GLAUCONITIZED SPONGE SPICULES FROM THE YELI FORMATION AT THE DAYANGCHA CAMBRIAN-ORDOVICIAN BOUNDARY SECTION,JILIN

The sponge spicules occur commonly in Palaeozoic marine carbonate rocks.Under microscope,the spicules could be discerned in thin sections by their distinct external morphology and optical characters.The glauconitized spicules were scarcely documented in literatures.Eisenack(1978) reported the finding of a fragmentary glauconite mold of sponge spicules from the Lower Ordovician of Sweden.Although the siliceous spicules of the Dayangcha section have been well examined through thin sections(Zhang and Chen,1986),the glauconitized spicules have not been mentioned.When collecting conodont specimens after acetic acid maceration of carbonate samples,the authors recovered dozens of well-preserved glauconitized sponge spicules,including monaxons(Text-figure 2-A—C2),tetractines or(stauractines)(Text-figure 2-D—F),pentactines(Text-figure 3-A),hexactines(Text-figure 3-B1—E).This paper briefly describes the glauconitized spicules and analyzes the glauconitization mechanism.All the fossils under present investigation were collected from the Cambrian-Ordovician transition interval [from HDA11 to HDA14-1(Chen,1986),or from 16.7m to 21.8m(Zhang et al.,1996)] at the Dayangcha section,which is located at 1.5km NE of Dayangcha Town and 25km NE of Baishan(formerly called Hunjiang) City,Jilin Province(Text-figure 1).Lithostratigraphically,the interval belongs to the lower Yeli Formation.The Dayangcha section has been well-known for formerly being one of the candidate sections for the Global Cambrian-Ordovician boundary stratotype more than ten years,and further,it had been accepted as the only candidate section for Global Cambrian-Ordovician boundary stratotype by the International Cambrian-Ordovician Boundary Work-ing Group since 1991.Although the Cambrian-Ordovician Boundary GSSP was finally chosen at the Green Point section in Newfoundland,the Dayangcha is still an important section for regional and interregional correlations across the Cambrian-Ordovician transition interval.The glauconitized spicules documented herein were extracted from limestones with maceration of 10% acetic acid,and examined under SEM after being coated with Pt/Au.The original composition of the spicules is opal(SiO_2.nH_2O),which is an unstable mineral.Subsequently,opal alteration during diagenesis usually resulted in the formation of quartz spicules(Text-figures 2-G,3-F)through dehydration and recrystallization.The formation of glauconitized spicules was related to the slow deposition rate,and local alkai micro-environment that made the opal dissolved.At the same time the glauconites precipitated and replaced the opal.Replacement involved the simultaneous dissolution of opal and precipitation of glauconites.The original morphology of the spicules therefore is preserved,and the fine details remain intact.There should be slow,step-by-step dissolution of opal and immediate precipitation of glauconites on sea floor or in the sediments,and both processes should be in a delicate balance.The slow process could occur during the glauconitic facies(representing an environment with slow deposition rate),to allow the spicules stay on the sea floor long enough for the glauconitization.Four morphologic types of glauconitized spicules are represented in present collection:1)monaxons,2) tetractines or(stauractines),3) pentactines,4) hexactines.Monaxons may belong to demosponge spicules,while others represent the characteristic spicules of hexactinellids.Among them,the density of stauractines is high,that of pentactines is low.These types of spicules,commonly found in the strata ranging from early Cambrian to Quaternary(Bengtson et al.,1990;Zhao and Li,2006),are essentially of modern sponges.It is inappropriate to use their morphology to identify particular taxa.

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