四川盆地上三叠统遗迹化石及其沉积环境

Trace fossils and sedimentary environment of the Upper Triassic Xujiahe Formation in Sichuan Basin

通过岩心观察和露头描述,在四川盆地上三叠统发现了大量遗迹化石,为该盆地沉积相的确定提供了重要证据。遗迹化石可分为3大类,即形态清晰的动物遗迹化石、逃逸构造和古植物根迹(主要为须状根迹)。形态清晰的动物遗迹化石可分为3大类7属10种(包括2个未定种):①居住迹包括Skolithos linearis,S.verticalis,Arenicol itesichnosp.,Cylindrium ichnosp.,Ophiomorpha nodosa;②进食迹包括Planolites beverleyensis,P.montanus,Palaeophycus tubularis,and P.striatus;③牧食迹包括Cochlichnus anguineus。根据遗迹化石的分布及沉积环境分析,可识别出3个遗迹组合:①Cochlichnus-Planolites遗迹组合,主要由进食迹和牧食迹组成,形成于潮间泥坪沉积环境,局部地区发育沼泽;②Cylindricum-Planolites遗迹组合,以居住潜穴和进食潜穴为主,遗迹化石的丰度和分异度高,代表了潮间带沉积环境;③Skolithos line aris遗迹组合,由长的垂直或高角度倾斜的悬食居住潜穴构成,形成于潮间砂坪和水下分流河道,环境水体能量随着相对海平面的变化而发生周期性的变化。

The Upper Triassic Xujiahe Formation in Sichuan Basin has abundant trace fossils. In accordance with ecological and behavior features of trace makers, these fossils can be classified into three categories, including trace fossils of animals with clear shape, escape structures and Rhizolithos （predominantly fibrous Rhizolithos）. Trace fossils of animals with clear shape can be classified into ten species in seven genera in three categories （including two species indeterminata）, namely,（1）Domichnia. Skolithos linearis, S.verticalis, A renicolites ichnosp., Cylindrium ichnosp., Ophiomorpha nodosa; （2）Fodiniehnia. Planolites beverleyensis, P.montanus, Palaeophycus tubularis, and P.striatus; （3） Pascichnia. Cochlichnus anguineus. Most of trace makers for these trace fossils can be classified as arthropod （especially insecta）. There are also a small quantity of mollusks （predominantly gastropods and bivalves） and worms. They are preserved in fine sandstones, sihstones or silty mudstones as full relief and epirelief. In accordance with distribution of trace fossils and analyses of sedimentary environments, three ichnofabrics can be identified：（1）Cochlichnus-Planolites ichnofabric, which is predominantly composed of fodinichnia and pascichnia, formed in sedimentary environments of supratidal mudflat with swamps developed locally; （2）Cylindricum-Planolites ichnofabric, which is dominated by dwelling burrows and feeding burrows. High abundance and diversity of trace fossils may reflect their sedimentary environment of intertidal zone; （3） Skolithos linearis ichnofabric, which is dominated by long, vertical or high-angle dwelling burrows of suspension feeders, formed in subtidal zone, in which energies in surrounding water changed periodically with changes in relative sea levels.