Lithofacies Variability and Organic-Matter Preservation in a Clastic-Starved Intra-Shelf Basin: Upper Cretaceous Natih B Member, Northern Oman

Said A. Al Balushi¹, Joe H. MacQuaker¹, and Henk Droste^2
1School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester, United Kingdom
²Shell Technology Oman, Muscat, Oman

Lithofacies variability and organic-matter preservation (up to 15% TOC) in the intra-shelf basinal (circa 80 m water depth) sediments of the mid-late Cenomanian Natih B Member (Natih Formation) have mostly been interpreted in terms of either oxic-anoxic cycles or variations in primary productivity. Geologists have traditionally assumed that enhanced organic-matter preservation relied upon a combination of bottom-water anoxia and high primary organic productivity. More recently they have also appreciated that episodic and rapid burial of organic matter are also important. How these processes interact in any specific setting, however, are poorly known.

The Natih B Member of northern Oman is an ideal natural laboratory for studying the fundamental controls on organic-matter preservation. It is well exposed, and extensive subsurface data are available. A suite of Natih B sediments have been analyzed using combined optical and electron-optical (back-scattered electron imagery) techniques, as well as by conventional field and geochemical analyses. The Natih B interval comprises two main lithofacies that are interbedded with one and another: a) partially-bioturbated, organic-matter-, calcareous microplankton- (coccoliths and foraminifera), and nannoplankton-bearing mudstone with in-situ thick-shelled Exogyra sp. and b) bioturbated, shell-fragment-bearing (bivalves, brachiopods, echinoderm debris, calcispheres, and rare foraminifera), calcite-cement-rich mudstone.

Bioturbation throughout the succession means that organic-matter preservation in Natih B was probably not caused by simple oxic-anoxic cyclicity. Instead, it is likely that varying organic productivity (phytoplankton bloom), delivery to the sediment-water interface, and burial rates were the main factors underpinning organic-matter enrichment.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas