Stratigraphic Architecture Intermediate Between Channels and Lobes: Characterization and Insight from the Tres Pasos Formation, Magallanes Basin, Southern Chile

Pemberton, Erin A. L.; Hubbard, Stephen M.; Romans, Brian W.; and Fletcher, Sean D.T.
[email protected]

The characterization of stratigraphic architectures associated with deep-water depositional systems has historically focused on two end-members, slope channels and submarine fan lobes. The channel-lobe transition zone is located between these two end-members and is composed of sediments that exhibit characteristics of both slope channel and submarine fan lobe architectures. This zone marks the transition from confined to unconfined flow and with this shift in flow parameters (ie: velocity, depth, gradient etc.) comes a fundamental evolution of facies. Sea-floor and deposit architecture typical of the channel-lobe transition zone, including numerous scours, have been documented in other environments characterized by rapid flow expansion such as channels and proximal levees. Therefore depositional context must be utilized when attempting to interpret evidence for the channel-lobe transition zone in the rock record.

Using outcrop to study the channel to lobe transition zone offers insight into the complex stratigraphic morphology of the transition from channels to lobes through observation of fine-scale changes in facies associations and stratigraphic body architectures. This study presents preliminary observations and interpretations of an outcrop example of this intermediary setting preserved in the Late Cretaceous Tres Pasos Formation, Magallanes Basin, Southern Chile. The study is based on comprehensive analysis of a 1.4 km long and 100 m thick, two- to three- dimensionally exposed, oblique-strike oriented outcrop at Arroyo Picana, southern Chile. The units of interest are located stratigraphically beneath (and presumed downdip of) large slope channel complexes. The stratigraphic architecture in the Arroyo Picana outcrop is highly variable with respect to sandstone sedimentary body geometry, ranging from channelized (low aspect ratio with incisional relief up to 25 m) to broad and more tabular, (high aspect ratio; ~5 m thick and extending over 650 m laterally). Major sandstone packages and mappable surfaces were walked out and surveyed using differential GPS, revealing the stratigraphic juxtaposition of high- and low-aspect ratio sedimentary bodies characterized by abundant scours and low- to high-relief channelization.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013