The Forming Mechanism of Dolostone of Nengjiang Formation in Songliao Basin——Example from CCSD-SKII

The CCSD-SK is the first scientific drilling well of whole core Cretaceous rocks in the continent of China, which is composed of two drilling wells: CCSD-SKⅠ in the north and CCSD-SKⅡ in the south. Totally, 62 layers of bedded and ellipsoidal dolostone were identified in the Nenjiang Formation during the description of CCSD-SKⅡ. Dolostone nodular occurs as lens in the vertical section, and interior of the nodular is characterized by foliation which converges at both lateral edges. Under microscopic observation, dolostone is grey in the core and bright at the rim, and marl has been replaced, locally forming leopard spot dolomite. All these features indicate that dolomite of the Nengjiang Formation was the product of lacustrine facies limestone during penecontemporaneous replacement. The forming mechanism of dolostone of the Nenjiang Formation was that under the setting of the marine transgression of Late Cretaceous in the Songliao Basin, marls brought by turbidity flow and extinction of ostracoda began to deposit during the replacement, thus forming the dolostone.