Fundamental Controls on Organic-Matter Preservation in a Clastic-Starved Intra-Shelf Basin: The Upper Cretaceous Natih B Sediments (Natih Formation) of Northern Oman

Said A.K. Al Balushi¹*, Joe H.S. MacQuaker¹, and Henk Droste^2
1University of Manchester, UK
²Shell Technology Oman, Muscat
*[email protected]

It is often argued that enhanced organic-matter preservation (up to 15% total organic con-tent) in the Mid- to Late Cenomanian Natih B intra-shelf basin (maximum water depth circa 80 m) is caused by the presence of anoxic bottom-water conditions. Mechanisms that un-derpin organic-matter enrichment in intra-shelf basinal settings, however, are complex. They are controlled by a balance between primary (both organic and inorganic) productiv-ity, clastic input, rate of sedimentation and early diagenesis. In this study possible mecha-nisms for organic-matter preservation other than persistent bottom-water anoxia are investi-gated. The results of this study will help improve the understanding of the distribution of source rocks in intra-shelf basins. Natih B sediments (collected from both core and outcrop in northern Oman) have been investigated using a combination of optical and electron-optical (back-scattered electron imagery) techniques, which provide additional data to those gathered by traditional field and geochemical methods. Natih B lithofacies alternate be-tween two main types: organic-rich mudstones and cement-rich wackestones. The organic-rich mudstones are typically, fine-grained, dark-grey, unlaminated, and paler-coloured where burrowed. These units commonly contain planktonic foraminifera, coccoliths and organic matter. In addition, in-situ thick-shelled bivalves (including “Exogyra sp.”) are pre-sent. The cement-rich wackestones are lighter in colour and extensively bioturbated. This lithofacies comprises a mix of skeletal fragments, echinoderm debris, bivalves, brachio-pods, calcispheres and rare foraminifera, cemented by fine-grained carbonate. Given these observations, it is likely that bottom waters during deposition of the Natih B intra-shelf bas-inal sediments were oxygenated. Organic-matter accumulation here, therefore, cannot have been controlled by the presence of persistent bottom-water anoxia. Instead, short-term en-hanced organic productivity, rapid delivery of organic components to the sediment-water interface, optimal rates of sediment accumulation and episodic burial were probably the fundamental parameters that controlled facies variability.

AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain