Evolution of coarse-grained, upper slope channel fairway deposits, Paleocene El Rosario Formation, San Carlos, Baja California, Mexico
Anderson, Bryan James
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Study of the upper El Rosario Formation (UERF) (62% conglomerate, 11% sandstone, and 27% mudstone) documents the stratigraphic evolution of Paleocene deepwater channels and related deposits of a conglomerate-rich lithosome. The ~1.7 km² (1.9 km-long by 0.9 km-wide) study area represents a segment of a 7 km-long by 1+ km-wide upper slope channel fairway within an overall mudstone-rich slope succession. Collection of measured sedimentologic sections (746 m), interpretation of photo-panel coverage of outcrop cliff-faces (3.7 km), and geologic mapping facilitated the identification of 28 hydrodynamic facies, 5 sedimentary bodies, and 3 sedimentation regions within the fairway. Facies, bodies, and sedimentation regions are organized and analyzed within a threefold stratigraphic hierarchy of one 3rd-, five 4th-, and eleven 5th-order cycles. Conglomerate facies were dominantly deposited from hyperconcentrated, nonturbulent, multipartite subaqueous flows characterized by a wide range of behaviors and sediment support mechanisms. They reside within an organized fourfold channel hierarchy that shows organized changes in diversity and abundance that correlates to the threefold stratigraphic hierarchy. Sedimentary bodies reflect variations in scale, stacking pattern and confinement, and include channelform, wedgeform, lobeform, mass transport deposit, and sedimentary drape. Sedimentation regions, defined as spatial domains characterized by specific proportions of facies and sedimentary bodies, record variable degrees of subaqueous flow confinement. These include: (1) channel axes, which correlate to the thickest, most channelized and conglomerate-rich intervals, (2) channel margins, which show the highest facies and sedimentary body diversity, and (3) channelflanks, which correlate to the highest proportion of unchannelized deposits and thinbedded sandstones and mudstones. Results of this study indicate that: (1) minimal flow transformation influenced the dominance of ungraded conglomerate facies; (2) channel erosion was limited to the elementary and composite channel scales; (3) grain size is not a dominant control on channel dimension because overlap exists among conglomerate and sandstone channels at multiple scales; (4) a stratigraphic hierarchy is key when analyzing channels characterized by variable stacking pattern; (5) sedimentation regions exhibit compensational stacking upward through stratigraphy; and (6) there is an upward change from multistory to multilateral channel stacking that correlates to erosional- to depositional-confinement at multiple scales.