ZHANG H，JIANG D，ZENG L P，et al. Genesis of inter-salt dolostone: A case of the Feixianguan Formation at Jinzhuping，NE Sichuan[J]. Journal of Henan Polytechnic University( Natural Science), doi:10.16186/j.cnki.1673-9787.2024070023.

Received：2024-07-04

Revised：2024-10-12

Online：2024-05-22

Genesis of inter-salt dolostone：A case of the Feixianguan Formation at Jinzhuping，NE Sichuan (Online)

ZHANG Hang¹，JIANG Dong¹，ZENG Lingping¹，YANG Heng¹，LI Liang²，GAO Zhengji³，LU Zhipeng³，LUO Lianchao³，WEN Huaguo³

（1.Northeastern Sichuan Oil and Gas District，PetroChina Southwest Oil and Gas Field Company，Dazhou 635000，Sichuan，China；2. Zhanjiang Branch of China National Off shore Oil Corporation Ltd.，Zhanjiang 524057，Guangdong，China；3.State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation & Institute of Sedimentary Geology，Chengdu University of Technology，Chengdu 610059，Sichuan，China）

Abstract: Objectives Dolostone-evaporite associations are good subjects for studying the genesis of dolostone in saline settings，but rare research focused on the dolostone within dolostone-evaporite associations formed of interbedded dolostone and evaporite （i.e.， inter-salt dolostone）. Methods In this study，the petrological and geochemical characteristics of the inter-salt dolostone association of the Cycle Ⅱ of the lower Feixianguang Fm. at Jinzhuping （NE Sichuan） was therefore investigated with the aim to deterring the genesis of inter-salt dolostone. Results The results show that the inter-salt dolostone is anhydrite-bearing dolo-mudstone，while the inter-salt dolostone was characterized by layers of inter-salt dolostone （i.e.，anhydrite-bearing dolo-mudstone） alternating with anhydrite evaporite or dolomite-bearing anhydrite evaporite. Each layer can reach near ten meters in thickness. No exposed structures were observed. The studied inter-salt dolostone association yields δ³⁴S of anhydrite between 14.6‰ and 18.3‰ V-CDT，falling into the δ³⁴S range of lower Triassic marine sulfate evaporites. The thick anhydrite evaporite layers，the low-porous rocks，the absence of exposed structures，and the similar δ³⁴S between the studied inter-salt dolostone association and lower Triassic marine sulfate evaporites suggest that the inter-salt dolostone association was largely deposited in low-energy evaporative lagoons and the dolomitizing fluids might be closely related to the hypersaline fluids within the lagoons. The inter-salt dolostone displays δ¹³C_dol from −1.5‰ to 2.2‰ V-PDB and δ¹⁸O_dol from −6.8‰ to −2.5‰ V-PDB. The REE patterns of the inter-salt dolostone were partially contaminated by the siliciclastic detritus. When excluding the samples with few or no detritus contamination，the inter-salt dolostone shows MREE-enrichment and significant negative Ce anomaly. The computed temperatures of the dolomitizing fluids based on traditional oxygen isotope thermometers show that the fluids dolomitizing the lower part of the Cycle Ⅱ have temperatures only slightly higher than those of normal open-marine seawaters. This indicates that the evaporative lagoon water is probably significantly linked with the formation of inter-salt dolostone. However，the inter-salt dolostone from the middle-upper part of the Cycle Ⅱ displays dolomitizing fluid temperatures significantly higher than those of the lower part. This is interpretated to be the result of the input meteoric water and/or normal seawater. The MREE-enrichment and significant negative Ce anomaly of the inter-salt dolostone reflect that the dolomitizing fluids might be closely linked to the （pene）contemporaneous seawater. Conclusions An integrated investigation disclosed that the inter-salt dolostone had a close relationship to the evaporative lagoon seawater and was largely originated from the seepage dolomitization of （pene）contemporaneous evaporative seawater. The findings of this study may help us better understanding the genesis of dolostone within dolostone-evaporite associations.

Key words：dolostone-evaporite association; dolo-mudstone; anhydrite evaporite; geochemistry; Feixianguang Fm; NE Sichuan

CLC: P618.13