Geochemical and Microfacies Implication of the El Pujal Section at the Close of Carbon Isotope Segment C5: Organyà Basin, Catalunyà, Spain

Abstract

Unusual accumulation of organic matter (OM)-rich intervals collectively known as oceanic anoxic events (OAEs) are the hallmark of the Cretaceous Period. In the Organyà Basin (NE Spain), the El Pujal section exposes an expanded sedimentary record of OAE 1a, characterized by lithologies with OM enrichments (TOC) up to 1.83%. Here we report the results of a multi-proxy study, integrating petrographic and geochemical results, for the upper 85.82 m of the sequence to assess temporal variations in OM burial and attempt to correlate this period with other sequences deposited in neighboring petroliferous basins (Basque-Cantabrian basin). The basal 24.72 m is composed of alternating limestones, argillaceous limestones and marlstone beds. In this interval, enrichments in redox sensitive trace elements (RSTEs) (V, Cr, Co, Ni, Cu, Mo, U), bio-limiting (P, Fe) and terrestrially derived elements (Al, Si, Ti) typify the marlstone layers concurrent with enhanced OM preservation and microfacies marked by higher pyrite concentration, reduced benthic fauna and lower degree of bioturbation. Thus, suggesting short-lived, intermittent pulses of fluvial fluxes, which led to mesotrophic surface waters and episodic dysoxic bottom waters. A large expanse (57.17 m) of marlstones ensues, with relatively higher TOC, RSTEs, P, Fe, Al, Si and Ti values. Planktonic foraminiferal analysis reveals the sparse presence of Leupoldina cabri, whose Zone parallels OAE 1a. Further corroborating evidence from stable carbon isotope (δ¹³C_org) data with unvarying values (x̅ -24.27‰) and a pronounced dip to -25.31‰ in the upper 1.4 m, permits the precise chemostratigraphic correlation within the closure of δ¹³C_org segment C5. We postulate that throughout this interval concurrent with segment C5 of OAE1a there was prevalent increase in runoff associated with extant humid continental conditions with strong, quasi-permanent eutrophic surface waters. In contrast, a stark change in lithology is registered for the upper 3.93 m, with hard limestones (TIC ≥ 85.63%) impoverished in TOC (< 0.27%), Al, Si, Ti, P and Fe, indicating a reduction in fluvial fluxes and surface water fertility. Simultaneous changes in microfacies, with coarser grain packstones, devoid of planktonic foraminifera and increase in benthic taxa, imply a shallowing of the basin. Similar shallowing sequences have been reported in the Basque-Cantabrian Basin of northern Spain synchronous to the negative δ¹³C_org excursion heralding the end of segment C5.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019