EVIDENCE FOR COOLING ACROSS THE DUBUQUE/MAQUOKETA CONTACT (UPPER ORDOVICIAN) USING CONODONT PALEOTHERMOMETRY

Oxygen isotope (d¹⁸O_phos) conodont paleothermometry was used to estimate changes in water temperatures across the transition between the Dubuque and Maquoketa Formations at the western margin of the Upper Ordovician epeiric seaway in North America. Seven locations were sampled in a north-south transect in what is now eastern Iowa and Minnesota with three of the seven locations spanning the lithologic transition between the limestone-rich Dubuque Formation, and the shalely Maquoketa Formation. We have been targeting the conodonts Drepanoistodus suberectus and Panderodus gracilis for analysis due to their relatively high abundance in both formations. By utilizing species specific separates we gain a higher level of potential resolution as mixed separates may average conditions across the niches each species preferred. Preliminary results indicate cooling across the lithologic boundary with values of d¹⁸O_phos ~18‰ _V-SMOW in the Dubuque Formation and d¹⁸O_phos ~20‰ _V-SMOW in the basal portion of the Maquoketa Formation.

Sampling density is still low, but these preliminary results are consistent with a quasi-estuarine circulation model for the seaway with transgression of deep open ocean waters through time. In the model, fresh-water runoff from the Taconic highlands and surface winds drove a quasi-estuarine gyre that resulted in surface currents flowing basinward and out of the epeiric sea; while cool ocean water flowed into the epeiric sea through the Sebree Trough. The cooling trend seen at the Dubuque-Maquoketa boundary could be a reflection of the cool ocean waters coming in through the Sebree Trough. With additional data spanning a longer interval and additional sections, we hope to test whether this cooling event seen at the Dubuque-Maquoketa boundary is a single event, or one of many temperature fluctuations in the Upper Ordovician.