MIDDLE UPPER ORDOVICIAN TRILOBITE BIOFACIES OF THE NORTHWESTERN MARGINAL AREA OF THE YANGTZE BLOCK, CHINA

摘要

1 INTRODUCTIONIn the northwestern marginal area of the Yangtze Block,the Pagoda Formation is well exposed in southern Shaanxi and northern Sichuan area (Text-fig. 1), where it is of latest Llanvirn-Caradoe age, and composed chiefly of grey and purplish-red, thin- to thick-bedded,condensed, polygonal reticulate biocalcilutites, with thickness varing from 10 m to 36.2 m (Zhou et al., 2000b;Zhou et al., 2003). Previous work on the Pagoda Formation (Ji, 1985; Chen and Qiu, 1986; Zhou and Xue,2000) and its equivalent Pa Kae Formation (Fortey, 1997)in Thailand has suggested that carbonates of this sort were deposited on an oxygenated sea floor under uninterrupted, subtidal, deep-water conditions.