Aggradation versus By-Pass in Coarse-Grained Deep-Water Channel Fills: Characteristics and Differences Based on Field Examples (Cerro Toro Formation, Chile, and Rosario Formation, Mexico)

Abstract

The architecture of submarine channel fills records both erosion and deposition related to variations in gravity flow characteristics (e.g. magnitude, composition, density), combined with changes in the slope morphology induced by tectonics, channel avulsions, and aggradation (Kneller, 2003; Pirmez et al., 2000). Understanding outcrop in deep-water channelized systems is essential to define sub-seismic facies architecture, and predict reservoir quality. Based on architectural elements, this work uses logs and photo-interpretation to define main differences in depositional features and flow regimes of two coarse-grained end members deposited in similar slope settings; Cerro Toro Formation, Magallanes Foreland Basin, Southern Chile; and Rosario Formation, Peninsular Ranges Fore-arc Basin, Baja California, Mexico. Extensive mapping and logging combined with photo/image treatment and interpretation have been used to understand in greater detail the main differences in terms of processes and deposits of highly turbulent Newtonian flows (turbidity currents), non-Newtonian, potentially laminar flows (debris flows), and hybrid flows that may be transitional flows between the two. Identification of lithofacies, facies associations and architectural elements allow the understanding of the system vertically (overall waning or waxing), and laterally (axis to margin). The results obtained to date, mostly based on mapping and logging show major differences in terms of lateral continuity of the beds, and in texture of the deposits, between the bypass-dominated Rosario Formation and the aggradation-dominated Cerro Toro Formation. The beds in the Cerro Toro Formation are laterally extensive for hundreds of meters; their thickness is up to 5 meters; clast/matrix ratio is low but variable, typically decreasing upwards through a bed; in terms of processes is clear that non-Newtonian flows were responsible for a high proportion of the channel fill. The Rosario Formation is marked by laterally discontinuous beds due to intense “cut and fill”; beds are hard to define, since the deposits consist largely of erosional remnants of tractional bedforms and bars, typically with high clast/matrix ratios; highly turbulent flows were responsible for the bedload transport. We discuss possible causes for these dramatic differences in facies and architecture, and the implications for reservoir prediction.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015