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Architectural Elements and Implications for Deepwater Overbank Deposition of Fine-Grained Lithofacies in the Cerro Toro Formation (Cretaceous), Silla Syncline, Chile

Campion, Kirt *1
(1) Marathon Oil Corporation, Houston, TX.

Fine-grained lithofacies in the Cerro Toro Formation exhibit two architectural patterns: 1) broad (>200 m) undulating or wavy-bedded elements that laterally terminate by onlap, truncation (toplap) and downlap patterns, and 2) laterally persistent (> 400 m), horizontal, thin-bedded mudstone and sandstone. These facies exhibit a stratigraphic transition from horizontal to wavy and curved beds concurrent with pronounced aggradation of laterally equivalent, and possibly coeval, channel facies. Sandstone and mudstone beds within the wavy-bedded facies exhibit turbidite lithofacies that include current-ripple lamination (Tc), planar lamination (Tb) massive, graded intervals (Ta), and laminated to structureless silt- and clay-rich beds (Tde). Typically, these beds are a few centimeters thick, but locally, sandstone beds form bedsets over 1m thick. These thick sandstone bedsets display inclined bedding associated with mudstone rip-up clasts and are confined to swales within large-scale, wavy-bedded units. Erosion surfaces within this thin-bedded fine-grained lithofacies are spaced vertically at 10-15 m, commonly associated with the crest of curved bedding and display at least 5 m of relief. The fine-grained lithofacies located adjacent to and eroded into by a channel facies are interpreted as a coeval levee facies. Planar, tabular bedsets located at the base of this fine-grained lithofacies are interpreted as the initial overbank deposition associated with development of a channel complex, whereas curved, wavy-beds are interpreted as sediment waves developed on the backside of a levee. The curved bedding, lenticular, medium-grained sandstone bedsets, scattered erosional surfaces, and onlap-downlap stratal geometries in the Cerro Toro bear resemblance to sediment waves associated with Quaternary coarse-grained, channel-levee systems. Turbidity currents spilling onto a levee surface are the main processes invoked for deposition of these sediment waves. Significantly, the volume of sand and silt in the system may be adequate for hydrocarbon reservoirs, particularly gas, but the processes that shaped these rocks were not conducive to bed continuity and did not enhance reservoir potential.


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