THE SANTA BEACH RIDGE COMPLEX - SEA-LEVEL AND PROGRADATIONAL HISTORY OF AN OPEN GRAVEL COAST IN CENTRAL PERU

Over 4 km of coastal progradation produced the Santa beach ridge compl ex in central Peru. The complex is comprised of more than 15 individua l beach ridges that can be divided into four morphostratigraphic group s on the basis of ridge spacing and geometry. Each morphostratigraphic group accreted during periods of glacial contraction and increased me ltwater discharge in the headwaters of the drainage basin. The cross-s ectional morphology of individual ridges is a result of oceanographic conditions at the time of ridge accretion. Ridge crest height reflects the wave energy of the last storm event before the accretion of the s ubsequent ridge. The earliest ridges have subdued morphology indicatin g frequent overtopping and deposition during a period of rising sea-le vel. Ridge height peaks subsequent to sea-level stabilization and drop s off gradually indicating a relative decrease in wave height through time. A zone of lower slope at the base of the beach face of individua l ridges indicates the apparent tidal range during ridge formation. Ap parent tidal range varies from 1.00 to 2.12 m during ridge accretion. A sea-level curve was created using apparent tidal range elevations th rough the complex and radiocarbon age estimates on in situ and detrita l shell debris and driftwood. This model produces a sea-level curve th at peaks between 7.0 and 6.0 ka at about Im above present mean sealeve l and then remains relatively stable falling to the present level some time within the past 0.5 ka.