found on these clastic sections. There are 5 crystalline clastic rhyolite layers and 1 albite granite vein with 3 thermal metamorphic rocks.
|Types of rock||Total||Types and quantity|
|Siltstone||23||2 calcareous siltstones, 4 Quartzose siltstones, and 17 polymictic siltstones|
|Sandstone||235||5 quartzose sandstones, 122 quartzo-lithic sandstones, 2 quartzo-lithic wacke stones, 1 quartzo-feldspathic wacke stones, 12 lithic sandstones, 65 litho-quartzose sandstones, 7 feldspatho-quartzose sandstones, and 21 feldspatho-lithic sandstones|
|Conglomerate||15||6 polymictic conglomerates, 3 calcareous conglomerates, and 6 volcanic conglomerates|
|Gravel||98||65 limestones, 4 tuffs, 5 litho-quartzose sandstones，1 quartzo-lithic sandstones， 9 quartzose sandstones, 3 siltstones, 1 metamorphic siltstone, 1 marble, 1 trachyte, 2 granitoids, 5 andesites, and 1 rhyolite|
|Mixed siliciclastic and carbonate rock||2||2 mixed siliciclastic and carbonate rocks|
|Calcareous nodule||6||6 micrite marl rocks with veins|
|Pyroclasticrock||14||8 tuffs, 3 rhyolitic volcanic ash, and 3 sedimentary pyroclastic rock|
|Igneous rock||6||5 rhyolite rokcks and 1 albite granite|
|Metamorphic rock||3||2 metamorphic sandstones and 1 metamorphic gabbro|
Sandstone is one of the most important carriers of sedimentary provenance analysis and the most common research object related to sedimentation. However, there are very few reports on the statistical results of the subdivision types of sandstone. The statistical results of composition identification of 250 pieces of Cretaceous sandstone (including sandstone gravel) from terrestrial, delta, or shallow sea environment showed (Figure 6) that there were six kinds of sandstone and two kinds of wacke rocks in the central-northern Lhasa terrane. Quartzo-lithic and litho-quartzose sandstones are the main ones, accounting for 49% and 28% of all sandstones, respectively, whereas neither quartzo-lithic wacke rocks, quartzo-feldspathic wacke rocks, nor feldspatho-quartzose sandstones were more than 3% of all sandstones. In view of the small amount of sandstone samples included in the dataset, the current statistical regularity is not very obvious. Further research is needed after the more sandstone-related databases are shared.
Although this dataset included GPS of the measured sections, the GPS of a single rock sample was not provided due to the intensive sample sites on one section. However, field photos of sections in the dataset and the sample sites in stratigraphic columns, combined with the obvious differences between the field units, the thickness of the strata, and essential marker layers could help find the sampling positions and sections in the field.
The 0.03-mm thickness of rock thin sections met international standards. During this photomicrograph imaging and thin section identification process, the interference colors of quartz particles observed in the same batch of thin sections were all first-class interference colors, indicating that the thickness of the sections met the national standard of 0.03 mm.
The photomicrographs were high-definition and no difference in color. In the process of microscopy imaging, automatic exposure and white balance were used to make the color of the naked-eye observation and system images as consistently as possible. The resolution of microphotographs adopted the highest value of 4908×3264 pixels in the camera system, saved in JPG format. Therefore, the quality and clarity of the photomicrograph fare reliable.
The thin section identification report was independently reviewed and checked by Dr. Ma Anlin from Chengdu University of Technology. Dr. Ma Anlin has been engaged in the identification of clastic rocks and study of sedimentary tectonics. After the independent verification, the reliability of the identification results was further confirmed.
The microscopic images of this dataset contain clastic rocks, including the continuous evolutionary sequence from the shallow sea continental shelf deposits of the land remnant sea to the (fan) delta, then to the continental river deposits, and then to the piedmont alluvial fan near the source-braided river sedimentary sequence; diverse sedimentary environments and varying provenance areas are some of the essential characteristics of this batch of samples. In addition, this batch of data include the Lhasa–Qiangtang collision, demise of the Bangong–Nujiang Ocean[4,7], uplift of the BNSZ, initial uplift of the northern Lhasa Plateau[8-9], and other paramount geological processes. The microscopic images of these samples still have much potential and scientific value waiting to be found.
In addition to being used for geological research, the photomicrograph dataset can be used for social production, such as assisting in finding suitable building materials and mineral exploration. Besides, the typical shallow marine, delta, or terrestrial sedimentary microscopic characteristics shown in the thin section images can be used in teaching and popular science.
Finally, these high-definition microscopic images can be used as machine learning samples, image verification or graphic password design materials, and data for future joint research on artificial intelligence identification of thin sections. The individual and appreciative microscopic images can also provide materials for artistic appreciation or advertising.
Although the format of the dataset is simple, please pay attention to the following points when using the dataset.
(1) All thin sections appearing in the dataset are stored by Prof. Hu Xiumian’s research group, Nanjing University. If the micrographs provided in the dataset cannot meet the needs of further research, you can contact the author of this article to apply for further use of these thin sections.
(2) Various academic papers have been published on sedimentary geology research and related areas based on the thin section identification results of the dataset. You can read the relevant literature for more details.
(3) If you wish to simply use the images, you can download them directly from the database. However, if it is necessary to further solve the scientific problems related to geosciences, please combine the geographical location provided in the data information table, as well as the formed age and sample background.
Authors and contributions
Lai Wen, born in Ganzhou, Jiangxi Province, Ph.D., assistant researcher, research direction is tectonic sedimentology. Mainly responsible for sample collection, thin section identification, paper writing.
Zhang Yiqiu, born in Huaihua, Hunan Province, undergraduate student, research direction is sedimentary geology. Her research interests are sedimentary geology. She is mainly responsible for thin section photography, thin section identification and data collation.
Hu Xiumian, born in Nanchang, Jiangxi Province, PhD, professor, research direction is sedimentology. Mainly responsible for the design of field plan, design of dataset and paper writing.
Sun Gaoyuan, born in Huai'an City, Jiangsu Province, PhD, Associate Professor, with research interests in tectonic sedimentology. Mainly undertake part of the field work and sampling, paper revision.
Thanks Dr. Ma Anlin for reviewing the thin section identification report. Thanks for the contributions of Wang Jiangang, Han Zhong, Ma Anlin on fieldwork and sample collection.
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LAI W, ZHANG YQ, HU XM, et al. Photomicrograph dataset of Cretaceous siliciclastic rocks from the central-northern Lhasa Terrane, Tibet. Science Data Bank, 2020. (2020-03-23). DOI: 10.11922/sciencedb.j00001.00013.
How to cite this article
LAI W, ZHANG YQ, HU XM, et al. Photomicrograph dataset of Cretaceous siliciclastic rocks from the central-northern Lhasa Terrane, Tibet. China Scientific data , 2020, 5 (3). (2020-04-05). DOI: 10.11922/csdata.2020.0009.zh.