Control Factors of Organic Matter Sedimentation in a Modern Mud-Rich Turbiditic System: A Sedimentological and Geochemical Approach

Abstract

Abstract

Burial of organic matter (OM) along continental margins is influenced by several factors that often modify the quantity and quality of OM preserved. Understanding the interactions between the different processes involved in early deposition of OM may help to better understand the multi-scaled heterogeneities found in source rocks and the reasons for unusual OM concentration. This study examines the distribution of OM off the Gabonese margin (W-Africa) in the recent turbidite system of the Ogooué River. This work used a transect of 10 Kullenberg cores located along the system from the continental shelf to the distal lobes. Geochemical characterization including the carbon content, carbon isotopes of organic matter, XRF and XRD measurements were conducted to define main geological controls on OM accumulation. Our results show a high lateral and vertical variability in concentration of OM (0.7% to more than 9%) but also in the origin of the organic fraction. The most proximal part of the system has on average a higher content in OM (>2%), however some turbidites containing woody detritus and well preserved fragmentary leaf debris contribute to the enrichment in OM of the most distal parts of the system. This organic sedimentation is highly sensitive to the variations of the sea level. Lowstands are characterized by high OM accumulation rates in hemipelagic deposits with a high contribution of continents, and by the presence of frequent organic-rich sandy turbidite. Conversely, during sea level highstands, very few turbidites are recorded and the OM in hemipelagic sediments is mainly of marine origin and in lesser quantity. We propose that during sea level highstands, sediments delivered by the river are deposited on the shelf and mobilized by the strong northward coastal drift currents. During lowstands the margin is dominated by downdip transport of OM enriched sediments from the shelf to the deep basin, through a series of canyons and channels. Coarse-grained turbidites are deposited and their tops show laminae with high OM content (>10%). Moreover, the primary marine productivity is also certainly enhanced by the input of fresh continental OM. This input of OM is also associated with clay minerals allowing a more efficient preservation of both marine and terrestrial OM. OM distribution is mainly related to sea-level variations along the Gabonese margin, even though, spatial context locally influenced the distribution of potential organic-rich mudstones.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016