MIDDLE AND LATE CAMBRIAN ACRITARCHS FROM THE INNER CLASTIC BELT OF LAURENTIA

Middle to Late Cambrian rocks deposited around the cratonic margin of Laurentia in the United States preserve a record of non-marine and perhaps shallow marine palynomorphs. Palynomorphs are found in both outcrop and in cores. These include cryptospores, problematic microfossils, “leiospheres” and acritarchs. Although cryptospores and similar problematic forms dominate most assemblages, acritarchs from cores from the Conasauga Group in Oak Ridge Tennessee can comprise 1 % or more of total palynomorph count. These Middle Cambrian assemblages are typified by small (8 to 20 µm) acanthomorphic forms including Asteridium tornatum, Celtiberium cf. C. sp. B (Downie 1982), Cymatiosphaera luminosa, Lophosphaeridium sp., several probable species of Skiagia, and a possible new acritarch genus with a partially developed reticulum and distinctive processes. Samples of later Cambrian age from the western US contain monospecific assemblages of Polygonium. Acritarchs from the Middle Cambrian Conasauga Group are a close match to a basinal assemblage from the Burgess Shale (Kevin Gostlin, University of Toronto). In the US, they are found in near-shore and shallow marine carbonate settings. Given that these tiny spiny acritarchs are distributed over such a wide range of depositional settings, perhaps they represent a general trend for the Middle Cambrian within Laurentia, rather than simply marking variation between depositional settings. These observations support claims by European workers have proposed that some Middle Cambrian acritarch species are widely distributed between continental blocks.