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Abstract

AAPG Bulletin, V. 88, No. 11 (November 2004), P. 1471-1500.

Copyright copy2004. The American Association of Petroleum Geologists. All rights reserved.

Deep-water leveed-channel complexes of the Cerro Toro Formation, Upper Cretaceous, southern Chile

R. T. Beaubouef1

1ExxonMobil Exploration Co., 233 Benmar, Houston, Texas 77060; [email protected]

AUTHORS

Rick Beaubouef earned a Ph.D. in geology from the University of Houston in 1992. Since then, he has worked as a geologist for Exxon and later, ExxonMobil. He was previously a senior research specialist at ExxonMobil Upstream Research Co. (formerly Exxon Production Research Co.) involved in investigations of a wide range of depositional environments and basins using seismic data, well logs, core, and outcrops. However, most of his work has focused on the stratigraphy and depositional facies of deep-water reservoirs. Since 2002, he has been a stratigraphy advisor for ExxonMobil Geosciences. In this role, he is responsible for stewardship of technologies related to deep-water reservoir characterization and is involved in a wide range of exploration, development, production, research, and training activities on a global basis.

ACKNOWLEDGMENTS

I acknowledge present and former Exxon geologists who participated in this study and contributed to our understanding of the Cerro Toro Formation. These are R. Bissell, M. DeVries, K. Glaser, R. Lindholm, P. McLaughlin, and C. Rossen. However, responsibility for the content and conclusions drawn in this paper is mine. I am grateful to my many colleagues who encouraged me to finally write this paper. In particular, I cite M. Sullivan, C. Rossen, D. Ying, and B. Dixon for inspiration. The reviews of R. J. Miola, M. S. Ryer, and H. J. White improved the paper and are much appreciated. Thanks go to ExxonMobil URC for allowing me to publish this work. Finally, a very special thanks is extended to my family for putting up with me while I pulled this off.

ABSTRACT

The Cerro Toro Formation in the Torres del Paine National Park, southern Chile, contains a series of deep-water channel complexes deposited in an elongate Andean foreland basin during the Late Cretaceous. This stratigraphic interval represents an essentially continuous depositional record of migrating, leveed-channel complexes. Collectively, the channel-fill units in the study area form a belt approximately 5 km (3 mi) wide and several hundred meters thick. Within the study area, four sets of channel complexes are identified. This paper focuses on the best exposed of these channel-complex sets (channel-complex set 3). The channels are filled by bedded conglomerate and amalgamated sandstones interpreted to represent the deposits of high-concentration turbidity currents and debris flows. Large-scale cross-beds in some of the conglomerates indicate significant bed-load transport of gravel- and cobble-forming bars in the channels. Channel axis to margin facies changes between clast-supported conglomerate and either (1) thick-bedded sandstone or (2) matrix-supported conglomerate are observed. Channel-fill facies lie on erosional surfaces that cut into adjacent interchannel facies. Beds thin and onlap these surfaces toward the channel margins. Shale or siltstone drapes of the channel cuts are uncommon and laterally discontinuous. Bed continuity between channel and adjacent, interchannel facies is not observed. The interchannel strata are interpreted to represent levee successions that bound the channels. Stratigraphy in the levee units is defined to include (1) basal, sandy lobe deposits comprised of medium- to thick-bedded turbidites and (2) overbank facies consisting primarily of packages of fining- and thinning-upward, fine-grained, thin-bedded turbidites. This vertical succession is transitional. Distal levee facies include mudstones with thin-bedded, laterally continuous sandstones. Proximal levee facies include mudstones with both thin- and thick-bedded sandstones; however, the thick-bedded sandstones have lower lateral continuity. The proximal levee facies have a higher sandstone percentage than the distal levee, but also have greater depositional and postdepositional complexity, with sand-filled crevasses, erosional truncation, and slumped beds. Field observations suggest that these leveed channels formed in stages that are represented by depositional and/or erosional events. In chronological order, these are (1) an initial stage of relatively unconfined, sand-rich deposition; (2) aggradation of a mud-rich, confining levee system resulting from overbank deposition as turbidity flows bypass the area; (3) erosion as the channel becomes entrenched or as the channel migrates; and (4) filling of the channel-margin relief by onlap of channel-fill sediments. These stages appear to have repeated several times during the formation of a series of channel complexes. In these ways, the Cerro Toro Formation appears analogous to leveed-channel systems observed in late Pleistocene submarine fans and subsurface examples.

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