Petrographic and chemical traits of Cenomanian platform carbonates (central Lebanon): implications for depositional environments

Abstract

This paper provides new data on the Cenomanian (Late Cretaceous) carbonates in Lebanon, which, due to their location and considerable thickness, represent an important segment of the broader Eastern Mediterranean platform. Detailed field and microfacies investigations have shown that the Late Albian–Cenomanian succession (i.e., the Sannine Formation in Lebanon) consists of a number of lithologically distinct strata. These are: micritic limestone, silicified micrite, bioclastic limestone, chalky limestone (foraminifera-wackestone), marl and marly limestone, dolostone and dolomitic limestone. The laminated micritic limestones are typically associated with chert nodules and bands, and are interpreted to have been deposited in deep marine environments. The bioclastic limestones have abundant skeletal fragments suggesting deposition in relatively shallow marine settings, an interpretation that is further supported by the presence of local palaeo-channels within the limestones. Geochemically, the formation includes five distinct chemical groups, each characterized by certain chemical traits reflecting the lithological nature of the various facies. The micritic limestones have relatively low concentrations of most major and trace elements (Mg, Sr, Si, Al, K, P, Ti and Rb). In sharp contrast, the marly limestone samples have the highest concentrations of most of these elements. The silicified micrites have a wide range of Si and Sr contents, but abundances of most elements are intermediate between those of the micritic limestones and the marly limestones. The chalky limestones have relatively low Si and Al, but relatively high P contents. The dolomitic limestones have the lowest Rb and Sr concentrations. The normalized REE patterns of representative samples of most of these rocks are parallel to subparallel and nearly flat. Based on facies analysis and chemical traits, we have constructed a carbonate platform ramp model for this succession, in which the dolostones and marly limestones were deposited in back ramp to subtidal environments, the bioclastic limestones accumulated in shallow, wave-dominated to deep ramp environment, and the micritic limestones and silicified micrites formed in a deep ramp environment. This ramp was influenced by sea-level fluctuations and tectonic activities. The proposed model has wider implications for the geodynamic evolution of the eastern Mediterranean region during the Cretaceous Period, and is consistent with interpretations of some other tectonically active margins worldwide.