The sedimentary structure of the continental terrace of the Costa de Nayarit, on the west coast of mainland Mexico, has been investigated geophysically by means of continuous acoustic reflection profiling. Facies interpretations are made from these records and profiles on the basis of shape, attitude, nature of internal reflecting horizons, and relationship to adjacent facies.

The framework of this continental terrace is composed of a sequence of wedges of deltaic sediments, interspersed with strata of other paralic and open-shelf facies, which grade seaward into continental-slope facies. The deltaic platforms are correlated and matched with flanking erosional shorelines to obtain a chronology of deltaic progradation to the edge of the terrace. Low positions of sea level below -65 fathoms are postulated for early Wisconsin time. Younger sequences, believed to be of late Wisconsin age, were formed during fluctuations of sea level between -35 and -68 fathoms. The youngest low stand of sea level at -68 fathoms is dated 17,600 years B.P.

The northern part of this terrace is nondeltaic and consists of a thick section of open-shelf sediments grading seaward over a deep, rounded shelf-break into a thick section of slope sediments. Faulting in the reflection records shows that this thick section was deposited on a subsiding platform. It is concluded that shelf and slope deposits are volumetrically very important in regions like the Costa de Nayarit.

The break in slope at the edge of the southern and central Costa de Nayarit shelf is very complex in origin, but is primarily controlled by Pleistocene deltas. The edge of the northern subsiding portion of the shelf is also depositional in origin, but is only indirectly controlled by lowered sea level. Long-term balance between deposition and removal of sediment, controlled by wave action well below surf-base (about 5 fathoms), and by surf erosion during past transgressions and regressions, must form terrace structures of this northern type. These may have been processes of geological importance in pre-Quaternary time. Thus the explanation for the world-wide origin of the shelf-break as caused primarily by surf erosion during lowered sea level appears oversimplified.