Journal of Conference Abstracts

Stratigraphic Architecture, Sediment Flux and Isotope Curves of the Cenomanian Carbonate System in Northern Oman (Natih Fm.)

F. S. P. van Buchem (Frans.VAN-BUCHEM@ifp.fr)¹, P. Razin², J. Casanova³ & F. Walgenwitz⁴

¹ Institut Francais du Petrole, Rueil-Malmaison, France

² University of Bordeaux, France

³ BRGM, Orleans, France

⁴ ELF, Pau, France

The purpose of this paper is to investigate the possibilities of linking the evolution of the sedimentary system, with the changes in depositional geometries, biota, and sediment flux, to the changes that happen at the scale of the global biogeochemical cycles, such as expressed in the typical excursions in the carbon stable isotope curve. This excercise is motivated by the underlying hypothesis that the sedimentary system is controlled by a complex feedback system of physical, chemical and biological processes that interact and are linked at different time scales.

The case study is the Cenomanian/Turonian carbonate system which is exposed in seismic scale outcrops in northern Oman. Two outcrop transects are constructed based on 16 detailed fieldsections, covering an area of 120 x 60 km, and one regional subsurface well correlation. The thickness of the Natih Formation varies between 250 and 350 m. Selected parts of six sections were measured for carbonate and TOC content. Three sections have been systematically sampled for bulk carbon and oxygen isotope measurements in both the intra-shelf facies and the shallow water carbonates.

The stratigraphic record displays a hierarchical organisation at three orders of depositional sequences. This hierarchy facilitates the establishment of a high resolution time framework, which is an essential step towards the quantification of ecological change, and variations of the sediment flux through time. At each one of these orders specific control mechanisms are identified that influence the sedimentation processes, and the stratigraphic architecture.

Three 3rd order sequences are distinguished, of which the first and the third sequence have very similar geometries and facies. They show the development from a flat, ramp type setting to an intrashelf basin topography with an adjacent very low angle shallow water carbonate platform. The intrashelf basin topography is formed during the 3rd order transgressive phase. The facies is characterised in the basin by organic-rich/organic-poor carbonate mudstone couplets with abundant oysters, and on the shelf by a normal marine fauna. During regression, the basinal facies is rich in chert derived from dissolved sponge spicules, the upper slope is formed by bioclastic grainstones, and the platform top is dominated by rudists. The top boundary of sequence 1 is marked by incised channels, and related to a global eustatic sea level fall. The top boundary of sequence 3 is a diachronous surface caused by teconic uplift. Both cycles are almost purely formed by carbonate, with the exception for the organic matter deposited during the transgression in the basin.

The second sequence is essentially flat-bedded, has no geometries of any importance, and contains a high amount of clay. Clayey facies are dominated by Orbitolinas, with locally some rudist biostromes. These levels alternate with carbonate beds containing a more open marine fauna, which are often topped by ferruginous hardgrounds. In combination with the frequent Thallasinoides burrows this suggests a reduced accomodation potential at that time. The top boundary of sequence 2 was probably subaerially exposed.

Systematic measurements of the carbonate, organic matter and clay content has been performed in 3 outcrop sections and 1 well. Plots of these measurements show a dilution relationship between organic matter and clay versus carbonate in the transgressive phase of 3rd order sequences 1 and 3. This pattern is interpreted as a contiuous background sedimentation of organic matter and (little) clay, and a strongly varying carbonate flux (immature TOC varies between 0.5 and 14%). In sequence 2 no organic matter is preserved, and the clay content can be upto 80%.

In the carbon isotope curves, four types of signals are registered: (1) a positive excursion at the Cenomanian/Turonian boundary that can be correlated at the scale of the Tethys ocean, 2) two additional positive excursions that are correlatable at the scale of Oman, 3) large negative shifts that correspond to the source rock levels or meteoric diagenesis, and 4) small negative shifts that may be a result of temporary subaerial exposure. Significant positive shifts of the carbon isotope curve are supposed to correspond to times of widespread organic matter accumulations. In sequence 1, the shallow water carbonate rocks that are time-equivalent to the source rock levels in the intra-shelf basin, do indeed show a significant positive shift of the curve. This is less evident in sequence 3. In sequence 2 a positive shift occurs, also linked with a transgression, but no source rock deposition in the studied area.

In conclusion, the main controlling factors of this system are:

At the 3rd order scale: in both sequences 1 and 3 the organic-rich intrashelf basin formed during (rapid) eustatic sea level rise, possibly in combination with atmospheric conditions (CO₂ pressure?) and nutrient supply (mesotrophic setting?) that favoured organic matter production and preservation. The regression in sequence 1 is eustatically controlled, while the regression in sequence 3 is tectonically enhanced. Sequence 2 is a phase when relatively little accommodation space is created, characterised by a lower sedimentation rate, due to the frequent arrivals of clay inhibiting the carbonate production. Both climatic change and regional uplift may have been responsable for this change in sediment flux. The value of the carbon isotope curve as a reflection of the organic carbon and mineral carbon storage needs evaluation at a bigger scale.

At the 4th/5th order scale: 4th order, m to decameter scale, cycles are the result of strong lateral shifts of facies belts; high frequency, dm to m scale cycles have been observed in the intra-shelf basin and at the platform top, they are the result of high frequency climatic variations.

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