Lowermost Cambrian Ichnofabrics from the Chapel Island Formation, Newfoundland: Implications for Cambrian Substrates

Bioturbation long has been `blamed ' for eliminating late Proterozoic-style sedimentary structures and fabrics. While the presence of diverse and complex burrows in lowermost Cambrian strata is indisputable, analysis of Precambrian± Cambrian successions in southeast Newfoundland demon-strate that this burrowing style did not produce typical Phanerozoic-style ichnofabrics. Three hundred meters of the siltstone/sandstone facies of member 2 of the Chapel Island Formation were examined in the area of the Precambrian±Cambrian boundary stra-totype. Gyrolithes, Planolites, and Skolithos occur as sand in®lls ubiquitously throughout siltstone beds, most com-monly without direct contact with an overlying sandstone bed, as if ``¯oating' ' in the siltstone. In contrast, Treptichnus pedum occurs as sand in®lls adhering onto the base of thin sandstone beds that have different grain size and texture than the burrow in®lls. Both of these burrow types repre-sent a style of preservation in which the burrows are unat-tached to an overlying bed of the casting sediment. These styles of preservation occur frequently in the Treptichnus pedum Zone and continue into the Rusophycus avalonen-sis Zone in spite of an increase in trace fossil diversity. The sandstone beds are bioturbated only very rarely. The resul-tant fabric produced by ¯oating and, in particular, adher-ing burrows in these shallow marine deposits appears to be characteristic of many Lower Cambrian rocks. Silt layers appear to have been ®rm enough to have supported open burrows, likely as a result of a negligible mixed layer. This line of reasoning would predict that preservation of this type would be uncommon in younger strata deposited in open marine settings.