Observations from Model Channel-Levee Systems and their Utility for Interpretation of Ancient Levee Systems in the Cerro Toro Formation (Cretaceous), Chile

K. M. Campion
Marathon Oil Corporation, Houston, TX, [email protected]

Thin-bedded mudstone and sandstone lithofacies in the Cerro Toro, interpreted as ancient levees, exhibit two architectural patterns: 1) broad (>300 m) undulating or wavy-bedded elements that laterally terminate by onlap, truncation (toplap), and downlap patterns, and 2) laterally persistent (>500 m), horizontal, thin-bedded mudstone and sandstone. These facies exhibit a transition from horizontal to wavy beds concurrent with pronounced aggradation of a coeval channel facies. The horizontal beds are interpreted as initial overbank deposition, whereas the wavy-bedded facies are interpreted as sediment waves. Sandstone and mudstone beds within the sediment waves exhibit turbidite lithofacies including current-ripple lamination, planar lamination, massive, graded intervals, and laminated or structureless mudstone beds. Typically, these beds are a few centimeters thick, but locally, sandstone beds form bedsets over 1 m thick. Erosion surfaces within this thin-bedded lithofacies are distinctive, and locally they exhibit over 5 m of relief, have a wavy pattern, and are spaced vertically at 10-15 m.

Observations from the Quaternary Var Ridge provide insight that assists the interpretation of ancient levee systems. Like the Cerro Toro, the Var system has coarse-grained material within the fan valley, pebble- to cobble-size, and thin-bedded mud and sand in the levee with documented sand bed thickness up to 40 cm. Architectural details described from the Var Ridge indicated sediment waves make up a significant portion of the levee material. Beds within the sediment waves terminate laterally by onlap, downlap, and toplap, similar to the Cerro Toro. Deposition of sediment on the Var levee and growth of the sediment waves can be linked to turbidity currents that were generated via overbank flows from the Var fan channel. Growth of the sediment waves in the Var system represents active sedimentation and turbidity currents within a coeval channel rather than development during periods of channel inactivity or degradation.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009