HOLOCENE PALAEOENVIRONMENTS IN PLAYA CHICA LANDSCAPE, RÍO NEGRO NORTH COAST, BUENOS AIRES PROVINCE, ARGENTINA

Abstract

A sedimentary profile located on the north bank of the Negro river (Argentina) about 7 km from its mouth is studied. The column, 5 m of thickness, is composed by four differential units. The first unit shows a net domain of the clayey silt fraction, in brown reddish color, with good stratification. It possesses a fossil content of stem concretions and polihalobious to mesohalobious diatoms with Paralia sulcata domain. Depositional environment would be a tidal flat, formed under seasonal cold weather with temperate to warm oscillations. The second section, with sands lithology in pinkish grey to greenish grey colour, has a laminate structure with high organic matter and calcium carbonate content. Fossil algae are represented by polihalobious to mesohalobious species as Paralia sulcata, Cocconeis  scutellum and Rhaphoneis surirella. This unit ends with a deposit with palustrine characteristics. The depositional environment is considered a high tidal flat with sporadic marine influence. The climate would have oscillated from humid cold to warm arid. The third section dominated by sandy fractions, has brown yellowish color, with thick stratification and high percentage of organic matter and calcium carbonate. Fossil content is represented by similar diatoms to that of the previous section. The sedimentary environment begins with a swamp, little developed, continuing with an alluvial plain to conclude with a buried floor, under a variable climate between humid and arid. The fourth section of continental origin, presents massive sands of greyish color and with diatomologic contents dominated by oligohalobious indifferent species. Depositional environment would correspond to an eolic of interdune with cold and arid climate and interdigitation in west direction with deposits fluvial marine clayey silt, limited in their roof and bottom by sandy levels with fine stratification. The comparative age of the group is assigned to Holocene, and it would represent a marine–continental cycle that would have begun about 6.500 years ago.