Rocks under the Microscope Zone II Versions EN1 Vol 5 (3) 2020
Download
A photo photomicrograph dataset of the mid-Cretaceous rocks from Langshan Formation in northern Lhasa terrane, Tibet
 >>
: 2020 - 06 - 23
: 2020 - 09 - 18
: 2020 - 07 - 13
: 2020 - 09 - 27
224 0 0
Abstract & Keywords
Abstract: Microfacies analysis constitutes the basis for studying carbonate depositional environments. Although a scholarly tendency to categorize microfacies into distinct types is observed, there is a risk of ignoring continuity and congruity between different environments. To address this predicament, we studied the carbonate strata (Langshan Formation) in central Tibet. We collected the polarized photomicrographs of 559 thin sections from three stratigraphic sections of the Langshan Formation and described the location, stratigraphic age, lithology, and petrography of these thin sections. Results show that the Langshan Formation lithology comprises limestone and dolostone, and the limestone consists mainly of wackestone, mudstone, and floatstone. This photomicrograph dataset can be used to support basic geological research, petroleum exploration, carbonate facies modeling, education, and general science promotion.
Keywords: Lhasa terrane; mid-Cretaceous; Langshan Formation; Photomicrograph
Dataset Profile
TitleA photomicrograph dataset of the mid-Cretaceous rocks from Langshan Formation in the northern Lhasa terrane, Tibet
Data authorsXu Yiwei, Hu Xiumian, and Sun Gaoyuan
Data corresponding authorHu Xiumian (huxm@nju.edu.cn)
Time rangeThe rock samples were collected in 2016–2017, with their stratigraphic age attributable to the mid-Cretaceous (approximately 120–98 Ma). Polarized photomicrographs of the thin section were obtained in 2020.
Geographical scopeThe rock samples were collected from the Geji County and Geze County in the Tibet Autonomous Region.
Polarized microscope resolution4908 × 3264 pixels
Data volume24.9 GB
Data format*.png, *.xlsx, and *.pdf
Data service system<https://dx.doi.org/10.11922/sciencedb.j00001.00033>
Source of fundingNational Natural Science Foundation for Distinguished Young Scholar (Grant No. 41525007)
Dataset compositionThis dataset consists of three subsets. (1) “Polarized microscopic images of the rock thin sections” (*.zip) with a data size of 24.9 GB, containing 1134 images of rock thin sections from three stratigraphic sections. (2) “Measured stratigraphic columns” (*.pdf) with a data size of 431 kB, containing the records of the lithology, thickness, and age of the three measured sections. (3) “Thin section identification information table” (*.excel) with a data size of 190 kB, comprising basic information, microscopic characteristics, and other identification data of the 559 rock thin sections.
1.   Introduction
In the mid-20th century, the discovery of carbonate petroleum reservoirs on a global scale promoted an understanding of the origin of carbonate rocks and a detailed study of carbonate sedimentary environment, diagenesis, and facies models. A notable result is the concept of standard microfacies types that can be compared throughout the Phanerozoic[1]. Today, carbonate microfacies analysis has become an essential content of the carbonate facies model and sedimentary environment interpretation. Researchers have summarized several representative sedimentary microfacies from a large number of thin sections based on different research purposes and established regional facies models. However, the changes between microfacies are continuous and dynamic[2].
A categorized microfacies can only provide the framework of sedimentary environments, and the continuous changes between different sedimentary environments are ignored. This is particularly obvious for carbonate homoclinal ramps and the inner part of the rimmed platform because variations in sedimentary environments are gradual in these areas. Therefore, although there are many sedimentary environment studies based on microfacies analysis, considerable information is buried behind these methods. Additionally, different researchers focus on different parts of the same thin section for their demands. For example, researchers studying the evolution of sedimentary environments mainly focus on the particle combination and matrix type in thin sections. Alternatively, diagenesis and pore type analysis may be more important for researchers studying the physical characteristics of reservoirs.
Therefore, a complete collection of photomicrographs of thin sections is an ideal choice to address the aforementioned questions, fully understand the information in thin sections, and meet the needs of different research directions. This approach can objectively display continuous changes in the microfacies and provide background information for scientists in other research fields, reducing repetitive tasks.
The samples of this dataset were collected from the mid-Cretaceous carbonate strata in the northern Lhasa terrane. The carbonate stratum called the Langshan Formation is widely exposed in the northern Lhasa terrane, and its thickness is approximately 1 km. The Langshan Formation is the latest marine strata of northern Lhasa. Its origin and evolutionary history are important to constrain the collision time between the Lhasa and Qiangtang terranes, uplifting history of the northern Lhasa terrane, and petroleum exploration in the northern Lhasa terrane. However, research on the Langshan Formation is lacking and there are few biostratigraphy and paleontology studies on this formation[3-4]. The sedimentary environments and microfacies of the Langshan Formation have not been thoroughly investigated so far, and its depositional model has been described simply as a carbonate platform[5-8].
Therefore, three parallel geological sections of the Langshan Formation were measured in central Tibet (Figure 1) and systematically sampled with a sampling frequency of 2–4 m for carbonate. A polarized microscope was used to capture the photographs of 559 thin sections, and the details including their location, stratigraphic age, rock name, and other important features were described. This dataset can be widely used for basic geological research, regional geological surveys, and petroleum exploration in the northern Lhasa terrane. Moreover, the sedimentary characteristics of the typical mid-Cretaceous shallow-water carbonates shown in the photographs can be used for scientific research, education, and science promotion. They can also be used for subsequent image analysis research.


Figure 1   Simplified geological map of the northern Lhasa terrane (modified after Pan et al., 2004)
Red stars refer to the position of sections. BNSZ: Bangong–Nujiang suture; SNMZ: Shiquan River–Nam Co Mélange zone; LMZ: Luobadui–Milashan fault; IYZSZ: Indus–Yarlung–Zangbo suture; SGAT: Shiquan–Gaize–Amdo thrust; GST: Gaize-Selin Co thrust; ET: Emei La thrust; GLT: Gugu La thrust.
2.   Data collection and processing methods
Before collecting the field samples, we selected sections with complete outcrops, fresh rock outcrops, and representative areas. During the field survey process, the exposed stratum was observed and recorded. A sampling interval of 2–4 m was maintained for systematic sampling based on the thickness of a single layer, lithological variation, and sedimentary structures of the Langshan Formation. A Chinese geological company prepared the thin sections based on standard preparation procedures of thin sections. Photography and information collection of thin sections were performed uniformly according to the standards of “Rock photomicrograph Topics”[9].
3.   Data sample description
The dataset is divided into three folders for storage: stratigraphic log, thin section photomicrographs, and thin section description.
The stratigraphic logs show the thickness, lithology, age, sample location, and intervals of the Langshan Formation. The simplified stratigraphic logs are shown in Figure 2. The three measured sections are the Azhang and Letie sections in Gaize county and the Zulong section in Gegyai county. The position of the sections and their geographical distribution are shown in Table 1 and Figure 1, respectively.


Figure 2   Stratigraphic logs of three sections of the Langshan Formation
(a) Lithological and biostratigraphic correlation of the three sections of the Langshan Formation and (b) larger benthic foraminiferal biozones and their age. Abbreviation: TLK1, Tibetan larger foraminiferal zones of the Cretaceous.
Table 1   Global positioning system (GPS) and sample number of three sections of the Langshan Formation
SectionStart GPSEnd GPSNumber of samples
Azhang31°52′48.5″N; 84°55′16.5″E31°57′54.5″N; 84°51′12.7″E119
Letie32°5′20.476″N; 84°27′57.114″E32°5′39.162″N; 84°28′11.16″E80
Zulong32°36′38.64″N; 81°19′47.95″E32°37′51.24″N; 81°19′43.42″E360
The photomicrographs contain the photomicrographs of 559 thin sections. Each thin section was photographed under cross-polarized and plane-polarized lights. The resolution of the photomicrograph was 4908 × 3264 pixels, and the images were saved in the PNG format. The thin section description contains three carbonate tables the nomenclature and sample information of the limestone or dolostone from the three sections of the Langshan Formation.
Based on the results of the thin section identification (Fig. 3), the samples from the three sections of the Langshan Formation can be lithologically divided into carbonate and volcanic rocks, including 556 carbonate and 3 volcanic rocks. Among the carbonate rocks, 443 samples are limestone and 113 samples are dolostone. Limestone in the Langshan Formation is dominated by wackestone, accounting for 34%. Mudstone, floatstone, and rudstone are subordinate, accounting for 15%, 14%, and 9%, respectively. Packstone, grainstone, and boundstone are the lowest, accounting for <9%. Dolomite in the Langshan Formation accounts for 20% and is distributed only in the Zulong section. These dolomitic rocks are dominated by completely recrystallized dolostone, some of which are recrystallized dolostone with a residual grain structure. Volcanic rocks are rare, occupy less than 1%, and are distributed only in the Azhang section (Table 2).


Figure 3   Rock types and their relative contents in the Langshan Formation
Table 2   Rock types and their numbers
Rock typesNumbersSubtypes and numbers
Carbonate556Mudstone 83, Wackestone 191, Packstone 19, Grainstone 18, Floatstone 78, Rudstone 52, Boundstone 2, Dolostone 113
Volcanic3Rhyolite 3
4.   Data quality control and evaluation
Owing to intensive sampling, the specific positions the samples were not recorded. However, the user can refer to the lithological differences shown in the stratigraphic logs to estimate the approximate horizon of each sampling. When collecting the field samples, fresh rock specimens were selected. The thickness of the thin sections was selected according to international standards, and the interference color of the thin slices was normal, indicating that the thin sections comply with international standards.
Photomicrographs have high resolution, and there is no color difference. We used autoexposure and autowhite balance functions during shooting to maintain the colors in the observations and photomicrographs as constant as possible. The resolution used for photo photomicrographs was set to the highest value on the camera(4908 × 3264 pixels), and all images were saved in the PNG format. Therefore, the quality and transparency of the photo photomicrographs are reliable. When describing thin sections, a comparison chart from Flügel[1] was used to estimate the grain content. However, there is still an error between the estimated and actual grain contents, which is less than 10%.
Additionally, the mainly types of grains of the Langshan Formation are bioclasts. We refers to the properties described by Flügel[1] and Scholle and Ulmer-Scholle[11] to identify the type of organism at the class-phylum level. Organisms can be partially identified down to the genus level. The identification results were consistent with the fossils reported in Xu[12] which were identified by our paleontologist collaborator Marcelle BouDagher-Fadel to ensure the reliability of the identification results.
5.   Data value
This dataset is the first photo photomicrograph dataset obtained using high-density sampling and high-resolution thin section shooting and provides a description of the Langshan Formation. The geographic location of the samples, stratigraphic age, composition, fabric, and structure of the carbonate rocks under plane-polarized and cross-polarized lights were systematically described. Therefore, this dataset can be used for the basic geological research of the northern Lhasa terrane and can provide essential data for petroleum exploration and reservoir evaluation. Moreover, the Langshan Formation contains shallow-water and deep-water carbonate deposits, which can be used to study the mid-Cretaceous carbonate depositional models. The photo photomicrographs show the sedimentary characteristics and biological composition of typical shallow-water carbonate platforms, which can also be widely used for scientific research, education, and scientific promotion.
6.   Data usage and recommendations
The data format of this dataset is simple, and the following points must be considered when using it:
(1) The photomicrographs in this dataset only reflect the lithology of the three sections of the Langshan Formation. Owing to the extensive regional distribution of the Langshan Formation, its lithology varies in different regions. In this article, it is unreasonable to assume that the lithology of the Langshan Formation is consistent.
(2) Although the selected photomicrograph of this dataset is typical, one photomicrograph cannot cover all the information pertaining to the thin sections. If the photomicrographs provided in the above dataset cannot meet the needs of further investigations, readers can contact the author of this article to request further use of the thin sections.
(3) An interpretation of carbonate microfacies based on the identification of thin sections in this dataset has been published in the literature by Xu et al.[12].
Acknowledgments
The authors thank Ye Jiapeng for his help in the field.
[1] FLÜGEL E. Microfacies of carbonate rocks: analysis, interpretation and application. second edition. New York: Springer-Verlag, 2010.
[2] SPENCE G H, TUCKER M. Modelling carbonate microfacies in the context of high-frequency dynamic relative sea-level and environmental changes. Journal of Sedimentary Research, 1999, 69: 947-961.
[3] BOUDAGHER-FADEL M K, HU X M, PRICE G D, et al. Foraminiferal biostratigraphy and palaeoenvironmental analysis of the mid-Cretaceous limestones in the southern Tibetan plateau. Journal of Foraminiferal Research, 2017, 47(2): 188-207.
[4] RAO X, SKELTON PW, SHA J, et al. Mid‐Cretaceous rudists (Bivalvia: Hippuritida) from the Langshan Formation, Lhasa block, Tibet. Papers in Palaeontology, 2015, 1: 401-424.
[5] CHEN M, WANG J, TAN FW, et al., The planar distribution and significance of the organic reefs in the Lower Cretaceous Nangshan Formation in the Coqen Basin, Xizang, Sedimentary Geology and Tethyan Geology, 2003, 23(4): 68-70.
[6] LI XH, ZENG QG, WU G, et al. Lower Cretaceous sequence stratigraphy of northern Coqen basin, Central Tibet, China. Journal Of Chengdu University Of Technology (Science & Technology Edition), 2005, 32(2): 111-119.
[7] QU YG, LIU GZ, ZHENG CZ, et al. Stratigraphic succession of the Lower Cretaceous Langshan, Kangqi and Yuqiong Formations in the Baingoin District, Northern Tibet. Journal of Stratigraphy, 2003, 27(3): 220.
[8] YU GM, WANG C S. People’s Republic of China Ministry of Geology and Mineral Resources, Geological Memoirs, Serious 3, Number 12, Sedimentary geology of the Xizang (Tibet) Tethys, Bejing, Geological Publishing House, 1990.
[9] HU XM, LAI W, XU YW, et al., Standards for digital photomicrograph of the sedimentary rocks. China Scientific Data, 2020. (2020-03-02). DOI: 10.11922/csdata.2020.0008.zh.
[10] PAN GT, DING J, YAO D S, et al. Guide Book of 1:1,500,000 Geologic Map of the Qinghai-Xizang (Tibet) Plateau and Adjacent Areas, Chengdu, China, Cartographic Publishing House, 2004.
[11] SCHOLLE P A, ULMER-SCHOLLE D S. A Color Guide to the Petrography of Carbonate Rocks: Grains, Textures, Porosity, Diagenesis, AAPG Memoir 77. Canada: AAPG, 2003.
[12] XU YW, HU XM, BOUDAGHER-FADEL M K,et al. The major late Albian transgressive event recorded in the epeiric platform of the Langshan Formation in central Tibet. Geological Society, London, Special Publications, 2020, 498: 211-232.
Data citation
XU YW, HU XM, SUN GY. A photo photomicrograph dataset of mid-Cretaceous rocks from Langshan Formation in northern Lhasa Terrane, Tibet. Science Data Bank, 2020. (2020-07-13). DOI: 10.11922/sciencedb.j00001.00033.
Article and author information
How to cite this article
XU YW, HU XM, SUN GY. A photo photomicrograph dataset of mid-Cretaceous rocks from Langshan Formation in northern Lhasa Terrane, Tibet. China Scientific Data, 2020, 5(3). (2020-09-16). DOI: 10.11922/csdata.2020.0049.zh.
Xu Yiwei
Contribution: Data collection, analysis, and original draft.
PhD student from Luan City, Anhui Province, majoring in carbonate sedimentology.
Hu Xiumian
Contribution: Field work, data collection, and draft editing.
huxm@nju.edu.cn
Doctor, professor from Nanchang City, Jiangxi Province, majoring in sedimentology.
Sun Gaoyuan
Contribution: Field work, data collection, and draft editing.
Doctor from Huaian City, Jiangsu Province, majoring in sedimentation and tectonic.
Publication records
Published: Sept. 27, 2020 ( VersionsEN1
Released: July 13, 2020 ( VersionsZH2
Published: Sept. 27, 2020 ( VersionsZH3
Updated: Sept. 27, 2020 ( VersionsZH4
References
中国科学数据
csdata