Depositional Environment Research Of Ping Chau Formation In Tung Ping Chau In Hong Kong

Ping Chau formation,in Tung Ping Chau of Hong Kong,is a set of over 450 meters thick flaggy dolomitic and calcareous siltstone containing a few flint layers.In this paper,the author employs field sampling and laboratory analysis to carry out a systematic study of the depositional environment of the Ping Chau formation from the sedimentary structure,rock composition,pollen characteristics and rock elements.By measuring field strata,the measured strata can be divided into 20 layers from bottom to top,the total thickness is about 76.3 meters.Field observation and analysis manifest:(1)the types of sedimentary structures of Ping Chau formation include horizontal bedding,wavy bedding,rhythmic bedding,ripple marks,mud cracks,convolute bedding and other sedimentary structures;(2)Terrestrial sand crumbs is composed of feldspar,quartz and lithic,the content varies from 55%-70%,the content of clay varies from 30%-45%;(3)The pollen assemblage of this fossil formation are mainly gymnosperm pollen,accounting for 80.88%;angiosperm pollen content is extremely low,only 1.19%;and pteridophytes pollen is accounting for 18.78%.The main component of the pollen assemblages reflects that the terrain of the margin of sedimentary basin is undulating;the main vegetation type is coniferous forest with a small amount of evergreen broad-leaved forest,pteridophytes flourish in understory;the climate is subtropical semi-humid and semi-arid,the temperature is lower than today's;(4)The content of main and rare elements of all clastic rocks sample from Ping Chau formation are closer and the standardization diagram of samples display that its main and rare elements are in general closer to the upper crust and the North American shale combination.At last,we can get several conculsion by analying sedimentary enviroment of Ping Chau formation:(1)Comprehensive analysis of its provenance indicates that the clastic rocks of Ping Chau has crustal nature,or a similar source region to the North American shale combination.All the evidence shows that the clastic rocks of Ping Chau formation come from the source region mixed by felsic or mafic provenance,the source rocks may be a mixture of granite,metamorphic and sedimentary rocks;(2)Comprehensive analysis shows that the climate of deposition zone is gradually droughty.The climate of the 2nd layer,the 5th layer,the lower part of the 13th layer,the 14th layer,the 16th layer and the.18th layer are relatively hot and dry.From bottom to top,Ping Chau Formation can be roughly divided into 6 climate cycles:the 1st layer to the 2nd layer,the 3rd layer to the 5th layer,the 6th layer to the lower part of the 13th layer,the upper part of the 13th layer to the 14th layer,the 15th layer to the 16th layer and the 17th layer to 18th layer.In the lower part of the climate cycles,the content of xerophytes pollen is low,the content of Ti and Zr is high,the content of Mn is lower and Fe/Mn ratio is higher,Mg/Ca,Al2O3/TiO2,K2O/TiO2,K2O/Na2O,Al2O3/Na2O ratios are lower,indicating the climate is relatively warm and humid but the upper part of climate cycles is often opposite;(3)The boron content of the sample as well as changes in boron gallium ratio shows paleosalinity is belongs to continental half salt water to freshwater,The boron content is increasing with bottom-up profile indicating that water salinity is increasing and gradually salinization;(4)It combined with pollen era analysis and chert layers era analysis showed that the sedimentary era is Late Cretaceous.