OCEAN-BASIN FLOOR PROVINCES AND CRUSTAL STRUCTURE

The results of seismic-refraction measurements in the ocean-basin floor can be divided into two categories depending on whether the measurements were made (1) in the abyssal floor, or (2) on an oceanic rise. Measurements in the abyssal floor of the western Atlantic (Ewing, Sutton, and Officer, 1954) revealed the simple pattern shown in Figure 35b and e—namely, that beneath 4-5 km of water lies .5-1 km of sediments and sedimentary rock with a compressional-wave velocity of about 2 km/sec, overlying 3-4 km of oceanic crustal rocks (6.5 km/sec); beneath this the sub-M mantle rocks show a velocity of about 8.1 km/sec. This pattern has been observed by most workers in the abyssal floors of other oceans (Raitt, 1957; Hill, 1956).

Officer, Ewing, and Wuenschel (1952) and Katz and Ewing (1955) have reported on the structure of the Bermuda Rise. The topographic change from abyssal floor to the Bermuda Rise is accompanied by a corresponding change in crustal structure (Fig. 35b and d).

A typical column measured on the Bermuda Plateau is shown in Figure 35d. Here the sub-M velocity appears to be lower, or possibly a new intermediate-velocity layer is inserted between the oceanic crust and the true mantle. In the Bermuda Rise seismic velocities in the oceanic crust differ somewhat from the typical abyssal-floor values. In the Bermuda Apron and adjacent parts of the Bermuda Plateau (Fig. 35c) above the oceanic crust a thick (up to 4 km) section of 4.5 km/sec velocity is found which has been quite reasonably identified as sedimentary and volcanic rocks.

Seismic-reflection measurements in the smoother parts of the ocean-basin floor fall into two general groups. Reflection records from the oceanic rises generally show a succession of reflections which can be correlated for considerable distances. Reflection records from the abyssal plains in general show many reflections which are usually impossible to correlate even between adjacent shots. This difference has been explained by Ericson, Ewing, and Heezen (1952) in terms of the distribution of turbidity-current deposits. In the abyssal plains relatively frequent turbidity flows have deposited an alternating sequence of clays and silts which return a great number of reflections to the reflection seismograph. In contrast the rises receive only pelagic sedimentation, and thus the layering of their sedimentary cover is simple and widespread since it relates to major changes in pelagic sedimentation of past geologic ages.

Figure 35.—Crustal sections in various physiographic provinces, determined by seismic-refraction measurements