Classification of deep-marine architectural elements with palynofacies

Modelling of deep-marine depositional systems is notoriously complex, especially in the subsurface, and new approaches to understanding stratigraphic organization and hierarchy are important. Here we present a new,... [ view full abstract ]

Modelling of deep-marine depositional systems is notoriously complex, especially in the subsurface, and new approaches to understanding stratigraphic organization and hierarchy are important. Here we present a new, high-resolution classification of deep-marine architectural elements, based upon their palynofacies. The objective of this study is to determine the distribution of particulate organic matter in turbidite systems, for which integrated sedimentological, ichnological and palynological datasets were developed from a series of world class outcrops, where confidence in placement within a system is high.

Twenty-eight architectural elements were described from slope channel-levee outcrops of the Rosario Fm., Mexico; confined mini-basins of the Grès d'Annot Fm., France; and basin-floor submarine fans of the Marnoso-Arenacea Fm., Italy. Elements encompass a wide variety of depositional settings, ranging through upper-slope channels and related over-bank deposits; highly confined fill-and-spill turbidites; to basin-floor fans with constituent lobe elements. Six hundred samples were collected, in conjunction with sedimentary logging, from turbidite mudstones; 10 g of each were processed using non-oxidising acids to examine their organic matter. Three hundred pieces of matter were counted per sample, organized by major groups (phytoclasts, palynomorphs, amorphous organic matter) and subgroups (according to their features). Samples are typically dominated by allochthonous terrestrial material, but also contain relatively autochthonous marine material, corresponding to sixteen categories. Variations in the type and proportions of organic matter observed were used to differentiate architectural elements. Multivariate and fuzzy c-means cluster statistical analysis were used to explore the variations. Results show a decrease in sorting, particle size and density of particles moving away from sediment conduits, with the least hydrodynamic fragments being deposited in the most proximal settings. This result demonstrates the role of primary sediment dispersal mechanisms in controlling density stratification and distribution of particulate organic matter in deep-marine systems.

Our observations have permitted the construction of a palynofacies classification scheme for a range of architectural elements encountered in turbidite systems. This classification scheme is now being integrated with existing geophysical, petrophysical and sedimentological criteria for improved identification of subsurface architectural elements and reservoir hierarchy, understanding of which is crucial for exploration and development of prospects.