Abstract

The Miocene Monterey Formation and deepwater channels of the Mio-Pliocene Capistrano Formation crop out at Dana Point Harbor (California) and are locally separated by a series of extruded sandstone bodies. Stratigraphic columns, photo panels, and high-resolution lidar data are used to describe the stratigraphy of the 840-m-wide, 30-m-thick exposure. The studied exposure contains: (1) a lower unit composed of Monterey slope mudstone, (2) an up to 13-m-thick, discontinuous middle unit of structureless sandstone bodies, and (3) an upper sandstone unit composed of the laterally and vertically stacked breccia-, conglomerate-, and sand-filled channels of the Capistrano Formation.

Several observations collectively suggest that the structureless sandstone bodies in the middle unit are remobilized: (1) The lower mudstone unit contains centimeter- to meter-scale injections of structureless, poorly sorted, coarse-grained sandstones. (2) The facies in the middle sandstone unit are identical to the injected sandstones facies observed in the lower mudstone unit. (3) The structureless sandstone bodies of the middle unit form disconnected bodies below the upper sandstone unit and have an average aspect ratio of 4:1 whereas the Capistrano channels have an aspect ratio greater than 10:1. (4) Quartz and feldspar grains in the injected sandstone of the lower unit and the structureless sandstone of the middle unit are fractured. (5) The structureless sandstone bodies of the middle unit contain meter-scale angular mudstone clasts of composition similar to that of the Monterey mudstones. (6) These clasts contain internal sandstone injections and are deformed. (7) The Monterey strata around the structureless sandstone bodies display distinct variations of strike and dip relative to the regional trend.

The remobilized sandstone bodies forming the middle unit are interpreted as extruded sandstone bodies that incorporated large mudstone clasts (rafts) during the migration from a deep sand source interpreted as being a lower sandy Monterey interval. These sand extrusions were emplaced at the seafloor in multiple phases, following slope channel incision and preceding channel-fill deposits. These extruded fluidized sands triggered local syndepositional deformation of the channel fills in the form of large load structures. The processes that caused the source sands to fluidize are interpreted to be related to deep fluid migration toward a structural high located below the study area. The mechanism triggering the upward migration of sand is likely related to slope channel incision.