Calcretes and Cryogenic Structures in the Area of Puerto Madryn (Chubut, Patagonia, Argentina)

Abstract

lcretes closely linked with fossil ice-wedge pseudomorphs in Pleistocene alluvial deposits are widespread in Patagonia from Rio Negro (Lat. 40°S) to Rio Gallegos (Lat. 53°S). As carbonate rocks are absent in the region, the questions to be asked are: where did the calcium carbonate come from, how did it accumulate, what does this relationship between calcium carbonate and cryogenic structures mean?Two Pleistocene alluvial levels (Puerto Madryn. at 90–100 m asl, Montemayor, 270 m asl) are described here using sedimentological, micromorphological, and chemical analyses. By comparing the characteristics of the alluvial bodies, ice-wedge cast infilling, and upper massive calerete it appears that there is no common origin of the alluvial deposit and the carbonate accumulation. This one is a mix of allochtonous silt and calcareous dust of littoral origin, blown and deposited by wind. Its close association with cryogenic structures as well as some 14C datings demonstrate that it accumulated during cold periods, corresponding to marine regressions and the emergence of the highly carbonated continental platform. In South America, for a sea level lowering 100–120m, the continental shelf widened 400 to 1000km. The rim of strong easterly katabatic winds migrated northwards, sweeping the exposed platforms.This interpretation fits in well with the results obtained by geophysicists working on Antarctic ice cores, which show that during cold maxima, characterized by an aridification of climate, an increase of wind strength and the emergence of the continental platform, the aerosol amount increases 5 to 30 times. Isotopic analyses demonstrate that a large part of the aerosols collected in East Antarctic ice-cores came from South American continental shelves.If these conclusions cannot be systematically extrapolated to all calcretes, they should at least be taken into consideration for the explanation of the origin of Pleistocene calcretes.