湖南新邵巨口铺泥盆纪棋梓桥组的层孔虫及其造礁特征

STROMATOPOROIDS FROM DEVONIAN CHITZECHIAO (QIZIQIAO) FORMATION OF JUKOUPU IN XINSHAO,HUNAN AND THEIR REEF-BUILDING CHARACTERISTICS

湖南新邵巨口铺泥盆纪生物礁出现在中泥盆统棋梓桥组中上部,根据沉积特征的差异,礁体分上下两部分;下部成层性明显,以厚层生物格架灰岩为主;上部以灰白色块状徽晶一亮晶灰岩为主,灰岩成层性很差,生物也较单调。礁体可划分8种沉积微相,其发展过程有明显阶段性。这些阶段性与礁体群落的发展和演替相联系。它是开阔浅海与台地浅海相变过渡带上的一种层状礁。礁体内有丰富的层孔虫,计16属近40种。该文选择了较为常见、保存较好的10属18种进行了描述和研究。

The present paper describes 10 genera and 18 species of stromatoporoids with such most importantforms as Actinostroma clathratam Nicholson; A. cf. crassepilatum Lecompte, Clathrostroma bifarium(Nicholson), Synthetostroma acttnostromoides Lecompte, Stromatopora hiipschii Bargatzky, S. cf. cima-censis (Lecompte), S. cf. maiUieuxi (Lecompte), Parallelopora ostiolata Bargatzky, P. goldfussii Bar-gatzky and Stachyodes insignia Yavorsky. These species may be correlated with those of West and SouthChina, and also with those occurring in the equivalent beds of many European countries and Canada,providing evidence for determining the Chitzechiao Formation is late Middle Devonian in age. The Jukoupu reefs occur in the Middle Devonian Chitzechiao Formation, a transition belt with afacies change from open shallow-sea marl to platform shallow-sea carbonate rocks. Based on the dif-ference in sedimentary features, the reefs may be divided into two parts. The reefs in the lower partare clearly stratified and consist of thick-bedded framestones with stromatoporoids and massive corals.The thick-bedded limestones are intercalated with thin-bedded yellow mudstone, marl or mud shalewith a thickness of 20--30 cm, indicating short-term and periodic changes in sedimentary environ-ment. The reefs in the upper part are separated from the lower reefs by a nearly 10 m thick layer ofcrinoidal limestone. The limestone with a total thickness of 120 m is poorly stratified, containingmonotonous fossils mainly with crinoidai stems,dendritic or straticulate stromatoporoids and aiveolites. Based on field observation and indoor identification, the Jukoupu reefs may be divided into 8 sed-imentary microfacies, namely, micrites with massive stromatoporoids frames; mudstone-shale facies;micrites with massive compound coral frames; sparites with crinoidal fragments; biomicrosparites withbiogenic incrustation; biobaffling micrites; oncolite micrites; and bioelastic micrites. The developmental process of the Jukoupu reefs may be as follows: nearshore normal shallow-sea environments--stromatoporoid reefs developed in the transitional belt--closed or semiclosed environ-ments of back-reef facies abnormal salinity. The characteristics of the life assemblages and sedimenta-ry facies generally vary with changes in sedimentary environments.