BAOTA FORMATION: A MIDDLE ORDOVICIAN CONDENSED SECTION

The Baota Formation consisting of micritie limestones and biomieritie 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 facies 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 hard-ground 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.