A 11.5 ka paleo-seacliff left behind by Melt Water Pulse 1B sea-level rise

The 30-m thick mid-shelf mudbelt of the Santa Cruz continental shelf covers a notch in the underlying eroded bedrock that appears to be the remnant of a 11.5 ka seacliff. This paleo-seacliff is located midway between the present coastline and the shelf break, about 7 km offshore from Santa Cruz. Calcareous shell and wood debris that occur in the coarse material at the base of the muds on the outer shelf are dated at 14-15 ka Cal yr BP and confirm the mud deposit as Holocene. The depth range of the hypothesized paleo-seacliff, reflecting the former height of the cliff, is from 40 m at top to 70 m at base. Meltwater Pulse (MWP) 1B has been documented as quickly raising sea level to the -43 m level at 11.5 ka. We believe this sea-level event caused a 12-ka seacliff to be left behind and subsequently buried. The seismically-discernable gradient of the cliff face is 0.04, which is greatly diminished from its presumed original vertical slope. The implied cliff-retreat rate, from 11.5 ka to the present, for the area east of Santa Cruz where the bedrock consists of the relatively soft Purisima Formation is 600 mm/yr. West of Santa Cruz where the bedrock is the more resistant silica-rich Santa Cruz Mudstone, the paleo-seacliff is closer to shore and the implied retreat rate is 130 mm/yr. Today's cliff-retreat rates in the Santa Cruz area average about 200 mm/yr. In the simplest model, if the sea-level rise rate were equal to the cliff retreat rate during transgression of the cliff, the resulting cliff would be attenuated from vertical to 45°. The transgressive sea-level event that left the paleo-seacliff behind probably proceeded at a rate significantly less than the cliff retreat rate to account for the greatly attenuated slope. We therefore estimate a rate on the order of 100 mm/yr or less for the sea-level rise associated with MWP 1B. High-resolution seismic surveys of the world's shelves will undoubtedly reveal similar evidence of sea-level stands beneath today's high-stand muds, and these sea-level stands may reveal much about climate for the mid-late Holocene by showing shoreline configurations at locations seaward of present coastlines.