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ABSTRACT

This paper refines the chronostratigraphic definition of the Shu’aiba Formation using profiles of 13C/12C ratios integrated with available 87Sr/86Sr ratios and biostratigraphic data from subsurface and outcrop sections of Oman and Abu Dhabi. It is the aim to provide a reference record for the Shu’aiba Formation, which enhances stratigraphic resolution and aids regional correlation. The shape and exceptional resolution of the carbon-isotope profiles, combined with lithologic data, provide further insight into the underlying cause for changes in the global carbon-isotope record. In particular, the impact of methane hydrate dissolution and oceanic anoxic events are discussed as well as changes in the biosphere.

The δ13C profiles of shallow-marine carbonates from the Aptian Shu’aiba Formation faithfully track the evolution of δ13C through time established from pelagic sections elsewhere in the World with the majority of carbon-isotope values ranging between 0.0 to +6.0‰ δ13C. The degree of correspondence indicates that overall a diagenetic overprint of the δ13C is minor or absent despite whole-scale recrystallization of the rock indicated by the resetting of δ18O values, which range between -1.0‰ and -8.0‰ δ18O. The vast majority of δ18O ratios of the Shu’aiba Formation are significantly more negative than those in equilibrium with Cretaceous seawater and surprisingly uniform. This suggests that diagenesis occurred on a regional scale and likely in a confined burial setting after deposition of the Nahr Umr Formation at slightly elevated temperatures.

The δ13C profiles have a very high-resolution due to the apparent lack of diagenetic overprint, and the relatively high sedimentation rates of shallow-water carbonate systems. Anchored by biostratigraphic and Sr-isotope age data the δ13C profiles can readily be correlated with all major trends recognized in pelagic profiles from elsewhere in the World. Minor but distinct trend reversals of 0.3–0.5‰ δ13C add significant character to the isotope curve.

Utilizing data from high-resolution expanded sections a reference curve has been constructed for the Shu’aiba Formation. It is based on δ13C profiles from different localities anchored by dates from Sr-isotope samples and biostratigraphic data from benthic foraminifera, nannofossils and rudists. This reference curve has furthermore been linked to GTS 2004, oceanic anoxic events, northern Neo-Tethys carbon-isotope stages and pelagic marine biozones. The high-resolution reference curve has been used to guide lateral correlation both on production and exploration scale. Correlations reveal a complex stratigraphic architecture for the Shu’aiba Formation providing further support to published interpretations based on seismic data and high-resolution sequence-stratigraphic correlations. The recognition of architectural complexity bears significant implications for the distribution of properties in many Shu’aiba reservoirs and exploration plays.

Negative trends in Early Aptian δ13C profiles are interpreted to relate, at least in part, to the catastrophic release of methane from methane hydrate deposits due to ocean temperature warming. The occurrence of several spikes may indicate that methane hydrate defrosting occurred in several pulses. Following the initial input of light carbon at the Barremian/Aptian boundary the global ocean system responded with a prolonged period of organic carbon withdrawal that lasted several million years. The Livello Selli/Goguell level, which is often equated with Oceanic Anoxic Event 1a is but one interval in this period. System disequilibrium may also have had a significant impact on the bio-composition and lithology of the Shu’aiba Formation. The widespread but short-lived predominance of the fossil Lithocodium-Bacinella during the Early Aptian is possibly related to the impact of catastrophic methane release on the biosphere.

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