COMPARATIVE GENESIS OF BIOCONCRETIONS: PALEOENVIRONMENTAL IMPLICATIONS. The aim of this study is to describe and morphogenetically and environmentally compare bioconcretions found in a continental environment at Quequén Salado with those found in a coastal environment at Playa Chica; both localities are in Buenos Aires Province. The first section —on the bank of the Quequén Salado river— is a 1.80 m thick bed of a late Pleistocene—Holocene siltstone with a sand to clay grain-size. These bioconcretions reach up to 25 cm long and variable diameters ranging from 1 to 5 cm. They are vertically oriented with ascending branches and associated with frustules of freshwater diatoms. The second analyzed profile is located at Playa Chica, on the northern shores of the Río Negro. The 2.40 m thick layer with bioconcretions is divided into 5 to 10 cm with subhorizontally laminated rhythmite-like beds at very low angle. The BC are yellowish to dark-reddish brown in color, subcircular, well-consolidated, accompanied by marine to brackish-water diatoms, euryhaline ostracods and foraminifera. Regarding the origin of the bioconcretions it was possible to establish that the Quequén Salado concretions were originated in a seasonally flooded, fluvial environment dominated by groundwater fluctuations. The Playa Chica bioconcretion deposits show wet periods with a stable water table. The sequence of several bioconcretion layers with successive growth is evidence of changing conditions with flooding and drainage of marine water levels in the basin, within a period of increasing aridity.
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Vol. 50 • No. 5