REGIONAL FLUCTUATING ENVIRONMENTAL CONDITIONS AND SHALLOWING EVENT IN THE NORTH AND NORTHEASTERN IBERIAN PENINSULA DURING THE END OF CARBON ISOTOPE SEGMENT C5: ORGANYÀ BASIN, CATALUNYÀ, SPAIN

The Cretaceous marine sedimentary record is punctuated by brief occurrences of intervals unusually enriched in organic carbon associated with severe oxygen depletion termed oceanic anoxic events (OAEs). Here we report petrographic and geochemical results for the top 85.82 m segment (86.07 m - 171.89 m) of the expanded El Pujal section in the Organyà Basin, previously reported to contain the sedimentary record of OAE 1a. Carbon isotope (δ¹³C_org) values for this segment hover at an average of -24.27‰ with a negative shift to -25.31‰ (1.04‰) in the uppermost 1.4 m, consistent with the pattern reported elsewhere for the denouement stages of δ¹³C_org segment C5. Concurrent presence of Leupoldina cabri further supports the timing of the section to within OAE 1a.

The basal 24.72 m reveals fluctuating lithologies of limestone, argillaceous limestone and marlstone. Microfacies of marlstone interbeds show higher pyrite concentration, reduced benthic fauna and lower bioturbation index. These interbeds also show periodic increase of redox sensitive trace elements (RSTEs) (V, Cr, Co, Ni, Cu, Mo, U), bio-limiting (P, Fe) and terrestrially-derived elements (Al, Si, Ti) with enhanced organic carbon preservation (TOC). These results suggest short-lived, intermittent pulses of fluvial fluxes, and episodic dysoxic bottom waters.

The upper 57.17 m consist essentially of marlstones, enriched in TOC (≥ 1.29%), and higher contents of RSTEs, P, Fe, Al, Si and Ti. We postulate that an increase in runoff because of probable established humid continental conditions prevailed in this interval, and that the high TOC facies was mainly controlled by high surface productivity coupled with terrigenous input further enhanced by clay preservation.

In contrast, the uppermost 3.93 m of hard limestone (TIC ≥ 85.63%) with microfacies characterized by partly recrystallized biomicrite, absence of planktonic foraminifera and increase in benthic taxa, suggests a reduction in fluvial fluxes and surface water fertility. This facies shows a marked decline in TOC, Al, Si, Ti, P and Fe, synchronous to the negative δ¹³C_org excursion (1.04‰) heralding the end of segment C5. These data are indicative of a shallowing development similar to regressive sequences reported in northern Spain (Basque-Cantabrian Basin).