A Predictive Framework for Intra-Channel Deposits from Slope Settings Based on Sedimentology and Ichnology

Individual slope channel fills in the outcropping Tres Pasos Formation of southern Chile are 10-25 m thick and 200-400 m wide. The intra-channel facies association contains a detailed record of sedimentary processes; trace fossil suites also record variation in paleoenvironmental conditions across the narrow channels. Axial deposits are easily recognized as thick successions of amalgamated, thick-bedded turbidites. Other intra-channel facies are more difficult to discern with sedimentological characteristics alone, however, trace fossil suites can be diagnostic. The purpose of this study is to utilize ichnology and sedimentology to decipher slope channel processes, and ultimately intra-channel facies. Using extensive data collected from exceptional outcrops, a model is formulated that has the potential to help constrain reservoir distribution in cores that contain analogous slope channel units.

Non-amalgamated thin- to thick-bedded turbidites are characteristic of a series of intra-channel facies: (1) units that drape erosional surfaces, deposited from the tails of high-energy flows that largely bypassed channels, and successive dilute turbidity currents; (2) axial abandonment units deposited from waning flows at the top of channel fill successions; and (3) units deposited at the margins of conduits. These marginal deposits are separated into “proximal margin” where beds transition from amalgamated axial units, “distal margin” units that onlap channel edges, and “distal abandonment” beds that are present at the uppermost margins of channel fills. Out-of-channel deposits, including levee units, are not considered in the study.

The various intra-channel facies are each characterized by unique trace fossil assemblages. Drape deposits have low bioturbation intensity, with uncommon grazing and deposit feeding ethologies noted. Axial abandonment deposits record opportunistic behaviors, with only crustacean burrows present (ie, Ophiomorpha and Thalassinoides). Proximal margin deposits are largely unbioturbated, except for passively filled Diplocraterion up to 1 m long that descend from exhumed surfaces within the channel fill (Glossifungites Ichnofacies). Distal deposits are characterized by elevated trace fossil numbers that record a range of grazing, deposit feeding and opportunistic ethologies. Distal abandonment deposits are associated with the most diverse trace fossil assemblage of the intra-channel setting (up to 15 forms).

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California