Multiple Scales
of Confinement for Conglomeratic
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