A Method to Estimate Water Depth of the Pennsylvanian Late Paleozoic Midcontinent Seaway in North-Central Texas Using Stable Isotope Stratigraphy

A majority of central North America was periodically flooded from 310 - 272 mya by the Late Paleozoic Midcontinent Seaway (LPMS). Earth’s icehouse climate had waxing and waning glaciers that drove eustatic sea level fluctuations. Scientific arguments persist regarding the water depths of the LPMS. The sea-level changes resulted in cyclic sedimentation, producing strata known as cyclothems. Cyclothems contain a variety of rock types, each characterized by a suite of fossils. Together, rocks and fossils can be used to analyze paleoecological conditions within the LPMS. Conodont fossils are common to all cyclothem marine facies and have been used to interpret the LPMS as significantly deep. Conodonts are complexly distributed into distinct biofacies that suggest that different water masses were stacked atop one another in the LPMS.

Joachimski and Lambert (2015) found that all conodont species recovered from the maximum flooding horizons have the same oxygen isotope ratios. Thus, all conodonts lived in the same water mass – the upper part of the water column, or surface waters. This study is designed to compare the oxygen isotope values derived from conodonts with previously researched oxygen isotopes of brachiopods, which dwelled on the sea floor. North Central Texas cyclothems that were used in previous brachiopod studies have been resampled to directly evaluate the surface and bottom water isotope ratios from both fossil groups. These ratios can be used to infer paleowater temperatures. Through comparing oxygen isotopic values from brachiopods and conodonts, this study shows that the North Texas Eastern Shelf could have been as deep as 250 m at maximum sea level.