Acoustic horizons previously described in the Argentine Basin, southwestern Atlantic Ocean, have been correlated with the stratigraphic sequence drilled at Deep Sea Drilling Project site 358. Horizon B is associated with the transition from basal marly chalks to siliceous mudstones in the middle Eocene at 730 m subbottom. Horizon A, at 460 m subbottom, occurs in the upper Oligocene section and is correlated with a diagenetic boundary between diatom-radiolarian mudstones above and siliceous mudstones below in which biogenic opal has been converted to amorphous silica. Similarly, horizon F at 155 m subbottom correlates with a middle Miocene change from diatom-radiolarian mud above to siliceous mud below. Horizon A at site 358 in the Argentine Basin differs in age from a similar horizon in the Malvinas Outer Basin (site 328) and differs in age and character from horizon A in the North Atlantic. It is possible that silica diagenesis responsible for acoustic horizons A and F at site 358 occurred at or near the sediment-water interface during times of relatively slow sedimentation or that the degree of diagenesis is controlled by the composition of the host sediment.