Sedimentary characteristics and origin of dolomitic ooids of the terminal Ediacaran Dengying Formation at Yulin (Chongqing, South China)

Journal Pre-proof Sedimentary characteristics and origin of dolomitic ooids of the terminal Ediacaran Dengying Formation at Yulin (Chongqing, South China)

Dongfang Zhao, Guang Hu, Lichao Wang, Fei Li, Xiucheng Tan, Min She, Wenji Zhang, Zhanfeng Qiao, Xiaofang Wang PII:

S0031-0182(19)30778-3

DOI:

https://doi.org/10.1016/j.palaeo.2020.109601

Reference:

PALAEO 109601

To appear in:

Palaeogeography, Palaeoclimatology, Palaeoecology

Received date:

19 August 2019

Revised date:

10 January 2020

Accepted date:

10 January 2020

Please cite this article as: D. Zhao, G. Hu, L. Wang, et al., Sedimentary characteristics and origin of dolomitic ooids of the terminal Ediacaran Dengying Formation at Yulin (Chongqing, South China), Palaeogeography, Palaeoclimatology, Palaeoecology (2020), https://doi.org/10.1016/j.palaeo.2020.109601

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© 2020 Published by Elsevier.

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Sedimentary characteristics and origin of dolomitic ooids of the terminal Ediacaran Dengying Formation at Yulin (Chongqing, South China) Dongfang Zhaoa, b, Guang Hua, b*, Lichao Wanga, b, Fei Lia, b, Xiucheng Tana, b, Min Shec, d,

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Wenji Zhange, Zhanfeng Qiaoc, d, Xiaofang Wangc, d

a. State Key Laboratory of Oil and Gas Geology and Exploitation, Southwest Petroleum

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University, Chengdu, Sichuan, 610500;

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University, Chengdu, Sichuan, 610500;

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b. Division of Key Laboratory of Carbonate Reservoirs, CNPC, Southwest Petroleum

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c. PetroChina Hangzhou Research Institute of Geology, Hangzhou, 310023;

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d. CNPC Key Laboratory of Carbonate Reservoir, Hangzhou, 310023;

400021.

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e. Chongqing Gas Field, PetroChina Southwest Oil and Gas Field Company, Chongqing,

* Corresponding author: [email protected]

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Abstract: The Ediacaran was a crucial transition period in the evolutionary history of the Earth. Ediacaran ooids in the Dengying Formation from the Yulin section in southwestern China are important archives of paleoenvironmental information. Although these ooids have been totally dolomitized, the cortical fabric characteristics are locally preserved well, indicating that these ooids were mimetically replaced. The common occurrence of

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concentric–radial fabrics and low level of Sr concentrations (60.3 ± 8.8 ppm) in the studied ooid cortex is more likely suggestive of a primary low-Mg calcite mineralogy and that a

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calcite sea might exist during deposition of the second member of the Dengying Formation. In

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situ geochemical data for the ooid cortices obtained by laser ablation inductively coupled

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plasma mass spectrometry show that these ooids have middle rare earth element enrichment

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(PrN/SmN = 0.7 ± 0.1 and SmN/YbN = 1.5 ± 0.4) and high Fe contents (8200 ± 1800 ppm).

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This suggests ferruginous pore waters, which were related to the selective or preferential

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absorption and release of REEs by Fe–Mn oxyhydroxides.

Keywords: Primary mineralogy, LA-ICP-MS, Mimetic dolomitization, Pore waters, Sedimentary environment

1. Introduction The late Ediacaran (ca. 550–541 Ma) was a crucial period for biological evolution (e.g., Hua et al., 2005; Wood, 2011; Cui et al., 2016; Cai et al., 2017). The ocean was an important place for Ediacaran life (Shu et al., 2014) and, as such, seawater chemistry has been a large focus of Ediacaran paleoenvironmental reconstructions (Catling et al., 2005). It has long been 2

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recognized that seawater Mg/Ca ratios had a significant influence on biomineralization (Stanley, 2006) and the mineralogy of biological skeletons (Porter, 2007; Zhuravlev and Wood, 2008). The chemical composition of Ediacaran seawater remains controversial, despite extensive investigations (e.g., Sandberg, 1983; Zhu et al., 2007; Scott et al., 2008; Frei et al., 2009; Li et al., 2010; Cui et al., 2015; Wood et al., 2017; Jin et al., 2018; Liu et al., 2019).

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With regards to seawater Mg/Ca ratios, most previous studies have considered that this period was characterized by an aragonite sea (e.g., Sandberg, 1983; Hardie, 1996, 2003), whereas

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some studied have proposed an aragonite–dolomite sea (Hood et al., 2011; Hood and Wallace

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2012; Wood et al., 2017) and dolomite might have precipitated directly from seawater.

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As a type of coated grains, ooids are diverse in composition (such as carbonate,

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ferruginous, siliceous and phosphorous) and are widely distributed in strata of various

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geological ages (Flügel, 2004). So far, ooids have been widely applied in geoscientific research, such as sedimentary environment analysis (e.g., Sumner and Grotzinger, 1993;

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Flügel, 2004; Trower et al., 2017), paleoeclimate and paleoenvironment (e.g., Opdyke and Wilkinson, 1990; Li et al., 2013, 2015b), seawater chemistry (e.g., Li et al., 2019), interaction between microbes and carbonate (e.g., Pacton et al., 2012; Li et al., 2017). The fabric-retentive oolites that developed in the upper part of the second member of the Dengying Formation (hereafter the DF2 interval), in the Yulin section in southwestern China, are good materials for paleoenvironment research. In this study, the petrological and geochemical characteristics of the ooids from the upper DF2 interval were investigated, in order to determine the sedimentary environment of deposition, timing of oolitic dolomitization, and primary mineralogy of ooids, so as to provide research examples and 3

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discuss the geological implications for the terminal Ediacaran paleoenvironment. 2. Geological setting and study section characteristics South China developed into a series of platforms on a passive continental margin in the late Neoproterozoic (Wang and Li, 2003; Zhao et al., 2011). Most of the upper Yangtze region was flooded by an extensive transgression after the Marinoan glaciation (Nantuo Formation) (Jiang et al., 2011), and Ediacaran and Cambrian sequences were widely deposited (Jiang et

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al., 2003, 2011; Cao et al., 2013; Shi et al., 2014, 2015, 2017). The Ediacaran strata (635 −

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541 ± 1Ma) in the upper Yangtze region consist of the older Doushantuo and younger

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Dengying formations (Stratigraphic Committee of the China, 2014). The Doushantuo

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Formation consists of dolomites, dolomitic limestones, and shales, which were deposited in

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shore, tidal flat, shelf, and basin sedimentary settings, with an average thickness of 100 − 200

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m (Liu et al., 2015). The Dengying Formation consists of microbial dolomites, crystalline dolomites, and dolograinstones, which were deposited in a carbonate platform environment,

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with thicknesses of 200 − 1600 m (Liu et al., 2015; Liu et al., 2014, 2016). The basin deepened towards the northeast, and the sedimentary facies changed from a carbonate platform in the southwest to a deep basin in the northeast. The Yulin area was located in the transition zone from carbonate platform to basin (i.e., the platform edge) during the deposition of the DF2 interval (Fig. 1). The studied section (31° 40′ 11.3″ N, 109° 27′ 29.6″ E) is located in Yulin Township, Wuxi County, northern Chongqing, South China (Fig. 2). The Ediacaran outcrop in the Yulin section is approximately 124.1 m thick, and consists of the Doushantuo Formation (15.1 m) and the Dengying Formation (109.0 m) (Fig. 3). The lower part of the Doushantuo Formation 4

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is composed of light gray, thin-bedded, calcareous mudstones, and the upper part of dark gray, thick-bedded to massive micritic limestones (Fig. 4a) which are interbedded with gray-black shales. The Dengying Formation consists mainly of crystalline dolomites (Fig. 4b, c) and grainstone dolomites (Fig. 4d, e). In detail, the DF2 interval comprises: (1) tidal flat, dark gray, medium- to thick- bedded finely crystalline dolomites (Fig. 4c); (2) beach-deposited, light

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gray, medium-bedded dolograinstones (Fig. 4d); and (3) shoal-deposited, medium- to thick-bedded oolitic dolomites(Fig. 4e) interbedded with thin- to medium-bedded, silt

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crystalline dolomites (Fig. 3).

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Dark gray, middle- to thick-bedded oolitic dolomites are developed in the upper DF2

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interval in the Yulin section (Fig. 5c, e). There are two oolitic segments (Fig. 5a, b) that are

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separated by a 2.4 m thick mud-silt crystalline dolomite horizon (Fig. 5d). The upper oolitic

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3. Materials and methods

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segment is ~ 1.4 m thick, and the lower segment is ~ 5.5 m thick.

In the Yulin section, a total of 37 samples were collected from the two oolitic segments and adjacent strata (Fig. 3). One or two thin sections were prepared of each sample (a total of 56 thin sections). Petrographic characteristics were observed and photographed with a German Leica DM4P DFC450C microscope system, focusing on the fabric of the ooid cortices and ooid diameters. Prior to microscopic observations, one-third of each thin section was stained with alizarin red-S and potassium ferricyanide (after the process of Dickson, 1965, 1966). In situ trace element analyses of the ooid cortices were conducted by laser ablation 5

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inductively coupled plasma mass spectrometry (LA-ICP-MS) at the Wuhan SampleSolution Analytical Technology Co., Ltd. After detailed microscopic analysis, five samples with well-preserved ooid cortices (i.e., XWL-04, XWL-05, XWL-07, XYL-08 and XYL-09) were selected for in situ LA-ICP-MS analyses. Before analysis, sample preparation included inspection of sample quality, washing with ultrapure water, drying, and sealing in ziplock

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bags. An Agilent 7700e ICP-MS instrument was used to acquire the ion signal intensities, with a laser spot size and frequency of 44 µm and 5 Hz, respectively. The operating

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conditions of the laser ablation system, as well as ICP–MS data acquisition and reduction

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procedures, followed Zong et al. (2017). Trace element compositions were calibrated against

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various reference materials (BHVO−2G, BCR−2G and BIR−1G) without using an internal

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standard (Liu et al., 2008). Each analysis involved a background acquisition of approximately

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20 – 30 s followed by 50 s of data acquisition during ablation. Microsoft Excel-based software ICPMSDataCal was used to perform off-line data analysis (Liu et al., 2008). After

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LA-ICP-MS analysis, the positions of the laser spots were again carefully examined under a microscope. The data were discarded where the ablation sites were located on small fractures or beyond the boundary of the ooid cortices. The trace element contents of the ooid cortices were calculated from the mean values of spots in each ooid (generally five spots). Major element concentrations of the ooid cortices were determined with an EPMA-1610 electron microprobe at the PetroChina Hangzhou Research Institute of Geology. The electron beam had a resolution of 5 nm and was operated at an accelerating voltage of 30 kV. In addition, same samples (XWL-04, XWL-05, XWL-07, XYL-08 and XYL-09) were selected for whole-rock ICP-MS solution analyses. Trace element analysis of the whole-rocks 6

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was conducted on an Agilent 7700e ICP-MS at the SampleSolution Analytical Technology Co., Ltd., Wuhan, China. The detailed sample digestion procedures were as follows: (1) sample powders (200 mesh) were placed in an oven at 105℃ for 12 h; (2) ~100 mg of sample powder was accurately weighed and placed in a Teflon bomb; (3) 5 mL of 20% acetic acid was slowly added into the Teflon bomb; (4) the Teflon bomb was water bath heated at 60℃

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for 24 h; (5) after cooling and centrifugation, the supernatant and the undissolved residue were obtained. The following procedures were then carried out on the supernatant and

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undissolved residue: (1) samples were evaporated to incipient dryness, and then 1 mL of

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HNO3 was added and evaporated to dryness; (2) 2 mL of 50% HNO3 was added, and the

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Teflon bomb was resealed and placed in the oven at 190℃ for 12 h; (3) the final solutions

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were transferred to a polyethylene bottle and diluted to 100 g by the addition of 2% HNO3.

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For in situ analyses, the relative standard deviations were <8% based on repeated measurements (n = 45) of the MACS-3 standard during this study (Table S1). For whole-rock

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ICP-MS solution analyses, we conducted two duplicate measurements on the same solution, and a mean value was calculated with relative errors less than 5%.

4. Results 4.1 Petrology More than five types of ooids were observed in the Yulin section. These ooid types included micritic ooids, concentric–radial ooids, concentric ooids, radial ooids, and other ooids, using the terminology of Sorby (1879). Generally, the micritic ooids were the most abundant, followed by the concentric–radial ooids, whereas the other ooid types were less 7

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abundant. The ooids in the Yulin section are totally dolomitized. Different types of ooids co-exist in each segment, and the relative abundance of different types of ooids vary from bottom to top. Micritic ooids have variable shapes, including spherical, ellipsoidal and irregular ooids. Their diameters range from 0.1 to 1.1 mm (mainly 0.2 − 0.3 mm). Although most of the

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cortices and nuclei of these ooids were indistinguishable because of the mosaic crystal structure (Fig. 6a, m), some showed diffuse concentric–radial laminae.

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Concentric–radial ooids are nearly spherical and very abundant. Their diameters vary

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between 0.6 and 0.9 mm, and the ratio of the nucleus diameter to cortex thickness is

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generally >1. Ooid nuclei are comprised mainly of aggregations of dolomite crystals or

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microspheres. According to the cortex characteristics, the concentric–radial ooids can be

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divided into two categories. One type of ooid consists of two subcortices, and the external concentric subcortices are typically thinner than the internal radial subcortices (Fig. 6b). The

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other type of ooid is characterized by a relatively uniform interaction between the concentric and radial laminae (Fig. 6c, n).

Concentric ooids are spherical with peloid nuclei (Fig. 6d, o). The diameters of these ooids vary from 0.6 to 0.8 mm. The ratio of the ooid nucleus diameter to cortex thickness is <1. The exquisitely preserved cortices consist of bright microcrystalline dolomite and dark organic matter-rich laminar couplets, in which a single lamina is usually thinner than 10 μm. The number of laminae ranges from a few to tens, and the interfaces between the bright and dark laminae are smooth and sharp. Radial ooids were rarely observed. They are spherical and ellipsoidal in shape, relatively 8

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uniform in size (~ 0.6 mm), and the ratio of the ooid nucleus diameter to cortex thickness ~1. The ooid nuclei are comprised mainly of aggregations of microspheres. The cortices consist of radial arrangements of dolomite crystals extending from the nuclei. The radial cortices are interrupted by concentric banding (Fig. 6f, n), which is a common feature in ancient ooids (Sandberg, 1975; Wilkinson et al., 1985).

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There are four remaining types of ooids (i.e., compound ooids, broken and regenerated ooids, and deformed ooids) in the Yulin section, although their abundances are very low. The

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compound ooids have two or more sub-ooids and relatively thin cortices (Fig. 6g, r). The

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diameters of the compound ooids vary from 0.4 to 0.8 mm. The broken ooids and regenerated

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ooids are oolitic fragments or regenerations coating thin cortices (Fig. 6h, i, s). The deformed

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ooids are mainly characterized by deformation or distortion, and have variable shapes and

4.2 Geochemistry

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linear contacts between the ooids (Fig. 6j, k, q).

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Concentrations of major elements in the ooid cortices are listed in Table 1. The Mg and Ca contents in the cortices were 49 ± 1 mol.% (± 1σ standard deviation) and 48 ± 1 mol.%, respectively. All the ooid cortices have low contents (<1 wt.%) of SiO2, TiO2, Al2O3, MnO, Na2O, K2O, and P2O5, but relatively high values of Fe (FeOcortex = 0.8 ± 0.2 wt.%). The rare earth elements (REE), yttrium (Y), and redox-sensitive element data for in situ analyses, whole-rock acetic acid leachate and the undissolved residue fractions are given in Tables 2, 3, and Table 4 respectively. The REE data are normalized to Post-Archean Australian Shale (PAAS, Taylor and McLennan 1985), and REE anomalies are calculated according to Lawrence et al., (2006). The REE patterns of the ooid cortices from the in situ 9

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analyses and acid acetic leachates of the whole-rock have many similarities (Fig. 7), including: (1) enrichment in the middle REEs (MREEs); (2) negative Eu anomalies (Eu/Eu*cortex = 0.47 ± 0.05 and Eu/Eu*acetic acid leachate = 0.39 ± 0.05); (3) negligible La anomalies (La/La*cortex = 1.10 ± 0.10 and La/La*acetic acid leachate = 1.08 ± 0.07); (4) no positive Y anomalies compared with modern seawater (Y/Hocortex = 27.4 ± 1.4, Y/Hoacetic acid leachate = 31 ± 1). Both the in situ

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and acetic acid leachate data have significantly different REE characteristics as compared with the undissolved residues, which are characterized by obvious enrichment in light REEs

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(LREEs) and high total REE contents (∑REE+Yundissolved residue = 225 ± 84 ppm) (Fig. 7g). The

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in situ data are characterized by relatively high total REE concentrations (∑REE+Ycortex = 57

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± 11 ppm) as compared with the Early Triassic ooid cortices from South China (∑REE+Y =

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5.4 ± 2.9 ppm) and modern Bahamas ooid cortices (∑REE+Y = 1.2 ± 0.5 ppm) (Li et al.,

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5. Discussion

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2017).

5.1 Sedimentary environment

The ooid diameter is an important indicator of hydrodynamic conditions (Flügel, 2004; Trower et al., 2017). Larger ooids are generally formed under more agitated conditions or over a relatively longer time period (Sumner and Grotzinger, 1993; Flügel, 2004; Trower et al., 2017). In the Yulin section, the diameters of the concentric–radial ooids are generally larger than those of the micritic ooids (Figs 3; and 8a–d). This suggests that the concentric–radial ooids might have formed from more agitated water and over a longer time period than the micritic ooids (Flügel, 2004; Trower et al., 2017). This hypothesis was confirmed by detailed 10

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microscopic examination, which showed that most concentric–radial ooids were fragmented and incomplete due to abrasion (Fig. 6c) (Flügel, 2004). In addition, all the oolitic fragments in the broken ooids and regenerated ooids had concentric–radial laminae (Fig. 6h, i), which further suggests that these ooids were formed under strong hydrodynamic conditions. The micritic ooids with smaller sizes were likely transported into a relatively quiet environment in

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which the precipitation period was relatively short, and then underwent micritization. In addition, the preservation of ooid fabrics might also be an indicator of the

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hydrodynamic conditions. Borings on the ooids caused by microbes and subsequent

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micritization destroyed the ooid fabrics (Flügel, 2004). However, boring microbes cannot

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easily attach to the ooid surface in strong hydrodynamic conditions. Thus, agitated conditions

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with a longer time period of precipitation result in better preservation of ooid fabrics. In the

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Yulin section, the cortices of the concentric–radial ooids were exquisitely preserved, whereas the cortices of the micritic ooids were are less well-preserved (Fig. 6c, d).

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Moreover, the distribution of ooids may also provide information on the environment of deposition. Different types of ooids are mixed and present in each segment, and it appeares unlikely that different types of ooids were caused by variable original mineralogies. The relative abundance of concentric–radial broadly correlates with the ooid diameter (Fig. 3), and thus it is reasonable to attribute this to the hydrodynamic conditions. Given that some micritic ooids also have diffuse concentric–radial laminae, the original mineralogies of all the ooid types were likely the same. Therefore, it can be concluded that the larger concentric–radial ooids were formed under stronger hydrodynamic conditions, whereas the smaller micritic ooids were formed in a 11

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relatively quiet environment (Sellwood and Beckett, 1991). The ooid diameter in the Yulin section was mainly determined by the hydrodynamic conditions. The stratigraphic changes in relative abundance of different types of ooids were mainly determined by the differences in preservation (micritization), rather than the original ooid mineralogy. It is likely that the

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micritic ooids also had concentric–radial laminae before micritization.

5.2 Primary mineralogy of the ooid cortices

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Previous studies have confirmed that the primary mineralogy of ooids is mainly

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dependent on the seawater chemistry, even if microbial facilitation is involved (Riding, 2006;

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Ries et al., 2008). Thus, ooid mineralogy has been demonstrated as a reliable indicator of

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seawater chemistry (e.g., Sandberg, 1975, 1983; Wilkinson et al., 1985; Hardie, 1996).

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Numerous studies have investigated the original mineralogy of ooid cortices (e.g., Assereto and Folk, 1976; Lohmann and Meyers, 1977; Sandberg, 1983; Tucker, 1984, 1992; Given and

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Wilkinson, 1985; Wilkinson et al., 1985; Zempolich and Baker, 1993; Algeo and Watson, 1995; Li et al., 2015a; Hood and Wallace, 2018). Precambrian ooids are commonly mimetically dolomitized, and ooid textures can be partially preserved with distinctive features that can be used to determine the original mineralogy (Zempolich and Baker, 1993; Corsetti et al., 2006; Hood and Wallace, 2012, 2018). Most

previous

studies

have

considered

the

terminal

Ediacaran,

the

final

chronostratigraphic unit of the Precambrian, as featuring an aragonite sea (e.g., Sandberg, 1983; Hardie, 1996, 2003; Porter, 2007; Cui et al., 2019; Ding et al., 2019). However, several lines of evidence suggest that the primary mineralogies of the studied dolomitic ooids need to 12

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be reconsidered. Recognition of the primary mineralogy of ooid cortices requires assessment of (1) primary cortical fabric, (2) manner of preservation of cortical layers, and (3) elemental concentration data, especially of Sr content (Algeo and Watson, 1995). Generally, primary aragonite ooids exhibit poorly preserved morphologies and concentric fabrics, and high concentrations of Sr ( generally > 2000 ppm); primary low-Mg calcite ooids are featured with

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relatively well-preserved radial (or concentric–radial) cortex and low Sr concentrations (Tucker, 1984, 1992; Given and Wilkinson, 1985; Wilkinson et al., 1985; Heydari et al., 1993;

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Algeo and Watson, 1995; Li et al., 2015a).

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In this study, concentric–radial ooids accounted for more than 95% (n = 915; 902 of

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those ooids showing concentric–radial cortices) of the ooids whose cortices could be

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identified. The dominance of concentric–radial ooids imply that the primary mineralogy of the

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ooids are more commonly linked to low-Mg calcite. Moreover, Sr concentrations could be an effective method to identify the primary mineralogy of ooids (Algeo and Watson, 1995; Li et

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al., 2015a), since the distribution coefficient of Sr in aragonite is significantly larger than that of calcite (Kinsman and Holland, 1969). But, in some cases, for dolomitic ooids, Sr concentration data should be applied with caution in the identification for the primary mineralogy. It is generally recognized that dolomitization is a process roughly characterized by the loss of Sr (e.g., Sibley et al., 1987; Brand and Veizer, 1980; Gregg et al., 2015), so the Sr concentrations in dolomitic ooids could be modified by dolomitization. The Sr concentrations in ooid cortex (60.3 ± 8.8 ppm) are really low, which is more typical of originally low-Mg calcite than aragonite ooids, though the Sr concentration data are somewhat modified by diagenesis (Fig. 9a, b). Therefore, it is more likely that the studied 13

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ooids were originally low-Mg calcite in composition, though there is no way to be sure about the original mineralogy. The terminal Ediacaran-Early Cambrian was a switch period from aragonite seas to calcite seas (Sandberg, 1983), but whether a calcite sea existed during the terminal Ediacaran in the Yangtze area needs further study.

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5.3 Mimetic dolomitization The ooids in the Yulin section have been almost completely dolomitized (Fig. 6a–l), and

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some ooids show exquisitely preserved laminae (Fig. 6b–d), which are similar to those in the

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oolitic dolomite in the Neoproterozoic Beck Spring Formation (Zempolich et al., 1988) and

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the Johnnie dolomitic ooids (Summa, 1993; Trower and Grotzinger, 2010). These perfectly

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preserved primitive fabrics of the ooids in the Yulin section provide important evidence for

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mimetic dolomitization during early diagenesis (Sibley, 1991). Mimetic dolomitization refers to the preservation of the form and internal structure of a primitive allochem or cement during

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replacement (Sibley, 1991).

Mimetic dolomitization is promoted by the presence of sediment-capping microbial mats (Corsetti et al., 2006), and seawater with high supersaturation (Zempolich and Baker, 1993) and low sulfate conditions (Baker and Kastner, 1981). Previous studies have shown that the Dengying stage was characterized by highly supersaturated seawater with respect to carbonate (Higgins et al., 2009), widespread distribution of microbial mats (Hagadorn and Bottjer, 1999; McIlroy and Logan, 1999; Seilacher, 1999), and low-sulfate conditions (Hurtgen et al., 2002; Frank and Fielding 2003). Therefore, mimetic dolomitization for Yulin ooids would have been facilitated by such conditions. 14

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In summary, the exquisitely preserved laminae suggest that the ooids were early mimetically dolomitized (e.g., Tucker, 1983; Sibley, 1991; Corsetti et al., 2006), and the dolomitization fluid could largely trace the seawater chemical composition during the depositional period.

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5.4 Ferruginous pore waters The Yulin ooid cortices have relatively high values of Fe (FeOcortex = 8200 ± 1800 ppm),

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as compared with Early Triassic ooid cortices from South China (227 ± 171 ppm) (Li et al.,

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2017), modern Bahamas ooid cotices (202 ± 33 ppm) (Li et al., 2017), and Neoproterozoic

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marine carbonates deposited in ferruginous conditions (~ 2000 ppm) (Hood and Wallace,

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2015). To clarify whether the high Fe contents are contaminations from terrigenous detritus,

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we investigated the occurrence of Fe in the ooid cortices. The studied ooid cortices were stained light blue (Fig. 6k) with alizarin red-S and potassium ferricyanide, which identifies

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ferroan dolomite (Evamy, 1963; Dickson, 1965), because of the reaction between ferrous iron and potassium ferricyanide in dilute hydrochloric acid that forms the pigment Turnbull’s blue (Evamy, 1963). Given its similar ionic radius (Hendry, 2003), Fe2+ enter the dolomite lattice and partially displaces Mg2+ to form [Ca(Mg, Fe, Mn)CO3] (Hendry, 2003; Gregg et al., 2015). The lack of positive correlations between Fe and Si (Fig. 9d), Al (Fig. 9e), and Mn contents (Fig. 9f) suggest the high Fe contents in the ooid cortices were not derived from terrigenous debris but instead result from Fe in the dolomite lattice (i.e., ferroan dolomite). The exquisitely preserved laminae of the ferroan-oolitic dolomites (Fig. 6k) suggest that Fe2+ entered the dolomite lattice during marine diagenesis and not during burial. 15

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The REE patterns of the ooid cortices are strikingly different from modern seawater (Webb and Kamber, 2000; Nothdurft et al., 2004; Webb et al., 2009). All of the ooid cortices have distinctive MREE-enriched patterns (Fig. 7a~f). The enrichment of MREEs could be produced by early diagenetic, suboxic to anoxic pore waters (Haley et al., 2004; Chen et al., 2015; Zhang et al., 2016) and microbial activity (especially for Sm and Eu; Takahashi et al.,

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2007). However, microbial activity would also result in HREE enrichment (Takahashi et al., 2007). While the heavy REE enrichment was not found in the ooid cortices in the Yulin

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section, the possible effects of microbial activity can be excluded. Therefore the

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MREE-enriched patterns indicate suboxic to anoxic pore waters. The details of the

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MREE-bulge pattern remain controversial (Chen et al., 2015; Smrzka et al., 2019). Given that

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ooid cortices have relatively high values of Fe (FeOcortex = 8200 ± 1800 ppm), the enrichment

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of MREEs was more likely related to the selective or preferential absorption and release of

Huh, 2013). 6. Conclusions

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REEs by Fe–Mn oxyhydroxides (Haley et al., 2004; Prakash et al., 2012; Soyol-Erdene and

(1) Multiple types of ooids were identified (i.e., micritic ooids, concentric-radial ooids, concentric ooids, radial ooids and other types of ooids) in the Yulin section. The ooids were deposited in an oolitic shoal setting, and the larger concentric and concentric-radial ooids were formed in more agitated hydrodynamic conditions than the smaller micritic ooids. (2) The fabric-retentive dolomitic ooids were mimetically dolomitized. The dominant concentric–radial ooid fabrics, and low level of Sr concentrations in ooid cortex imply primary low-Mg calcite ooid mineralogy, and that a calcite sea might exist during deposition 16

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of the DF2 interval. (3) High Fe contents and MREE-enriched patterns in the ooid cortices suggest ferruginous pore waters, which were related to the selective or preferential absorption and release of REEs by Fe–Mn oxyhydroxides.

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Acknowledgements

We thank Anping Hu and Yujuan Qin (PetroChina Hangzhou Research Institute) for

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assistance with electron microprobe analyses. Sicong Luo and Qian Pang are thanked for their

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help with field work. Ashleigh Hood provided helpful comments on an earlier version of our

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manuscript, and Huan Cui, an anonymous reviewer, and journal editor Thomas Algeo

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provided thoughtful and constructive reviews of our manuscript. This work was jointly funded

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by the National Science Foundation of China (Grant Nos. 41872155 and 41602148), the National Science and Technology Major Project (Grant No. 2016ZX05004002-001), the Key

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Laboratory of Carbonate Reservoirs, CNPC (Grant No. 2018D-5006-35), and Major Project of CNPC (Grant No. 2016B-0403).

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Journal Pre-proof Figure and Table Captions. Fig. 1. Paleogeography of the Sichuan Basin during deposition of the second member of the Dengying Formation (adapted from Zhou et al., 2015; Gu et al., 2016; Zhou et al., 2017). U-Pb zircon ages adapted from Condon et al. (2005) and Cui et al. (2016).

Fig. 2. Geological setting and location of the Yulin section. (a) Sketch map of South China

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showing the main tectonic units and location of the study area. (b) Enlarged map of the study

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area in (a) showing the regional tectonic features. (c) Geological map showing Ediacaran

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outcrops in the study area and the location of the studied Yulin section.

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Fig. 3. Schematic diagram of the stratigraphy, sample locations, and ooid diameters in the

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Yulin section from the Wuxi area.

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Notes: extreme outliners represented by open circles are between ±1σ and ±3σ errors; mild

yellow rectangle.

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outliers represented by filled stars are beyond ±3σ errors. The DF2 interval is marked by a

Fig. 4. Photomicrographs of different lithological intervals in the Yulin section. (a) Micritic limestone with scattered euhedral pyrite (plane-polarized light; upper part of the Doushantuo Formation). (b) Micritic dolomite (plane-polarized light; first member of the Dengying Formation). (c) Finely crystalline dolomite with sporadic subhedral–euhedral pyrite (plane-polarized light; lower part of the second member of the Dengying Formation). (d) Dolograinstone (plane-polarized light; upper part of the second member of the Dengying Formation). (e) Oolitic dolomite containing fissures infilled with late-stage calcite 31

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(plane-polarized light; upper part of the second member of the Dengying Formation). (f) Argillaceous siltstone (plane-polarized light; lower part of the third member of the Dengying Formation).

Fig. 5. Field photographs of the Dengying Formation oolites in the Yulin section. (a) Outcrop

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photograph and (b) line drawing showing oolitic segments in the upper part of the second member and lower part of the third member of the Dengying Formation. Close-up view of the

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horizon, and (e) upper oolitic segment.

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(c) lower oolitic segment, (d) sample XYL-11 in intervening mud–silt crystalline dolomite

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Fig. 6. Oolite types and their microscopic features. (a) Micritic ooids (plane-polarized light;

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XWL-6). (b) Concentric–radial ooids (plane-polarized light; XYL-8). (c) Concentric–radial ooids (plane-polarized light; XYL-9). (d) Concentric ooid (plane-polarized light; XYL-9). (e)

Jo ur

Same view as (d) (cross-polarized light; XYL-9). (f) Radial ooid (plane-polarized light; XYL-8). (g) Compound ooids (plane-polarized light; XYL-9). (h) Broken ooids (plane-polarized light; XYL-9). (i) Broken and regenerated ooids (plane-polarized light; XWL-4). (j) Deformed ooids (plane-polarized light; XYL-9). (k) Deformed ooids (plane-polarized light; XWL-7). (l) Ooids stained light blue (plane-polarized light; XYL-9). Schematic diagrams of (m) micritic, (n) concentric–radial, (o) concentric, (p) radial, (q) deformed, (r) compound, and (s) broken and regenerated ooids, and (t) brick-like laminae.

Fig. 7. PAAS-normalized REE patterns for the (a–e) ooid cortices, (f) acetic acid leachate of 32

Journal Pre-proof

the whole-rock, and (g) undissolved residue of the whole-rock.

Fig. 8. Histograms and probability curves for the oolite diameters in the Yulin section from the Wuxi area.

of

Fig. 9. Plots of (a) Sr vs. Mg/(Mg+Ca), (b) Sr vs. Mn, (c) Th vs. ∑REE+Y, (d) FeO vs. SiO2, (e) FeO vs. MnO, (f) FeO vs. Al2O3 for the ooid cortices in the Dengying Formation from the

-p

ro

Wuxi area.

lP

re

Table 1. Concentrations of major elements in the ooid cortices from the Yulin section.

na

Table 2. Concentrations of trace elements (ppm) in the ooid cortices from the Yulin section.

whole-rocks.

Jo ur

Table 3. Concentrations of trace elements (ppm) in the acetic acid leachates of the

Table 4. Concentrations of trace elements (ppm) in the undissolved whole-rock residues.

33

Journal Pre-proof Tab. 1 Sample

Sample

Mg

Ca

SiO2

TiO2

Al2O3

FeO

MnO

MgO

CaO

Na2O

K2O

P2O5

source

number

mol%

mol%

wt%

wt%

wt%

wt%

wt%

wt%

wt%

wt%

wt%

wt%

1

48.94

47.29

0.93

0

0.26

0.84

0.18

21.08

28.34

0.38

0.14

0.05

2

49.69

47.90

0.53

0

0.16

0.96

0

21.67

29.07

0.06

0.12

0.01

3

49.23

47.32

1.19

0

0.21

0.67

0

21.17

28.31

0.18

0.11

0.04

4

49.97

47.85

0.56

0

0.19

0.54

0.02

21.05

28.05

0.03

0.10

0.02

5

49.69

47.21

0.74

0

0.27

0.70

0.03

21.19

28.00

0.12

0.11

0.02

6

50.26

46.85

0.85

0.01

0.22

0.69

0

21.25

27.56

0.09

0.15

0.03

7

49.01

47.27

0.94

0

0.38

1.24

0

21.37

28.68

0.04

0.11

0.06

8

48.69

47.03

1.04

0

0.58

1.12

0

21.04

28.29

0.10

0.11

0.01

9

48.99

47.92

0.77

0.03

0.45

0.92

0

21.31

29.01

0.09

0.08

0.04

10

49.50

47.73

0.58

0

0.24

0.85

0.04

21.70

29.11

0.05

0.10

0

11

49.18

47.97

0.70

0.18

0.22

0.75

0.03

21.70

29.45

0.07

0.12

0.01

12

49.30

47.47

0.81

0

0.40

0.81

0.07

21.35

28.60

0.10

0.09

0.04

13

49.09

49.12

0.37

0.21

0.64

0.03

20.93

29.14

0.03

0.06

0

14

49.32

48.40

0.52

0

0.19

0.75

0

21.29

29.07

0.09

0.10

0.11

15

50.09

16

48.87

rn

0

17

49.97

18

49.17

19

XWL−4

l a

XWL−7

u o

r P

47.95

0.41

0

0.25

0.71

0

21.78

29.01

0.04

0.09

0.01

48.13

0.79

0

0.27

0.82

0.15

21.37

29.28

0.05

0.15

0.05

47.88

0.33

0

0.14

0.86

0

21.71

28.94

0.06

0.06

0.02

47.72

0.65

0

0.31

1.08

0.10

21.37

28.86

0.03

0.11

0.05

49.55

47.77

0.47

0

0.20

0.97

0

21.26

28.52

0.08

0.09

0.03

20

49.48

48.23

0.38

0

0.33

0.87

0.02

21.40

29.03

0.07

0.07

0.02

21

47.40

48.40

1.41

0.17

0.49

0.64

0

20.05

28.49

0.08

0.22

0.03

22

50.58

47.49

0.35

0.13

0.18

0.55

0.05

20.42

26.68

0.04

0.05

0.07

/

/

62.8

0.99

18.90

/

0.11

2.19

1.29

1.19

3.68

0.16

XYL−8

XYL−9

o r p

e

XWL−5

f o

PAAS

J

Note: PAAS data from Taylor and McLennan (1985).

34

Journal Pre-proof Tab. 2 Sample source

XWL−4

XWL−5

XWL−7

XYL−8

XYL−9

Sample number

Mg/(Mg+Ca)

Mn

Sr

∑REE + Y

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Y

Ho

Er

Tm

Yb

Lu

Ce / Ce*

Eu / Eu*

La / La*

Y/Ho

PrN / SmN

SmN / YbN

Th

4A−01

0.363

277.8

53.5

66.15

11.10

25.24

2.76

11.18

1.93

0.39

1.90

0.23

1.40

8.23

0.30

0.76

0.09

0.55

0.09

1.07

0.48

1.03

27.43

0.90

1.78

2.53

4A−03

0.368

296

51.4

65.92

11.44

25.53

2.77

10.74

2.19

0.38

1.74

0.28

1.39

7.77

0.27

0.71

0.09

0.55

0.08

1.03

0.44

0.97

28.78

0.80

2.02

2.43

4A−04

0.357

298.5

59.1

64.94

11.00

25.05

2.68

10.51

2.05

0.44

1.78

0.26

1.44

7.91

0.32

0.67

0.11

0.64

0.08

1.06

0.52

0.99

24.72

0.82

1.63

2.43

4B−01

0.360

304.8

50.8

67.40

11.58

26.35

2.83

10.81

2.16

0.46

1.76

0.26

1.41

8.06

0.30

0.63

0.09

0.61

0.10

1.03

0.54

0.94

26.87

0.82

1.79

2.51

4B−02

0.359

295.2

50.7

70.25

11.88

26.82

2.85

12.02

2.35

0.41

2.02

0.30

1.56

8.21

0.32

4B−03

0.365

308

51

71.11

12.39

26.93

3.14

12.26

2.18

0.43

2.19

0.27

1.48

7.99

0.29

4B−05

0.369

300.2

50.7

65.27

10.63

25.11

2.63

11.00

2.22

0.42

1.80

0.25

1.39

7.94

0.27

5−01

0.365

295

50.6

66.92

11.44

25.55

2.86

10.84

2.30

0.39

1.78

0.26

1.49

8.15

5−04

0.361

293.3

51.4

60.78

10.07

22.48

2.44

10.21

2.10

0.42

1.77

0.27

1.42

5−05

0.361

314.8

49.6

74.36

12.93

28.83

3.15

12.70

2.25

0.44

1.92

0.24

5−07

0.361

303

52.2

74.33

13.37

29.39

3.18

12.10

2.32

0.42

1.99

5−08

0.362

282.3

50

69.10

11.95

26.77

2.92

12.04

2.15

0.41

5−09

0.363

289.1

51.8

75.32

13.81

30.02

3.26

12.59

2.27

7−02

0.37

287.9

55.1

36.67

5.64

12.08

1.37

5.66

7−05

0.362

296.4

54.9

38.47

5.85

13.08

1.40

5.90

7−06

0.362

294.1

52.5

36.70

5.43

7−07

0.364

278.5

57.9

36.62

5.61

8−01

0.368

204.3

71

59.89

8−02

0.37

205.6

70.6

54.82

8−03

0.37

223.3

68.3

51.66

J

8−04

0.369

216.8

68.4

8−05

0.374

218.5

8−06

0.367

8−07

0.72

0.09

0.08

1.15

0.46

1.16

25.66

0.76

1.88

2.87

0.77

0.09

0.62

0.09

0.97

0.51

0.94

27.55

0.90

1.80

2.71

0.78

0.09

0.67

0.09

1.16

0.50

1.11

29.41

0.74

1.68

2.53

0.33

f o

0.63

0.72

0.10

0.63

0.10

0.98

0.46

0.90

24.70

0.78

1.85

2.56

7.84

0.29

0.71

0.10

0.59

0.08

1.12

0.48

1.13

27.03

0.73

1.81

2.82

1.64

8.49

0.30

0.71

0.10

0.57

0.08

1.07

0.52

1.05

28.30

0.88

2.00

2.42

0.24

1.50

7.93

0.26

0.74

0.09

0.71

0.09

1.02

0.50

0.95

30.50

0.86

1.65

2.17

1.71

0.25

1.23

7.81

0.28

0.73

0.09

0.66

0.10

1.09

0.48

1.09

27.89

0.85

1.67

2.95

0.42

1.80

0.28

1.46

7.70

0.28

0.71

0.09

0.57

0.09

1.03

0.49

0.99

27.50

0.90

2.02

2.76

1.19

0.24

1.34

0.19

1.16

6.35

0.22

0.59

0.08

0.49

0.07

1.06

0.34

1.10

28.86

0.72

1.23

1.46

1.29

0.27

1.38

0.21

1.21

6.59

0.25

0.49

0.07

0.43

0.07

1.13

0.36

1.16

26.36

0.69

1.51

1.42

ro

p e

r P

11.99

1.36

5.72

l a 1.26

0.29

1.31

0.19

1.18

6.51

0.22

0.60

0.08

0.50

0.07

1.07

0.41

1.11

29.59

0.68

1.27

1.36

12.30

1.36

5.77

1.19

0.27

1.29

0.20

1.03

6.20

0.22

0.57

0.08

0.46

0.06

1.10

0.37

1.16

28.18

0.72

1.31

1.46

19.61

2.14

8.89

2.32

0.48

2.10

0.33

2.00

10.55

0.37

1.01

0.15

0.94

0.13

1.10

0.52

1.12

28.51

0.58

1.26

2.36

17.49

1.89

8.15

1.92

0.41

1.94

0.33

1.91

10.41

0.39

1.00

0.13

0.88

0.12

1.15

0.46

1.21

26.69

0.62

1.11

3.07

7.41

16.74

1.76

7.54

1.64

0.44

2.03

0.32

1.82

9.68

0.36

0.93

0.13

0.76

0.12

1.18

0.52

1.21

26.89

0.67

1.10

1.90

55.62

8.07

18.40

1.99

8.35

1.93

0.37

2.10

0.31

1.83

9.84

0.39

0.95

0.13

0.86

0.11

1.12

0.43

1.12

25.23

0.65

1.14

2.13

63.1

56.80

8.26

18.85

1.93

8.21

1.99

0.43

1.94

0.33

1.94

10.34

0.39

0.97

0.14

0.96

0.13

1.20

0.49

1.21

26.51

0.61

1.05

2.05

215.6

71.5

56.56

8.30

18.61

1.93

8.25

1.97

0.44

2.04

0.32

1.95

10.25

0.36

1.00

0.13

0.87

0.13

1.19

0.50

1.23

28.47

0.62

1.15

2.09

0.364

207.8

68.3

57.34

8.46

18.98

2.04

8.60

1.96

0.41

2.25

0.29

1.68

10.28

0.35

0.97

0.12

0.82

0.13

1.13

0.48

1.16

29.37

0.65

1.21

1.94

9−03

0.372

190.9

69.2

48.52

7.00

15.93

1.69

7.06

1.64

0.41

1.74

0.29

1.60

8.91

0.34

0.87

0.12

0.81

0.11

1.13

0.49

1.13

26.21

0.65

1.02

2.22

9−04

0.369

197.4

64

49.43

7.24

15.61

1.79

7.40

1.85

0.46

1.87

0.29

1.71

9.03

0.35

0.84

0.12

0.75

0.12

1.05

0.54

1.08

25.80

0.61

1.26

1.91

8.88 7.87

rn

u o

35

Journal Pre-proof 9−05

0.365

185.5

73.9

53.84

7.64

17.41

1.87

8.09

1.98

0.44

1.89

0.32

1.86

9.95

0.37

0.94

0.13

0.84

0.11

1.17

0.49

1.20

26.89

0.59

1.20

2.12

9−06

0.369

108.3

75.1

51.22

7.43

16.98

1.79

7.66

1.89

0.39

1.72

0.28

1.73

9.21

0.35

0.86

0.12

0.72

0.11

1.17

0.46

1.19

26.31

0.60

1.33

2.08

9−07

0.367

108.6

71.8

53.14

7.71

17.13

1.91

8.04

1.92

0.41

1.94

0.31

1.92

9.54

0.36

0.87

0.13

0.83

0.13

1.09

0.48

1.12

26.50

0.63

1.18

2.12

9−08

0.371

109.4

69

49.41

7.24

16.27

1.71

7.36

1.71

0.38

1.77

0.27

1.60

8.84

0.32

0.92

0.13

0.78

0.11

1.18

0.46

1.23

27.63

0.63

1.11

1.83

9−09

0.374

106.3

68.1

49.89

7.11

16.12

1.69

7.27

1.91

0.38

1.83

0.26

1.68

9.40

0.33

0.88

0.12

0.78

0.12

1.19

0.46

1.22

28.28

0.56

1.24

1.80

9−10

0.374

194.5

65.3

49.97

6.89

16.13

1.77

7.12

1.91

0.40

2.01

0.30

1.72

9.33

0.33

0.97

0.12

0.85

0.12

1.06

0.46

0.99

28.27

0.58

1.13

1.93

f o

l a

e

o r p

r P

n r u

o J

36

Journal Pre-proof Tab. 3 Sample source

∑REE + Y

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Y

Ho

Er

Tm

Yb

Lu

Ce / Ce*

Eu / Eu*

La / La*

Y / Ho

PrN / SmN

SmN / YbN

Th

XWL−4

25.62

3.81

10.20

1.11

4.95

1.30

0.29

1.46

0.22

1.36

7.99

0.26

0.70

0.09

0.55

0.08

1.18

0.39

1.07

30.99

0.54

1.21

0.69

XWL−5

30.98

3.36

8.67

0.97

4.41

1.19

0.27

1.37

0.21

1.25

7.73

0.24

0.66

0.08

0.50

0.07

1.17

0.37

1.11

31.72

0.51

1.21

0.77

XWL−7

36.67

3.07

7.69

0.92

4.36

1.22

0.28

1.50

0.25

1.48

8.66

0.27

0.73

0.09

0.55

0.07

1.14

0.37

1.17

32.30

0.47

1.13

1.31

XYL−8

29.74

2.74

6.98

0.81

3.67

1.01

0.23

1.12

0.19

1.12

6.52

0.21

0.54

0.07

0.41

0.06

1.13

0.33

1.08

30.64

0.50

1.25

1.16

XYL−9

46.31

4.56

12.30

1.35

5.74

1.62

0.40

1.91

0.32

1.82

10.60

0.36

0.98

0.13

0.80

0.11

1.17

0.47

0.96

29.60

0.52

1.03

1.39

N.A. = not analysed

o r p

l a

e

r P

n r u

o J

37

f o

Journal Pre-proof Tab. 4 Sample source

∑REE + Y

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Y

Ho

Er

Tm

Yb

Lu

Ce / Ce*

Eu / Eu*

Y / Ho

PrN / SmN

SmN / YbN

Th

XWL−4

380.55

80.1

177

19.0

68.4

9.94

1.26

5.54

0.60

2.65

12.8

0.48

1.24

0.19

1.17

0.18

0.97

0.74

26.91

1.20

4.31

17.5

XWL−5

217.45

44.9

102

10.7

38.3

5.41

0.73

3.19

0.35

1.63

8.16

0.29

0.81

0.12

0.75

0.11

0.98

0.60

28.33

1.25

3.68

11.0

XWL−7

182.70

39.4

84.4

8.79

30.9

4.36

0.57

2.69

0.30

1.43

7.69

0.28

0.80

0.13

0.84

0.12

0.98

0.53

27.89

1.27

2.63

10.0

XYL−8

129.67

27.5

58.9

6.41

22.3

3.09

0.44

1.93

0.22

1.16

6.08

0.21

0.58

0.10

0.65

XYL−9

215.66

44.5

101

10.7

38.4

5.33

0.68

3.07

0.34

1.63

8.02

0.29

0.76

0.11

0.71

l a

e

o r p

r P

n r u

o J

38

f o 0.10

1.02

0.50

29.13

1.31

2.42

6.2

0.12

0.99

0.57

27.33

1.26

3.82

10.8

Journal Pre-proof

Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.

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E-mail: [email protected]

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State Key Laboratory of Oil and Gas Geology and Exploitation, Southwest Petroleum University, Chengdu,

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610500, China

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Highlights

• The fabric-retentive dolomitic ooids were mimetically dolomitized.

• Concentric–radial fabrics and low Sr contents imply primary low-Mg calcite ooids.

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• Ooid cortex preserved ferruginous pore water signals.

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