DON’T FORGET THE FAUNA: THE IMPORTANT ROLE OF CAMBRIAN-ORDOVICIAN TRILOBITES IN STRATIGRAPHIC AND TECTONIC SYNTHESIS IN THE APPALACHIAN OROGEN

Given the significant role that the contrast in Cambrian faunas across the Iapetus suture in the northern Appalachians played in the emergence of Plate Tectonics as the unifying paradigm in the geological sciences, it should come as no surprise that Cambrian-Ordovician trilobite faunas in the Appalachians continue to provide critical constraints for modern stratigraphic and tectonic models proposed for this region. Increasingly refined trilobite-based correlations within outer shelf and off-platform facies in the central Appalachians is clarifying stratigraphic relationships both along and across depositional strike, despite profound changes in thicknesses of many rock units over short distances. Refined biostratigraphy for Cambrian carbonates in the Conestoga Valley reveal dramatic thickening of rock units westward across the valley that is consistent with the pronounced difference in thermal subsidence that would have characterized crust on opposite sides of a transform fault offsetting the margin in vicinity of the Susquehanna River. However, available data are insufficient to reject an alternative model that attributes eastward thinning of these units to higher sedimentation rates in the proximal (near-slope) toe-of-slope setting, as opposed to more distal fan and starved basin settings to the east. Comparable thinning of the similar, off-platform deposits of the Cambrian Frederick Formation eastward across the Frederick Valley of Maryland provides some support for that alternate interpretation. Other significant results of recent work on central Appalachian faunas include discovery of faunas previously reported from the northern Appalachians, expediting recognition and more precise dating of some significant regressions and submergence events that affected the entire southern Laurentian margin, and areas outside the Appalachians. Recognition of such widespread events is a critical first step in disentangling the sedimentological signal of eustacy from that of local tectonics. Moreover, some endemic trilobite species, restricted to certain parts of the Appalachians, are proving useful for identification of tectonically detached segments of the southern Laurentian margin that now reside in such far-removed locations as the Argentine Precordillera and the North Slope of Alaska.