Controls on the Organic- Rich Mudstones Development across the Cenomanian- Turonian Oceanic Anoxic Event (OAE2) in Moroccan Basins

Abstract

Organic matter (OM)-rich black mudstones were widely developed across Morocco during the Late Cenomanian/Early Turonian (C/T) and have significant source rock potential. The global Oceanic Anoxic Event (OAE2) has commonly been considered to be associated with substantial organic matter preservation in a range of palaeogeographical marine settings, but the interplay of controlling factors is still debated. This study investigates the distribution and characterisation of OM-rich mudstones associated with the OAE2 and younger Turonian interval in well-exposed C/T sections across Morocco and assesses the controls and mechanisms for organic matter enrichment. Extensive fieldwork was undertaken on sections exposed in the West Moroccan coastline Agadir and Tarfaya basins and the inland Pre-African Trough Basin (East Morocco). A sampling programme allowed detailed petrological observation, sedimentological analysis, stratigraphic correlation and geochemical analysis. New biostratigraphic and high-resolution δ13C stratigraphic data are presented that provides a more precise definition of the OAE2 interval and C/T boundary location. OM-rich mudstones associated with anoxic bottom water conditions, were recorded during the OAE2 in the deep-water sediments exposed in the Tarfaya Basin. In the Agadir Basin no OM-rich mudstones were observed related to the OAE2, but moderately thick beds of OM-rich black mudstones identified in both basins post-dating the OAE2 interval, related to the Early Turonian marine transgression. Trace element analysis suggests organic matter content can be correlated with increased sea surface productivity and oxygen-depleted bottom water conditions, within an environment with relatively low background terrigenous input. This suggests that in both the OAE2 and Early Turonian intervals transgression played a significant control on the deposition and preservation of OM-rich black mudstones in the studied basins. In the Pre-African Trough Basin (East Morocco), influenced by the Tethys Ocean, biostratigraphic and carbon isotopic dating indicate that mudstone sedimentation was diachronous across the OAE2 interval, extending from the upper OAE-2 to post-OAE2 interval. The sedimentology, water conditions and palaeoproductivity of the palaeoenvironments were studied to characterise the lithofacies and geochemical signature. The OAE2 interval recorded a dominantly shallow carbonate platform environment in the Errachidia-Goulmima Basin, with a lack of widespread anoxic facies that lack organic matter accumulation. Restricted marine environments were not recorded until the Late Cenomanian / Early Turonian marine transgression that allowed the development of anoxic to euxinic conditions in basinal settings, controlling organic carbon preservation in the Errachidia area. Five dominant mineral assemblages in the potential source rocks were recognised across these basins. The microstructure of mudstones and organic matter particles were investigated. Organic matter is preferentially developed in calcite-rich mudstones and predominantly distributed in the matrix. Two geometries of organic matter particles are recognised: elongated shapes in the matrix with good connectivity and spherical shapes in the foraminiferal tests with poor connectivity. The distribution and source rock quality of the OM-rich mudstones were controlled by a combination of the OAE2, marine transgression, palaeogeographical settings and palaeoenvironments. Specific controlling factors show a difference between the Atlantic and Tethyan basins. During the OAE2 interval, thick organic-rich mudstones were widely developed in the deeper Atlantic influenced basins, which is interpreted to have the greatest hydrocarbon potential. In most marginal interior basins influenced by Tethys, the presence of locally distributed, thinner organic-rich mudstones indicates a more complex hydrocarbon potential, restricted to local deeper troughs. Understanding the controlling fac

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Kevin Taylor (Supervisor) & Jonathan Redfern (Supervisor)