FACIES AND SEQUENCE CONTROLS ON THE APPEARANCE OF THE CAMBRIAN BIOTA IN SOUTHWESTERN MONGOLIA - IMPLICATIONS FOR THE PRECAMBRIAN-CAMBRIAN BOUNDARY

Neoproterozoic-Cambrian rocks of the Zavkhan Basin (Govi-Altay, wester n Mongolia) comprise large-scale alternations of siliciclastic- and ca rbonate-dominated units (cf. 'Grand Cycles'). Analysis of such deposit ional sequences near the base of the Cambrian confirms that the distri bution of trace fossils, small shelly fossils and calcimicrobial struc tures was strongly controlled by ecology and taphonomy, corresponding to specific points in a sea-level cycle. Evolution of the Cambrian bio ta is thus viewed through a series of narrow time windows, once only f or each depositional cycle. Correlation of the Precambrian-Cambrian bo undary level on the basis of the first appearance of the Phycodes pedu m assemblage is also fraught with difficulty, since stratigraphic reso lution may be limited to a single sea-level cycle (c. 1-5 Ma). It is s uggested that, in many cases, basin analysis will need to be undertake n before this boundary can be drawn.