The Lower Cambrian Longwangmiao Formation shoal dolostone reservoir in Sichuan Basin is currently an exploration and research highlight in China. Reservoir rocks mainly consist of crystalline dolomite with residual gr...The Lower Cambrian Longwangmiao Formation shoal dolostone reservoir in Sichuan Basin is currently an exploration and research highlight in China. Reservoir rocks mainly consist of crystalline dolomite with residual grain texture, and dolarenite of which the arene is mainly composed of muddy to micritic dolomite with some crystalline grain directionally aligned. The trace element indicates that the dolomites of Longwangmiao Formation may be related to the high salinity of sea water. The oxygen isotope values of crystalline dolomite and dolarenite are both similar to that of the Early Cambrian marine dolomites, and the carbon isotope values of every kind of dolomites are completely overlapped with that of the seawater in Early Cambrian, indicating the dolomitization fluid is originated from the Early Cambrian sea water. The restricted marine biological communities and a small amount of gypsum pseudonodule seen in muddy to micritic dolomite indicate that the sea water in Early Cambrian was restricted and evaporated. However, the general lack of massive evaporite mineral and gypsum karst breccia indicates that the salinity of sea water during dolomitization was lower than the value of gypsum＇s precipitation. The Longwangmiao Formation consists of several high-frequency sedimentary cycles, indicating frequent sea level changes. This study indicates that massive dolomitization may also occur in underwater palaeohigh in carbonate platform through the reflux of penesaline sea water driven by a combination of high-and low-frequency sea-level changes. This kind of dolomitization can explain the generation of massive dolomites in the absence of evaporite precipitation, and further indicates that replacement dolomites can be produced by sea water with a wide range of salinity（normal, penesaline to hypersaline）.展开更多
Abstract The study examines a Lower Jurassic （late Liassic） cyclic lagoonal-peritidal stratigraphic unit outcropping in central western Crete （Tripolis unit）, which corresponds to the eastern （internal） part of ...Abstract The study examines a Lower Jurassic （late Liassic） cyclic lagoonal-peritidal stratigraphic unit outcropping in central western Crete （Tripolis unit）, which corresponds to the eastern （internal） part of the mainland Gavrovo-Tripotis platform, the most significant external platform of the Hetlenides. The studied Tripolis carbonate sequence consists of meter-scale, shallowing-upward successions of restricted inner- carbonate platform facies, including cyclically repeated subtidal, intertidal and supratidal facies, that are separated by erosion surfaces （elementary cycles）. Each cycle starts with relatively open-marine facies, which are overlain by shallower, more restricted facies （tidal fiat progradation）. The lithofacies association includes dotomitic intraclastic-peloidal-bioclastic wackestones-packstones/floatstones and grainstones/rudstones dominated by a restricted shallow-marine fauna （bivalves, gastropods, ostracods and seldom benthic fora- minifers）, representing a shaUow subtidal to intertidal, moderately high-energy environment within an inner- platform setting （peritidal environment to restricted lagoon）. This lithofacies association has been intermit- tently subaeriaUy exposed and has undergone diagenetic processes in an inter- or supratidal environment, exhibiting features of vadose diagenesis and pedogenesis due to long-lasting exposure along certain horizons. The peritidat facies are capped by dolocretes controlled by root-activities （laminar dolocretes, petoi- dat-pisoid dolocretes and massive dotocretes）, marking the end of each depositional cycle, and, thus, dis- tinguishing the successive episodes of a prolonged subaerial exposure period and birth of pateosol horizons. Dotocretes consist a diagenetic facies, characterized by several vadose and pedogenic fabrics, including fenestral cavities with geopetal structures, ＂flower spar＂ to blocky sparry cement in primary pores, micritic coatings, crudely pelleted watts, atveotar-septat texture, infilts of rootlet moulds, inter-granular micritic bridges, meniscus cement, sinuous desiccation cracks, circum-granular cracking and in-situ brecciation. The stratigraphic distribution of the cyclothems, suggests relative sea-level control on the peritidal cyclicity controlled by uniform tectonic subsidence and eustasy （attocyctic processes）, in an inner- ramp-to-shelf carbonate setting with tidal fiat and restricted lagoon depositional environment. However, a combination of attocyctic with autocyctic processes controtting accommodation space and sediment accumutation, shoutd be atso taken in consideration. The studied high-frequency cyctes are interpreted to have been formed during the fatting stage and correspond to towstand systems tract （LST） sediments, which consists part of a regressive peak of the Lower/htiddte Jurassic Transgressive-Regressive Facies Low Frequency Cycte （2nd order）. Due to the absence of massive evaporites, dolomitization has been attributed to reflux of stightly increased-satinity marine fluids （i.e. penesatine）, that are driven into the undertying tagoonat-peritidat carbonate sediments during periods of tong term sea-fever fatl in a marginal marine setting.展开更多
基金supported by the National Natural Science Foundation of China (No.41602166)
文摘The Lower Cambrian Longwangmiao Formation shoal dolostone reservoir in Sichuan Basin is currently an exploration and research highlight in China. Reservoir rocks mainly consist of crystalline dolomite with residual grain texture, and dolarenite of which the arene is mainly composed of muddy to micritic dolomite with some crystalline grain directionally aligned. The trace element indicates that the dolomites of Longwangmiao Formation may be related to the high salinity of sea water. The oxygen isotope values of crystalline dolomite and dolarenite are both similar to that of the Early Cambrian marine dolomites, and the carbon isotope values of every kind of dolomites are completely overlapped with that of the seawater in Early Cambrian, indicating the dolomitization fluid is originated from the Early Cambrian sea water. The restricted marine biological communities and a small amount of gypsum pseudonodule seen in muddy to micritic dolomite indicate that the sea water in Early Cambrian was restricted and evaporated. However, the general lack of massive evaporite mineral and gypsum karst breccia indicates that the salinity of sea water during dolomitization was lower than the value of gypsum＇s precipitation. The Longwangmiao Formation consists of several high-frequency sedimentary cycles, indicating frequent sea level changes. This study indicates that massive dolomitization may also occur in underwater palaeohigh in carbonate platform through the reflux of penesaline sea water driven by a combination of high-and low-frequency sea-level changes. This kind of dolomitization can explain the generation of massive dolomites in the absence of evaporite precipitation, and further indicates that replacement dolomites can be produced by sea water with a wide range of salinity（normal, penesaline to hypersaline）.
基金co-financed by the University of Athens Program KAPODISTRIAS/Special Account for Research Grants of the National and Kapodistrian University of Athens(S.A.R.G)
文摘Abstract The study examines a Lower Jurassic （late Liassic） cyclic lagoonal-peritidal stratigraphic unit outcropping in central western Crete （Tripolis unit）, which corresponds to the eastern （internal） part of the mainland Gavrovo-Tripotis platform, the most significant external platform of the Hetlenides. The studied Tripolis carbonate sequence consists of meter-scale, shallowing-upward successions of restricted inner- carbonate platform facies, including cyclically repeated subtidal, intertidal and supratidal facies, that are separated by erosion surfaces （elementary cycles）. Each cycle starts with relatively open-marine facies, which are overlain by shallower, more restricted facies （tidal fiat progradation）. The lithofacies association includes dotomitic intraclastic-peloidal-bioclastic wackestones-packstones/floatstones and grainstones/rudstones dominated by a restricted shallow-marine fauna （bivalves, gastropods, ostracods and seldom benthic fora- minifers）, representing a shaUow subtidal to intertidal, moderately high-energy environment within an inner- platform setting （peritidal environment to restricted lagoon）. This lithofacies association has been intermit- tently subaeriaUy exposed and has undergone diagenetic processes in an inter- or supratidal environment, exhibiting features of vadose diagenesis and pedogenesis due to long-lasting exposure along certain horizons. The peritidat facies are capped by dolocretes controlled by root-activities （laminar dolocretes, petoi- dat-pisoid dolocretes and massive dotocretes）, marking the end of each depositional cycle, and, thus, dis- tinguishing the successive episodes of a prolonged subaerial exposure period and birth of pateosol horizons. Dotocretes consist a diagenetic facies, characterized by several vadose and pedogenic fabrics, including fenestral cavities with geopetal structures, ＂flower spar＂ to blocky sparry cement in primary pores, micritic coatings, crudely pelleted watts, atveotar-septat texture, infilts of rootlet moulds, inter-granular micritic bridges, meniscus cement, sinuous desiccation cracks, circum-granular cracking and in-situ brecciation. The stratigraphic distribution of the cyclothems, suggests relative sea-level control on the peritidal cyclicity controlled by uniform tectonic subsidence and eustasy （attocyctic processes）, in an inner- ramp-to-shelf carbonate setting with tidal fiat and restricted lagoon depositional environment. However, a combination of attocyctic with autocyctic processes controtting accommodation space and sediment accumutation, shoutd be atso taken in consideration. The studied high-frequency cyctes are interpreted to have been formed during the fatting stage and correspond to towstand systems tract （LST） sediments, which consists part of a regressive peak of the Lower/htiddte Jurassic Transgressive-Regressive Facies Low Frequency Cycte （2nd order）. Due to the absence of massive evaporites, dolomitization has been attributed to reflux of stightly increased-satinity marine fluids （i.e. penesatine）, that are driven into the undertying tagoonat-peritidat carbonate sediments during periods of tong term sea-fever fatl in a marginal marine setting.
