Geochemical characteristics and paleoenvironmental implications of carbonate rocks at the Givertian and Eifelian boundary in northeast Guangxi
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摘要:
依托阳朔县龙岩口艾菲尔阶与吉维特阶界线剖面，采集碳酸盐岩样品10个，通过显微特征、元素组分含量及相关元素的比值特征、碳氧同位素数据等，对该界线附近的古海洋沉积环境进行了分析和探讨。研究表明：①样品YS3层位产牙形刺分子Polygnathus intermedius，可归属于艾菲尔晚期ensensis带，为该层位碳同位素偏移事件、Eiffelian与Givertian界线以及Kačák事件提供了准确的时限约束；②参考Wilson的标准微相沉积模式，结合岩性特征，识别出SMF23、SMF16、SMF4和SMF15共四种微相类型；③通过对碳酸盐岩主、微量元素及其比值分析，得出以下结论：YS1至YS2对应阶段为沉积水体快速变深，且具有一定物源供给的沉积环境，反映了海侵初期，海洋环境由局限－半局限台地相向斜坡相快速转变的地质过程；YS3至YS4阶段低含量的陆源组分反映了水体较深、海平面相对较高的远岸沉积环境，该阶段海洋环境以缺氧为典型特征；YS5至YS10阶段陆源组分含量逐渐增加，出现代表低能环境的放射状鲕粒灰岩，沉积区海平面呈现出明显下降的趋势，剖面中多个向上变厚的进积型层序是良好的沉积学响应；④依据氧同位素，重建了古海水温度演化曲线，研究表明该时期海水平均温度为21 ℃，为温暖的亚热带气候，与其位于赤道附近的古地理位置相吻合；⑤样品YS3层位碳同位素表现出的负偏移现象与摩洛哥全球界线层型剖面和北美加拿大地区同层位的δ13C值曲线特征相吻合，共同反映了全球尺度海侵背景下的缺氧沉积环境。
Abstract:The Devonian period, marking the initial phase of the Late Paleozoic era, has garnered considerable attention in recent decades. This heightened interest stems from its intricate climatic fluctuations, recurrent global shifts in sea levels, and a series of consequential biological catastrophes. Of particular captivation is the upper Devonian-lower Carboniferous in northeastern Guangxi, characterized by a multifaceted paleogeographic configuration known as platform-trench-basin facies. Notably, the stratigraphic progression from basin facies to open platform facies displays a notable continuity, culminating in the formation of a substantial kilometer-thick carbonate rock layer. This unique geological foundation underpins the development of a renowned karst geomorphic landscape, which is globally significant.
Furthermore, this distinctive karst landscape has facilitated the establishment of the international Devonian-Carboniferous boundary auxiliary layer profile, adding another layer of importance to the study of the Devonian system in this region. While prior investigations have primarily concentrated on sedimentary facies, paleontology, pivotal organisms, and event layers like the 'F-F' biological catastrophe event and the D/C boundary layer during the Late Devonian, certain critical events, notably the Kačak-Otomari event in the late middle Devonian, have received comparatively less attention. Moreover, a comprehensive global-scale comparative analysis has been lacking.
Given these gaps, the present study investigates a critical juncture, the boundary between the Eiffelian and Givertian systems, situated in Longyankou village, Yangshuo county. A meticulous examination was carried out, involving the collection of ten carbonate rock samples from this specific section. Through a comprehensive analysis encompassing microscopic features, elemental compositions, ratios, as well as carbon and oxygen isotope data, the study aimed to elucidate and discuss the ancient oceanic sedimentary environments in proximity to this location. Our findings can be summarized as follows:
(1) The presence of Polygnathus intermedius, a conodont fossil, is observed within the YS3 sample layer, attributed to the late Eifelian period. This fossil aids in establishing a precise temporal boundary for both the carbon isotope migration event and the Kačák event within this stratum. (2) By employing Wilson's established microfacies sedimentary model in conjunction with lithological traits, four distinct microfacies types, SMF23, SMF16, SMF4, and SMF15, have been successfully distinguished. (3) Through meticulous geochemical analysis, we draw several significant conclusions regarding the sedimentary environment during various stages: The sedimentary environment during the YS1 to YS2 transition demonstrates rapid deepening of sedimentary water and a consistent material source supply. This mirrors the geological process of swift transformation from limited platform facies to slope facies during the initial transgression phase; the low terrigenous component content during YS3 and YS4 stages indicates a distant shoreline sedimentary environment with deep waters and relatively elevated sea levels. Notably, this stage is characterized by widespread hypoxia, displaying global uniformity; Geochemical indicators for YS5 to YS10 stages exhibit minor fluctuations. Notably, the terrigenous component content gradually increases, while the appearance of radial oolitic limestone signifies a low-energy environment. Simultaneously, the sedimentary area witnesses a significant sea level decline, characterized by thicker upward progradational sequences that provide excellent sedimentary responses. (4) Reconstructing the paleoseawater temperature evolution curve based on oxygen isotope data reveals an average seawater temperature of 21 ℃ during the studied period. This temperature profile reflects a warm subtropical climate, aligning with the paleogeographic proximity to the equator. (5) The observed negative migration of carbon isotopes corresponds with the E/G boundary stratotype profile in Morocco and the carbon isotope curve characteristics observed in Canada within the same horizon as sample YS3. In summary, these findings emphasize the prevalence of a deep-water and anoxic sedimentary environment amidst a global-scale transgressive backdrop.
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Key words:
- northeast Guangxi /
- Kačák event /
- carbonate rocks /
- paleoenvironment /
- Devonian
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表 1 阳朔县龙岩口剖面碳酸盐岩元素及同位素数据
Table 1. Element and isotope data of carbonate rocks in Longyankou section of Yangshuo
样号 YS1 YS2 YS3 YS4 YS5 YS6 YS7 YS8 YS9 YS10 SiO2 1.194 6.041 0.701 0.439 0.483 0.361 0.545 0.662 0.570 0.740 TiO2 0.013 0.081 0.010 0.009 0.007 0.008 0.007 0.010 0.011 0.006 Al2O3 0.241 1.584 0.101 0.054 0.057 0.091 0.074 0.157 0.129 0.076 TFe2O3 0.115 0.632 0.047 0.026 0.069 0.047 0.063 0.088 0.068 0.038 MnO 0.008 0.008 0.004 0.005 0.005 0.007 0.005 0.005 0.003 0.004 MgO 3.862 4.863 0.663 0.562 0.771 0.540 0.712 0.676 0.864 0.665 CaO 50.306 44.675 54.313 54.775 54.543 54.908 54.446 54.402 54.267 54.541 Na2O 0 0.024 0.015 0.017 0.013 0.010 0.017 0.010 0.001 0.010 K2O 0.082 0.620 0.036 0.012 0.021 0.020 0.023 0.046 0.038 0.020 P2O5 0.017 0.047 0.018 0.017 0.015 0.011 0.012 0.016 0.010 0.010 LOI 43.717 40.567 43.480 43.618 43.577 43.678 43.545 43.601 43.599 43.526 V 5.06 26.10 5.06 3.94 2.18 2.03 1.80 6.33 3.68 2.09 Ni 6.41 12.40 6.70 6.33 6.60 6.22 5.93 7.23 6.32 6.03 Sr 161 186 238 204 270 197 258 275 252 210 La 1.02 3.75 0.59 0.40 0.92 0.62 0.74 1.09 0.63 0.62 Cr 1.62 7.84 0.91 0.75 1.00 0.73 1.22 1.55 1.04 0.86 Ce 1.92 7.82 1.24 0.72 1.77 1.12 1.46 2.18 1.32 1.08 δ13C −3.23 −2.83 −7.05 −2.72 −1.38 −2.88 −1.33 −1.09 −1.44 −1.86 δ18O −6.86 −5.82 −7.18 −7.34 −6.69 −7.75 −7.23 −8.47 −5.62 −6.41 MgO/CaO 0.077 0.109 0.012 0.010 0.014 0.010 0.013 0.012 0.016 0.012 Ce/La 1.88 2.09 2.10 1.80 1.92 1.81 1.97 2.00 2.10 1.74 V/Cr 3.12 3.33 5.56 5.25 2.18 2.78 1.48 4.08 3.54 2.43 古温度t/ ℃ 21 16 22 23 20 25 22 28 15 19 
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