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Characterization of a Seismic-Scale, Outcropping Channel-Levee Complex, Tres Pasos Formation, Southern Chile


Seismic reflection datasets enable large-scale characterization of deep-water slope channel systems, whereas outcrop studies are essential for investigating stratigraphic details. Consideration of both scales is crucial for development of conceptual models to characterize these systems. However, seismic resolution limits bed- to geobody-scale interpretation, whereas most outcrops are limited by their scale or stratigraphic context. Linking these scales of observation is important for understanding slope channel systems and characterizing subsurface hydrocarbon reservoirs.

We describe a previously undocumented, 750 m long by 300 m thick exposure of a slope channel system from the Tres Pasos Formation that crops out 25 km north of Puerto Natales, Chile. This formation records the southward filling of the Magallanes Basin axially, through a prograding clinoform system (800-1000 m relief) that connects shelf deposits of the Dorotea Formation with deep-water units. The present-day outcrop belt extends for >100 km from north to south (along-depositional-dip), offering a rare opportunity to consider fine-scale facies and architectural details together with seismic-scale stratigraphic context. The studied section is interpreted to be located ~40 km down-dip from a coeval shelf edge, and it is characterized with more than 1300 m of measured stratigraphic section, approximately 500 paleocurrent measurements, and GPS mapping of 100s of stratal surfaces.

Analysis of stratigraphic data reveals several composite channel-form bodies, each up to 40 m thick and ~250 m wide. The lower units show evidence for lateral migration and low aggradation whereas the upper units are characterized by greater aggradation and a laterally offset stacking pattern. This suggests that vertical connectivity between channel fill sandstone is higher in the basal part of the succession and the preservation of fine-grained out-of-channel deposits is prominent in the upper part. A distinctive, tens of m thick siltstone-prone succession occurs lateral to channel fills and is dominated by thin (mostly <5 cm), fine-grained beds that rarely exhibit amalgamation. We propose that this unit represents an inner levee or terrace deposit. The discovery of an outcrop with this unique range of channel system components in the Tres Pasos Formation provides key insight into the outcrop belt, as well as facies and stratigraphic architecture trends in petroliferous deep-water conduit deposits, worldwide.