摘要
分布于扬子区西北缘宁强—广元地区志留系宁强组以约三千米厚的浅水相泥页岩、海相红层夹灰岩为特征 ,部分灰岩层段中发育生物礁。通过对礁灰岩的微相分析表明 ,菌藻类可通过四种方式参与造礁过程 :1)叠层石 ;2 )凝块岩 ;3)核形石 ;4 )钙藻碎屑堆积。其中以叠层石和凝块岩最为常见 ,它们对灰泥基质起显著的粘结作用。而核形石和钙藻相对较少。
Copper and Brunton (1991), Copper (1994, 1997) and Brett (1991) noted that the Silurian reefs developed a wide spectrum across various environments and varied biota. Stromatolites generally occur in restricted hypersaline basins or nearshore embayments, and carbonate mudmounds mostly occur in outer shelf or slope environments. The Ningqiang Formation (late Telychian, Llandovery, Silurian), characterized by shales intercalated with carbonates, was formed between Ningqiang (S. Shaanxi Province) to Guangyuan (N. Sichuan Province) adjacent to the northwest margin of the Yangtze Platform. The sequence shows multiple horizons of reef building occurred within a relatively short geological interval and resulted in more than 30 patch reefs with the size up to 200 m in diameter and 1—50m in high, composed of abundant fossils. However, the rich muddy and low angle ramp environment is not the optimum setting for large scale Silurian stromatolites and carbonate mud mound growth. The evidence from Ningqiang Formation indicates that stromatolites occur in reef top shallow environments occasionally. High diversity metazoan associations are usually disrupted by the increase of stromatolitic structures, which are at least partly microbially cemented, near the reef top as the metazoan diversity declined. It is mostly attributed to lower energy conditions, indicating a reef under stress and calcimicrobial structures and micritic matrix takes up more than half of the rock, leading to an overall lithological character similar to carbonate mudmounds and stromatolites. Pelmatozoa have a wider adaptation potential and maintained growth in this setting while other framework biota and reef dwellers were relatively stressed. The reason for a calcimicrobial crinoid association in tops of the reefs is not clear; emersion of the reef tops has not been recognised, and a possible cause is higher nutrients as energy fell and mud input increased leading to an unfavourable setting for framework growth. The example of patch reef degradation features from this study has similarities with Spanish Jurassic reefs (Leinfelder et al ., 1994) and Wenlockian Hgklint patch reefs from Gotland, Sweden (Watts and Riding, 2000), in which calcimicrobial structures replace metazoan skeletal framework at reef top. Small scale stromatolites could also grow locally on the regional high energy belt.
出处
《微体古生物学报》
CSCD
北大核心
2002年第2期170-177,共8页
Acta Micropalaeontologica Sinica
基金
获国家自然科学基金 (4 9972 0 0 7)
中国科学院重点项目 (0 0 0 111)
中国科学院南京地质古生物研究所特支项目 (990 60 5 )
创新基金的资助
关键词
陕南
川北
志留系
特列奇阶
宁强组
礁相
微生物岩
钙藻
生物礁
microbilites, calcareous algae, reef, Ningqiang Formation, Silurian, South Shaanxi, North Sichuan
