Marginal basins include those within the body of the continental terrace (shelf and slope basins) and those flanking the terrace where it is separated from the deep sea by intervening topographic highs. Geographically widespread investigations with continuous reflection profilers suggest that continental slope basins are more common than previously suspected, particularly in tectonically active regions. Because of well-known topography and surface-sediment distributions, selected California continental borderland basins can be used as natural laboratories to study details of internal structures of basin deposits. These are compared with records

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of normal open shelf and slope deposits from other regions. Early results suggest that basin plains are underlain predominantly by ponded turbidites with internal reflecting horizons of near horizontal initial attitude which conform to their flat featureless surface. Lateral continuity of these reflectors appears to be large compared to those within the gently sloping aprons and sea fans of the basins. Reflection profiles of the peripheral regions of the Tyrrhenian Sea show horizontally-bedded, probably ponded turbidites in closed slope-basins and hemipelagic sediments blanketing and conforming to underlying topography in open-slope areas. Similar features are recorded in profiles from the continental terrace and marginal basin of the East China Sea and other regions. Filled marginal asins are believed to be quantitatively important in retaining terrigenous sediments within the continental framework.

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