TRACING SEQUENCE STRATIGRAPHIC SURFACES OF CYCLOTHEMIC BLACK SHALES USING NITROGEN ISOTOPES

In recent years sequence stratigraphic principles have been applied to Pennsylvanian cyclothems. It is a challenge to identify sequence stratigraphic surfaces (like the maximum flooding surface; MFS) within these regionally extensive black shales. The black shales are ~1 m thick and situated between two limestone beds. The basal limestone is interpreted to be a transgressive limestone while the upper limestone is interpreted to have been deposited during sea level fall (regressive limestone). Commonly, the transition from the transgressive limestone to the black shale facies is interpreted as a major flooding surface with the MFS following within the first ~30 cm of the black shale.

We sampled the Hushpuckney Shale (Swope Formation) from two cores from two areas (KS and IA) that represent a transect through the depositional basin. The KS site was situated closer to the open ocean while the IA site was situated closer to the shoreline.

Our results challenge the current understanding of the depositional environment of these deposits. In Kansas, the lowermost part of the black shale is characterized by a rapid rise of nitrogen isotopes (δ¹⁵N) from ~4‰ to ~14‰ within the first 20 cm. δ¹⁵N then slowly drop back to background values of ~4‰. The peak in δ¹⁵N is below the MFS which has been suggested to be at ~25 cm above the transgressive limestone. Previous work suggested that the peak in δ¹⁵N is related to the advection of denitrified waters from the south. Thus, if a similar peak can be detected further north, that peak would be time equivalent to the δ¹⁵N peak further south as the water was advected through the basin. We indeed find such a peak in the IA core. However, this peak (~15 ‰) is located much higher in the black shale in IA (~ 55 cm) and the lower 35 cm are characterized by relatively stable δ¹⁵N values (as compared to the rapidly changing δ¹⁵N values in the basal section in KS). In this case, the first 35 cm of the basal black shale in IA cannot be time equivalent to the lower part of the black shale in KS as it should also display a rising trend (which is only beginning after 35 cm in IA). Therefore, the MFS in IA should be in the upper part of the black shale rather than the lower part of the black shale like in KS. Our study suggests that nitrogen isotopes could help identify sequence stratigraphic surfaces in black shales.