LATE ORDOVICIAN TRILOBITE BIOFACIES OF EASTERN LAURENTIA: PALEOECOLOGICAL PATTERNS DURING FORELAND BASIN DEVELOPMENT

Trilobite distribution, abundance and diversity in the Late Ordovician of Laurentia must be understood in the context of environmental changes associated with the Taconic Orogeny. Establishment and infilling of the Taconic Foreland basin led to profound changes in the distribution of lithofacies and biofacies across the eastern half of the continent. Here, we report on trilobite faunas of the late Sandbian to Katian interval. In the late Sandbian, mid-continent regions such as Oklahoma, and foreland basin regions share similar deep-water faunas, dominated by raphiophorid and isoteline trilobites. Later, in the Katian, regions in the mid-continent, such as central Oklahoma, have a relatively continuous record of carbonate deposition and diverse platformal biofacies that pass down-ramp into deeper subtidal, low-diversity cryptolithine faunas. In the foreland basin, cryptolithine biofacies became widespread in the Katian, and expanded geographically as the clastic wedge prograded westward. Sedimentary evidence indicates that cryptolithines have a broader bathymetric range in the foreland basin and emerged at least locally into shallow subtidal, storm-influenced settings. Up-ramp, around the margins of the basin in such regions as southern Ontario, more diverse biofacies lack cryptolithines and share taxa with mid-continent faunas. Preliminary data also indicate that the distribution of trilobite biofacies reflects patterns in the carbonate isotope stratigraphy, suggesting a relationship with different water masses recorded by aquafacies. The emerging patterns of biofacies distribution demonstrate the influence of regional processes on trends in diversity, faunal turnover and replacement over a broad area of Laurentia.