The Integrated Study On Ping Chau Formation,Hong Kong

The geographical limitations of Tung Ping Chau of Hong Kong cause difficulties for geological researches,only a few numbers of studies have been carried out which are insufficient for a systematic and comprehensive understanding of the Ping Chau Formation.A detailed study on the sedimentary features and the origin of the rock formation is important and significant on reconstructing the geological history of Hong Kong.This paper studies on Ping Chau Formation in a broad scope,includes the rock types,sedimentary features,and its sedimentary environment through field investigations and regional geological surveys with application of the methodologies of petrology,sedimentology and geochemistry.Rock sampling has been carried out on a representative sedimentary profile at the Southwest of the Tung Ping Chau.Rock samples collected have been examined by thin section analysis,constants and trace elements analysis,pollen analysis and the data were processed by mathematical statistics.The major findings of this paper are as follows:(1)Ping Chau Formation features of rhythmic bedding includes siltstone,argillaceous siltstones,silty mudstone,inter-bedded mudstone and a single layer of rhyolitic tuff.Typical sedimentary structures have horizontal bedding,wavy bedding,rhythmic bedding,ripple marks,mud cracks,rain prints and convolute bedding etc.;the sedimentary facies being recognized are lake sedimentary facies and shore-lake subfacies.(2)Trace element content of rocks from Ping Chau Formation is generally closer to the shale of the upper crust and that of North American.The rock forming materials of Ping Chau Formation are terrigenous,no traces of biologenic materials were found and no hydrothermal influence has been detected.(3)The pollen assemblage of the rock formation is dominant by gymnosperms,vegetation is coniferous forest-based with a small portion of evergreen broad-leaved plants,with lush ferns understory;angiosperm pollen is rare.It is an arid to semi-arid environment,tropical-temperate climate,but colder than present days.(4)The lithology of the unique rock layer of Tung Ping Chau-the 'Dragon Diving into the Sea' is rhyolitic tuff,containing over-saturated silicon,Distribution patterns of REE shows negative Eu anomaly and displays a right shift v-shaped curve.The origin of the unique rock layer is possibly the result fine volcanic materials deposition produced form a nearby phreatomagmatic eruption.(5)Comprehensive analysis concludes Ping Chau Formation was formed in a brackish water body or under reducing environment in a freshwater shallow lake.A small amount of volcanic pyroclastic rocks indicated the crustal instability,littoral and shallow lacustrine deposition might frequently influenced by volcanic activities in proximity.Based on the pollen combination and unusual litholitic features of the'Dragon Diving into the Sea',the geological age of the Ping Chau Formation should be Late Cretaceous.