Dynamic Behavior in Deep-Water Gravelly Subaqueous Flows, El Rosario Formation (Upper Cretaceous), Baja California, Mexico

McNamara, Kelsey¹, Bryan Anderson¹, Jesus Ochoa-Rodriguez¹, Michael Gardner¹, James Schmitt¹ (1) Montana State University, Bozeman, MT

Conglomerate channel deposits confined to 2 km wide slope valleys record traction transport in and deposition from subaqueous flows. Examination of 0.4 to 2 m thick sedimentation units reveals associations of 7 defined facies, including clast-supported massive conglomerates with normal to inverse grading and fining- to coarsening-upward stratification, with regular massive ungraded beds. Framework clasts include rounded metamorphic, igneous and sedimentary cobbles and pebbles with rare angular bioclast sandstone boulders. Fine- to medium, moderately- to poorly-sorted sand matrix represents 5 to 50% of a sedimentation unit. These facies attributes show conspicuous lateral changes within individual beds over distances of one or more meters. Despite abrupt lateral variability, the vertical succession consists of similar facies reflecting restricted movement within channel confinements. Sedimentation was characterized by transitioning debris flows, hyperconcentrated flows, and high- and low-density turbidity currents acting coevally to produce a sedimentation unit. Lateral changes in texture and geometry of conglomerate beds reflect changes in the axis or margins of the depositing flow. Evidence of traction along flow axes reflects nonuniform turbulent flow, whereas massive deposits suggest cohesive behavior. Composition, grain size, and abundance of matrix are laterally consistent, with variation in grading and stratification. Lateral and longitudinal changes in conglomerates reflect transitions in sediment support mechanisms within subaqueous flows. Assigning a specific process to a sedimentation unit is problematic due to lateral variability occurring within the same unit. Designating dimensional attributes (e.g. length, width, shape) to deposits based on 1D interpretation of cohesive vs. turbulent flow behavior should be avoided.