Evidence of Ancient Freshwater Tidal Deposits
Evidence of Ancient Freshwater Tidal Deposits
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古代淡水潮汐沉积的证据
DOI：
10.2110/pec.98.61.0095
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发表时间：
1998
期刊：
影响因子：
--
通讯作者：
M. Mastalerz
中科院分区：
文献类型：
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作者：
E. Kvale;M. Mastalerz
在美国伊利诺斯盆地东部，与潮汐泥滩有关的发育良好的潮汐韵律层覆盖在一些含硫量最低的宾夕法尼亚煤之上。这种观察似乎与传统观点不一致，传统观点认为，在伊利诺斯盆地和其他地方，低硫煤通常与受海洋影响的顶板岩石无关。盐度海洋环境这种沉积物在现代系统中是已知的，但尚未从岩石记录中得到充分的记录。对紧接在低硫硫值1之上的潮汐沉积物的地球化学岩相学和沉积学分析证实，印第安纳州阿托坎-戴维斯县的宾夕法尼亚巴西组的下块煤段证实了在2000年保存广泛的480 kID 2区域淡水潮滩沉积物的潜力。岩石记录一个强烈的潮汐海洋信号，表现为保存小规模小潮春季周期的潮汐韵律，保存在下块煤正上方的层压泥岩和互层砂岩和泥岩中。这些周期表明沉积在一个以昼夜为主的混合潮汐系统中，其中沉积速率高达1 m1 yr。碳硫比显微组分类型和陆地有机标志物的优势。韵律岩相揭示了下块泥炭煤最初是由后滨水低盐度潮坪上覆的。潮汐韵律岩的存在表明，在海侵期间至少存在中潮条件。从下块煤到下一个较年轻煤的巴西组上块煤段的平面分布、沉积学、地层学、岩石学和地球化学序列表明，潮坪相形成于一个多海湾的海岸环境，经历了大量的降雨和径流，这是一个典型的潮湿气候。海湾可能是由沿海泥炭沼泽的坍塌和海侵形成的。下块煤，最终充满了泥滩和混合砂滩和泥滩相。非常高的降雨量和相关的径流导致了海岸拥抱淡水羽流的形成。这些羽流将盐水楔推到近海，阻止它进入。该研究结果可能对其他石炭纪盆地的古地理重建有意义。在欧洲和其他地方的一些石炭纪层序中，类似的潮汐控制相可能没有被识别出来，这些层序目前被解释为古代非海相泥质为主的热带体系。及其古地理意义
Well developed tidal rhythmites associated with tidal mudflats overlie some of the lowest sulfur Pennsylvanian coals in the eastern part of the Illinois Basin U S A Such an observation is seemingly at odds with the traditionally held view that low sulfur coals are not typically associated with marine influenced roof rock in the Illinois Basin and elsewhere This association can be explained however ifthe tidal mudflats formed in a freshwater or low salinity marine setting Such deposits are known from modern systems but have not been adequately documented from the rock record Geochemical petrographic and sedimentologic analyses of tidal deposits immediately above the low sulfur sulfur values 1 Lower Block Coal Member of the Pennsylvanian Brazil Formation Atokan Daviess County Indiana confirm the potential for the preservation of extensive 480 kID2 area freshwater tidal flat deposits in the rock record A strong tidal marine signal manifested as tidal rhythmites preserving small scale neap spring cycles is preserved within the laminated mudstone and interbedded sandstone and mudstone immediately above the Lower Block Coal These cycles indicate deposition within a mixed predominantly diurnal tidal system in which sedimentation rates were as high as I m1yr Carbon to sulfur ratios maceral types and the dominance of terrestrial organic markers within the rhythmite facies reveal that the Lower Block peat coal was initially onlapped by afreshwater low salinity tidal flat The presence of tidal rhythmites indicates that at least mesotidal conditions prevailed during transgression The areal distribution sedimentology stratigraphy petrography and geochemistry of the succession from the Lower Block Coal to the next younger coal Upper Block Coal Member of the Brazil Formation suggest that the tidal flat facies formed within an embayed coastal setting that experienced significant rainfall and runoff typical of an ever wet climate The embayments likely formed by the collapse and transgression of the coastal peat mires Lower Block Coal and ultimately filled with mudflat and mixed sandflat and mudflat facies Very high rainfall and associated runoffresulted in the formation of coast hugging freshwater plumes These plumes pushed the salt water wedge offshore and prevented it from entering the embayments until the coastal peats had already been covered by at least a meter of tidal flat mud The results of this study may have implications for paleogeographic reconstructions inother Carboniferous basins Similar tide dominated facies may have gone unrecognized in some Carboniferous successions in Europe and elsewhere that are currently interpreted to be ancient nonmarine mud dominated tropical systems Such recognition would have important sequence stratigraphic and paleogeographic implications