MAGWOOD, JAMES P. A., and ALLAN A. EKDALE, University of Utah, Salt Lake City, UT

ABSTRACT: Ichnofabrics and Ichnofacies: Stratigraphic Analysis of the Lower Cambrian Great Basin

The basal Cambrian transgressive sedimentary sequence in western North America consists of a thick basal sandstone overlain by a thin shale and sandstone sequence and capped by a thick package of carbonate rocks. Although the overlying carbonates contain a rich fossil assemblage, the basal sandstones are nearly devoid of body fossils. Reconstruction of regional stratigraphic sequences also is hampered by laterally variable, local-scale stratigraphy; monotonous rock types; and several generations of extensive faulting.

This stratigraphic package contains a rich assemblages of trace fossils and these can help resolve this dilemma in several ways. Several potentially isochronous trace-fossil horizons are used to correlate between widely spaced sections. They suggest that the traditional boundary between the Lower Cambrian Prospect Mountain quartzite and Pioche Formation cuts across major continuous lithologic units.

Ichnofacies and ichnofabrics also can assist in the construction of sea level curves. Skolithos pipe rock suggests a shallow-water environment. Dense, often intertwining Teichichnus occurs in all sedimentary facies including clean sandstone and may indicate widespread changes in water chemistry, such as dysoxia. The Cruziana/Planolites assemblage contains a rich diversity of ichnotaxa and suggests a quiet, deep-water environment. The Scolicia assemblage is dominated by large horizontal burrows and trails, produced by soft-bodied animals, preserved in clean sandstone. This assemblage probably was emplaced in an intertidal setting. The Psammichnites assemblage consists of dense, over-crossing horizontal burrows crosscut by thickly lined Skolithos. These burrows were emplaced in a loose, water-laden, sandy substrate, probably in deeper water.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.