Size Distribution Of The Framboidal Pyrites In The Black Shales Of The Lower Cambrian Niutitang Formation In Taoying, Guizhou, And Its Environmental Significance

Based mainly on a drilling log from the Taoying town, Guizhou, analyticalsamples were collected from the Laobao Formation of the Upper Sinian to theJiumenchong Formation of the Lower Cambrian. By the study of petrologicalcharacteristics, recognition of sedimentary structures and their sedimentaryenvironment significance, stratigraphical division and correlation as well as thedistribution of pyrite framboids in the rocks, we aimed at analyzing and discussing thesedimentary environmental changes at the studied area when those rocks weredeposited.On the basis of previous research, samples were collected in the field work andthen analyzed in the laboratory. With the combined knowledge of petrological,stratigraphical, sedimentary together with mineralogical compilations of the sampledsection, the following are the several achievements we have obtained:(1)The lithology of the studied section is relatively simple, with a series ofsiliceous rock containing organic matters, black mudstone and limestone as the maintypes of the rocks components. The most striking feature is that the31m-thick blackmudstone of the Niutitang Formation in the Lower Cambrian contain the mostabundant organic matters in the studied section, thus making it the most importantore-bearing strata in the Taoying area. Field observation of black colored rocks,development of horizontal lamina and the abundant presence of pyrites, has led up tothe preliminary conclusion that this series of rocks were deposited under hydrostatichypoxic environment.(2)The Cambrian strata outcropped well in Guizhou province, leading to the factthat adequate studies on black shale series have been undertaken, and yet most of the studies focused in the middle and northern part of Guizhou. In the contrast, studies onthe black shale series developed in eastern Guizhou, had been very scares. Aconfusing fact is that previous literature had not reached an agreement on the namingof the black shale from the lower Cambrian in the studied area, making it difficult tocorrelate these series of rocks with the strata in adjacent regions. With the descriptionof the petrological as well as of the stratigraphical features of the black shales,preliminary correlations with the middle and north parts of Guizhou were achieved.(3)The commonest sedimentary structures in the studied rocks include horizontallamina, phosphatic nodules, small-scaled cross beddings, carbonate nodules/lens andpyrite nodules/lamina, etc. When combine these sedimentary structures with rocktypes etc., it can be concluded that the sedimentary environment had gone through thechanges from bathyal basin through shallow water regions to outer continental shelf.(4) Abundant pyrites were observed in the analytical samples, in some of whichsamples pyrites can even be seen with naked eye scattering on the surface. Most of thepyrites are framboidal, with only a rare occurrence of euhedral pyrites. However, it isframboidal pyrites that are practical indicators of the redox conditions when thesediments were deposited. Sizes of the pyrite framboids in each samples weremeasured and counted under the scanning electron microscope, from the distributionof which, conclusion can be drawn that these strata with high organic matter contentsin the studied area had experienced a long period of hypoxic sedimentary conditions.