TIDAL CONTROLS ON THE FORMATION OF FINE-SCALE SEDIMENTARY STRATA NEARTHE AMAZON RIVER MOUTH

The Amazon river mouth provides a dynamic setting for studying the for mation of sedimentary strata under conditions where fluvial and marine processes merge. River-mouth anchor stations were occupied for diurna l tidal cycles during three stages of river flow, and reoccupied for c onsecutive spring and neap tides during two stages of river flow. At e ach anchor station, box cores were collected every two hours and compl ementary time-series measurements were made of water-column suspended- sediment concentrations, salinity, and current velocities. During high -energy periods in the fortnightly cycle (i.e. most spring tides), a c ross-laminated sand layer is present at the seabed surface that exhibi ts varying degrees of bi-directional current structure and contains lo w porewater salinities (10-15 parts per thousand). During low-energy p eriods in the fortnightly cycle (i.e., most neap tides), a mud bed for ms at the seabed surface in association with fluid muds in the water c olumn. This mud layer ranges in thickness from 2 to 15 cm, and exhibit s higher radionuclide activities, higher porewater salinities (15-25 p arts per thousand), and lower saturated bulk densities (1.18-1.30 g/cm (3)) than the sand beds (1.30-1.60 g/cm(3)). The mud beds are subject to resuspension and deposition by semidiurnal tidal currents that form thin sandy interlaminations. Interlamination and interbedding of sand and mud result from the combination of estuarine and tidal processes at the river mouth. Interlaminations (alternating layers of sand and m ud <1 cm in thickness) can be produced semidiurnally due to tidal fluc tuations, as well as fortnightly during periods of moderate tidal ener gy. Interbedding is entirely a fortnightly feature. Low-energy fortnig htly periods produce a more stratified water column, and fluid mud for ms as a result of estuarine circulation under conditions of decreased vertical mixing. High-energy fortnightly periods increase mixing and d estratify the water column, thus resuspending part or all of the fluid mud and producing bedload transport of sand. The fine- and coarse-gra ined beds are both preserved, forming interbedded (alternate layers >1 cm in thickness) muds and sands. The thickness of the fortnightly bed s is dependent upon monthly variations in spring/neap amplitudes. The processes active near the Amazon river mouth that form interlaminated and interbedded sediments operate in other fluvial-marine settings, an d produce similar types of interlayered sediments due to the presence of estuarine circulation, high suspended-sediment concentrations, and tidal energy.