Latest Llanvirn to early Caradoc trilobite biofacies of the north-western marginal area of the Yangtze Block, China

Representative collections have been made from latest Llanvirn to early Caradoc carbonate rocks at seven measured sections in northernmost Sichuan and southern Shaanxi provinces, China, on the north-western marginal area of the Yangtze Block. Northwards from the margin of the Hannan land-area four trilobite biofacies, based oil faunal composition. diversity, and relative abundance of genera, are present on the shallow to deep shelf area at this time. The Agerina Biofacies comprises a small number of benthic genera in tempestite sequences, indicating a shallow inner shelf environment. The Cyclopyge-rorringtonfid, Cyclopyge-Alceste and Cyclopyge-trinucleid biofacies are all characterized by the common mesopelagic cyclopygids, and occur successively distally along a deeper, steep outer shelf. along which there is a reduction in overall faunal diversity and an increase in the relative abundance of cyclopygids. Outer shelf benthic assemblages contain both normal eyed and atheloptic forms, but only the latter are present at the shelf edge. Rorringtoniids occur largely in the inner part of the outer shelf, while the illaenid Alceste is found mainly in carbonate buildups of the outer shelf.