Anatomy and facies distribution of terminal lobes in ephemeral fluvial successions: the Jurassic Tordillo Formation in the Neuquén Basin, Argentina

CORONEL M.D.; ISLA, MANUEL F.; VEIGA G.D.; MOUNTNEY N.P.; COLOMBERA L.

SEDIMENTOLOGY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2020

0037-0746

In terminal fluvial-fan systems, characteristic proximal-to-distal variations in sedimentary architectures are recognized to arise from progressive downstream loss of water discharge related to both infiltration and evaporation. This work aims to elucidate downstream trends in facies and architecture across the medial and distal zones of terminal-fan systems, which record transitions from deposits of channel elements to lobe- and sheet-like elements. This is achieved via a detailed characterization of ancient ephemeral fluvial deposits of the well-exposed Kimmeridgian Tordillo Formation (Neuquén Basin, Argentina). The fine sand- and silt-prone succession associated with the medial-to-distal sectors of the system has been studied to understand relationships between depositional processes and resulting architectures. Facies and architectural-element analyses, and quantification of resulting sedimentological data at multiple scales, were undertaken to characterize sedimentary facies, facies transitions, bed types, architectural elements and larger-scale architectural styles. Eight bed types with distinct internal facies transitions were defined and interpreted in terms of different types of flood events. Channelized and non-channelized architectural elements were also defined based on their constituent bed types and their external geometry. The most common elements are terminal lobes, which are composite bodies within which largely unconfined sandy deposits are stacked in a compensational manner; a hierarchical arrangement of internal components is recognised. Proximal feeder-channel avulsion events likely controlled the evolution of terminal-lobe elements and their spatiotemporal shifts. Stratigraphic relations between architectural elements record system-wide trends, whereby a proximal sector dominated by channel elements transitions to a medial sector dominated by sandy terminal-lobe elements, passing further downdip to a distal sector dominated by silty terminal lobe-margin and fringing deposits. This work enhances our understanding of the stratigraphic record of terminal fluvial systems at multiple scales, and provides insight that can be applied to predict the facies and architectural complexity of terminal fluvial successions.