Repeated enrichment of trace metals and organic carbon on an Eocene high-energy shelf caused by anoxia and reworking

Abstract

Abstract Petroleum source rocks are strongly enriched in organic carbon (OC), and their trace metal (TM) contents often reach low-grade ore levels. The mechanisms leading to these coenrichments are important for understanding how extreme environmental conditions support the formation of natural resources. We therefore studied organic-rich Eocene marls and limestones (oil shale) from the central Jordan Amzaq-Hazra subbasin, part of a Cretaceous–Paleogene shelf system along the southern Neo-Tethys margin. Geochemical analyses on two cores show highly dynamic depositional conditions, consistent with sedimentological and micropaleontological observations. Maximum and average contents, respectively, in OC (∼26 and ∼10 wt%), sulfur (∼7 and ∼2.4 wt%), phosphorus (∼10 and ∼2 wt%), molybdenum (>400 and ∼130 ppm), chromium (>500 and ∼350 ppm), vanadium (>1600 and ∼550 ppm) and zinc (>3800 and ∼900 ppm) are exceptional, in particular without any indication of hydrothermal or epigenetic processes. We propose a combination of two processes: physical reworking of OC- and metal-rich material from locally exposed Cretaceous–Paleogene sediments (as supported by reworked nannofossils), and high marine productivity fueled by chemical remobilization of nutrients and metals on land that sustained anoxic-sulfidic conditions. Burial of high-quality organic matter (hydrogen index 600–700 mgHC/gOC) was related to strongly reducing conditions, punctuated by only short-lived oxygenation events, and to excess H2S, promoting organic matter sulfurization. These processes likely caused the OC and TM coenrichments in a high-energy shallow-marine setting that contradicts common models for black shale formation, but may explain similar geochemical patterns in other black shales.