碧口群的层序及时代

PROBLEM ABOUT THE STRATIGRAPHIC CLASSIFICATION OF THE LOWER CRETACEOUS IN NORTHERN HEBEI

碧口群是一套发生在台地与裂谷之间的裂谷边缘型沉积，各类岩石之间具有规律性的相变特征，按岩石类型及组合特征可划分为７个组级岩石地层单位。根据该群不整合在志留纪茂县群或水晶组之上，被中泥盆统—石炭系略阳组连续沉积覆盖，以及自身所产化石（Ａｍｐｈｉｓｉｔｅｓｐｕｌｃｈｅｒ，Ｂａｉｒｄｉａｓｐ．，Ｄｅｓｑｕａｍａｔｉａｓｐ．，Ｓｃｈｉｚｏｐｈｏｒｉａｓｐ．等），将其时代确定为早—中泥盆世。

In light of the wedge-like interlocking (interfingering) relationships between volcanic rock layers and sedimentary beds in continental volcanic sedimentary strata, the authors put forward a new idea about the lithostratigraphic classification and divide the Early Cretaceous strata in northern Hebei into the Xiguayuan Formation and Huajiying Formation as well as the overlying Qingshila Formation. The Huajiying Formation was formed by superimposition of intermediate to intermediate basic and small amount of acid subalkaline volcanic rocks in the basin during the deposition of the Xiguayuan Formation. So the two formations are contemporaneous but heteropic deposits and are listed in juxtaposition. The paper also discusses some major problems existing in the previous classification schemes.