Abstract: At the end of the Caledonian orogeny, most parts of Southern China were exposed to and suffered from variable levels of weathering and denudation. The Devonian transgression submerged most of the Yangtze block, through which terrigenous siliciclastic rocks of similar lithology were widely deposited, in unconformable contact with the underlying strata. The provenance and sedimentary environmental information on siliciclastic rocks reveal the paleogeomorphology of the lower Yangtze region in Late Caledonian tectonic movement and the paleogeographic pattern of sedimentary lattices in the early Late Paleozoic. Existing petrological data of the Wutong group in the lower Yangtze region is scarce, and basic petrological work remains ongoing. To make this data more widely accessible, we collected and analyzed the characteristics of 200 siliciclastic rock samples under a polarized microscope. The samples were taken from 12 sections and 12 outcrops of the upper Devonian and lower Carboniferous Wutong group in the southern part of the lower Yangtze River. We also collected a total of 212 microimages of 12 siliciclastic thin rock sections of Wutong group from existing literature. This dataset includes information related to sampling location, stratigraphic age, rock name, and the specific characteristics of the samples. This information can be used to support basic geological research of the Wutong group as well as interdisciplinary research related to intelligent image recognition. Moreover, it supplements basic data services to support social production.
Keywords: Southern lower Yangtze; Late Devonian–Early Carboniferous; Wutong group; siliciclastic rocks; micrograph dataset
|Title||Micrograph dataset of terrigenous siliciclastic rocks of the Late Devonian–Early Carboniferous Wutong group in the southern lower Yangtze River|
|Data corresponding author||Hou Mingcai (firstname.lastname@example.org)|
|Data authors||Cai Wenpeng, Hou Mingcai, Chen Hanzhi, Liu Yanpeng|
|Time range||The rock samples were collected during 2018–2019 with a stratigraphic age attributable to the Late Devonian to Early Carboniferous (~382.7 to ~360 Ma). Polarized photomicrographs of the thin section were taken in 2020.|
|Geographical scope||Nanjing City, Yixing City, Jiangsu Province; Changxing County, Hangzhou City, Zhejiang Province; Guangde City, Ningguo City, Jingxian County, Tongling City, Chizhou City, Lujiang County, Wuwei County, Chaohu City, Anhui Province.|
|Polarized microscope resolution||2560*1920 pixels|
|Data volume||2.85 GB|
|Data format||*.jpg, *.xls|
|Data service system||<http://www.dx.doi.org/10.11922/sciencedb.j00001.00103>|
|Source of funding||Science and Technology for Production Program (Grant No. CCL2018SHPS004RSI).|
|Dataset composition||This dataset comprises three parts; the first includes 29 files that store a total of 2892 photos of 12 sections, with a data volume of 2.85 GB. The second is an information table sheet with a data volume of 95 KB. The third part is a table wherein the abbreviation code for each debris type is recorded (data volume, 15 KB).|
At the end of the Caledonian tectonic cycle, most areas of southern China were exposed to and suffered from weathering and denudation to varying degrees. In the Early Devonian, seawater intruded into the South China plate from the Qinzhou-Fangcheng trough[1-2] and reached the lower Yangtze region. This trend continued until the early Late Devonian, resulting in nonconforming contact between the Devonian clastic rocks and underlying strata. The Lower Devonian Lianhuashan formation (D1l ) in Qinzhou–Yulin and Liuzhou of Guangxi exhibit unconformity on the Cambrian. The Shiqiao formation (D1s ) of the Lower Devonian in the Guilin area exhibits unconformity on the lower Cambrian Qingxi formation (Є1q ). The Lower Devonian Pingyipu formation (D1p ) in the western part of the upper Yangtze and the Middle Devonian Xiaoxiyu formation (D2x ) in the eastern Sichuan area exhibits parallel unconformity on the Middle Silurian Huixingshao formation (S2hx ). The Yuankou (D1y ), Banshan (D1b ), and Tiaomajian (D2t ) formations of the lower Devonian were gradually deposited from the bottom to the top in South Central Hunan and southwest Jiangxi, which exhibit unconformity on the Chayuantou (Є2C ) of the Middle Cambrian. The Yuntaiguan formation of the Middle Devonian, the Xiaoxi formation (S2x ) of the Middle Silurian in northwest Hunan and southwest Hubei, and the Yuntaiguan formation of the Middle Devonian exhibit parallel unconformity above the Middle Silurian Xiaoxi formation (S2x ). In the middle–lower Yangtze area, the upper Devonian–Lower Carboniferous Wutong group (D3C1w ) exhibits parallel unconformity in the Middle Silurian Funtou formation (S2f ) and the upper Silurian Maoshan formation (S3ms ). The developments of the strata are shown in Table 1.
From the upper to the lower Yangtze region, although differences can be observed in the position of the Devonian above the unconformity, significant similarity exists in terms of the lithological composition. A layer of basal conglomerate developed at the bottom of the set of siliciclastic rocks and evolved into medium-fine clastic rock dominated by quartz sandstone. The Wutong group is broadly exposed in the southern part of the lower Yangtze (see Fig. 1) and can be divided into the Guanshan and Leigutai formations from the bottom to the top. The boundary between the Devonian and Carboniferous is located in the upper part of the Leigutai formation [11-15]. The lithology of the Guanshan formation primarily comprises quartz conglomerate, gravelly quartz sandstone, and quartz sandstone intercalated with siltstone and mudstone. The lithology of the Leigutai formation primarily comprises quartz sandstone, siltstone, and mudstone intercalated with fine quartz sandstone.
Fig. 1 Field profile and sampling locations of the Wutong group[7-9]
1: The strata of the Wutong group in their exposed position; 2: fault; 3: section and sampling positions; 4: toponym; 5: coastline; 6: Paleo-Qinling oceanic subduction zone. I: Lower Yangtze block; II: Qinling–Dabie orogenic belt; III: North China Craton; IV: the Cathaysia block.
The provenance and sedimentary environment information of Devonian siliciclastic rocks above the unconformity surface are key to revealing the paleogeomorphology of the Yangtze area at the end of Caledonian tectonic movement as well as the sedimentary and paleogeographic pattern of early Late Paleozoic. A total of 200 siliciclastic rock samples from 12 sections and 12 outcrops in the southern part of the lower Yangtze were observed and photographed under a microscope, and detailed identification and analyses were performed. Meanwhile, 12 thin-section images of siliciclastic rocks in the Wutong group [16-19] were collected from existing research results. Data for a total of 212 rock samples were collected and sorted into a detailed dataset. This dataset can provide details regarding provenance information and sedimentary environment evolution during the Devonian period in the lower Yangtze area.
Following substantive literature research and detailed field investigation, 24 representative sections and outcrops from the upper Devonian–lower Carboniferous Wutong group in the south of the lower Yangtze region were selected for field measurement and observation. Detrital rock samples from the Wutong group were systematically collected from a sampling location shown in Fig. 1, and 200 rock samples were selected for ordinary thin-section sampling. To improve the reusability of these thin sections, only two-thirds of each thin section was prepared. Microscopic observations were completed in the sedimentary geology laboratory of the Institute of Sedimentary Geology, Chengdu University of Technology, China. A polarizing microscope (Nikon LV100 P0L) was used in a test environment temperature of 25℃ and 45% humidity.
The thin-section photography and information collection were conducted following the “special subject of rock microscopic image” standard. The microscopic rock images were systematically collected and the relevant information of thin sections were obtained. Accordingly, systematic observation, analysis, and identification were conducted, wherein the description of thin sections and the naming of siliciclastic rocks were all in accordance with the “special subject of rock microscopic image” standard.
The dataset comprises 212 polarizing micrographs of rock slices. Among these, only 12 rock slices had previously been collected in literature, and only orthogonal polarized images are available; the compiler of the dataset has prepared 200 rock slices, each of which show at least one field of view; additionally, each field of view includes one orthogonal polarizing micrograph and one monopolar micrograph. Automatic exposure and automatic white balance were adopted in the photography process to ensure that the color of the photographed micrograph was to the greatest extent consistent with naked-eye observations under the polarizing microscope. The components in the microscopic images are the same as those described in the identification report.
The numbering principle was as follows: thin sections number + "m" + digital serial number of camera field + "+" or "−." For example, slice number WT-01 includes three fields of view, each of which has two orthogonal polarization images and a single polarization image, respectively, denoted as WT-01m1+, WT-01m1−; WT-01m2+, WT-01m2−; and WT-01m3+, WT-01m3−. The antepenultimate "m" in these identifiers represents “micrograph”, while "+" refers to orthogonal polarization and "−" indicates monopolar light (Fig. 2). Field of view 1 primarily includes quartz clasts with siliceous cementation among the quartz grains and a small number of argillaceous fillings. For field of view 2, the clasts are primarily quartz with a small amount of tourmaline, and quartz grains are mainly siliceous cemented with a small number of argillaceous fillings. For field of view 3, the intergranular grains are silty mudstone fragments. The surface of quartz grains is clean and secondary enlargement is common.
The scale of the rock micrographs is set in the lower right corner of the image (unit, μm). When taking an image, the appropriate magnification was selected according to the size of the rock debris particles to ensure that the rock micrographs could directly display the microscopic characteristics of thin slices and ensure image clarity. Micrograph resolution was set to 2560 × 1920 pixels, and the images were saved in JPG format.
Thin-section identification content and rock sample details are stored in an Excel spreadsheet. The identification content primarily includes grain composition, matrix content, cement types, and diagenesis. For grain composition, only grain types with more than 10% quartz (Q), feldspar, (F), and lithics (L) were required as well as the size relationship and proportion of these three rock types. If the matrices contained more than 15% of the three rock types, the rock was regarded as greywacke. The information about rock samples primarily include data from existing literature, a detailed description of the geographical sampling location, the section name, the longitude and latitude of the section from where the sample was taken, sample age, slice holder, etc. (Table 2).
|Serial no.||original no. of thin section rock||rock type||name of rock (Garzanti, 2016) ||grain composition description||impurity base content||Supplementary description||field of view||number of photos||publication document ID||Geographical location||section name||sample /profile longitude||sample / profile latitude||group / formation||age||thin section owner|
|More than 10% of the main particles||QFL content relationship,||cuttings type & situation||cement type||diagenesis||other||national||province||city/county/village/mountain/river/lake|
The rock samples in this dataset include the following: one gravel sample, nine conglomerate samples, 165 sandstone samples (154 pure sandstone and 11 greywacke), 15 siltstone samples, and 22 mudstone samples. These details are shown in Table 3.
|Rock types||Number||Rock types and Number|
|Gravel||1||Metamorphic Quartzite gravel (1)|
|Conglomerate`||9||Quartz conglomerate (1), Lithic quartz conglomerate (8)|
|Sandstone||Pure sandstone||165||154||Quartz sandstone (89), Quartz lithic sandstone (1), Lithic quartz sandstone (63), Feldspathic quartz sandstone (1)|
|Greywacke||11||Quartz Greywacke (9), Lithic quartz Greywacke (2)|
|Siltstone||15||Sandy quartz siltstone (4), Quartz siltstone (3), Argillaceous quartz siltstone (8)|
|Mudstone||22||sandy mudstone (7), Silty mudstone (14), mudstone (1)|
All the samples in this dataset were collected from field sections and outcrops by the authors of the paper. Stratigraphic boundary accuracy was ensured by comparing our results with those of existing studies. Our dataset provides coordinates for the starting point of the profile and the sampling outcrop. Considering the high sampling density of the profile, the coordinate position of a single rock sample was not recorded.
In the process of taking micrographs and identifying thin sections, the interference color grade of quartz particles observed in all rock thin sections was designated as grade I, indicating that the thickness of the thin sections meets the national standard of 0.03 mm. All micrographs were taken using automatic exposure and automatic white balance settings, which helped ensure that the color observed by the naked eye and photo images were as consistent as possible. The resolution of micrographs was set at the highest available value of the photographing system (2560 × 1920 pixels), and the images were saved in JPG format. Accordingly, the quality and clarity of the micrographs are considered reliable. To ensure the accuracy of this dataset, following the initial identification of all rock thin sections, the datasheet information was checked and verified several times to ensure the reliability of the information.
All the rock samples in this dataset are terrigenous siliciclastic rocks. The sampling sites for siliciclastic rock samples almost covered the entire southern part of the lower Yangtze region. Some sections comprise medium/high-density continuous sampling and high-resolution imaging of the thin sections. Additionally, the thin-section identification report is completely open. The appraisal report includes detailed geographic information about the location and horizon of sampling sites/sections. This dataset can be used in basic geological research involving the Wutong group, e.g., for identifying sedimentary environments through grain-size analysis as well as identifying provenance characteristics through single siliciclastic characteristics and multi-siliciclastic Dickinson triangle diagrams. Furthermore, it can be applied to interdisciplinary research, e.g., intelligent image recognition. Concurrently, it can be used to assist social production, e.g., finding high-quality building stone, quartz sand, and clay mines.
The format of the dataset is simple; however, attention should be paid to the following points when referencing it.
(1) All the slices in this dataset are stored by the research group of Professor Hou Mingcai of the Chengdu University of Technology. If the micrographs provided in the above data do not meet the needs of future research studies, the authors of this paper can be contacted to arrange the application for the use of these thin sections.
(2) If a researcher only requires the use of one image set, it can be directly downloaded from the database. However, if additional solutions are required for scientific obstacles related to geosciences, combining the geographic locations provided in the data table as well as the geological age and tectonic background of rock formations will be necessary.
The authors would like to thank Ms. Chang Xiaolin for help in the process of image collection and Qi Zhe for help in sample preparation.
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How to cite this article
CAI WP, HOU MC, CHEN HZ, et al. A micrograph dataset of terrigenous clastic rocks of Upper Devonian Lower Carboniferous Wutong Group in southern lower Yangtze. China Scientific data, 2020, 5(3). (2020-09-25). DOI: 10.11922/csdata.2020.0068.zh.