Multi-event organization of deepwater sediments into bedforms: Long-lived, large-scale antidunes preserved in deepwater slopes

Long-lived, multi-event bedforms hundreds of meters in wavelength in outcrop are interpreted as deepwater antidunes, the first recognized of this nature. Deepwater sediment gravity flows commonly reach a Froude supercritical flow state, but interpretation of their deposits largely excludes antidunes, which are commonly assumed to be ephemeral. Well-exposed, extensive slope turbidites of the Fish Creek-Vallecito Basin of Southern California (USA) are organized into 3-10-m-thick bedsets of 20-30 distinguishable lenticular backset beds that build low-angle (<10 degrees), undulating geometries and accrete opposite to paleoflow. Bedsets lack high-angle geometries, deep scour surfaces, and structureless facies intrinsic to cyclic steps. Instead, bedsets are differentiated by rhythmic down-dip transitions from thin, subparallel fine-grained beds into thicker, inclined coarser-grained beds and back into thinner, flattening, and, in cases, downflow-dipping finer-grained beds. Within bedsets, compensationally stacked waveforms have similar to 3-7 m amplitudes and similar to 75-215 m wavelengths that increase upsection and are comparable to modern upstream-migrating sediment waves. Bioturbated fine-grained caps of each sand bed indicate that antidune bedforms evolved across multiple flow events. Recognition of antidunes in deep water can have important implications for paleoflow reconstruction.