High Resolution Sequence and Event Stratigraphy of Mid Silurian Strata in Eastern Laurentia and Avalonia

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Increasing evidence indicates that the Silurian, long viewed as "quiescent", was actually a very dynamicperiod from the standpoint of tectonics and ecological evolutionary events. However, correlations of strataand events, particularly during the mid-Silurian, remain ambiguous even in well-studied areas. This projectproposes to establish a regional stratigraphic framework, paleogeography, and sea level history for theNiagaran (late Llandovery-Wenlock) in eastern North America (Laurentia), based on coupled K-bentonites,sequence stratigraphic, and biostratigraphic criteria. We will also evaluate the potential for internationalcorrelation by comparisons of newly discovered K-bentonite beds in North America and Avalonia. Thisresearch will: a) yield more precise correlation and bracketing of this interval in eastern North America,with emphasis on the Cincinnati Arch region; b) provide a test and extension of previous sequencestratigraphic interpretations; c) help to document the presence of and potentially, the position of volcanicactivity and basin dynamics during early phases of the Salinic Orogeny (late Telychian-Wenlock); d)provide zircon-based radiometric dating of parts of the Niagaran Series (North American) andLlandovery/Wenlock Series (Britain); and e) explore the possibility of matching K-bentonite horizons orintervals of genetically similar ash beds, between Laurentia and Avalonia. High-resolution correlations willpermit discrimination of regional sea level and tectonic history of late Llandovery-Wenlock strata withineastern Laurentia and provide tests for apparent synchroneity of various physical and biotic eventsregionally and globally.The specific benefits expected to emerge from this study can be summarized as follows:1) If K-bentonites and other event beds can be identified and traced within each paleogeographic area, asseems definite, this will permit a) high resolution stratigraphic correlation of late Llandovery-Wenlock stratawithin both eastern Laurentia and Avalonia; b) in turn, this will facilitate detailed linkage of stable platformareas with tectonically active basins; c) in both cases, but particularly in eastern North America (for whichrefined graptolite biostratigraphy is impossible). Such correlations will then permit tests for apparentsynchroneity of various physical and biotic events. For example, similarities of relative sea level curvesreconstructed from sequence stratigraphy suggest eustatic controls (Brett et al., 1990, 1998; Johnson et al.,1991); K-bentonites may provide independent high-resolution stratigraphic controls that will enable adefinitive test of this inference. Bentonite correlations can also test for synchroneity of tectonic events, suchas pulses of subsidence in the foreland and episodic uplift of the Findlay/Algonquin arch, and precursors ofthe Cincinnati arch (Ettensohn and Brett, 1998). Finally, refined correlations may provide a test of thecoeval vs. diachronous nature of bioevents, such as the turnover pulses that separate intervals of relativelystable faunas (or ecological-evolutionary subunits) in the Appalachian Basin (Brett and Baird, 1995), andelsewhere.2) At the very least, if synchroneity of stratigraphic sequences within each area can be established,and those patterns appear similar, then it will strengthen the case of allocyclic and probably eustatic, asopposed to strictly local autocyclic controls of facies fluctuations. Furthermore, if clusters of K-bentonites(as opposed to single fingerprinted beds) coincide approximately (within conodont biostratigraphicresolution) in Laurentia and Avalonia, this may corroborate models of shared tectonic histories, consistentwith other tectonic evidence (Torsvik et al. 1996). Patterns of K-bentonite dispersal and thickness in eachpaleocontinent will aid in location of the volcanic source terranes and reconstruction of paleowind directionsand paleogeography.3) Finally, the establishment of trans-Iapetus K-bentonite and sequence stratigraphic correlations: a)permit the first really detailed correlation of the Wenlock stratotype with the classic Niagaran provincialseries of eastern North America; b) provide a test for global eustatic sea level curve recently proposed byJohnson and others (1991); c) provide a test for synchroneity of possible global bioevents.This project will provide a post-doctoral position for Michael DeSantis and give him furtherresearch experience in an area related to, but distinct from his doctoral studies. In addition, we intend toemploy one or more undergraduates. These students will benefit from the training in field and laboratoryresearch and may develop "capstone" projects resulting in senior theses and/or published abstracts andpapers.