Paleoecology of Early Ordovician Reefs in the Yichang Area,Hubei:a Correlation of Organic Reefs Between Early Ordovician and Jurassic

The Early Ordovician System is composed mainly of a series of carbonate platform deposits interbedded with shale and is especially characterized by a large number of organic reefs or buildups that occur widely in the research area.The reefs have different thicknesses ranging from 0.5 m to 11.5 m and lengths varying from 1 m to 130 m.The reef-building organisms include Archaeoscyphia, Recepthaculitids,Batostoma,Cyanobacteria and Pulchrilamina.Through the research of characteristics of the reef-bearing strata of the Early Ordovician in the Yichang area,four sorts of biofacies are recognized,which are(1) shelly biofacies:containing Tritoechia-Pelmatozans community and Tritoechia-Pomatotrema community;(2) reef biofacies:including the Batostoma,Calathium-Archaeoscyphia, Pelmatozoa-Batostoma,Archeoscyphia and Calathium-Cyanobacteria communities; (3) standing-water biofacies:including the Acanthograptus-Dendrogptus and Yichangopora communities;and(4) allochthonous biofacies:containing Nanorthis-Psilocephlina taphocoense community.The analysis of sea-level changes indicates that there are four cycles of sea-level changes during the period when reef-bearing strata were formed in this area,and the development of reefs is obviously controlled by the velocity of sea-level changes and the growth of accommodation space.The authors hold that reefs were mostly formed in the high sea level periods.Because of the development of several subordinate cycles during the sea-level rising,the reefs are characterized by great quantity, wide distribution,thin thickness and small scale,which are similar to that of Juassic reefs in northern Tibet.The research on the evolution of communities shows that succession and replacement are the main forms.The former is favorable to the development of reefs and the latter indicates the disappearance of reefs.

Application of Stable Strontium Isotope Geochemistry and Fluid Inclusion Microthermometry to Studies of Dolomitization of the Deeply Buried Cambrian Carbonate Successions in West-Central Tarim Basin, NW China

Detailed petrographic,geochemical(O-C-Sr isotopes)and fluid inclusion studies of the deeply buried Cambrian carbonates in the West-central Tarim Basin revealed three types of crystalline dolomites(fine-crystalline,nonplanar-a(s),dolomite(RD1),fine-to medium-crystalline,planar-e(s)dolomite(RD2),and medium-to coarse-crystalline,nonplanar-a dolomite(RD3)),medium-to coarsecrystalline,nonplanar-a saddle dolomite cement(CD)and early and later-stage calcite cement.The occurrence of RD1 along low-amplitude stylolites points to link with pressure dissolution by which minor Mg ions were likely released for replacive dolomitization during early-to intermediate-burial seawater dolomitization.The increasing crystal sizes of RD2 and RD3 with irregular overgrowth rims suggests intense recrystallization and replacement upon the RD1 or remaining precursor limestones by dolomitizing fluids during late intermediate burial dolomitization.The overlap ofδ^(18)O,δ^(13)C and ^(87)Sr/^(86)Sr values of RD1-RD3 and CD dolomite with coeval seawater values,suggests that the principal dolomitizing fluids that precipitated these dolomites was connate(Cambrian)seawater preserved in the host limestones/dolomites.Their high ^(87)Sr/^(86)Sr ratios suggest influx of radiogenic strontium into the Cambrian seawater.Two regimes of fluid flow are recognized in the study area:firstly,influx of magnesium-rich higher-temperature basinal brines along deep-seated faults/fractures,resulting in cementation by CD dolomite.Secondly,the incursion of meteoric waters,mixing with ascending highertemperature basinal brines,and an increase in Ca^(2+)/Mg^(2+)ratio in the fluids probably results in the precipitation of calcite cement in vugs and fractures.

Machine learning methods for predicting the uniaxial compressive strength of the rocks:a comparative study

The uniaxial compressive strength(UCS)of rocks is a critical index for evaluating the mechanical properties and construction of an engineering rock mass classification system.The most commonly used method for determining the UCS in laboratory settings is expensive and time-consuming.For this reason,UCS can be estimated using an indirect determination method based on several simple laboratory tests,including point-load strength,rock density,longitudinal wave velocity,Brazilian tensile strength,Schmidt hardness,and shore hardness.In this study,six data sets of indices for different rock types were utilized to predict the UCS using three nonlinear combination models,namely back propagation(BP),particle swarm optimization(PSO),and least squares support vector machine(LSSVM).Moreover,the best prediction model was examined and selected based on four performance prediction indices.The results reveal that the PSO–LSSVM model was more successful than the other two models due to its higher performance capacity.The ratios of the predicted UCS to the measured UCS for the six data sets were 0.954,0.982,0.9911,0.9956,0.9995,and 0.993,respectively.The results were more reasonable when the predicted ratio was close to a value of approximately 1.