ABSTRACT Lower Cretaceous strata crop out in a series of closely spaced wadis in the Nakhl area of the northern Oman Mountains and document the evolution of the carbonate-dominated southeastern margin of the Arabian Plate. The Late Barremian to Early Aptian platform rim shows a rapid transition from bioclastic-oolitic high-energy ramp deposits to a constructed high-energy margin. The rigid construction is due to intense microbial activity rapidly binding and cementing bioclastic debris and isolated autochthonous organisms including rudists, corals and sponges. The outcrops allow investigating the geometries of these “debris reefs” and their lateral and vertical facies evolution. Here the initiation of the constructed margin is studied in detail. This stratigraphic interval represents a shelf-margin wedge of earliest Aptian age, which is interpreted as the lowstand of the Aptian Supersequence. On the platform (the Arabian Plate) this corresponds to a stratigraphic hiatus caused by subaerial exposure. At least four phases of buildup growth are distinguished. They show a low, irregular morphology, and locally have a mound-like shape, attaining heights of up to ten meters. Sealed Neptunian dykes have also been observed. A control by water depth and different energy levels along a coastal transect is inferred. The scale and shape of the constructions resembles that of modern spur and groove structures in wave-dominated reef environments. An important part of the binding and encrusting can be attributed to chasmolithic, cryptic microbialites and associations of Bacinella and Lithocodium s. i ., respectively. Stromatolitic and thrombolitic textures are common and patches and intervals entirely formed by cryptic microbialites do occur, but quantitatively are of minor importance. Environmental factors that may trigger such a sudden “bloom” in microbial activity include elevated input of nutrients onto the platform through increased rainfall, higher water temperatures or coastal upwelling. Such conditions stress oligotrophic carbonate producers and diminish their carbonate production potential and, thus, favor the microbial community. This microbial bloom predates the widespread microbial deposition on the platform during the Early Aptian transgression, which was time-equivalent with the Early Aptian Selli event and the OAE 1a.

Among various paleoenvironmental proxy records across the lower Aptian Selli Level (Umbria-Marche basin, Italy) and its equivalents (i.e., sedimentary expressions of oceanic anoxic event [OAE] 1a), one very intriguing feature is a prominent negative δ 13 C excursion at the base of this sedimentary unit, generally by as much as a few per mil in marine carbonates. This early Aptian δ 13 C event has received special attention as an important clue to the genesis of OAE 1a, but the exact amplitude of this relatively short lived δ 13 C variation is not precisely constrained, and this fact has been an obstacle in paleoenvironmental modeling. Particularly enigmatic is the large amplitude, by −7‰, in pelagic limestones at the central Pacific Deep Sea Drilling Project (DSDP) Site 463; it may be a primary δ 13 C signal, considering its deposition in a fully open-ocean setting and a conservative burial diagenetic environment. Nevertheless, new Sr isotope data help identify a peculiar diagenetic overprint on the lower Aptian interval of this site. While the majority of examined samples represent slightly shifted, but acceptable, 87 Sr/ 86 Sr ratios for marine carbonates of this age, markedly unradiogenic 87 Sr/ 86 Sr ratios are recorded within and just below the Selli Level–equivalent interval. In particular, extremely unradiogenic 87 Sr/ 86 Sr ratios (0.70623–0.70642) are detected at exactly the same interval as the most negative δ 13 C values and a smectite-rich lithology. It is therefore proposed that hitherto unknown diagenetic process, or processes, under peculiar interstitial-water geochemistry, resulting from volcanic ash alteration, played a role for the paired δ 13 C- 87 Sr/ 86 Sr anomaly at DSDP Site 463. By removing the δ 13 C data from samples that possess the highly unradiogenic 87 Sr/ 86 Sr ratios, the amplitude of early Aptian negative δ 13 C excursion is reevaluated to be −2.7‰ or −3.3‰, facilitating comparison with published δ 13 C records from other sections. The refined Site 463 δ 13 C profile further implies that the early Aptian negative δ 13 C event is better described as a twin excursion.

ABSTRACT This paper refines the chronostratigraphic definition of the Shu’aiba Formation using profiles of 13 C/ 12 C ratios integrated with available 87 Sr/ 86 Sr 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 δ 13 C profiles of shallow-marine carbonates from the Aptian Shu’aiba Formation faithfully track the evolution of δ 13 C through time established from pelagic sections elsewhere in the World with the majority of carbon-isotope values ranging between 0.0 to +6.0‰ δ 13 C. The degree of correspondence indicates that overall a diagenetic overprint of the δ 13 C is minor or absent despite whole-scale recrystallization of the rock indicated by the resetting of δ 18 O values, which range between -1.0‰ and -8.0‰ δ 18 O. The vast majority of δ 18 O 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 δ 13 C 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 δ 13 C 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‰ δ 13 C 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 δ 13 C 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 δ 13 C 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.