Résumé
Rhythmites observed and cored in the hypertidal (12 m tidal range) Santa Cruz -Chico River estuary (Southern Patagonia, Argentina) are described. The estuary is made of two estuarine arms (the Chico River arm; the Santa Cruz River arm), both converging into an intermediate subtidal basin connected to the Atlantic Ocean. Tidal dynamics are dominant along the low fluvial discharge Chico River arm. In contrast, the water discharge of the Santa Cruz River is much higher so that the influence of tidal dynamics quickly drops down a few kilometers upstream. Extensive sandy to muddy tidal flats develop along the outer portion of the two arms. Upper intertidal sediment successions frequently reveal well-developed rhythmic layering, resembling tidal rhythmites. Cores (1m long) were retrieved in these successions, one from the tidedominated Chico River arm, one from the tide-influenced Santa Cruz River arm. Core analyses show that the Chico River sediment succession is made of a fairly regular stacking of typical semi-diurnal sand-mud couplets and neap-spring-neap tidal rhythmites, and represents a 3-year continuous record. A seasonal modulation is probably superimposed to the tidal signal. Semi-diurnal tidal couplets are visible on the Santa Cruz arm succession, but the fortnightly cyclicity is altered, due to the combined influence of fluvial dynamics, westerlies-induced wave agitation and aeolian sand transport. 210Pbexc measurements confirm for both cores very high sedimentation rates. Although 137Cs data are difficult to exploit due to the very low fallout in the Southern Hemisphere, they could suggest that the successions are several decades old, meaning that the studied tidal flats are no longer aggrading.