--> Abstract: Multiple Scales of Confinement for Conglomeratic Slope Valleys: El Rosario Formation (Upper Cretaceous), Baja California, Mexi; #90063 (2007)

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Multiple Scales of Confinement for Conglomeratic Slope Valleys: El Rosario Formation (Upper Cretaceous), Baja California, Mexico

 

Anderson, Bryan J.1, Jesus A. Ochoa1, Kelsey McNamara1, Jesse J. Melick1, Michael H. Gardner1, James G. Schmitt1 (1) Montana State University, Bozeman, MT

 

Slope valley-fill deposits (200 m thick; 2 km wide) of the El Rosario Formation are encased in slope mudstone and record evolution from erosional to depositional confinement. Longitudinal and cross-sectional exposures of degradational, accretionary, and aggradational conglomerate channel fills are flanked by, or interbedded with, thin-bedded sandstone and mudstone. These changes are documented across four canyons which dissect the 2.5 km long study area. Preliminary results indicate <10% of the multistory fill is erosionally confined. This slope valley succession changes upward from an isolated 30 m thick by 300 m wide degradational composite channel incised into slope mudstone to a 90 m thick succession consisting of three channel complexes (30 m thick; 600 m wide) interdigitated with thin-bedded levees. Two laterally offset channel complexes (15 m thick; 200 m wide) cap the succession. Three models proposed to explain the evolution of slope valley systems are: (1) coeval channel-levee deposition, (2) levee formation prior to back-filling, or (3) proportional change with more levee than channel deposition, followed by more channel than levee deposition. These models impact predictions of down-profile sediment bypass, internal connectivity, and heterogeneity. Model 1 describes deposition and aggradation with sediment bypass occurring within large flows that transmit their fine fraction down-profile. Model 2 requires development of large-scale erosional surfaces recording levee deposition preceding back-filling unrelated to down-profile deposition. Model 3, most consistent with these data, combines the first two by modulating the proportion of filling. Erosion at the elementary and composite channel-scales is the dominant mode of confinement at the scale of the slope valley. Hence, slope channel classification on the basis of erosion has meaning only if channel elements are hierarchical.

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California