塔里木板块巴楚晚奥陶世藻礁滞后演化型式分析

The Great Ordovician Biodiversification Event and widespread epicontinental sea promoted reef thriving and episodic evolution of reef communities. Diverse and high abundance coral-stromatoporoid-bryozoan-calcareous algae biota remarkably expanded in the Late Ordovician reefs and gradually replaced microbial-dominated types of the Middle Ordovician. Two reefal ecosystems are respectively recognized in the Katian Lianglitag Formation (Late Ordovician) from the southeastern well blocks of the Tazhong Oil Field and northwestern Bachu outcrop, Central Tarim Uplift, NW China. The Tazhong Type boundstones dominantly are formed by novel frameworks of the coral-stromatoporoid-bryozoan thus match to the mainstream of reef development. In contrast, Bachu Type, which is mainly formed by calcimicrobal bindstones, shows primary style and likely betray the macroevolutionary pattern timely. The present proposal focus on the mechanism of the evolutionary retardation of the Bachu Type. Lithological structures are applied for investigation the reef components from the reefal limestones of the Bachu Type. Chemostratigraphic profiles of the δO18 values from two palaeoecological units of the Bachu and Tazhong Oil Field respectively are further significant in dating of marine-water temperature differentiations. Warming process of the ocean currents on the carbonate platform of the Central Tarim Uplift caused formation of the two biogeographic realms from Tazhong windward and Bachu leeward. Perspective comparisons on the regional evolutionary pattern to its global coeval analogues imply that the reef communities of the Upper Ordovician Lianglitag Formation are both stenotropy.

奥陶纪生物大辐射事件和广布的温暖浅海环境为生物礁繁盛和造礁群落阶段性演化提供了优良条件。晚奥陶世开始珊瑚-层孔海绵-苔藓虫-钙藻丰度和分异度剧增，逐步取代了中奥陶世钙质微生物造礁群落。塔里木板块中央隆起区上奥陶统凯迪阶良里塔格组存在两种礁群落古生态分区，分别是东南部塔中油田区新型珊瑚-层孔海绵-苔藓虫绑结岩构建的“塔中型”和西北巴楚露头区原始型钙质微生物粘结岩构建的“巴楚型”，巴楚型的礁与全球晚奥陶世后生动物礁群落宏演化模式相比具有显著的滞后性，造成这种现象的原因值得深入探讨。本项目通过巴楚型礁灰岩岩相学分析，细化原始钙质微生物礁群落的组成；结合δO18同位素分析巴楚型和塔中型古水温差异与古生态分区的耦合关系，论证洋流在中央隆起区运移时的增温过程影响迎风面和背风面生长不同的造礁群落；将区域生物礁实例与全球同期生物礁演化模式进行对比，凸显上奥陶统良里塔格组两种造礁群落对温度的狭适性。

中-晚奥陶世之交，后生动物礁逐渐繁盛，逐步取代了小型钙质微生物藻形成新的造礁群落。这一时期的生物礁主要以珊瑚、层孔虫、苔藓虫礁为主，是礁灰岩演化进程中重要的转折期。但塔里木板块表现出与全球奥陶纪生物礁演化模式不完全一致的特点，尤其以中央隆起区最为显著。中央隆起区不仅发育了与全球晚奥陶世生物礁演化同步的塔中型苔藓虫、珊瑚、层孔虫、钙藻障积岩和粘结岩，也仍然存在滞后于全球演化主流的巴楚型钙藻-钙质微生物礁群落。巴楚露头区良里塔格组北礁－南滩的分布主要由北深－南浅的缓坡地貌控制。主要造礁生物为钙质微生物Renalcis和Girvanella，以及钙藻Vermiporella，Halysis，Subtifloria，Wetheredella，Sphaerocodium，Mastopora，表现出对晚奥陶世全球生物礁宏演化主流明显的迟缓性。斜坡类型及洋流模式控制的水温和含氧量变化是导致“塔中型”和“巴楚型”造礁群落分异的重要因素，晚奥陶世处于迎风面的塔中油田陡斜坡边缘形成了水温偏低且能量偏高的环境，适宜于后生动物礁生长；洋流在塔里木中央隆起区向西北的巴楚缓斜坡背风区运移数百千米过程中会发生水温升高、能量减弱且含氧量降低的变化，有利于钙质微生物造礁。这一理论模式合理的解释了中央隆起区两种造礁群落的差异性。.同时，塔里木上奥陶统良里塔格组礁滩复合体是塔里木油田的主要储集层。对该时期礁滩体沉积环境以及埋葬环境的研究确定了礁滩灰岩的展布规律，从而进一步确定了岩性特征孔隙度、连通性指标，这些又进一步控制着对储量的评估，该研究可为综合评价预测有利油气储集区带的分布和寻找理想的油气沉积储存区块提供基础资料。