Two 1.3 m gravity cores of cohesive clayey silt with very high plasticity were raised from the north and south parts of the deep basin in Golfo San Matias, a gulf located about 900 km southwest of Buenos Aires, Argentina. Rivers do not flow into this gulf, which has a maximum water depth of about 194 m. Movement of water between the gulf and the adjacent ocean is restricted by a sill at a depth of 60-66 m. Sediment grain-size averaged 47% clay, 50% silt, and 3% sand; montmorillonite is the predominant clay mineral. Water content ranged from 211% dry weight at the surface to 163% at the bottom of the cores, and averaged 186%. Liquid and plastic limits averaged 130% and 55%, respectively. Bulk density varied from 1.26 to 1.32 Mg/m 3 , and averaged 1.28 Mg/m 3 (1.28 g/cm 3 ). Shear strength ranged from 0.78 to 3.8 kPa, and averaged 2.2 kPa. Sediment sensitivity varied between 3 and 11, and averaged 6. Calcium carbonate content averaged 6%. Consolidation behavior, shown by sedimentation-compression curves, is unusual because the void ratio remained relatively constant at a value of about 5 with respect to increasing effective overburden pressure. From one laboratory consolidation test the compression index was about 2.4. The over consolidation ratio was computed to be 2.3, which is usual for a small burial depth. The distribution of the isobaths indicate probable movement of sediments originating from the Rio Negro or the Rio Colorado from west to east into the basin, which may be filling rapidly.