Controls on submarine channel architecture, upper Miocene -lower Pliocene Capistrano Formation, San Clemente State Beach, California

Abstract

Outcrops of the upper Miocene - Pliocene Capistrano Formation in San Clemente State Beach provide near continuous exposures of nested sandstone channel bodies. Numerous outcrop studies have reached varying conclusions on fundamental characteristics of the outcrop. Varying interpretations of channel stacking, confinement, hierarchy, and gradient profile position reflect the uncertainty associated with three-dimensional interpretation of a two-dimensional outcrop. The northwest - southeast oriented cliffs range from 3 to 15 m in height and 2.3 km in length. The study focuses on three outcrop segments oriented sub-parallel to northwest (341°) paleoflow. The southernmost segment has been the subject of the most intense study while the two northern outcrops have only recently been incorporated. Eleven sedimentological sections totaling 124 meters, and two kilometers of outcrop photo panel were integrated and interpreted within a hierarchy of sedimentary attributes. A new channel hierarchy for Capistrano Formation is proposed where the outcrop belt at San Clemente State Beach in interpreted to represent a single upper-slope channel fairway. Three sedimentation regions are defined by sedimentary facies, sedimentation units, and sedimentary bodies and surfaces, which record varying degrees of confinement on the upper-slope profile position. Four channel complexes represent allogenic phases of sedimentation recording the initiation, growth, and retreat of the channel fairway. Each channel complex is composed of multiple composite channels that represent repeated cutting and filling episodes of autogenic sedimentation. Within the composite channel, elementary channels represent channel thalweg migration within an open channel course.