Sequence
Stratigraphy in Tectonically Active Basins-is it Possible? An Example from the Ridge Basin, Southern
California
Sullivan, Morgan D.1, Kenneth
D. Ehman2 (1) Chevron Energy Technology Company, Houston, TX (2)
Skyline Ridge, Inc, Los Gatos, CA
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.