“干化蒸发”与“回灌重溶”——对鄂尔多斯盆地东部奥陶系蒸发岩成因的新认识

"Evaporation drying " and "reinfluxing and redissolving" ——a new hypothesis concerning formation of the Ordovician evaporites in eastern Ordos Basin

鄂尔多斯盆地东部奥陶系发育巨厚的蒸发岩地层 ,已有不少学者对其成因提出了不同的假设。本文通过对该区蒸发岩微相分布规律、剖面结构及地球化学等特征的分析 ,提出该区蒸发岩形成于成盐盆地与外海周期性隔绝的“干化蒸发”条件下的新认识。首次提出用“回灌重溶”机制来解释厚层石盐岩中的泥质薄夹层的成因、各种石盐岩颜色的成因及蒸发岩剖面中缺乏干化蒸发晚期阶段的钾镁盐层的原因 ,并由此提出“溶积层”的概念。微相分析表明 ,在形成蒸发岩的盐坳盆地东部的水下古地形隆起上 ,发育有堤坝状的礁或生物丘等生物建隆 ,其在地形上的隆起构成了盐坳盆地与外海的障壁 ,在海平面下降期间可使盐坳与外海完全隔绝而进入干化蒸发阶段 ,从而形成了石盐沉积 ;由海平面上升所造成的突发性海水回灌事件又使先成的石盐层部分溶解 (回灌重溶 ) ,并由于不溶残余物的富集成层 (溶积层 )而阻止了下伏石盐层的进一步溶解。这是该区蒸发岩地层中缺少典型干化证据的主要原因。本区奥陶系马家沟组碳酸盐岩与蒸发岩间互分布的地层层序结构特征 ,主要受控于区域海平面变化旋回 ,碳酸盐岩主要形成于海侵期和高水位期 。

A great thickness of evaporite strata were developed in the Ordovician in eastern Ordos Basin Quite a few researchers have proposed different views about their genesis This paper presents another point of view in which the evaporites were deposited under the condition of the saline basin's periodical isolation from the open sea This hypothesis is based on distribution patterns of the microfacies,profile architecture and geochemistry characters of the evaporites The idea of 'reinfluxing and redissolving',is developed for explaining the origin of the thin black clay layers in thick saline rocks and the origin of various colors of the evaporites The lack of sylvite carnallite layers is also considered A new concept,'dissloving deposition bed',is proposed to explain a special mud layer in evaporite strata The microfacies of the Ordovician in this area show that some bio buildups including reefs or bio mounds,grew on the elevated areas underwater along the eastern edge of the basin During the period of sea level falling,it acted as a barrier between the halite depression and the open sea that kept the halite depression isolated from the open sea,and brought it into the stage of evaporation drying to form halite beds The abrupt event of seawater reinfluxing,as a result of sea level rising,could make part of the formerly formed halite bed dissolving(reinfluxing and redissolving) At the same time,the undissolved residues,dispersed in halites,gathered to form a bed(dissolving deposition bed),which may prevent the underlying halite from further dissolvation The process of reinfluxing and redissolving may be the main reason for the lack of evidence of typical evaporation drying The sequence distribution style of carbonate and evaporites,were mainly controlled by regional sea level change cycle Carbonates were formed during transgressive period or highstand period,while evaporites were formed during lowstand period