EVIDENCE OF A PRONOUNCED MID-IBEXIAN SUBMERGENCE EVENT ON THE SOUTHERN LAURENTIAN PLATFORM

New conodont and trilobite data from Lower Ordovician strata deposited along the southern margin of Laurentia, from the Appalachians to the southern Basin and Range, reveal a consistent signal of onlap near the middle of the Stairsian Stage. First recognized in Maryland, where slope facies replace shelfbreak microbial reefs and grainstone at the top of the Grove Formation, a marine incursion that occurred during deposition of the Macerodus dianae conodont Zone caused lithologic changes across the length and breadth of the carbonate shelf. The deepening event briefly interrupted microbial reef growth and triggered deposition of oolite in distal to intermediate platform sites in western Maryland ("oolitic member" of the Rockdale Run Formation) and southern New Mexico ("Jose" member of the Hitt Canyon Formation). A concomitant shift in trilobite biofacies from a bathyurid-dominated fauna to an asaphid-rich assemblage supports interpretation of the dark, burrow-mottled oolite as a slightly deeper/cooler facies. In proximal shelf settings the onlap produced thin limestone intervals within dolomite sequences (e.g., Kingsport Formation in the southern Appalachians) and caused a return to carbonate-dominated sedimentation following deposition of nearshore clastics within the Roubidoux Formation in Missouri. The asaphid Aulacoparia? huygenae dominates the trilobite fauna in the "Jose" oolite in the Big Hatchet Mountains, Florida Mountains, and Cooks Range, suggesting synchronous deposition of the unit across southwestern New Mexico. However, the interval mapped as the "Jose" in the Caballo Mountains of south-central New Mexico yields a different fauna that lacks A.? huygenae, calling into question whether that deeper water package is coeval with the "Jose" to the west. Physical correlation between the Caballo Mountains and the Franklin Mountains of west Texas produced encouraging results in which thicknesses between similar event horizons compare closely between the two areas. This correlation aligns the "Jose" in the Caballo Mountains with a thin interval of dark lime mudstones and shales, the deepest water facies present in the Hitt Canyon Formation in the Franklin Mountains. Additional sampling is planned to obtain biostratigraphic data to test this correlation.