The Substrate Revolution And Small Skeletal Fossils From The Cambrian Series 2 – Miaolingian (Houjiashan Fm. To Zhangxia Fm.) Of North China

The Cambrian substrate revolution(CSR)is one of the most important geological events in the early evolution of life during Cryptozoic-Phanerozoic.As the increasing of bioturbation,both in intensity and depth,the previously dominant Ediacaran mat grounds were fully disturbed,transforming the micro-mat substrate into mixed softground.The existing concept of CSR does not,however,emphasize the appearance of some trace fossils,such as structural engineer-type trace fossils and coprolites,focusing instead on the mixing of the sediment.Besides,the absence of fossil material in the Longwangmiao Formation and later shallow marine deposits in South China,resulting from intense dolomitization,constrains our understanding of the shallow marine biota of South China during the Cambrian.Many Burgess shale type fossils have been recovered from middle Cambrian,North China.But only few researches on small skeletal fossils are well presented.In order to increase our understanding of this poorly known biota and how structural engineer-type trace fossils and coprolites affect on the substrate.I conducted researches on investigating both burrows with a diagenetic halo from the Houjiashan Formation,Cambrian Series 2 and fecal pellets from early to middle Cambrian deposits,and reported,for the first time,how the nature of the substrate effected preservation of small skeletal fossils in the extensively exposed carbonates from the Cambrian Series 2 to Miaolingian of North China.Using thin sections,quantitative statistic of fecal pellets and the bioturbation volume,and quantitative statistics of small skeletal fossils,micro-X-ray fluorescence test on fossils and slabs,and stable carbon and oxygen isotope analysis,the following new insights were obtained:1)Intense occurring burrows with a thick diagenetic halo were first reported from the Houjiashan Formation,Cambrian Stage 4.Those 3D maze-like burrow systems act as conduits for fluids during synsedimentary,allowing the formation of firm diagenetic halo around the burrow.Based on the quantitative statistics,I found that with the increasing of bioturbation and associated halo,the substrate was modified from softground into firmhardground.Burrows from the Houjiashan Formation indicate that bioturbation not only can disturb and mix the substrate,but also can subsequently harden the substrate and increase stability.2)I first proposed criteria for distinguishing burrow with a micrite halo and boring occurring in micrite pebble within breccia based on materials from early to late Cambrian,North China.Borings in breccia occupy small portion of the pebble but burrows with a micrite halo occupy large portion.Boring may cross cut micro laminae or encrusted by pyrite or glauconite.Using these criteria,I have identified more burrows with a halo from early Cambrian to modern deposits,indicating that this form of bioturbation is much common than previously thought.3)Pellet aggregations and burrow filled with fecal pellets were reported from the Guanshan Biota.This work found those differences in pellets density are representative of different methods of digestion.The studied burrows,filled with pellets,were very similar to those found from modern tidal flats.Investigations of the pellets reveal that both the pellets and their associated source organisms promote cycling and concentrating specific elements,minerals and nutrients in the surrounding sediments.4)On the basis of interaction between substrate and organisms,the preservation of small skeletal fossils from Cambrian Series 2 to Miaolingian were reported.I found out that the carbonate from the Houjiashan to Zhangxia formations can be divided into normal bioclastic grainstone microfacies,hardground bioclastic grainstone microfacies,flat-pebble conglomerates with bioclasts microfacies,hummocky cross stratification and glauconite bioclastic grainstone microfacies.5)Large numbers of small skeletal fossils were recovered from normal bioclastic grainstone microfacies,but diversity levels are low and preservation quality is poor,with the exception of phosphatic brachiopod shells,which are well preserved.Diversity in hardground bioclastic grainstone microfacies is also low with brachiopod ornament poorly preserved,but these layers include a phylum unique to this microfacies(palaeoscolecidians).Flatpebble conglomerates with bioclasts and hummocky cross stratification microfacies both preferentially preserve brachiopods,but with most larger shells are broken and diversity is low.Bioclastic grainstone with authigenic glauconite favors the preservation of all bioclast types.Depositional environments that result in the formation of authigenic glauconite also facilitates the growth of apatite.The availability of apatite allows for the preservation of delicate microstructures and shell ornament,and thus the higher diversity and superior preservation quality found in glauconite bioclastic grainstone microfacies is to be expected.6)I found amounts of small skeletal fossils(n=31991)during this research.Those fossils are divided into Brachiopoda,including Paterinate(with 2 genus and 4 species),Lingulida(with4 families);Mollusca,including Helcionelloida(with 1 genus and 2 species)and Hyolitha(with 5 genus and 5 species);Echinodermata;Spongia;Chancelloriida;Arthropoda;and three species belong to unknown phyla.Among those small skeletal fossils,phosphatic brachiopods are the most abundant constitute in almost all of the samples and are the most common constitute in different settings thus could provide reliable evidence for intercontinental stratigraphic correlation in the future.