Ups and downs of the Mississippi Delta

Abstract

During the last glacial period, when sea level was low, meltwater discharge drove incision of the lower Mississippi valley, with valley filling and delta construction during Holocene sea-level rise. Isostatic modeling shows that sediment volumes removed and replaced were sufficient to induce uplift of >9 m along valley margins followed by subsidence of the same magnitude, with effects dissipating only over distances of >100–150 km along the Gulf of Mexico coast. Recognition of cyclical uplift and subsidence refutes recent interpretations of delta stability, and suggests that late Holocene relative sea-level curves from the delta region are instead a record of subsidence of the pre-Holocene depocenter. More broadly, incised valley cutting and filling is a common fluvial response to glacioeustasy, and cyclical uplift and subsidence should be common to large alluvial-deltaic systems elsewhere.