Tidal controls on the formation of fine-scale sedimentary strata near the Amazon river mouth

The Amazon river mouth provides a dynamic setting for studying the formation of sedimentary strata under conditions where fluvial and marine processes merge. River-mouth anchor stations were occupied for diurnal tidal cycles during three stages of river flow, and reoccupied for consecutive spring and neap tides during two stages of river flow. At each anchor station, box cores were collected every two hours and complementary 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 cross-laminated sand layer is present at the seabed surface that exhibits varying degrees of bi-directional current structure and contains low porewater salinities (10–15‰). During low-energy periods in the fortnightly cycle (i.e., most neap tides), a mud bed forms at the seabed surface in association with fluid muds in the water column. This mud layer ranges in thickness from 2 to 15 cm, and exhibits higher radionuclide activities, higher porewater salinities (15–25‰), 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 mud < 1 cm in thickness) can be produced semidiurnally due to tidal fluctuations, as well as fortnightly during periods of moderate tidal energy. Interbedding is entirely a fortnightly feature. Low-energy fortnightly periods produce a more stratified water column, and fluid mud forms as a result of estuarine circulation under conditions of decreased vertical mixing. High-energy fortnightly periods increase mixing and destratify the water column, thus resuspending part or all of the fluid mud and producing bedload transport of sand. The fine- and coarse-grained beds are both preserved, forming interbedded (alternate layers > 1 cm in thickness) muds and sands. The thickness of the fortnightly beds 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, and produce similar types of interlayered sediments due to the presence of estuarine cir