鄂尔多斯盆地东部马家沟组含钾蒸发岩系沉积机制及保存条件

鄂尔多斯盆地中央古隆起东侧奥陶系是海相固体钾盐矿的重要靶区。本文在岩心、测井和地震等资料的基础上,开展了该地区马家沟组中组合(马五~5~马五~(10)亚段)的沉积微相、蒸发岩分布、蒸发岩形成机制和保存条件研究。结果表明:马家沟组中组合总体为局限台地环境,可进一步划分出潮坪、潮缘滩、外潟湖、内潟湖等沉积微相,各微相沿古隆起呈环带状展布,内潟湖是含钾蒸发岩的主要形成环境;内潟湖沉积具有明显的旋回性,马五~6、马五~8、马五~(10)时期为海退蒸发环境,以蒸发岩和白云岩为主,马五~5、马五~7、马五~9时期为海侵环境,以灰岩为主;海退期,向心浓缩作用形成了大型米脂蒸发盐盆,进一步在"深水成盐"机制下沉积了大量厚层含钾蒸发岩系;岩溶地貌与含钾蒸发岩系分布的关系表明含钾蒸发岩系主体分布于岩溶盆地内幕。印支期岩溶作用的强度和深度分析进一步显示古岩溶作用在岩溶盆地范围内基本未破坏到马家沟组中组合的含钾蒸发岩系,具备了钾盐矿的沉积和保存条件。

The sedimentation mechanism and preservation condition of the potassium evaporate series in the Majiagou Formation, east of Ordos Basin

The Ordovician strata constitute an important target of marine potassium ore in the east of Central paleo-uplift, Ordos Basin. Based on core, logging and seismic data, the authors conducted researches on the middle combination of Majiagou Formation (from 5th to 10th intervals of the fifth member of Majiagou Formation), in the aspects of sedimentary microfacies, evaporate distribution, formation mechanism and preservation condition of evaporate. The results indicate that the middle combination of Majiagou Formation was deposited in a restricted platform environment which can be further divided into four sedimentary microfacies, i.e., tidal flat, tide marginal beach, outer lagoon and inner lagoon. These microfacies are distributed around the central paleo-uplift orderly. The inner lagoon is the main depositional environment of the potassium evaporate. The sediments of the inner lagoon present obvious cyclicity. The sedimentary environment of the sub-member 6, 8, 10 were under the regressive evaporation condition and produced evaporite and dolomite, while the sub-member 5, 7, 9 were of transgressive setting and formed limestone. In the regressive period, the centripetal enrichment effect formed a large salt sag in Mizhi area, Shaanxi Province. Furthermore, under the mechanism of"salt deposited in deep water", a lot of thick potassium evaporate series were deposited. These potassium evaporate series were mainly distributed under the post-karst basin coincidentally. The analysis of the intensity and depth of the Caledonian karst indicates that karstification did little damage to these potassium evaporate series.