Slope failures and timing of turbidity flows north of Puerto Rico

The submerged carbonate platform north of Puerto Rico terminates in a high (3,000–4,000 m) and in places steep (>45°) slope characterized by numerous landslide scarps including two 30–50 km-wide amphitheater-shaped features. The origin of the steep platform edge and the amphitheaters has been attributed to: (1) catastrophic failure, or (2) localized failures and progressive erosion. Determining which of the two mechanisms has shaped the platform edge is critically important in understanding landslide-generated tsunami hazards in the region. Multibeam bathymetry, seismic reflection profiles, and a suite sediment cores from the Puerto Rico Trench and the slope between the trench and the platform edge were used to test these two hypotheses. Deposits within trench axis and at the base of the slope are predominantly composed of sandy carbonate turbidites and pelagic sediment with inter-fingering of chaotic debris units. Regionally-correlated turbidites within the upper 10 m of the trench sediments were dated between ∼25 and 22 kyrs and ∼18–19 kyrs for the penultimate and most recent events, respectively. Deposits on the slope are laterally discontinuous and vary from thin layers of fragmented carbonate platform material to thick pelagic layers. Large debris blocks or lobes are absent within the near-surface deposits at the trench axis and the base of slope basins. Progressive small-scale scalloping and self-erosion of the carbonate platform and underlying stratigraphy appears to be the most likely mechanism for recent development of the amphitheaters. These smaller scale failures may lead to the generation of tsunamis with local, rather than regional, impact.