Seismic Stratigraphy and Global Changes of Sea Level: Part 3. Relative Changes of Sea Level from Coastal Onlap: Section 2. Application of Seismic Reflection Configuration to Stratigrapic Interpretation

Relative changes of sea level can be determined from the onlap of coastal deposits in maritime sequences. The durations and magnitudes of these changes can be used to construct charts showing cycles of the relative rises and falls of sea level. Such charts summarize the history of the fluctuations of base level that control the distribution of the sequences and the strata within them. A relative rise of sea level is indicated by coastal onlap, which is the landward onlap of littoral and/or nonmarine coastal deposits. The vertical component, coastal aggradation, can be used to measure a relative rise, but it should be adjusted for any thickening due to differential basinward subsidence. During a relative rise of sea level, a transgression or regression of the shoreline, and a deepening or shallowing of the sea bottom may take place. A common misconception is that transgression and deepening are synonymous with a relative rise, and that regression and shallowing are synonymous with a relative fall. A relative stillstand is indicated by coastal toplap; intermittent stillstands between rapid rises are characteristic of a cumulative rise. A relative fall of sea level is indicated by a downward shift in coastal onlap from the highest position in a sequence to the lowest position in the overlying sequence. After a major relative fall of sea level, the shelf tends to be bypassed, and the coastal onlap may be restricted to the apex of a fan at the basin margin. Seismic sections provide the best means of determining the onlap and toplap patterns within the depositional sequences, and well control can provide the determinations of coastal and marine facies. Each cycle is plotted on a chart in chronologic order, dating and measuring the relative rise by increments of coastal aggradation, dating any relative stillstands by the duration of coastal toplap, and dating and measuring the relative fall by the downward shift of coastal onlap. Seismic examples illustrate the procedures and some of the problems encountered.