Abstract: Late Cretaceous-Eocene carbonate rocks (with some sandstone or mixed siliciclastic-carbonate interlayers) are widely exposed in the western Tarim Basin, Xinjiang Uygur Autonomous Region, China. The study of these carbonate rocks allows for the description of the Cretaceous-Paleogene paleogeographical evolution of the Tarim Basin, the tectonic evolution of the northern margin of Tibetan Plateau, the aridification process of Central Asia, and informs oil and gas exploration. Although these strata have been researched for over 30 years, their publicly available petrological data remains scarce, which results in the inefficient repetition of work. In light of this, this paper presents a polarized light micrograph dataset of thin sections of 682 rocks collected from the Late Cretaceous-Eocene paralic facies strata in the front thrust zone of the West Kunlun Mountains and the Tianshan in the western Tarim Basin. Among these, there were sections of 436 limestones, 129 terrigenous clastic rocks, 91 mixed carbonate-siliciclastic rocks, 14 dolomites, 11 evaporites, and 1 tuff. The micrographs contain repeatable information, and the dataset also includes information such as sampling location, stratigraphic age, rock name, and rock characteristics. We anticipate that this dataset will be especially valuable for geological research concerning the Late Cretaceous-Eocene strata in western Tarim Basin and to guide petroleum exploration and engineering surveys. The typical shallow marine sedimentary characteristics of the thin sections could also be applied to scientific research, teaching, and public outreach.
Keywords: thin section; polarized light micrograph; carbonate rocks; western Tarim Basin; Late Cretaceous-Eocene
|Title||Polarized light micrograph dataset of Late Cretaceous-Eocene rock thin sections from western Tarim Basin, Xinjiang, China|
|Data corresponding author||Hu Xiumian（firstname.lastname@example.org）|
|Data authors||Zhang Shijie, Hu Xiumian|
|Time range||The rock samples were collected during 2015–2016 from the Late Cretaceous-Eocene (~100–34 Ma) strata. The polarized light micrographs were taken in 2019.|
|Sampling site||In terms of administrative division, the study areas are located in the Akto and Wuqia counties of Kizilsu Kirghiz Autonomous Prefecture, Xinjiang Uygur Autonomous Region, China. Geographically, they are located in the front thrust zone of the West Kunlun Mountains and the Tianshan, along the northeastern margin of the Pamir in the western Tarim Basin, with longitudes and latitudes between 74°32′E–76°24′E and 39°52′N–38°25′N.|
|Spatial resolution||4908 × 3264 pixels|
|Data volume||2.06 GB|
|Data format||*.zip, *.jpg, *.xls, *.pdf|
|Data service system||< https://dx.doi.org/10.11922/sciencedb.00035>|
|Source of funding||National Science Fund for Distinguished Young Scholars (41525007).|
|Dataset composition||The dataset includes five data files, namely: polarized light micrographs.zip, regional geological map.pdf, field photos of the measured sections.pdf, stratigraphic columns.pdf, and identification report.xls. (1). Polarized light micrographs.zip is a dataset made up of 1364 polarized light micrographs (*.jpg) of rock thin sections (2.06 GB). (2). Regional geological map.pdf contains the simplified geological maps of the six measured section areas (533 KB). (3). Field photos of the measured sections.pdf shows the field photos of the six measured sections and the characteristics of the outcrops (2.39 MB). 4. Stratigraphic columns.pdf describes the stratigraphic thickness and age of the six measured sections and all sampling locations in the stratigraphic columns (557 KB). 5. Identification report.xls is a data sheet containing identification information of 682 rock thin sections (412 KB).|
During the Late Cretaceous to Paleogene, a shallow epicontinental sea extended across Eurasia from the Mediterranean Tethys to the Tarim Basin in western China, which is referred to as the proto-Paratethys Sea and its eastern region in the Tarim Basin referred to as the Tarim Sea [1-4]. The Tarim Sea experienced five transgressive-regressive cycles during this time, with five units of carbonate rocks (Figure 1) deposited in the neritic-transitional environments. These strata document the palaeogeographic evolution of the western Tarim Basin and the tectonic evolution of the northern margin of the Tibetan Plateau in the Cretaceous-Paleogene and are important for studying aridification in Central Asia [3-5]. The bioclastic and oolitic grainstone in the Late Cretaceous-Eocene carbonate rocks is rich in dissolved and intergranular pores, which are good oil and gas reservoirs. For example, these lithofacies have provided oil and gas reservoirs in the Kekeya structural belt. Therefore, studying these carbonate rocks is attractive for future oil and gas exploration in the Southwest Depression of the Tarim Basin.
The far-reaching implications of understanding these carbonate rocks have long attracted research attention. Between the 1970s and 1980s, petrological, stratigraphical, and paleontological studies in the region determined the basic stratigraphic framework. From the 1980s to the 1990s, widespread fundamental geological research for oil and gas exploration was carried out, as well as further paleontology, sedimentology, and geochemistry . Due to serious sandstorms across eastern China, the focus of the 21st century has been addressing the aridification of Central Asia, which acts as a source of such storms. The regression of the proto-Paratethys Sea is one of the most important factors influencing this aridification. Therefore, research of these carbonate deposits is currently flourishing.
Regardless of the historic research into this area, there remains a lack of basic petrological data and related available atlases of the Late Cretaceous-Eocene carbonate rocks. A unique example exists in Marine Sedimentary Characteristics and Environments from Late Cretaceous to Early Tertiary in the West Part of Tarim Basin of Xinjiang, where Tang et al. (1992) appended some black and white micrograph and marked rough sampling sections and locations . Other studies only show typical thin section polarized light micrographs related to the main topic of discussion or only show the name of the survey sections and the sampling formations [4,6,9-11]. This phenomenon results from the precise sampling locations are not clear, the published thin section micrographs are representative of a very small proportion of the overall data, which reduces its utilization efficiency. As petrology studies are the basis of much regional research, this inaccessibility to the available data means that repeated research is common and useful data is lost.
To counter this effect, this article presents a polarized light micrograph dataset of the thin sections of 682 rocks sampled from six significant Late Cretaceous-Eocene sections in the western Tarim Basin (Figure 2). The data records in detail the sampling location, stratigraphic age, rock type, etc., and the identifications and descriptions of the thin sections. It can be used for basic geological research, regional geological surveys, mineral exploration, and engineering surveys in the western Tarim Basin. The typical neritic-transitional facies sedimentary features shown in the polarized light micrographs can also be used in scientific research, teaching, and public outreach, as well as future image analysis and research.
Prior to the fieldwork, it was necessary to investigate the sections in the proposed study area and select those with complete exposure, fresh rock outcrop, and regional representation. The measuring process involved the outcrop being recorded meter by meter to obtain its true thickness. A sampling interval of 1–3 m was maintained in accordance with the thickness of a single layer of the stratum, the diversity of the lithological changes, and the frequency of changes in sedimentary characteristics. The rock thin sections were produced using the standard process at Chengxin Geology Service Co. Ltd., Langfang, Hebei Province, China.
Photographs and other information were collected in accordance with previously published standards.
3.1 Sample demonstration
This dataset includes five data files, which are regional geological maps, field photos of the measured sections, stratigraphic columns, polarized light micrographs, and an identification report.
The regional geological maps show the locations, geological background, and coordinates of the sections in this dataset (Figure 3). The six measured sections were Akqiy, Bora Tokay, Tuoyunduke, and Qimugen in the northern front thrust zone of the West Kunlun Mountains, and Bashibulake and Kuzigongsu in the southern front thrust zone of the Tianshan range (Figure 2). In terms of administrative division, these sections are located in the Akto and Wuqia counties of the Kizilsu Kirghiz Autonomous Prefecture, Xinjiang Uygur Autonomous Region, China. The precise starting point coordinates of the sections are shown in Table 1.
|Section name||Segment of the|
|c||39°51′22.72″N||74°33′44.08″E||Kalataer Formation – Bashibulake Formation|
|Kuzigongsu||a||39°46′01.65″N||75°16′45.37″E||Kukebai Formation – Dongba Formation|
|b||39°45′45.43″N||75°17′24.69″E||Qimugen Formation – Kalataer Formation|
|Akqiy||a||39°20′20.98″N||74°56′31.42″E||Kukebai Formation – Wuyitake Formation|
|b||39°20′09.06″N||74°57′51.90″E||Yigeziya Formation – Tuyiluoke Formation|
|Bora Tokay||a||39°30′53.62″N||75°04′48.87″E||Qimugen Formation – Wulagen Formation|
|Tuoyunduke||38°32′00.78″N||76°16′00.22″E||Kukebai Formation – Tuyiluoke Formation|
|Qimugen||a||38°25′48.33″N||76°23′17.42″E||Lower part of the Qimugen Formation|
|b||38°25′56.26″N||76°23′41.86″E||Upper part of the Qimugen Formation|
|c||38°26′02.15″N||76°23′58.96″E||Kalataer Formation – Wulagen Formation|
The field photos of the measured sections shows the far view photos, the outcrop exposure conditions, the strata boundaries, and the names of the strata units, as shown in Figure 4.
The stratigraphic columns show the stratigraphic thicknesses, the stratigraphic units, the sampling intervals, and the sampling locations at the columns (as shown in Figure 5). The color of each lithologic column represents the rock color in the field.
The polarized light micrographs dataset is composed of micrographs of 682 rock thin sections, with each section represented by one cross-polarized and one plane-polarized light micrograph in the same field of view. The micrographs are in JPEG format with a resolution of 4908 × 3264 pixels and their colors are consistent with naked-eye observations through the polarized light microscope. The information displayed in the micrographs is consistent with the corresponding descriptions in the identification reports, shown as in Figure 6. The micrograph naming consists of three parts: the number of the thin section; “+” or “−” for a cross or single-polarized micrograph, respectively; and a number, which represents the objective lens magnification, e.g., 15TY05+2.5.
Figure 6 Examples of micrographs, their numbers, and brief descriptions of representative rock thin sections in the western Tarim Basin
|15TY05, shoot under 2.5x objective lens, cross-polarized micrograph (+)||15TY05, shoot under 2.5x objective lens, single-polarized micrograph (−)|
|Bivalves wackestone, matrix-support, the particle content is about 20%, dominated by the broken bivalves, poorly sorted, and oriented arrangement. The matrix is marl. Dolomite crystalline grain fills along the crevice.|
|15QM73, shoot under 2.5x objective lens, cross-polarized micrograph (+)||15QM73, shoot under 2.5x objective lens, single-polarized micrograph (−)|
|Sandy micrite, grain-supported, poorly sorted. Limestone grains are mainly fragments of bryozoans (15%) and crinoids (15%), followed by intraclasts (5%). Terrigenous grains are dominated by quartz, content is about 15%, and the size is concentrated in 100–300 μm. The matrix is marl.|
|15TY01, shoot under 5x objective lens, cross-polarized micrograph (+)||15TY01, shoot under 5x objective lens, single-polarized micrograph (−)|
|Feldspatho-quartzose sandstone, well-sorted, medium rounding, grain-supported and point contact between the grains. The matrix is calcareous mud and microcrystal. The grain size is concentrated in 200–300 μm, with ~70% quartz, ~20% feldspar and ~10% lithic fragment. The lithic fragments are felsic volcanics.|
The identification report of the thin sections is composed of six carbonate rock and one sandstone identification sheets. The carbonate rock identification sheets contain the identification content of carbonate rock, siltstone, dolomite, shale, evaporite, and peperite. The sandstone identification sheet shows the classification, naming, identification description, and sample information of sandstone. The rock classification and thin section identification standards used are detailed elsewhere.
3.2 Statistical analysis
Among the 682 recorded rock thin sections, there were 436 limestone thin sections (Figure 7), of which 6 were crystalline limestone. Their primary textures and structures were alterated by diagenesis, and only their recrystallized textures were described in the identification report. The remaining 430 thin sections were normal limestone. The identification reports described their important sedimentological information, such as the clastic particle type, biological species, textures and structures, interstitial material, and grain-supported frameworks. Among the 430 limestone thin sections, there were 150 mudstones, 113 wackestones, 111 packstones, and 56 grainstones.
Among the 129 terrigenous clastic rock thin sections, there were 9 argillites, 66 siltstones, and 54 sandstones. Due to the lack of fine sand and coarser grain size in shale and siltstone, it was difficult to identify sandstone types in more detail based on their grain composition; therefore, they were retained in the carbonate rock identification sheets. In accordance with the supplementary description information in the carbonate rock identification sheet, their clastic particle type, content, textures, and structures were observed and described. The 54 sandstone thin sections did not provide any information other than their rock type on the carbonate rock identification sheets and so were described using the sandstone identification sheet, which included important identification information, such as rock name, grain type and content, textures, structures, and cement. Among these 54 sandstone thin sections, there were 40 litho-quartzose, 11 feldspatho-quartzose, 2 quartzolithic, and 1 lithic sandstones.
In addition, among all of the thin sections, there were 91 mixed carbonate-siliciclastic rocks, 14 dolomites, 11 evaporites (10 gypsum rocks and 1 sandy gypsum rock), and 1 tuff. The micrite matrix of the mixed carbonate-siliciclastic rock was suitable for detailed recording using the carbonate description method, and the clastic grain of the peperite was also recorded in the supplementary description column of the carbonate rock identification sheet. The dolomite and evaporite were infrequent, but their depositional textures, structures, and recrystallized textures were also described using the carbonate rock identification sheet. Among the 91 mixed carbonate-siliciclastic rock thin sections, there were 57 sandy micrites, 14 micritic sandstones, 9 muddy micirtes, 6 allochemic sandstones, 2 sandy allochem limestones, 2 micritic mudrocks, and 1 muddy allochem limestone.
The high sampling density of this study means that only the starting coordinates of the measured sections are provided; however, this dataset also includes field photos, sampling formations, and the sampling locations in the stratigraphic columns. This means the user can combine the differences between the stratigraphic units, the thicknesses of each formation, and the marker bed to constrain the sampling location within 10 m of the fieldwork.
The standard procedure for producing rock thin sections is well established and so is not repeated here. The interference colors of the minerals in the thin sections of this study were normal during the micrograph production and identification processes, which indicated that the thickness of the thin sections met the relevant standards.
Automatic exposure and white balance were used to ensure consistent micrograph color between the imaging process and observations with the naked eye. The highest available resolution was used for the micrographs (4908 × 3264 pixels) and they were uniformly saved in the JPEG format to maintain their quality and clarity.
The identification report of the thin sections was checked by PhD Candidate Xu Yiwei, a student at the School of Earth Sciences and Engineering, Nanjing University, who is involved in the identification, microfacies analysis, and paleoenvironment research of carbonate rocks. This confirmation process ensured the reliability of the identification reports.
On the basis that the information obtained in this study is independently verified, this dataset is an important, publicly available compilation of high-resolution polarized micrographs and identification reports of carbonate thin sections obtained by continuous high-density sampling (sampling interval 1–3 m) from six sections in the western Tarim Basin. The sampling locations, stratigraphic ages, components, textures, and structures of the rocks studied using polarized light microscopy were systematically described in their identification reports; therefore, this dataset has far-reaching implications in basic geological research, regional geological surveys, mineral exploration, and engineering surveys in the western Tarim Basin. The typical neritic-transitional facies sedimentary features shown in the polarized light micrographs of the rock thin sections can also be used in scientific research, teaching, and public outreach.
The format of this dataset is simple, but special attention should be shown to the following:
(1) This dataset provides the coordinates, regional geological maps, and field photos of the measured sections; however, user-performed field investigations must be carried out to verify the information prior to using these data.
(2) All the rock thin sections shown in this dataset are stored in the Hu Xiumian Sedimentary Geology Research Group at Nanjing University. If the micrographs provided in this dataset do not meet the needs of the user, please contact the authors to apply for the thin sections.
(3) The interpretation of carbonate microfacies based on the identification results of the carbonate thin sections in this dataset was published in 2018 . The identification results of terrigenous clastic rock and mixed carbonate-siliciclastic rock, sandstone clastic grain statistics data, and detrital zircon U-Pb dating of some sandstone samples in this dataset were published in 2019 . Users can refer to these articles for further information.
We thank PhD Candidate Xu Yiwei for carefully examining the identification report and Li Wei and Mahemuti Waili for the sample collection. This work benefited from discussions with Dr. Ma Anlin and Dr. Lai Wen.
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How to cite this article
ZHANG S J and HU X M. Polarized light micrograph dataset of Late Cretaceous-Eocene rock thin sections from western Tarim Basin, Xinjiang. China Scientific Data, 2020, 5(3). (2020-07-29). DOI: 10.11922/csdata.2020.0010.zh.