Abstract: The paper presents a dataset of marine carbonate rocks from the Sinian Dengying Formation on the northwestern margin of the Yangtze, which was the first large-scale carbonate formation deposited in the Upper Yangtze region after the Nanhua Ice Age. It records not only the Sinian response to global warming, but also information about the evolution of marine microorganisms and the growth of carbonate rocks derived from the Early Cambrian explosion when there was a sudden appearance of skeletal animals in the fossil record. Recently, mound-shoal dolomite has become an important indicator of deep-seated ancient carbonate oil and gas in Sichuan Basin. Microscopic images of the 119 rock thin sections collected from Qingping and Zhenba sections in the northwestern margin of the Yangtze Platform revealed that the composition and structural characteristics of the rocks change with time. The sections generally have a sequence of siliceous rock, micritic dolomite, algal dolomite, and granular dolomite. This sequence reflects the development process at the edge of the carbonate platform margin while affected by secondary sea level fluctuations along the northwestern margin of the Yangtze Platform during the slow sea level rise of the epeiric sea. The dataset presents detailed information such as the location, approximate stratigraphic age, rock composition, and structural characteristics of the samples. It can be applied to the study of rock microfacies of the Sinian Dengying Formation on the northwestern margin of the Yangtze Platform, and to interdisciplinary research, such as intelligent image recognition. It also provides basic information services for productive activities such as oil and gas reservoir characterization and formation mechanism analysis, teaching, and science outreach.
Keywords: northwestern margin of Upper Yangtze; Dengying Formation of the Sinian; carbonate rocks; polarized photomicrograph
|Title||A microscopic image dataset of Sinian carbonate from Dengying Formation on the northwestern margin of Sichuan Basin|
|Corresponding author||Hou Mingcai (email@example.com)|
|Data authors||Qi Zhe, Hou Mingcai, Xu Shenglin, He Liang, Tang Zhijiang, Zhang Mingxuan|
|Time range||The rock samples were collected in between January and February 2019, attributable to a stratigraphic age of Sinian period (~551–541 Ma). The polarized light micrographs of the rock thin section were taken in July 2019.|
|Geographical scope||Regarding administrative divisions, the study areas are located in Qingping Town, Sichuan Province and Zhenba County, Shanxi Province. Geographically, they are along Longmen Mountain and the Micangshan Tectonic Zone.|
|Polarized microscope resolution||2560*1920 pixels|
|Data volume||6.57 GB|
|Data format||*.jpg; *.xls; *.png.|
|Data service system||<http://www.dx.doi.org/10.11922/sciencedb.j00001.00105>|
|Source of funding||National Science and Technology Major Project (Grant No. 2017ZX05008005009)|
|Dataset composition||1. The dataset includes five data files: (1) “polarized light micrographs” is a dataset of polarized light micrographs (*.jpg, *.png) of the rock thin sections, with a data volume of 4.97 GB; (2) “field photo of the measured section” shows the field photos (*.jpg, *.pptx) of the measured sections and the characteristics of outcrop, with a data volume of 1.75 GB; (3) “stratigraphic columns” (*.jpg) shows the stratigraphic thickness, age, and sampling location of the measured sections, with a data volume of 24.4 MB; (4) “field geological record table” (*.xls) records in detail stratification, stratigraphic productivity, field description, microscopic description, among others, with a data volume of 100 KB; (5) “identification report” (*.xls) is the data sheet for rock thin section identification, with a data volume of 36 KB.|
During the Neoproterozoic, global plate tectonics were in the stage of the Rodinia supercontinent breakup. At the end of the Sinian, the South China plate drifted 25°N to the west side of the Gondwana continent, which was a relatively independent landmass (Fig. 1a) . During the Sinian, the northwest margin of the Upper Yangtze Platform was located at the margin of the South China plate. The northwest of the Upper Yangtze Platform was facing the vast Paleo-Tethys ocean, and its back was at the Upper Yangtze Platform. It was a marginal sea environment and highly sensitive to sea level change (Fig. 1) . As the first large-scale carbonate strata deposited on the Upper Yangtze Plate after the Nanhua glacial period, the Dengying Formation records the co-evolution of the environment and life in the Late Neoproterozoic. Previous researchers have done extensive research on it, including the discovery of the earliest fossil skeleton cloudina  and the embryonic fossils of Metazoa . Some scholars believe that the shallow sea in the northern margin of Yangtze River was an oxidative  and warm [6-7] environment during the Dengying period, and believed there was a causal relationship between deep-sea oxidation and the explosion of Early Cambrian skeletal animals . At the same time, this layer has shown good oil and gas reservoir potential in the oil and gas exploration of the Sichuan Basin. Some scholars believe that Hill Beach was formed as a result of microbial growth, while late tectonic uplift and atmospheric fresh water leaching were the main factors influencing reservoir formation [9-11].
Fig. 1 Palaeogeographic pattern of the Dengying period in the Upper Yangtze region and Sinian stratigraphic column of the Qingping section in the Western Sichuan Basin
A. Global ancient block at the end of the Dengying period ；B. C. Paleogeographic pattern of the Upper Yangtze area during the deposition of the third and fourth members of the Dengying Formation ; D. Sinian stratigraphic column of the Qingping section in the Western Sichuan Basin.
Based on the above situation and limitations, 119 samples were collected in the Qingping section and the Zhenba section on the northwest margin of Yangtze Platform. They were identified and photographed under the polarizing microscope, to systematically display the microscopic rock characteristics of the sedimentary rocks in this period, allowing the sedimentary environment and reservoir characteristics to be identified in petrographic detail. Finally, the dataset was created by sorting out section information and the rock thin sections, to be ready to be shared with scholars who are interested in this area and stratum.
Based on the existing geological understanding of the area, through literature research, and the comprehensive study of the regional geological map, we selected several geological sections for reconnaissance. According to the characteristics of the lithofacies paleogeographic map of the Dengying period, two sections with continuous strata and good outcrops were selected for field measurements. During the measurement process, layers were divided according to lithofacies changes, then the thickness, lithologic changes, sedimentary structures, and other characteristics of each layer were recorded in detail, and systematic sampling was completed.
In order to distinguish dolomite from calcite and facilitate further testing of the thin sections, one third of the thin sections were dyed with alizarin red, and the other half were covered with the stained area. Thin section observations were conducted in the Laboratory of Sedimentary Geology, Institute of Sedimentary Geology, Chengdu University of Technology, and the identification, naming and shooting of thin sections were carried out in strict accordance with the standards published on the subject of rock microscopic image data .
3.1 Sample display
This dataset is comprises five parts: a geological record table of measured sections, field photo atlas, comprehensive stratigraphic column, thin section photo dataset, and thin section identification report.
The specific horizon information, GPS coordinates, and the number of slices in the two sections collected are shown in Table 1. Regarding their administrative division, they are located in Qingping Town, Mianzhu City, Deyang City, Sichuan Province, and Zhenba County, Hanzhong City, Shaanxi Province. The specific geographical locations are shown in Figure 1.
Table 1 Information table of measured sections of the Sinian Dengying period, northwestern margin of the Sichuan Basin.
|Time||Formation||Section code||Section name||coordinate||Number of thinsection|
|Sinian Dengying period||Tianqiao formation|
|Zbt、Zbs||Measured stratigraphic profile of the Sinian Tianqiao formation shaoyaogou formation in Xiaomuling, Qingping Township, Mianzhu City, Deyang City, Sichuan Province||Starting point: 31°35′55″N ， 104°03′41″E|
End point: ：31°35′57.183″N,
|Sinian Dengying period||Dengying formation||Pm3||Measured stratigraphic section of Dengying Formation of Sinian in Luchi, Zhenba County, Hanzhong City, Shaanxi Province||Starting point: 32°17′8″N ， 108°9′44″E|
End point: 32°17′11″N ， 108°9′60″E
The stratigraphic conditions, occurrences, field descriptions, microscopic descriptions, and other information are shown in detail in the field geological record table. The macroscopic features and typical sedimentary structures of each layer are introduced in detail in the field photo atlas. The comprehensive stratigraphic column chart shows the stratigraphic age, stratified stratum thickness, lithologic changes, sampling location and typical microscopic characteristics of the section (Fig. 2). The color of the stratum is consistent with the actual field observation color.
The whole rock micrograph dataset contains 119 rock slices, each of which has multiple photos with various multiples and different visual fields. In the same field of view, one orthorhombic photo and one unipolar photo have been collected. The color of the micrograph is consistent with that observed by the naked eye under microscope, and multiple photos are taken according to typical characteristics. The resolution of the rock micrographs is 2560 × 1920 pixels, and the image formats are JPG and PNG. The naming principles of the photos are as follows: slice name + m + photographing horizon serial number + “+ or −,” for example, the corresponding photo numbers of rock slices numbered zbt0−1 are zbt0−1 m1−, zbt0−1 m1+; zbt0−1 m2−, zbt0−1 m2+, where m is the abbreviation of the micrograph, “−” is monopolar and “+” is orthogonal polarization. The scale of magnification of the photo is placed in the lower right corner of the photo, with a white background and a solid red line (Fig. 3).
The thin section identification report comprises a carbonate rock identification form, which contains the original number of the thin section. It is named according to Zeng Yunfu’s classification and naming method for carbonate rocks , and records the specific characteristics of carbonate rocks with different structural types in detail, describing the diagenesis, cement fabric and other information, including the sample location, age, etc. The specific information collection standards and requirements of the identification form have been detailed in “standards for collection and information collection of sedimentary rock microscopic image data sets”  and will not be repeated here.
3.2 Data statistics
A total of 119 micrographs of rock thin sections have been collected in the dataset, including 112 dolomite samples and 7 siliceous rock samples. Dolomite can be divided into allochthonous dolomite, in-situ dolomite, and crystalline dolomite. Among them, there are 35 samples of allochthonous dolomite (with a granular structure). The grain types of allochthonous dolomite are mainly sand debris (38%), agglomerate (38%) and spherulite (21%). The particle size is mostly within the range of 0.2–5 mm. Among the support types, the particle support accounts for 49%, as 66% of the samples are bright crystal cement, and the content of interstitial material is between 50%. There are 17 samples of in-situ dolomite (with biological structures), and all of them are algal, while 94% of them have algal laminar structures. The rocks recrystallized during dolomitization, and the grain size is mud microcrystalline; with 60 samples of crystalline dolomite (with recrystallization structure), including 3 ultra-fine crystal dolomite, 20 muddy crystal dolomite, 12 mud microcrystalline dolomite, and 1 microcrystalline dolomite. There are seven pieces of fine-grained dolomite, five pieces of fine-grained dolomite and three pieces of grain dolomite with a residual structure. Siliceous rock is the secondary lithology of the section, constituting a relatively small proportion. It mainly appears in the profile in the form of interlayers and nodules.
After microscopic analysis, it was found that the contact relationship between siliceous rock and dolomite is complex and diverse; mainly composed of microcrystalline quartz, most of which are authigenic. Presumably, it was formed in the synsedimentary stage through late filling and metasomatism. The sedimentary structures revealed in the thin section are mainly laminar in structure with phantom structures; the sedimentary structures involved include bird’s eye structures, grape-like structures, and suture structures. Through observation, it was found that diagenesis involved recrystallization, metasomatism, filling, and pressure dissolution. Based on the dataset, proportions of the rock type, dolomite type, grain type, and crystalline dolomite grain size are shown in Fig. 4.
The coordinates of the starting point and the end point of the profile are provided in this dataset. However, the sampling position is recorded in detail in the geological record table of the measured section and the comprehensive stratigraphic column chart. The user can limit the sampling position within 3 m according to the stratification position recorded in the field and the sampling position of the layer.
The grinding, dyeing, and glass covering of rock thin sections were carried out in accordance with relevant industry standards. No mineral interference color abnormality was found during the process of photomicrography shooting and thin section identification, indicating that the thickness of the thin section meets required standards.
During the process of image acquisition of the rock slices, the image color of each micrograph was consistent with the picture observed under the microscope by adjusting the white balance and exposure time. The maximum resolution of the equipment was used to retain a sufficient level of detail in the micrographs.
The designation of rock slices and the collection of information were strictly in accordance with the requirements of “standards for collection and information collection of sedimentary rock microscopic image datasets” , and the information in the appraisal table was checked and verified numerous times, to ensure the reliability of the information.
This dataset is the first micrograph dataset to release rock samples of the Sinian Dengying Formation carbonate rocks in the northwest margin of Sichuan Basin in the form of continuous high-density sampling, thin section high-resolution photography, and a full disclosure of the appraisal report, based on the premise of accurate information and repeatable observations. In the appraisal report, the geographic location and stratigraphic information of the samples collected, and the composition, and structure of carbonate rocks in the thin sections have been systematically described. Therefore, this dataset can be applied to the study of rock microfacies in the northwest margin of the Sichuan Basin, and can also be used in interdisciplinary research such as intelligent image recognition. At the same time, it can provide fundamental information for society, such as in oil and mineral exploration, teaching and science outreach.
The following points should be noted when using this dataset:
(1) Although the dataset provides detailed profile information, it is still necessary to check the data in the field before using this information.
(2) All the slices involved in the dataset are preserved by Professor Hou Mingcai of Chengdu University of technology. If you need to observe the thin sections, you can contact the author of this paper.
(3) The microfacies analysis of Zhenba section in this dataset has been published in 2020, and the relevant contents can be found in the reference  .
Thanks to Li Songzhuo and Huang Shuguang for their guidance on the paper and identification report, and for the contributions made by Zhang Lei, Mu Caineng, and Liu Yanpeng in field profile measurement and indoor sample sorting.
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How to cite this article
QI Z, HOU MC, XU SL, et al. A microscopic image dataset of Sinian carbonate from Dengying Formation on the northwestern margin of Upper Yangtze. China Scientific data, 2020, 5(3). (2020-09-25). DOI: 10.11922/csdata.2020.0069.zh.