TECTONIC CONTROLS ON SEDIMENTATION AND STACKING PATTERNS: AN EXAMPLE FROM THE RIDGE BASIN, SOUTHERN CALIFORNIA

Upper Miocene sedimentary rocks of the Ridge Basin display a distinct cyclicity of sedimentation which has been interpreted to reflect deposition contemporaneously with movement along the San Gabriel Fault. Observed changes in stratal stacking patterns further suggests that the units of the Ridge Basin can be divided into depositional sequences or “kinematic cycles” related to cyclic fault movement. The beginning of each kinematic cycle is marked by a period of uplift and extensional related to fault movement followed by tectonic quiescence. This cyclic tectonic activity produces distinct parasequence set stacking patterns due to its influence on accommodation and sediment supply. Kinematic cycles can also vary in appearance within a given basin if relative sediment supply and accommodation are not constant, such as in the asymmetrically subsiding Ridge Basin. On the rapidly subsiding portion of the basin, the Violin Breccia is comprised on a lower progradational interval reflecting an initial period of high subsidence related to fault movement, but extremely rapid erosion of the uplifted footwall produced more sediment than accommodation created by tectonic activity. Retrogradation of the Violin Breccia occurred in response to waning tectonic subsidence and diminishing sediment supply from the uplifted footwall. In contrast, the kinematic cycles within the Ridge Route Formation, which is derived from the less tectonically active margin, appear out of phase. Kinematic cycles in the Ridge Route Formation are comprised of a lower retrogradational interval that reflects the initial tectonic subsidence. On this less tectonically active side of the basin, however, sediment supply does not exceed accommodation. Finally, in response to waning tectonic subsidence, the members of the Ridge Route Formation prograded into the basin due to decreasing accommodation.