Mitchell 9627978 Sequence stratigraphic interpretations of facies development have come into wide application. Yet this model has seldom been tested against an independent set of high precision chronostratigraphic correlations. The proposed research seeks to directly examine two of the central assumptions of sequence stratigraphy: that unconformity bounded onlap/offlap cycles are synchronous across broad regions, and that maximum flooding surfaces within cycles can serve as regional isochrons. We propose to test these predictions by comparing the timing of late Middle Ordovician depositional sequence along two regionally extensive transects across eastern North America (Fig 1). The first will extend to the southwest within the Taconic foreland basin, from the New York Promontory through the Pennsylvania Reentrant to the Virginia Promontory. Along this transect, we will examine sections in east central Pennsylvania, western Virginia, eastern Tennessee, and northern Alabama. The second transect will extend from the Taconic foreland basin through the Jessamine Dome, in North central Kentucky, and into the cratonic interior, where we will examine sections in Iowa, Illinois, and Missouri. These areas are widely separated geographically and are known to have experienced different tectonic histories during this interval. Along these two transects, we will examine the time-space distribution of the upper-most Middle Ordovician third order stratigraphic sequence relative to an independently constructed high-precision chronostratigraphy. We will focus in particular on the equivalents of 1) the lower Trenton Group-Utica Shale transgressive and highstand systems tract that developed in the Taconic basin of New York during the Corynoides americanus Chron and 2) the succeeding lowstand fan (Dolgeville Formation), which developed during the Orthographus ruedemanni Chron. Predicted sequence equivalents of this cycle will be taken from published literature. Our previous research on the Middle Ordovic ian Trenton Group and Utica Shale in central New York State has produced a high resolution chronostratigraphy that integrates data from geochemical fingerprinting of K-bentonites with data from graptolite and conodont biostratigraphy. we propose to extend this chronostratigraphic framework by a combination of 1) graptolite and conodont biostratigraphic studies, and 2) high precision electron microprobe analysis of apatite phenocrysts and glass inclusions within quartz phenocrysts obtained from K-bentonites at the study sites. Chronostratigraphic resolution based on graphic correlation within this system should be high (greatly exceeding that of biostratigraphy alone or of radiometric dating) and sufficient to provide a definitive test of the synchroneity of the rapidly onlapping transgressive system track, the overlying maximum flooding surface/marine hiatus, and the upper sequence boundary/lowstand fan. By extending this set of time lines into the relatively stable cratonic interior and south into regions that experienced collisional loading during the earlier, Blountain Phase, of the Taconic Orogeny, it will be possible 1) to gain a precise and independent measure of synchroneity among the transgressive-regressive cycles documented in this interval in eastern North America and, therefore, 2) to use this system of correlations to examine the relative contributions of changing eustatic sea-level and regional tectonic events to the formations of late Middle Ordovician stratigraphic sequences. The work will provide important and otherwise inaccessible information about he physical and temporal connections between geological events on the craton and those along a major orogenic belt.
