Abstract: The Late Ordovician is a crucial period in the long geological history, during which carbonates deposited record the changes of the earth’s environment, as well as provide a foundation for the study of extinction events at the end of the Ordovician. In this study, we collected thin-section images of the Late Ordovician carbonate and identified all fossil fragment contents via a biological polarizing microscope. We focused on Kalpin of Northwestern Tarim and Yichang of South China and covered the stratigraphic range from the Sandbian to Katian stage with sampling intervals of 0.5–2 m and 0.2–0.3 m, respectively. The dataset can be compared with those from other regions to provide more details for subsequent studies on the paleoceanography and paleoenvironments of the Ordovician, as well as establish the spatial variation model of carbonate rocks deposited during the Late Ordovician.
Keywords: Upper Ordovician; Northwest Tarim; Middle Yangtze region; carbonate; fossil fragment
|Title||A micrograph dataset of the Late Ordovician carbonate rocks in Northwest Tarim and South China|
|Data authors||Chang Xiaolin, Hou Mingcai, Liu Xinchun, and Fan Taiyuan|
|Data corresponding author||Hou Mingcai (firstname.lastname@example.org)|
|Time range||Late Ordovician (Sandbian–Katian, ~458.4–448 Ma); Polarized photomicrographs of thin section were obtained in 2017 and 2019, respectively.|
|Geographical scope||The Dawangou section is situated about 75 km southwest of the city of Akesu, 40°43′14.93″N, 79°32′8.29″E. The Puxihe section is situated on the North Bank of Puxi River Bridge, Fenxiang, Yichang City, Hubei Province, China, 30°55′35″N, 111°25′45″E.|
|Data volume||1.96 GB|
|Data format||*.jpg, *.xls|
|Data service system||<http://www.dx.doi.org/10.11922/sciencedb.j00001.00043>|
|Source of funding||National Natural Science Foundation of China (41672102)|
|Dataset composition||The dataset includes 3 data files, which are 1. photomicrographs.zip: 348 polarized photomicrographs (*.jpg) of rock thin sections, with a data volume of 741 MB; 2. field photos of the measured section.zip: it has the field images of the measured section and the characteristics of the outcrop, with a data volume of 1.24 GB; 3. information table of database.xls: it contains all picture data and corresponding identification name, and the excel document includes sample number, corresponding level, and time ownership, with a data volume of 29.6 KB.|
The Ordovician (485.4–443.8 Ma) is a crucial period in the long geological history. During this period, a series of significant changes in the earth’s environment occurred. Among them, the great Ordovician biodiversification event (GOBE) and Late Ordovician mass extinctions (LOME) have become the research focus of the Ordovician. The GOBE is characterized by a rapid increase in the diversity of marine organisms, coupled with a rise in ecosystem complexity ; meanwhile, the LOME is the first mass extinction event in the Phanerozoic, and the effect of biological extinction is second only to the mass extinction at the end of the Permian. However, the mechanism of and environmental changes during these events are controversial [4–6]. Most studies have focused only on the last 1–2 million years of the Ordovician. Paleoenvironmental conditions during the Late Ordovician between the GOBE and LOME have remained poorly understood, despite several efforts . Therefore, it is crucial to minutely examine redox variations across the entire Late Ordovician to elucidate the environmental changes that caused the LOME.
Besides, it is well known that carbonate rocks record the information about the paleo-ocean when deposited, wherein abundant bioclasts can reflect changes in the abundance and diversity of organisms in the paleo-ocean, thereby reflecting the changes of the paleo-ocean environment. In this study, we selected the Dawangou section (Kalpin, Northwest Tarim) and Puxihe section (Yichang, middle Yangtze region) as the research areas. The Dawangou section became an auxiliary global stratotype section and point (GSSP) for the base of the Upper Ordovician (Sandbian) in 2001, with a well-exposed Ordovician sedimentary, and has been intensively studied regarding stratigraphy and paleontology. The lower–middle Upper Ordovician is best exposed at the Puxihe section of the middle Yangtze Block, and it records a deep ramp setting between a carbonate platform and a shelf basin. The Upper Ordovician strata are characterized by a low sedimentation rate; therefore, the stratigraphic thicknesses are moderate compared with the Dawangou section. During the Late Ordovician, the Tarim Basin, a peri-Gondwanan palaeo-plate, was situated close to the eastern edge of the Proto–Tethys Ocean at low to middle latitudes in the Southern Hemisphere, whereas the Yangtze Block was located in the northwest of South China, as part of a rifted microcraton off the northeastern margin of Gondwana. Based on this, we used the polarizing microscope to analyze and identify the rock slices from the Dawangou and Puxihe sections. These microscopic images provide detailed evidence for exploring the evolution of paleo-ocean environment and response of organisms to environmental changes.
The source of this dataset was rock samples. Data acquisition could be divided into two steps—rock sample collection and carbonate rock microscopic image acquisition.
2.1 Sample collection and processing
The collection of rock samples needs to follow these steps: literature review, previous research results summary, target horizon and profile locking, clarification of section stratigraphic units, field measurement, and sample collection. A total of 106 samples were collected from the Dawangou section with a sample interval of 0.5–2 m, including Sargan Formation, Kanling Formation, Qilang Formation, Yingan Formation, and Tierekeawati Formation; a total of 34 rock samples were collected from the Puxihe section with a sample interval of 0.2–0.3 m, including the Miaopo Formation and Pagoda Formation. The samples were collected away from the surface weathered rocks, and fresh samples were taken as far as possible. The samples were collected from fresh surfaces without veins, roots, or strongly weathered surfaces, which include microcrystalline limestones, micritic limestones, argillaceous limestones, shales, and siltstones.
After obtaining the rock samples, it is necessary to perform indoor processing to prepare thin slice samples.
2.2 Microscopic image collection
A total of 140 rock slices were imaged and analyzed using Nikon Ci-POL polarizing microscope. The observation and identification were performed simultaneously, focusing on the characteristic fossil fragments. Then, unidentified fossil fragments were searched and identified.
This dataset includes two sections—the Dawangou section in Kalpin of Northwest Tarim and the Puxihe section in Yichang of South China. The dataset is composed of three parts—the outcrop photos, thin-section photos, and information table of thin-section photos.
The field outcrop photos of the profile in the dataset mainly include lithology (Table 1), boundary contact relationship, fossil characteristics, and sedimentary structure. Among them, there are 72 photos from the Dawangou section and 16 photos from the Puxihe section.
The dataset contained 348 microscopic images (Fig. 1), including 93 and 255 images from the Dawangou and Puxihe sections, respectively. The abundance of bioclasts in carbonate rocks of the two sections was quite different, and the biological abundance and diversity of the Puxihe section were significantly higher than those of the Dawangou section. The information on the sections include initial of the section and number (the sample number of the Dawangou section was DWG- the order of sample; the sample number of the Puxihe section was PXH-layer number-intralayer number), photo number, photo content, two magnification factors, and plane-polarized light (−) or perpendicular-polarized light (+) (Table 2).
|Time||Group/Group name||Section name||Coordinate latitude||Coordinate longitude||Number of thin-section photos|
|Katian||Kanling Fm.||Dawangou section, Kalpin, Northwestern Tarim||40°43′14.93″N||79°32′8.29″E||42|
|Katian||Qilang Fm.||Dawangou section, Kalpin, Northwestern Tarim||40°43′14.93″N||79°32′8.29″E||51|
|Sandbian||Miaopo Fm.||Puxihe section, Yichang, South China||30°55′35″N||111°25′45″E||7|
|Katian||Pagoda Fm.||Puxihe section, Yichang, South China||30°55′35″N||111°25′45″E||248|
The information table of the database was consistent with the microscopic images, including the sample number, photo number, photo content, magnification factor, optical characteristics, lithostratigraphic units, and the ages of the samples.
The accuracy of the research data mainly depended on the following three aspects— the accuracy of the stratigraphic boundary, sampling location, and thin-section identification.
First, the Dawangou section (Kalpin, Northwestern Tarim) became the auxiliary GSSP for the base of the Upper Ordovician (Sandbian) in 2001, and the Puxihe section in Yichang was also in the same area of GSSP in the Middle and Upper Ordovician (Huanghuachang and Wangjiawan sections). In addition, the predecessors have conducted extremely strict and meticulous stratigraphic work in these study areas [9,10]. On this basis, in this study, a high-resolution sampling could ensure the accuracy of chronostratigraphic and geological chronostratigraphic boundaries. Further, the process of profile measurement and sampling was extremely strict, and the measurement and calculation of bed thickness are performed according to the fieldwork standard to minimize errors. Finally, the identification of thin sections was completed by a paleontologist with the guidance of experts. The data were carefully contrasted with previous data, and the acritarchs were recorded to ensure the accuracy of identification.
This dataset shows the abundance and diversity of organisms in the Late Ordovician for Northwest Tarim and South China, which can be combined and compared with the research data of other areas in the Late Ordovician. In addition, it can provide more detailed data supplement for subsequent paleo-ocean and paleoenvironment research on Ordovician. Further, it provides a spatial change model for paleo-ocean carbonate rock deposition of the Late Ordovician for data support. During usage, it is suggested to refer to the articles of the author's team. Based on request, more information can be obtained from the authors.
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CHANG XL, HOU MC, LIU XC, et al. A micrograph dataset of Late Ordovician carbonates in Northwest Tarim and South China. Science Data Bank, 2020. (2020-08-04). DOI: 10.11922/sciencedb.j00001.00043.
How to cite this article
CHANG XL, HOU MC, LIU XC, et al. A micrograph dataset of Late Ordovician carbonates in Northwest Tarim and South China. China Scientific Data, 2020, 5(3). (2020-09-18). DOI: 10.11922/csdata.2020.0059.zh.