Rocks under the Microscope Zone II Versions ZH1 Vol 5 (3) 2020
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A photomicrograph dataset of the Cretaceous siliciclastic rocks from the Xigaze forearc basin, southern Tibet
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: 2020 - 07 - 12
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: 2020 - 07 - 29
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Abstract & Keywords
Abstract: The Cretaceous Xigaze forearc basin in southern Tibet along the southern margin of the Lhasa terrane indicates the northward subduction of the Neo-Tethys to the Lhasa terrane. The Xigaze forearc basin mainly comprises siliciclastic rocks interbedded with minor marls and limestone, which can be divided into Chongdui, Sangzugang, Ngamring, Padana, Qubeiya, Quxia, and Jialazi formations. The final two formations were deposited later, following the initial India–Asia collision. While the forearc basin has been studied well over the decades for its intact preservation and good exposure, petrological data are difficult to obtain, resulting in repetitive petrological studies. In this study, the dataset includes polarized microphotographs of 191 Cretaceous clastic rock sections from the Chongdui, Ngamring, and Padana formations, covering 5 shale, 2 siltstone, 158 sandstone, 11 conglomerate, 7 gravel, 1 pyroclastic rock, and 7 siliceous rock sections. For more effective petrological data sharing, the dataset also comprises detailed information of 10 stratigraphic sections, where these rock sections were sampled. It provides petrological data for conducting basic geological research as well as other interdisciplinary research such as artificial intelligence recognition.
Keywords: Southern Tibet; Xigaze forearc basin; Cretaceous; siliciclastic rocks; thin section; polarized photomicrograph
Dataset Profile
TitleA photomicrograph dataset of the Cretaceous siliciclastic rocks from the Xigaze forearc basin, southern Tibet
Data corresponding authorAn Wei (anwei@hfut.edu.cn)
Data authorsZhang Yiqiu, An Wei, and Hu Xiumian
Time rangeSamples correspond to the Cretaceous stratigraphic age (approximately 120–75 Ma). Rock samples were collected in 2009–2010, and polarized photomicrographs of thin sections were obtained in 2020.
Geographical scopeThe sections are located in the Xigaze area of southern Tibet, between the Gangdese mountain to the north and the Yarlung–Zangbo ophiolite to the south. Longitude and latitude: 86°34′27.0″E–89°03′39.8″′E; 29°08′03.6″N–29°27′30.5″N.
Polarized microscope resolution4908 × 3264 pixels
Data volume8.90 GB
Data format*.png, *.jpg, and *.xls
Data service system<http://www.dx.doi.org/10.11922/sciencedb.j00001.00088>
Source of fundingNational Natural Science Foundation for Distinguished Young Scholars of China (Grant No. 41525007) and National Natural Science Foundation of China (Grant No. 41972106).
Dataset compositionThe dataset contains four data files, namely, (1) polarized microscope image set (PNG format) of the rock thin sections with a data size of 81 GB, containing 388 photos of the thin sections; (2) measured stratigraphic columns (jpg format) with a data size of 6.99 MB, containing data pertaining to the lithology, thickness, age, and samples of the three measured sections; (3) field photo data file (jpg format) with a data size of 78 MB, including 12 photos showing the field macrocharacteristics of 9 measured sections and the basic characteristics of outcrops; (4) thin section identification information table (excel format) with a data size of 37.4 kB, including the basic information of 191 rock thin sections and their microscopic characteristics identified therefrom.
1.   Introduction
The northward subduction of the Neo-Tethys and subsequent India–Asia collision resulted in the overall uplift of the Tibetan Plateau, during which numerous Cretaceous strata were deposited on the southern margin of the Lhasa terrane. The Xigaze forearc basin (Fig. 1) in the Xigaze–Zhongba area, southern Tibet, formed between the Gangdese magmatic arc and subduction complex during the northward subduction of the Neo-Tethys to the Lhasa terrane, is the main depositional repository for detritus from the Lhasa terrane; this repository directly indicates the geomorphic evolution of the Lhasa terrane in the Cretaceous and the northward subduction process of the Neo-Tethys [1-2].


Figure 1   Geological sketch map of the Xigaze forearc basin and some sections
a.Geologicalsketch map of the Xigaze forearc basin and its adjacent areas (revised from the literature [3]), GCT—Great Counter thrust and ZGT—Zhongba–Gyangze thrust; S1–S10 indicate the specific locations of the sampling sections in this study; b. Geological map of the S1 section; and c. Geological map of the S8 and S9 sections
The Chongdui, Sangzugang, Ngamring, Padana, and Qubeiya formations were successively deposited in the Xigaze forearc basin from bottom to top. During the India–Asia collision, the forearc basin evolved into a syncollisional basin, obtaining the deposition of the Quxia and Jialazi formations [4-5]. The Chongdui formation is directly deposited on the Xigaze ophiolite [6-7], including the lower part of siliceous rocks (Aptian period [8]) interbedded with tuff layers (approximately 119–113 Ma [7]) and the upper part of turbidites. The subsequent successions formed a shallowing-upward sequence from deep-water submarine fan facies and shelf facies to fluvial-delta facies, with complex origin changes [4,6,9-11]. At approximately 113–99 Ma, clastics in the forearc basin mainly came from the Cretaceous Gangdese magmatic arc, with a small amount originating from the early strata of the central or southern Lhasa terrane [6, 10]. At approximately 99–88 Ma, almost all the detritus in the forearc basin was provided by the Gangdese magmatic arc, extending from the Cretaceous to Jurassic rocks [6]. At approximately 88–76 Ma, detritus from the central Lhasa terrane significantly increased, indicating the existence of an ancient drainage system connecting the central Lhasa terrane to the forearc basin [6,10]. Moreover, studies conducted in the Xigaze and Zhongba areas [10] show that origin notably varies between the east and west parts of the forearc basin, which may be attributed to different sediment routing systems in the lateral direction of the basin.
These studies indicate that the Xigaze forearc basin preserves the completed strata formation by oceanic subduction and records key information about the demise of the Neo-Tethys, uplift and erosion history, and paleogeographic features of the Lhasa terrane. Therefore, this basin is a critical research area to study the demise of the Neo-Tethys and the uplift history of the Tibetan Plateau before the India–Asia collision. Furthermore, it is one of the ideal areas to investigate the evolution of forearc basins.
Therefore, the Xigaze forearc basin has attracted increasing attention and numerous researchers have participated in related studies. However, the basic petrographic data that are publicly accessible are insufficient and the microscopic image data are rarely recorded in academic works in a completed form. Published articles usually provide only limited microscopic images of typical thin sections [6,10-11]. Thus, obtaining specific measured section names and other sampling information becomes inconvenient for other researchers. The microscopic images data of rock slices are the original materials for basic petrography research; however, currently, only some basic information of thin sections can be shared. Related research in other areas of geoscience necessitates a significant investment in both human and material resources to conduct repeated investigations and basic field petrological work in the same area or even the same section. Therefore, it is difficult to improve the productivity of researchers.
In this study, the microscopic image data of 191 pieces of clastic rock samples are provided from 10 measured sections from the Xigaze forearc basin. A large amount of unpublished data, such as petrological and sampling information of the samples or parts of the samples, is shared with geological colleagues who are interested in this study area. Basic information such as detailed global positioning system coordinates, stratigraphic units, and rock slices contained in the 10 sections provided in this dataset is listed in Table 1. Figure 2 shows a comprehensive strata log diagram of the Xigaze forearc basin, in which the sampling locations of some samples are indicated.
Table 1   Information table of the measured sections in the Xigaze forearc basin
TimesGroup/Formation NamesSection CodesSection NamesLatitude Coordinates (N)Longitude Coordinates (E)Number of Slices
Upper CretaceousPadanaS10Ngamring County—Miga Section29°22′36.1″86°34′27.0″7
S9Ngamring County—Padana Ⅱ Section29°21′32.4″86°44′13.6″49
S8Ngamring County—Padana Ⅰ Section29°21′47.8″86°44′30.8″13
Early Upper CretaceousNgamringS7Ngamring County—Sangsang Section29°27′30.5″86°47′51.3″34
S6Ngamring County—Ngamring Formation Section29°16′51.1″87°11′55.6″15
S5Lhatse County—Ngamring Formation Section29°14′20.5″87°44′48.9″12
S4Xigaze City—Jiding Section29°18′08.0″88°24′55.7″7
S3Xigaze City—Xigaze Section29°18′07.0″89°03′07.3″16
Early CretaceousChongduiS2Xigaze City—Qunrang Xishan Section29°08′47.5″89°02′10.5″11
S1Xigaze City—Naxia Section29°08′03.6″88°26′39.4″27


Figure 2   Comprehensive strata log diagram of Xigaze forearc basin (revised from the literature [6])
2.   Data collection and processing
After determining preliminary scientific problems, we measured the representative stratigraphic sections based on relevant scientific assumptions according to systematic literature research and a field survey. Additionally, a detailed description of the sections and systematic sampling, stratigraphic division, sedimentary characteristics, and other original data were directly obtained and recorded from the field.
All rock samples collected from the field were sent to Langfang Chengxin Geological Service Co., Ltd., Hebei Province, for standardized thin section grinding. Then, they were processed into 0.03-mm optical rock slices. The microscopic images of all thin sections were obtained according to the standard of Rock Microscopic Images Project [12] as sample description shows. The relevant information of the thin sections was also collected according to the Rock Microscopic Images Project based on a systematic collection of rock microscopic images. The description of thin sections and the naming of related clastic rocks were according to the standards specified in the project.
3.   Sample description
This dataset consists of four parts: thin section micrograph folder, measured section column folder (S1, S8, and S9), field photo folder (S1–S2 and S4–S10 profiles), and thin section identification information table. Users can inquire about these components by connecting them in series in a complete database using the measured section codes and thin section sample numbers presented in Table 1.
3.1   Thin section micrograph folder
The thin section micrograph folder contains 388 polarizing micrographs of 191 rock slices. Among them, each rock slice has at least one monopolarized micrograph and one cross-polarized micrograph with the same field of view. The color of the micrograph is controlled such that it is visible to the naked eye under the microscope. The line scale of a red letter on a white background is embedded in the lower right corner of the micrographs. In this dataset, the highest resolution of 4908 × 3264 pixels is adopted for the micrographs. The magnification is selected based on the clear and recognizable particles in the image. Generally, there are 30–80 debris particles in the field of view. The micrographs are provided in the PNG format.
The microscopic images are numbered in a unified manner, and the format follows the rule of “slice number” + “m” (short for micrograph) + “digital serial number of the microscopic field” + “monopolarized light symbol − or cross-polarized light symbol +” [15]. Considering the rock slice with sample number 09PDN12 as an example, the monopolarized and cross-polarized images captured from the first view are saved as 09PDN12m1 − and 09PDN12m1 +, respectively (Fig. 3). If the representativeness of the first field of view is insufficient, the second field of view is selected to record the microscopic image again and the images are saved as “NO.samplem2 −” and “NO.samplem2 +”, i.e. 10SS19m2- and 10SS19m2+.




Figure 3   Microscopic images of 09PDN12 quartz lithic sandstone under monopolarized light (top) and cross-polarized light (bottom)
3.2   Measured section column folder
All the images in the measured section column folder are in the JPG format. This folder contains the basic information of S1, S8, and S9 sections in Table 1, including the section position, layer thickness, and related sample position (Fig. 4).


Figure 4   Measured lithological diagram of the S1 and S8 profiles obtained from the Chongdui and Padana formations, respectively (contains the section positions and sample information of the relevant sections)
3.3   Field photo folder
The field photo folder contains field photos (JPG format) of 9 measured sections (except S3), with a total of 12 photos (Fig. 5). Each photo file is named in the format of “section code stratigraphic unit name + section name.” A user can obtain the overall information of the section and establish a preliminary macrounderstanding. According to the field photos and thin section identification information table, the specific position of the section and sample position can be accurately determined to perform relevant fieldwork and further research.


Figure 5   Field photo of the South East Naxia section (S1) in Xigaze city
3.4   Thin section identification information table
The thin section identification information table is a clastic rock identification form. This table mainly contains the basic information and microscopic characteristics of shale, siltstone, sandstone, conglomerate, gravel, siliceous rock, and pyroclastic rock present in the 10 measured sections, i.e., S1–S10.
The thin section identification results are shown in Table 2. Overall, 191 rock samples are obtained. Because this dataset only contains the information of terrigenous clastic rocks in the Xigaze forearc basin south of the Lhasa terrane, samples of other rock types such as igneous rock and limestone have not been considered.
Table 2   Summary table of rock types and lithology information contained in the dataset
RocksNumbersRock types and numbers
Mudstone/Shale5mudstone 1 and shale 4
Siltstone2calcareous siltstone 2
Sandstone158lithic sandstone 2, quartz lithic sandstone 129, feldspar lithic sandstone 14, and lithic quartz sandstone 13
Conglomerate11calcareous conglomerate 9, basaltic breccia 1, and basaltic conglomerate 1
Gravel7gravel comprising limestone 1, quartz lithic sandstone 1, basaltic 4, and igneous rock 1
Siliceous rocks7siliceous rock matrix 1 and siliceous rocks 6
Pyroclastic rock1tuff 1
4.   Quality control and evaluation
This dataset records and provides the coordinates of the sampling point of each rock slice. Owing to the high sampling density of thin rock slices in the fieldwork, the specific coordinate positions of a few thin sections are excluded from the dataset, and only the starting coordinates of the section where they are located are recorded. However, the dataset provides some field photos and measured section columns, which can be used to search in the field by combining obvious differences between different stratigraphic units and positions and using the limited information such as stratum thickness and important marker strata.
The thickness of thin rock samples collected in this dataset meets the national and international standards, where the thickness of the slice was 0.03 mm. In the process of obtaining the micrographs and identifying the thin sections, the interference color of quartz particles in the same batch of rock slices was observed to correspond to the first-order interference.
In the process of photographing the samples under the microscope, automatic exposure and white balance are adopted using the photographing software connected with the microscope to ensure a consistent color for naked eye observations and system photo as much as possible. In terms of resolution, the highest value of the camera system is used and the resolution is 4908 × 3264 pixels, with pictures being saved in the PNG format. Thus, using these steps, the quality and definition of the micrograph can be guaranteed to be reliable, have a high definition, and exhibit no color differences.
5.   Value and significance
The micrographs of this dataset contain clastic rocks that include a continuous evolution sequence from deep-water submarine fan facies at the shelf location to fluvial-delta facies. The clastic grains come from the Gangdese magmatic arc and central Lhasa terrane, with diverse sedimentary environments and variable origins. Moreover, these data contain many important geological processes pertaining to the northward subduction of the Neo-Tethys to the initial India–Asia collision and the gradual uplift and erosion of the Lhasa terrane. The geological significance and scientific value of the dataset still need to be further developed.
In addition to its application value in the field of basic geology, this dataset can play an important role in social production practices such as the search for building stones, selection of nearby road-building stones, and mineral exploration. Additionally, the microscopic characteristics of typical abyssal facies-transitional facies-continental facies shown in the thin section images have great application prospects in the field of professional teaching.
Finally, these high-definition microscopic image sets can also be combined with artificial intelligence technology as machine learning samples, providing rich materials for image verification, feature capturing, and recognition training as well as the possibility for realizing artificial intelligence identification of rock slices in the future. Some microscopic images with unique aesthetic value can also be used as materials for art appreciation and advertising, presenting wonderful microscopic images of rock slices to a broader audience outside academia.
6.   Usage notes
The data form of this dataset is simple, and the following points should be noted when using it.
(1) Currently, all the thin sections in the dataset are stored by the research group of Professor Hu Xiumian of Nanjing University. If the micrographs or other information provided in the above dataset cannot meet the needs of further research, the authors of this paper can be contacted to apply for the further use of these thin sections and obtain relevant data.
(2) A series of sedimentary geology studies and related interpretations have been published based on the identification results of thin sections in this dataset, which can be referred to in the report by An et al. [6]. Users can download and read the relevant literature.
(3) If the dataset is needed only for using the images, it is sufficient to download and use the images directly; however, note that if further research on specific scientific issues related to geosciences is needed, analysis should be conducted in conjunction with the geographic location provided in the data information table as well as the specific geological age and tectonic background of rock formation.
Acknowledgments
Thanks to Dr. Lai Wen for his help in writing the paper and to Wang Jiangang and Guo Ronghua for their contributions in the field section measurement and sample collection.
[1] Einsele G, Liu B, Dürr S, et al. The Xigaze forearc basin: evolution and facies architecture (Cretaceous, Tibet). Sedimentary Geology 90(1994): 1-32.
[2] Wang C, Liu Z. Xigaze Forearc Basin and Yarlung Zangbo Suture Zone, Tibet. Beijing, Geological Publishing House (1999).
[3] Pan J, Ding J, Yao D, et al. The guide book of geologic map of the Qinghai-Xizang (Tibet) plateau and adjacent areas, scale 1:1,500,000. Chengdu, Chengdu Cartographic Publishing House (2004).
[4] Wang C, Li X, Liu Z, et al. Revision of the Cretaceous–Paleogene stratigraphic framework, facies architecture and provenance of the Xigaze forearc basin along the Yarlung Zangbo suture zone. Gondwana Research 22(2012): 415-433.
[5] Hu X, Wang J, Boudagher-Fadel M, et al. New insights into the timing of the India – Asia collision from the Paleogene Quxia and Jialazi formations of the Xigaze forearc basin, South Tibet. Gondwana Research 32(2016): 76-92.
[6] An W, Hu X, Garzanti E, et al. Xigaze forearc basin revisited (South Tibet): Provenance changes and origin of the Xigaze Ophiolite. Geological Society of America Bulletin 126(2014): 1595-1613.
[7] Wang J, Hu X, Garzanti E, et al. The birth of the Xigaze forearc basin in southern Tibet. Earth and Planetary Science Letters 465(2017): 38-47.
[8] Ziabrev S V, Aitchison J C, Abrajevitch A V, et al. Precise radiolarian age constraints on the timing of ophiolite generation and sedimentation in the Dazhuqu terrane, Yarlung-Tsangpo suture zone, Tibet. Journal of the Geological Society 160(2003): 591-599.
[9] Wu F Y, Ji W Q, Liu C Z, et al. Detrital zircon U–Pb and Hf isotopic data from the Xigaze fore-arc basin: Constraints on Transhimalayan magmatic evolution in southern Tibet. Chemical Geology 271(2010): 13-25.
[10] Orme D A, Carrapa B, Kapp P. Sedimentology, provenance and geochronology of the upper Cretaceous-lower Eocene western Xigaze forearc basin, southern Tibet. Basin Research 27(2015): 387-411.
[11] Orme D A, Laskowski A K. Basin Analysis of the Albian–Santonian Xigaze Forearc, Lazi Region, South-Central Tibet. Journal of Sedimentary Research 86(2016): 894-913.
[12] Hu XM, Lai W, Xu YW et al. Standards for digital micrograph of the sedimentary rocks (2020). China Scientific Data 3(2020). DOI: 10.11922/csdata.2020.0008.zh.
Data citation
1. Zhang Y, An W, Hu X. A photomicrograph dataset of Cretaceous siliciclastic rocks from Xigaze Forearc basin, southern Tibet (2020). Science Data Bank, 2020. (2020-07-29). DOI: 10.11922/sciencedb.j00001.00088.
Article and author information
How to cite this article
Zhang Y, An W, Hu X. A photomicrograph dataset of Cretaceous siliciclastic rocks from Xigaze Forearc basin, southern Tibet. China Scientific Data 4, 2020, 5(3). (2020-09-14). DOI: 10.11922/csdata.2020.0064.zh.
Zhang Yiqiu
Contribution: thin section photography, thin section identification, data sorting, and paper writing.
Undergraduate; research area: sedimentary geology.
An Wei
Contribution: field section measurement, sample collection, and paper writing.
anwei@hfut.edu.cn
Ph.D., Lecturer; research area: geotectonic sedimentology.
Hu Xiumian
Contribution: field plan design, data set design, and paper writing.
Ph.D., Professor; research area: sedimentology.
Publication records
Published: Sept. 16, 2020 ( VersionsZH1
Released: July 29, 2020 ( VersionsZH2
Published: Sept. 16, 2020 ( VersionsZH3
References
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