宝塔灰岩——中奥陶统密集段

广泛发育于中、上扬子地区的宝塔组马蹄纹灰岩是中奥陶统一密集层段，其依据是：宝塔组沉积时水深最大，沉降幅度大，沉积速率低，分布广泛；在成岩早期可能是一种海底硬地，马蹄纹是海底硬地发生收缩的结果。这些特征与密集层段的特征非常接近。

BAOTA FORMATION: A MIDDLE ORDOVICIAN CONDENSED SECTION

The Baota Formation consisting of micritic limestones and biomicritic limestones is widely distributed in Middle and Upper Yangtze area and noticed by horseshoe cracks (mud cracks or syneresis cracks developed on the surface of the rocks. It is substantiated to be a condensed section in terms of the following evidences. (1)The factes analysis shows that the Baota Formation was deposited in quiet and relatively deep water environment below the wave base,while the fossil studies probably indicate a depositional environment of more than 150 m,with a maximum depth during the Ordovician. (2)The Bubnoff subsidence curves have revealed that the subsidence rates range between 30 and 40 m/Ma,much larger than those of the Ordovician (about 20 m/Ma). (3)The depositional rates for the Baota Formation vary from 1 to 3 m/Ma,much lower than those of Early Ordovician (10-50 m/Ma ). (4)The formation occurs on a wide range of scales (about 5, 000, 000 km2 ) in the study area. (5)The horseshoe cracks might be originated from the carbonate hardground of the Baota Formation during early diagenesis because of the similarities of these cracks to those of modern carbonate hardground in the Persian Gulf,as described by E. A.Shinn.