The hypertidal Santa Cruz–Chico River estuary (South Patagonia, Argentina): A hybrid ria-type system under extreme tides, arid climate and active uplift

Abstract

The present study focuses on themorphosedimentary organization and sediment infilling stratigraphy of one of thelargest estuaries of southern Patagonia in Argentina.With a tidal range up to 12 m, the area is subject to extreme tidalconditions, combined with moderate offshore wave climate, strong and constant westerly winds, and contrastedwater and sediment discharges fromthe two tributaries of the estuary, the Santa Cruz and Chico rivers. The estuarinevalley is entrenched in the Patagonian coastal plateau due to significant uplift. On the basis of sediment facies(sedimentary structures, grain size, geochemistry, mineralogy), meiofauna (foraminifera and testate amoebae),morphological changes and shallow geophysics (high-resolution seismic reflection, ground-penetrating radar)data, the Santa Cruz–Chico River system is defined as a hybrid system comprising a tide-influenced fluvial mouth(the Santa Cruz River) and a tide-dominated estuary (the Chico River estuary), both converging toward an elongatedsubtidal ria-type estuarine basin. River-supplied sands and muds by-pass the estuarine basin and are exported offshorewherethey settle and form an ebb-tidal delta. Sediments in the Santa Cruz–Chico River valleymainly consist ofPleistocene lowstand fluvial gravels resting on the regional Miocene substrate, and thin earlyHolocene transgressivedeposits, deeply incised by a tidal ravinement surface that developed during the highest Holocene sea-level at ca7500 y. BP. After the maximum stillstand, relative sea level fell and a competition occurred between erosion, promotedby water depth decrease, and deposition, favored by tidal prism reduction. At present, sediment by-passingand offshore sediment export are the dominant processes. The very large size of the ebb-tidal delta, which expandson the continental shelf, suggests that this situation has prevailed for a very long time.