Abstract:

The Longmen Mountains region is one of the key areas for Devonian geological research in China. However,controversies persist concerning stratigraphic correlation and paleoenvironmental studies in this area. This study focuses on the Middle Devonian Guanwushan Formation and the Lower Carboniferous Zongchanggou Formation in the northwestern Sichuan Basin. Systematic field surveys of outcrop sections were conducted,complemented by thin-section petrographic analyses. A total of 210 carbonate rock samples were analyzed for carbon and oxygen isotope compositions. The results indicate that the middle part of the Guanwushan Formation in the northwestern Sichuan Basin was deposited under relatively higher paleosalinity and shallow-water conditions,representing a favorable stage for dolomite formation. The Guanwushan Formation exhibits conformable contacts with the overlying Shawozi Formation and the underlying Jinbaoshi Formation. The Devonian-Carboniferous boundary is marked by a regional unconformity,resulting in the absence of the upper Guanwushan Formation and the lower Zongchanggou Formation. Within the Guanwushan Formation,δ¹³C values range from -3.72‰ to 3.19‰(average: 0.61‰),while δ¹⁸O values vary between -7.30‰ to-1.11‰(average: -4.49‰). In the Zongchanggou Formation,δ¹³C values range from-4.47‰ to 2.08‰(average: -0.92‰),and δ¹⁸O values range from-5.83‰ to-0.64‰(average: -3.52‰). Paleoenvironmental reconstruction indicates that both the Guanwushan and Zongchanggou formations were deposited in typical marine settings. During deposition of the Guanwushan Formation,seawater temperatures ranged from 15 ℃ to 40 ℃,with an average of 25.32 ℃,suggesting a subtropical to tropical climate. In contrast,during deposition of the Zongchanggou Formation,seawater temperatures varied between 15 ℃ and 34 ℃,averaging 23.71 ℃,reflecting a temperate-tropical climate transition and revealing a cooling event across the Devonian-Carboniferous boundary. Furthermore,the multi-stage positive and negative excursions in δ¹³C were controlled by high-frequency sea-level fluctuations and periodic exposure of continental shelves,reflecting perturbations in the paleo-oceanic carbon cycle. The overall trends in carbon isotope variations across three measured outcrops in the study area show consistency and are broadly comparable to those documented in sections from Guangxi(China),France,and Germany,demonstrating their utility for global Devonian-Carboniferous stratigraphic correlation and subdivision. This study reveals the carbon isotope geochemical characteristics of the Devonian-Carboniferous strata in the northwestern Sichuan Basin,providing key evidence for regional isochronous stratigraphic correlation. It reconstructs the evolution of paleo-seawater temperatures and the mechanisms of carbon cycle fluctuations,identifies the favorable period for dolomite development,and holds significant implications for understanding paleoenvironmental evolution along the eastern Paleo-Tethyan margin and for hydrocarbon reservoir prediction.

Key words: carbon isotope, oxygen isotope, paleoenvironment, Devonian-Carboniferous boundary, Guanwushan Formation, Zongchanggou Formation, Sichuan Basin

摘要：

川西北地区(四川盆地西北部简称)是中国泥盆系研究的重要区域之一,但该地区的年代地层对比与古环境研究仍存在争议。本研究针对川西北地区中泥盆统观雾山组与下石炭统总长沟组系统开展野外露头剖面实测和薄片鉴定,并对其中210个碳酸盐岩样品开展碳、氧同位素分析。结果显示: 川西北地区观雾山组中部古盐度较高,水体较浅,是白云岩发育的有利层段,与上覆沙窝子组、下伏金宝石组呈整合接触; 泥盆系与石炭系之间为区域性不整合,表现为观雾山组上部与总长沟组下部缺失; 观雾山组δ¹³C分布在-3.72‰~3.19‰之间,平均为0.61‰,δ¹⁸O分布在-7.30‰~-1.11‰之间,平均为-4.49‰;总长沟组δ¹³C分布在-4.47‰~2.08‰之间,平均为-0.92‰,δ¹⁸O分布在-5.83‰~-0.64‰之间,平均为-3.52‰。古环境恢复表明,观雾山组与总长沟组均属于典型的海相沉积,观雾山组沉积期海水温度主要分布在15~40 ℃之间,平均25.32 ℃,属于亚热带—热带气候,而总长沟组海水温度主要介于15~34 ℃之间,平均23.71 ℃,属于温带—亚热带气候,也揭示了从泥盆纪到石炭纪存在降温事件。此外,δ¹³C的多期正—负漂移变化趋势受控于高频海平面波动及陆架周期性暴露,反映了古海洋的碳循环扰动。实测的研究区3个露头的碳同位素整体变化趋势基本一致,且与中国广西、法国和德国露头剖面的碳同位素变化趋势基本相同,故其可有效应用于泥盆系与石炭系的年代地层对比与划分。本研究揭示的川西北地区泥盆系—石炭系碳氧同位素地球化学特征,不仅为区域地层等时对比提供关键证据,而且重建的古水温演变及碳循环波动机制,有助于明确白云岩的有利发育时期,对认识特提斯东缘古环境演化及油气储层预测有重要意义。

关键词: 碳同位素, 氧同位素, 古环境, 泥盆系—石炭系界线, 观雾山组, 总长沟组, 四川盆地

CLC Number: 

• P597