The boundary of Larsen Basin on Tabarin Peninsula, Antarctica

The Larsen Basin is the depositional area including all the upper Mesozoic-lower Cenozoic sedimentary rocks on the continental shelf of the northern Antarctic Peninsula. The western boundary between the basin fill and the rocks forming its technical basement, is not exposed, and was defined on the basis of aeromagnetic data. On petrological grounds, the Mesozoic marine sedimentary rocks exposed on western Tabarin Peninsula, at the north-eastern extremity of the Antarctic Peninsula, are assigned to the lower part of the Aptian-Eocene regressive megasequence that forms the basin filling. These rocks are faulted against sediments of the Antarctic Peninsula magmatic arc, suggesting that post- depositional tectonic movements probably occurred. This compressional deformation is assigned to mid-Cretaceous times, when the Coniacian partial basin inversion was accompanied by westward-verging deformation at the western basin margin. Although the original extent of the basin cannot be discerned from these data, the lithostratigraphical evidence and geographical position of the basin sediments, together with terrestrial magnetic data, confirm that the Larsen Basin extends onto the southern part of Tabarin Peninsula, and indicate that the western limit of the basin is located northward of the previously proposed limit. © 2004 Asociación Geológica Argentina.Fil:del Valle, R.A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Scasso, R.A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Nanofósiles calcáreos, duración y origen de ciclos caliza-marga (Jurásico tardio de la Cuenca Neuquina) Calcareouos nannofosils, duration and origin of limestone-marl cycles (Late Jurassic, Neuquén Basin)

A nan-nofossil association corresponding to Zone NJ20b was defined in the rhythmically interbedded limestones and marls of the Los Catutos Member, Vaca Muerta Formation. It confirmed the Late Middle Tithonian age, previously assigned on the basis of ammonites, to the beds which crop out in the locality of Zapala, Neuquén Province, Neuquén Basin. The stratigraphical resolution of the nannoflora was no better than that of the ammonite fauna, but a good correlation between zones can be established. This represents a powerful and useful tool for future biostratigraphical studies in the Neuquén Basin. On the basis of the ammonite and nannofossil zones, a period of 1 my was estimated for the deposition of the 25 m thick sequence of limestones and marls. From these data a high sedimentation rate (5 m/ka on average) was calculated. As a consequence, the duration of the individual limestone-marl cycles is about 20 ka on average, in good agreement with the precession period of the earth axis. Thus, rhythmic changes in sedimentation would be related to climatic changes driven by orbital forcing. © 1999 Asociacián Gcológica.Fil:Scasso, R.A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Concheyro, A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Nanofósiles calcáreos, duración y origen de ciclos caliza-marga (Jurásico tardío de la Cuenca Neuquina)

Calcareouos nannofosils, duration and origin of limestone-marl cycles (Late Jurassic, Neuquén Basin). A nannofossil association corresponding to Zone NJ20b was defined in the rhythmically interbedded limestones and marls of the Los Catutos Member, Vaca Muerta Formation. It confirmed the Late Middle Tithonian age, previously assigned on the basis of ammonites, to the beds which crop out in the locality of Zapala, Neuquén Province, Neuquén Basin. The stratigraphical resolution of the nannoflora was no better than that of the ammonite fauna, but a good correlation between zones can be established. This represents a powerful and useful tool for future biostratigraphical studies in the Neuquén Basin. On the basis of the ammonite and nannofossil zones, a period of 1 my was estimated for the deposition of the 25 m thick sequence of limestones and marls. From these data a high sedimentation rate (5 m/ka on average) was calculated. As a consequence, the duration of the individual limestone-marl cycles is about 20 ka on average, in good agreement with the precession period of the earth axis. Thus, rhythmic changes in sedimentation would be related to climatic changes driven by orbital forcing. © 1999 Asociación Geológica Argentina.Fil:Scasso, R.A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Concheyro, A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

The Upper Cretaceous-Paleogene from the Río Bueno area, Atlantic coast of the Isla Grande de Tierra del Fuego

A composite section, 1400 m thick, is established for the Upper Cretaceous-Paleogene strata in the fold and thrust belt of the Andes Fueguinos, south of 54° 30′S. The basal Policarpo Formation, > 350 m thick, consists of tuffaceous sandstones and bioturbated sandstones and siltstones. The ammonites Maorites densicostatus and Diplomoceras sp.; foraminifera common to the Gaudryina healyi Zone; and the dynocysts Manumiella seelandica and Operculodium cf. azcaratei indicate a Maastrichtian age. The Paleocene to lower Eocene include four new formations. The Cabo Leticia Formation, c.150 m, Paleocene, consists of gravity flows deposits: brecchias; conglomerates; and massive, tuffaceous sandstones. La Barca Formation, c. 220 m, includes two members: LB1, tuffaceous sandstones and intercalated carbonaceous siltstones; and LB2, black mudstones with Palaeocystodinium golzowense and Spiroplectammina spectabilis. P. golzowense and the Bulimina karpatica Assemblage in LB1 indicate a late Paleocene age. Punta Noguera Formation, 380 m, is dominated by glauconite rich, massive, tuffaceous sandstones with interbedded turbidite packages. The dynocyst group Apectodinium, Deflandrea robusta, Palaeocystodinium sp., and Odontodinium askinae; the foraminifera Alabamina creta, Charltonina acutimarginata, Valvulineria teurensis and the first ocurrence of Elphidium and Cribrorotalia suggest an age near the Paleocene/Eocene boundary. The Cerro Ruperto Formation, 200 m, is dominated by glauconite rich, silty very fine sandstones and siltstones; dominance of Deflandrea dartmooria indicate an early Eocene age. Resting on angular unconformity, the Río Bueno Formation, c. 60-80 m, consists of carbonate rocks; its member RB1, rhythmically bedded grainstones, with the planktic foraminifera Planorotalites australiformis and Subbotina linaperta is of early middle Eocene age; and its member RB2, regular alternation of grainstones and bioturbated marls and micrites, with Elphidium saginatum and Bulimina cf. bortonica, is assigned to the mid middle Eocene. The sedimentary and stratigraphic features of the Maastrichtian/Eocene have a strong tectonic control, suggesting that the foreland phase of evolution of the basin has been active at least since the Maastrichtian.Fil:Olivero, E.B. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Malumián, N. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Scasso, R.A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Petrology and geochemistry of a limestone-marl rhythmite from the Southern Hemisphere: The Los Catutos Member (Vaca Muerta Formation). Middle Tithonian of Neuquén Basin

A 25 m thick, rhythmic, limestone-marl alternation of Los Catutos Member (Vaca Muerta Formation, Neuquén Basin), was studied bed by hed in several quarries near Zapala. Beds up to 1 m thick are of massive or laminated, lithographic, fossiliferous limestone, locally mottled or weakly bioturbated (?Chondrites), interbedded with thinner marl beds. Limestones and marls can be classified as pelbiomicrites and biopelmicrites, mainly composed of pellets, radiolaria, forams, ostracods, equinoids, spicles of sponges and gastropods, with minor amounts of peloids, feldspar, chert, muscovite, glauconite and phosphates. Values for δ18O and δ13C are similar to those of lithographic limestones from the northern hemisphere. δ13O values are most probably result of diagenetic modification of the primary signal but the δ13C agrees well with the published curves for the Tithonian sea water and with other records from Tethyan limestones. A preliminary analysis of negative excursions of δ13C point to productivity crisis in the sedimentary system with a periodicity of 400 ka, which can be a result of changes in the orbital eccentricity of the earth. Smectite and interlayers reflect pedogenic processes developed on volcanogenic source-rocks, whereas more abundant kaolinite in marls might reflect more humid conditions. With TOC up to 1.95% the rocks offer prospects as a regular to good source rock for hydrocarbons, although thermally immature. A low-energy, dysaerobic, open-marine environment is inferred for Los Catutos Member, with high sedimentation rates as a result of high productivity at the sea-surface and incoming plumes of suspended sediment from the inner shelf. Sediment accumulation in wide and gently dipping calcareous bulges in an outer ramp system is envisaged.Fil:Scasso, R.A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Alonso, M.S. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Lanés, S. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Lippai, H. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Marine Phosphates in the Paleogene oflsla Grande de Tîerra del Fuego

Phosphate prospecting in marine sedimentary rocks of the Marginal and Austral basins of the Isla Grande de Tierra del Fuego resulted in the first discovery of phosphatic beds in the Paleogene Rio Claro Formation. Phosphatic anomalies of up to 20.1% P2U5 in concretions and up to 2% PaOs in bulk rock offer promising opportunities for systematic prospection on the Isla Grande de Tierra del Fuego. The phosphatic beds are conglomerates, coquinas and conglomeratic sandstones interbedded with thick mudstone packets. They belong to the condensed-type phosphatic beds, which are the product of erosion and reworking of concretions developed in fine sediments. Phosphate concretions containing glauconite, together with abundant glauconite in the same and closely associated beds, indicate a common syn-sedimentary to earlydiagenetic origin for both minerals, via recycling of sedimentary iron. This process occurred on a shelf with low sedimentation rates, pointing to high sea-level stands and marked interruptions in the sedimentation of the thick Paleogene sedimentary sequence of the Austral Basin . The deposit point to the existence and extension of this early Cenozoic phosphogenic episode to high latitudes in the Southern Hemisphere. © 1998 Asociacidn Geoldgica Argentina.Fil:Olivero, E.B. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Scasso, R.A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Miretzky, P. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina