Abstract Several elongated and other crescent-shaped impressions interpreted as dinosaur tracks and preserved on Lower Cretaceous tidal flat deposits from the Agrio Formation (northern Patagonia, Argentina) are studied in detail. These tracks were documented on a palaeosurface showing palaeotopographic differences over a short distance, related to the lateral migration of a tidal channel. A genetic order between biostabilization and bioturbation of the palaeosurface is recognized. Ripples generated within the channel during the flood tide were first biostabilized; there, horseshoe crab trace fossils were produced. Then, the area bearing the tracks studied here was subaerially exposed, with the biostabilized substrate generating a slippery surface. Scanning electron microscope analysis of the rims indicates that the microbial mat was disturbed by the trampling. Finally, microbial mats recovered, and some tracks preserved wrinkle marks inside the tracks. Taking into account the substrate properties and track features analysed (morphology, size, depth and orientation), it is concluded that the tracks were produced by dinosaurs, probably small sauropods, trampling on a slippery surface and moving parallel to the channel margin. This study is an unusual case of slipping tetrapod tracks preserved on a biostabilized tidal flat developed in a mixed carbonate–siliciclastic environment.

Abstract Over the last decade, a new exploration frontier has opened up in Patagonia, Argentina, for gold mineralization. Important deposits, such as Cerro Vanguardia and Esquel, were discovered during this period. Despite these discoveries, basic geologic information for the vast Patagonia territory is still markedly scarce for generating targets for mineral exploration. In such a scenario, in which a prevailing arid climate and sparse vegetation often leave favorable bedrock exposed, multispectral satellite remote sensing is playing an important role in gold prospecting. This paper presents the results of a remote sensing project developed over northern Patagonia with the purpose of mapping potential areas of epithermal gold mineralization. A reconnaissance program using Landsat Thematic Mapper (TM) imagery to search for favorable targets on a regional scale was carried out, focusing on the mapping of possible hydrothermal alteration zones. The effectiveness of the results is evidenced by the fact that the majority of the hydrothermally altered targets pinpointed using Landsat TM showed anomalously high gold grades after follow-up geochemical analysis. Within the scenes analyzed, the region of Los Menucos stood out as having a high number of conspicuous altered areas carrying gold values greater than 0.1 g/t (some greater than 1 g/t). With the launch of Terra in 1999, carrying onboard the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), the Los Menucos region was further investigated to map alteration. A suite of advanced image processing methods was applied to the data, with the purpose of detecting and identifying key alteration minerals. The results accomplished in the Los Menucos region with Landsat TM and ASTER data demonstrate the capabilities of multispectral remote sensing for alteration mapping in frontier exploration regions, ranging from regional scale to district and deposit scales.

Abstract Ostracoda are small entomostracan Crustacea that are abundant in marine and nonmarine aquatic environments and that are excellent microfossils in paleoenvironmentalreconstruction (Whatley, 1983, 1988).They are extensively employed in this role in Quaternarynonmarine environments (Whatley, 1983;Aguirre and Whatley, 1995).In this paper, we present a review of studies of late Quaternary lacustrine ostracods from northern Patagonia, based on the analysis of numerous samples from lake bed cores and from outcrops along an approximately west-east transect near Mount Tronador (lat.41 00'S; long. 71 50'W) to Lake Cari-Laufquen (lat.41 12'S; long. 69 25'W). This transect has been the subject of a multidisciplinary study called the Lagos ComahueProject, which has the objective of contributing to our knowledge of the evolution of the climate, the environment, and the biota from the late Pleistocene through the Holocene of the cordilleran and patagonianmarginal areas as part of our understanding of global change. There is a large and growing body of literature on the Ostracoda of Argentina, but only a small part ofthis work is devoted to the ostracods of Quaternarylake deposits. Among such papers are those by Daday(1902), Ramirez (1967), Whatley and Cholich (1974),Zabert (1981), Zabert and Herbst (1986), Bertels andMartinez (1990), Aguirre and Whatley (1995), Cusminsky(1994, 1995), Cusminsky and Whatley (1995,1996),and Whatley and Cusminsky (1995a, b).

The magmatic and tectonic evolution of northern Patagonia are reviewed, and new data are presented from a transect through the Andes between 39 and 42°S. Four main episodes of calc-alkaline granitoid magmatism are recognized: ca. 450 to 370 Ma, ca. 350 to 280 Ma, 220 to 200 Ma, and 180 to 10 Ma (Andean). Only the Andean structural trend is north-south, parallel to the coastline and prominent active strike-slip faults. The three older phases are all best developed within the stable continental region of the North Patagonian Massif, where they were emplaced under the control of north-northwest–south-southeast to northwest-southeast structural lineaments. Their outcrops can be traced into the Andes, and the two younger belts appear to emerge on the western side, in the Chilean Lake Region and the Nahuelbuta Mountains, respectively. Most of the older granitoids are geochemically more evolved and have higher initial 87 Sr/ 86 Sr ratios (typically 0.706 to 0.710) than Andean granitoids, indicating greater crustal involvement in their pedogenesis. Neither the structural relationships observed, nor details of the timing and geochemistry of plutonism, provide definite support for the hypothesis of a pre-Mesozoic allochthonous Patagonian terrane separated from the South American continent to the north by subducting ocean floor.