TRACE FOSSILS AND BIOFABRICS AT THE PRECAMBRIAN-CAMBRIAN BOUNDARY INTERVAL IN WESTERN MONGOLIA

A small suite of trace fossils from the Zavkhan Basin (Govi-Altay) inc ludes many of the ichnotaxa typical of the Nemakit-Daldynian, Tommotia n and younger Cambrian stages, and other indeterminate forms. The trac es are almost entirely from the sandstone intervals of the large-scale alternations of carbonate and siliciclastic sediments, thus emphasizi ng the facies and taphonomic controls on trace fossil distribution, an d hence the inherent difficulties in using trace fossils in detailed g lobal correlation, especially in using first appearances. The assembla ge of traces and biofabrics is seen as resulting from the partly non-u niformitarian (non-actualistic) sedimentary environments that pertaine d over the boundary interval, rather than as evolutionary failures and subsequently vacated environments.