Onlap sequences

High and low stands of sea level are directly recorded as sedimentary coastal onlap sequences and as erosional terraces. These records are complicated in regions of crustal instability, and the rate and nature of crustal deformation determines whether evidence of short-term or long-term sea-level fluctuations are preserved and how easily this evidence is interpreted. Because continental basement warps and fractures through time, and because evidence of sea level is erased by erosion, the interpretation of this evidence to produce sea-level curves for the Phanerozoic has been a subject of considerable debate. 

Phanerozoic trends In the 1970 s, Vail and colleagues (e.g., Vail et al., 1977) using seismic stratigraphic data to determine the extent of onlap of coastal deposits in marine sequences derived a relative sea level curve for the Phanerozoic. The latest representation of this curve is shown in Figure 10.13, and compared with that... 

Fig. 2. The three basic types of reflection terminations for upper and lower boundaries of seismic sequences. Upper boundary type (a) toplap. Lower boundary t5q)es (b) onlap and (c) downlap.

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