Cracked limestones of polygonal stucture in the Middle Ordovician Baota Formation are of the so-called "Tortoise shell" patterns on their surface, and nodularlimestones are widely distributed in the upper reaches of the Yangtze River, whichattain about several handreds of thousand square kilometers in their distribution.They have got great attention for long time, but so far there is no satisfied answersto the interpretation of their. The cracked limestones are mainly developed withinthe Baota Formation, but in western Hunan Province they can also be found inupper Ordovician Linxing Formetion, Middle Ordovician Baota and GoniutanFormation and Lower Ordovician Dawan Formation. These cracks and nodulesusually occur in thin to thick bedded Limestones of Middle Ordovician carbonateplatform in South China, bait these limestones are quite different in their component and lithologies, of which skeletal and intraclastic (attaining about 5%-60% in their totol grains) micritic limestones are predominated. Therefore, crackswere not severely controlled by sedimentary environment, lithological factes andstratigraphic horizons.On the top and bottom of these limestones, as well as on their lateral sidesthey often demonstrate irreqular polygonal structure, but are predominated with tetragonal and pentagonal patterns. Under the microscope, these cracks are composedof complicated systems of many fine, curved fissures, which are quite differentfrom those formed by mud crack structures and those produces by subaqueous shrisikages. These cracks were formed during fracturing under the tectonic stress anddewatering-contracting of water-containing lime mud sediments in the course of rookforming process. Enormous evidences have indicated that the Yangtze plate hadundergone great compression not only from the Yunnan-Qinghai-Tibet plate, but also from the Pacific plate in Caledonian movement, thus suffering from intensivedeformation, faulting and uplift during late Middle Ordovician to early Late Ordovician time. These processes must affect the water-bearing carbonate sediments andproduce numerous crack systems. Therefore, these cracks are formed under the acactive setting of peculiar regional tectonics. Subsequently, they were further complicated by later diagenesis of pressure solution. Thus they were the products of tectonic and rock-forming processes.
Wang Yao. MIDDLE ORDOVICIAN CRACKED LIMESTONES OF POLYGONAL STRUCTURE IN THE UPPER YANGTZE RIVER AREA, SOUTH CHINA’A NEW INTERPRETATION OF THEIR ORIGIN[J]. Chinese Journal of Geology, 1995, (3): 268-274.