重庆开县上二叠统长兴组红花生物礁成礁模式

Reef-forming model of Honghua reefs of the Upper Permian Changxing Formation in Kaixian,Chongqing

通过对红花生物礁露头的精细解剖和微相分析,研究了礁的内部构成和成礁模式。红花生物礁发育3期礁体旋回:礁A、礁B和礁C。礁A由生屑泥晶灰岩和骨架岩构成;礁B由生屑泥晶灰岩、粘结岩、骨架岩和生屑灰岩构成;礁C由粘结岩、骨架岩和生屑灰岩构成。红花生物礁造礁生物有钙质海绵、钙藻类、苔藓虫和水螅类,附礁生物为有孔虫、腕足类、双壳类、腹足类和棘皮动物等。单个礁体内,由下往上的生物演化为:腕足类+双壳类+有孔虫组合→钙藻类→钙质海绵+水螅类+钙藻类+苔藓虫组合→生物碎屑;岩性演化为:生屑泥晶灰岩→粘结岩→骨架岩→生屑灰岩。礁B的生屑滩内生屑间为泥晶充填,生屑分选、磨圆较好,是由相邻的高能生屑滩侵蚀搬运到礁B侧翼低能区沉积形成。3期礁都发育在碎屑滩上,礁A为低能环境下形成的礁,礁B和礁C在礁A形成的高地上成礁,为高能环境礁;单个礁体的完整成礁模式为:在浅滩之上,钙藻类大量生长、粘结吸附颗粒固结基底,钙质海绵和钙藻类在硬质基底上繁茂生长,形成具有抗浪格架的生物礁,礁体暴露水面死亡后遭波浪、水流改造形成生屑滩。

A detailed study of internal architecture and reef-forming model of Honghua reefs has been conducted by outcrop observation and microfacies analysis. The Honghua reefs consist of three depositional cycles ： Reef A consisting of bioclastic micrite limestone and framestone, reef B consisting of bioclastic mierite limestone, bindstone, framestone and bioclastic limestone, and reef C consisting of bindstone, framestone and bioclastic limestone. Reef-building organisms are calcareous sponges, calcareous algae, bryozoans and hydrazoans. The organisms adhering to the reef are foraminifera, brachiopods, bivalves, gastropods and echinoderms. The evolution of organisms in an individual reef is from group of braehiopods, bivalves and foraminiferas to calcareous algae to group of calcareous sponges, calcareous algae, hydrazoans and bryozoans to bioclasts, and that of lithology is from bioclastic micrite limestone to bindstone to framestone to bioclastic limestone. Considering the good rounding and sorting of bioclasts in bioelastic bank of reef B, and the cementation by micrite, it is concluded that the bioclasts were transported from the adjacent high energy bank and deposited in reef B flank where the energy is low. Reef A was formed in low energy environment. Reef B and reef C were deposited over the highland made up by reef A in high energy environments. The individual reef-forming model is that calcareous algae binds grains and makes the basement hard, and calcareous sponge and calcareous algae abundantly buildup on the basement and form the reef that can resist waves, then the reef forms a bioclastic bank when it dies and is destroyed by waves and tide.