广西桂林南边村泥盆—石炭系界线候选层型剖面的综合评价

COMPREHENSIVE EVALUATION OF THE DEVONIAN-CARBONIFEROUS BOUNDARY SECTION IN NANBIANCUN, GUILIN, GUANGXI

本文从古生物、生物地层学、微相分析及沉积岩石学、稀土及微量元素地球化学分析,同位素年龄测定及磁性地层学等方面对广西桂林南边村泥盆—石炭系界线候选层型剖面进行综合评价。该剖面泥盆—石炭系界线符合界线定义,基本上具备界线层型的选择标准和条件,包含的化石门类多达14个,构成多种性化石带、组合带及综合性多门类化石的生物地层总貌,具备磁性地层及年代地层学研究的潜在优势条件,界线及点位在单相碳酸盐岩中,是稳定和连续的沉积,剖面位于交通方便的旅游城市,而且分布着不同相型的泥盆—石炭系界线地层,便于解决其间的对比问题,是一条不可多得的界线候选层型剖面。

This paper presents a comprehensive evaluation of the Devonian-Carboniferous Boundary (DCB) section in Nanbiancun, Guilin, Guangxi, from the biostratigraphic, lithostratigraphic, chronostratigraphic, sedimentological, geochemical, magnetostratigraphic and geochronometric viewpoints. The emphasis is laid on the confirmation of the former recommendations that the DCB containing the marker point in the Nanbiancun section is within a continuous and monofacial carbonate succession and that the base of the Carboniferous is coincident with the First Appearance Datum of Siphonodella Sulcata within the Siphonodella praesulcata to S. sulcata evolutional lineage. The rocks of the upper praesulcata Zone are 18.3 cm thick, and those of the sulcata Zone are 50.1cm thick in the Nanbiancun section It. Up to 14 fossil groups have been systematically described and illustrated, thus providing multispecies fossil zones and assemblage zones constituting the biostratigraphic framework and signature for GSSP guidance. The Rubidium-Strontium clay mineral method was applied to the dating of the DCB in the section Ⅱ within the sulcata Zone, yielding an isotopic age of 361.04±4.1 Ma. The illite is overwhelmingly of 1 Md type, indicating that it is not of diagenetic origin. The indices of crystallinity are within the non-metamorphic scale. The lowest ^(87)Sr/^(86)Sr isotopic ratio of carbonate has been determined at 0.708310±18, which is in accordance with the initial value of clay mineral isochron 0.70794±0.0002. Magnetostratigraphic measurement reveals that the mean magnetic inclination of the carboniferous and the Devonian strata can be determined as 4.1 and 4.36 respectively. The paleolatitude measured was 2.1N for the Carboniferous and was 2.2N for the Devonian. All of these indicate that the section bears potential amenability to magnetostratigraphic and geochronometric study in the future work. The microfacies study and the REE and trace element analysis prove that the boundary horizon with the marker point is in the deposits of favourable facies within the autochtonous deposits in a stable environment under continuous sedimentation. The event-stratigraphic boundary 74.3 cm below the DCB and the marker point is characterized by the existence of the thin layer of black shale with remarkable concentration of the trace elements and by the grainstone bed immediately above it considered as being of storm origin, both of which can serve as the auxiliary marker horizon. The section is located in a tourist city with convenient traffic, where various facies of DCB sections are developed, allowing correlation within a small region. As a whole the DCB in the Nanbiancun section is almostwholly in accordance with the operating definition of the DCB and meets most criteria for the Global DCB stratotype and stratotype point (GSSP).