湘鄂西奥陶纪宝塔组灰岩网纹构造成因及沉积环境探讨

On Polygonal Reticulate Structure of the Ordovician Pagoda Formation of the Western Hunan Hubei Area

网纹构造是宝塔组石灰岩地层中非常发育且分布广泛的一种构造形态 ,其成因一直存在争论和疑问。对湘鄂西地区宝塔组灰岩的研究表明 ,网纹构造的形成不是由于胶体凝缩、水下沉积物收缩或生物遗迹等原因 ,而是一种成岩早期形成的准同生变形构造。根据岩石性质及生物特征推断 ,宝塔组网纹状灰岩沉积于正常浪基面以下、风暴浪基面之上 (水深大约 5 0～ 15 0 m)

The polygonal reticulate structure is well developed and wide spread in the Pagoda Formation limestones in the whole Yangtze region. Till now, however, there have been different views on its origin and sedimentary environment implication. In the Western Hunan Hubei area, the limestones with the polygonal reticulate structure are mainly composed of wackestone bearing skeletal fragments of thin shelled organisms, such as ostracods, trilobites, gastropods, cephalopods and brachiopods, whereas the nets, embedded within the limestones but with no well defined boundaries, consist of clayey lime mudstone, especially rich in silt sized fragments of ostracods. Based on the studies of petrology and paleontology, this reticulate structure could not be interpreted as previously assumed, as dry cracks formed in an exposed environment, or shrinkage cracks formed on the sea bottom, or organic traces. In our opinoin, this structure is a kind of penecontemporaneous deformation structures. The original deposits of the reticulate limestone were a reverse density sequence composed of a thin, clay rich layer and an overlying thick bed of bioclasts and lime mud matrixes. Under loading process the clay rich layer intruded upwards into overlying carbonate bed, and the polygonal reticulate structure was finally formed through several stages, including the formation of load mold, and the formation of fire structure and sluicing structure(Fig.1). It is considered that the limestones of the Pagoda Formation with the polygonal reticulate structure were deposited in relatively deep environments of extra shelf and depression on the carbonate platform under normal wave base and over storm wave base. Therefore, the inferred water depth might be 50 to 150 meters.