Characterization of deep-marine channel-levee complex architecture with palynofacies: An outcrop example from the Rosario Formation, Baja California, Mexico

Abstract

Identification of specific elements of a deep-marine system currently relies on detailed sedimentological studies, which can be problematic with sub-surface targets. Here we propose identification of specific architectural elements with palynofacies, hypothesizing that organic matter will not be uniformly spread in turbidite successions. An integrated sedimentological and palynological study was conducted on outcrops of an Upper Cretaceous – Paleocene, slope channel-levee complex, of the Rosario Fm., Baja California. System architecture is well constrained by previous work, allowing certainty in sample placement. Architecture reflects a lateral progression from channel axis to distal elements, via overbank terrace; internal levees within the channel-belt; channel-bounding external levee, with an inner and outer component, grading into hemipelagites. Two hundred samples, placed in sedimentary logs, were collected from mudstones across the system. 10 g of sediment per sample was processed for analysis of three hundred pieces of organic matter. Samples display a range of both allochthonous terrestrial and relatively autochthonous marine matter. Results show a decrease in sorting of matter away from the channel axis, where dense humic materials are dominant. Lighter particles, e.g. plant cuticle, were retained in suspension at lower energy, being preferentially deposited in channel distal settings. Exploratory ordination analysis was used to explore these trends. The primary mechanism inferred for this distribution is hydrodynamic sorting, as the competence of turbidity currents to transport particles reduces with distance from the sediment conduit. Variation in palynofacies observed in the architectural elements allows a classification scheme to be developed, enabling recognition of depositional sub-environments within deepwater systems. This scheme can now be applied to subsurface samples to assist characterization of subsurface deepwater channel-levee complex architecture, understanding of which is vital for hydrocarbon exploration.