--> What’s That You Splay – A Depositional Model for the Humid Terminal Splay? A Comparative Analysis of the Modern Rio Seco, Argentina and the Cretaceous-Paleocene Raton Formation, Colorado

AAPG ACE 2018

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What’s That You Splay – A Depositional Model for the Humid Terminal Splay? A Comparative Analysis of the Modern Rio Seco, Argentina and the Cretaceous-Paleocene Raton Formation, Colorado

Abstract

Terminal splay depositional models are commonly associated with arid dryland basins, and alternatively, as depositing in the distal portion of distributive fluvial systems. While the facies models of modern and ancient terminal splay deposits are well defined in arid to hyper-arid conditions, they remain poorly understood in heavily vegetated humid to tropical environments. In this study, we present the facies architecture and depositional processes of terminal splays in humid to tropical conditions through a comparative analysis of deposits of the modern Rio Seco, Argentina with the Cretaceous-Paleocene Raton Formation, Colorado.

Humid terminal splay deposits are characterized by four architectural elements identifiable in the modern and ancient. Distributive channels are lenticular and composed of fine to medium grained quartzose sands up to 2m thick, with ripple, parallel and planar laminations, and trough bedding. Terminal splay sheets (TSS) are comprised of very fine to medium quartzose sands up to 50cm thick that are well-rounded well-sorted and up to 2.375km2 in extent. Sedimentary structures include parallel and planar bedding, low angle ripple laminations, and low angle sigmoidal cross bedding. In situ trees with their rootballs, well preserved organic debris, and vegetation induced sedimentary structures are abundant in TSS. Intersplay deposits are 10-20cm thick, comprised of silt, clay, and very fine sands with abundant organic material, rooting, and ripple laminations. Flood basin mudflats are pedogenically modified and organic to clay rich.

Humid terminal splay deposits form through the deposition of TSS and progradation of distributive channels. TSS deposition is characterized by unconfined flow onto subaerially exposed vegetated floodplain mudflats. Following TSS deposition is an abandonment phase and reestablishment of the flood basin mudflat, wherein pedogenesis begins and vegetation takes root.

Humid terminal splays have the potential to increase reservoir capacity and connectivity through cross cutting relationships while also juxtaposing organic rich source rock with sandy channel elements. Progradation of distributive channels through laterally extensive TSS connect otherwise discrete sandy reservoir elements, while fossilized trees through multiple TSS serve as permeability pathways between discrete TSS. In high-accommodation fluvial reservoirs, humid terminal splay deposits could prove key in the pursuit of thinly bedded fluvial plays.