Eduardo B. Olivero

Mesozoic-Cenozoic stratigraphy of the Fuegian Andes, Argentina

The stratigraphy of the Argentinean Fuegian Andes reveals contrasting Mesozoic-Cenozoic tectonic regimes. The Lemaire Formation, submarine complex of bimodal volcanites, breccias, and sedimentary rocks; the Yahgan Formation, volcaniclastic apron of deep-marine andesite-rich turbidites and mudstones; and the Beauvoir Formation, slope mudstones, all evidence late Jurassic early Cretaceous extension and consequent origination of the Rocas Verdes Marginal Basin. The basal late Cretaceous ductile deformation, isoclinal folding, and metamorphism of these rocks indicate a compressional tectonic regime that resulted in the closure of the marginal basin. The Fuegian Andes were uplifted by the late Campanian; subsequent propagation of the compressional deformation and subsidence by tectonic loading along the northern orogenic margin originated the Austral and Malvinas foreland basins. The Turonian-lower Campanian “Estratos de Buen Suceso” represents the final stages of the closure of the marginal basin and/or the beginning of the foreland basins. During the late Campanian to mid Miocene, the foraminiferal assemblages allow precise dating of the main tectonic events and with the associated trace fossils and sedimentary facies, the general characterization of the depositional settings. The thrust and fold belt of the Austral/Malvinas basins includes three depocenters: 1) the Bahia Thetis (upper Campanian-Maastrichtian), Policarpo (upper Maastrichtian/Danian), and Tres Amigos (upper Paleocene) Formations, mostly turbidite settings with cosmopolitan agglutinated foraminifers; 2) the Rio Claro Group, upper Paleocene-lower Eocene; and 3) the La Despedida Group, representing the extended upper mid Eocene-upper Eocene transgression with endemic foraminifers. A fourth depocenter, the Oligocene-mid Miocene Cabo Domingo Group mostly deposited below the calcite compensation depth and characterized by widespread agglutinated foraminifers, represents the last and deepest foredeep, originated just north of the fold belt during the waning compressional phase. Reduced exposures of mid Eocene bryozoan limestones, the Rio Bueno Formation, and fluvial upper Eocene lower Oligocene, the Punta Cactus and Sloggett Formations, conform reduced exposures within the fold belt. The late Miocene-Pliocene, shallow marine “estratos de la Maria Luisa” and Irigoyen Formation, the latter with foraminifers suggesting an Atlantic-Pacific connection, record Cenozoic transcurrency and pull-apart basins.

Lithofacies, biofacies, and ichnoassemblage evolution of a shallow submarine volcaniclastic fan-shelf depositional system (Upper Cretaceous, James Ross Island, Antarctica)

The Upper Cretaceous (Santonian-Campanian/lower Maastrichtian) Santa Marta Formation on James Ross Island, Antarctica, represents volcaniclastic shallow marine fan and shelf sedimentation adjacent to an active volcanic arc. A combined analysis of sedimentologic, paleoecologic, and ichnologic data allows for the recognition in this unit of six lithofacies associations, eight biofacies, and five trace fossils assemblages. Lithofacies are dominated by fine, massive, tuffaceous rocks; graded, turbidite-like tuffaceous sandstones; carbonaceous mudstones; resedimented conglomerates; coquinas; sandstones; silty sandstones; and minor stromatolite beds. Biofacies are defined by different composition and relative abundance of elements of the benthic fauna, mainly bivalves, gastropods, and serpulids, with minor elements represented by scaphopods, corals, brachiopods, and echinoids. Trace fossil assemblages include the most common elements of the Skolithos and Cruziana ichnofacies. A striking result of the analysis is that lithofacies, biofacies, and trace fossil assemblages form distinct, non-repetitive, vertically successive horizons, with their distribution boundaries roughly coincident. On this basis, seven major facies groups, showing a distinct combination of lithofacies, biofacies, and trace fossils, are distinguished in the Santa Marta Formation. These non-repetitive, vertically stacked facies groups reveal a one-way evolution of the depositional system during a transgressive-regressive cycle, with a new transgression at the top of the unit. The lower facies groups represent shallow marine settings with a very high rate of volcaniclastic sedimentation within subsiding basin. Shallow, volcaniclastic fan systems were probably formed at the base of delta slope and grew rapidly as a consequence of high sedimentary supply in equilibrium with basin subsidence. The upper facies groups probably represent sedimentation within the marine part of the envisaged deltaic system on a more stable shelf with diminished volcaniclastic sedimentary input.

Estratigrafía del Cretácico Superior-Paleoceno del área de Bahía Thetis, Andes fueguinos, Argentina: acontecimientos tectónicos y paleobiológicos

Se caracteriza la evolucion estratigrafica del Cretacico superior-Paleoceno de las cuencas de antepais Austral-Malvinas en la porcion mas interna de la faja plegada y corrida del extremo oriental de los Andes fueguinos. En la bahia Thetis se reconocen tres Formaciones: 1) Bahia Thetis, fangolitas oscuras, tobas, turbiditas arenosas y conglomerados resedimentados, con ammonites y foraminiferos del Campaniano tardio-?Maastrichtiano inferior; 2) Policarpo, fangolitas arenosas, tobaceas, bioturbadas con ammonites, foraminiferos y dinoquistes del Maastrichtiano en su parte inferior-media y con foraminiferos y dinoquistes del Daniano en su parte superior; y 3) Tres Amigos (n. nombre), conglomerados, areniscas y fangolitas, con dinoquistes y foraminiferos del Paleoceno austral. Estas tres formaciones, sumadas a los estratos de la bahia Buen Suceso (Santoniano-Campaniano inferior), de la caleta Falsa de Policarpo-Rio Bueno (Maastrichtiano a Eoceno medio basal), y del cabo Campo del Medio (Eoceno), integran sobre la costa atlantica una columna estratigrafica compuesta, relativamente completa del Santoniano al Eoceno superior. Dataciones paleontologicas e inferencias tectonicas en cuerpos conglomeradicos y discordancias, acotan la edad de tres pulsos principales de levantamiento de los Andes fueguinos en el Campaniano tardio-?Maastrichtiano temprano; Paleoceno medio-tardio; y Eoceno medio basal, respectivamente. Los ultimos inoceramidos hallados son del Campaniano temprano, pero probablemente su desaparicion en los Andes Fueguinos ocurra en el Maastrichtiano, conjuntamente con un marcado cambio de condiciones de fondo, de disoxicas-anoxicas en el Campaniano tardio-?Maastrichtiano inferior a oxicas en el Maastrichtiano tardio

Vegasaurus molyi, gen. et sp. nov. (Plesiosauria, Elasmosauridae), from the Cape Lamb Member (lower maastrichtian) of the Snow Hill Island Formation, Vega Island, Antarctica, and remarks on Wedellian Elasmosauridae

ABSTRACT A new elasmosaurid, Vegasaurus molyi, gen. et sp. nov., from Vega Island, James Ross Archipelago, Antarctica, is described. The holotype and only specimen of this species (MLP 93-I-5-1) was collected from the lower Maastrichtian Cape Lamb Member of the Snow Hill Island Formation. Vegasaurus molyi is the only Antarctic elasmosaurid and one of only a few Late Cretaceous elasmosaurids from the Southern Hemisphere whose postcranial anatomy is well known. Vegasaurus molyi is distinguished from other elasmosaurids by the following combination of characters: cervical region with 54 vertebrae with elongated centra, dumbbell-shaped articular faces and lateral ridge present in the anterior and middle parts of the neck but absent in the posterior-most cervical vertebrae; scapula with ventral ramus bearing a strong ridge in the anteromedial corner of its dorsal surface; ilium shaft with expanded distal end, divided into two parts forming an angle of 140° opening anteriorly; and humerus with anterior knee and prominent posterior projection with accessory posterior articular facet. Preliminary phylogenetic analysis places V. molyi within a clade that includes the Late Cretaceous Wedellian aristonectine elasmosaurids, Aristonectes and Kaiwhekea. This indicates a close relationship between Aristonectinae and non-Aristonectinae Late Cretaceous Weddellian elasmosaurids and suggests a Weddellian origin for the Aristonectinae.

Late Albian inoceramid bivalves from the Andes of Tierra del Fuego; age implications for the closure of the Cretaceous marginal basin

AT THE SOUTHERNMOST tip of South America, a thick pile of deep marine volcaniclastic rocks called the Yahgan Formation (Kranck, 1932) was deposited during the Early Cretaceous in a small marginal basin developed between the continent and a Pacific-facing volcanic arc (Katz, 1972; Dalziel et al., 1974). North and northwest of Tierra del Fuego, in the adjacent Austral or Magallanes basin, this unit is laterally replaced by coeval, fine-grained deposits representing basinal, slope, and platform marine settings (Winslow, 1982; Biddle et al., 1986; Wilson, 1991). The geometry of the basins changed markedly with a compressional event that produced the tectonic inversion of the marginal basin and the formation of a retroarc foreland basin in front of the rising cordillera. Closure of the marginal basin and strong deformation of the Yahgan Formation apparently occurred in the mid-Cretaceous (Halpern and Rex, 1972; Dalziel et al., 1974; Wilson, 1991); however, the timing of the opening and closing of the basin is poorly constrained because of the scarcity of fossil evidence. So far, a Late Jurassic-Neocomian age was favored for the Yahgan Formation on the basis of the record of belemnites and ammonites (Aguirre Urreta and Suarez, 1985; Halpern and Rex, 1972; Winn, 1978). Halpern and Rex (1972) mentioned the Hauterivian genus Favrella in Gardiner Island, but this record has been questioned by Thomson et al. (1982) who considered the ammonite imprint to be some kind of heteromorph. Also, the timing of the transition from marginal to foreland basin is not well documented. On the basis of indirect evidence the initiation of the foreland basin stage was assigned to the Albian in the Ultima Esperanza region of Chile (Wilson, 1991) and to the Late Cretaceous in northern Tierra del Fuego (Biddle et al., 1986). Recent field work by the authors in the area of No Top Mountain-Moat River, Tierra del Fuego (Figure 1), has resulted in the first record of diagnostic late Albian inoceramids in the Yahgan Formation. The objective of this note is to document this fauna and to briefly discuss its implications on the control of the timing of the transition from marginal to foreland basin in the area.

Control en la magnitud de desplazamiento de rumbo del Sistema Transformante Fagnano, Tierra del Fuego, Argentina

Constraints on the magnitude of strike-slip displacement along the Fagnano Transform System, Tierra del Fuego, Argentina. New fi eld data, specially the discovery of the contact between Upper Cretaceous-Paleocene rock units in a previously unexplored area in southeast Tierra del Fuego, allowed us to estimate the amount of strike-slip displacement accommodated by the Fagnano Transform System (FTS). This stratigraphic contact is exposed both to the north and south of the FTS and it is deformed by a NW-SE compressive regional structure that forms part of the Fuegian thrust-fold belt, called the Cerro Piramide-Cerro Malvinera Thrust (PMT). The compressive structure is laterally offset by the FTS. The precise mapping of the PMT and the Upper Cretaceous-Paleocene contact shows a horizontal offset of ca. 48 km along the transform system in the Isla Grande de Tierra del Fuego. A maximum age of ~7 Ma for the begin- ning of the strike-slip in this sector is estimated by combining the obtained offset with published data on the slip-rate along the FTS in Tierra del Fuego. This Late Miocene age coincides with the creation of the divergent plate boundary between Sandwich and Scotia Plates, which has been proposed as responsible for the beginning of the strike-slip activity between South America and Scotia Plates.

TAPHONOMY OF AMMONITES FROM THE SANTONIAN–LOWER CAMPANIAN SANTA MARTA FORMATION, ANTARCTICA: SEDIMENTOLOGICAL CONTROLS ON VERTICALLY EMBEDDED AMMONITES

Abstract Different morphotypes of vertically embedded ammonoid shells are abundant in the 1-km-thick deposits of the Santa Marta Formation, Antarctica, that record the evolution of a deep-water delta system. Vertical shells deposited in water depths well below the theoretical limit imposed by hydrodynamic and hydrostatic analyses are preserved as isolated specimens, dense concentrations associated with abundant wood fragments (pod preservation), or as dense concentrations inside and around large ammonites (sheltered preservation). Taphonomic analysis indicates that (1) vertical shell orientation is primary; (2) postburial reworking was minimum or absent, as indicated by consistent shell orientation parallel to regional paleocurrents and complete preservation of fragile shells with phragmocones filled with drusy calcite; and (3) vertical orientation is not biased towards a preferred morphotype. Rapid sedimentation, including deposition from high- and low-density currents, tempestites, and weak bottom currents carrying a dense suspension of ammonoid shells and wood fragments, was one of the main factors controlling the vertical preservation below the limits imposed by theoretical hydrostatic analyses. Another important factor was the plugging of the siphuncular tube with clay particles during transportation. The clay plug was stiff enough to resist the ambient hydrostatic pressure, avoiding or delaying the waterlogging of the phragmocone.

Progressive deformation of a Coulomb thrust wedge: the eastern Fuegian Andes Thrust-Fold Belt

Abstract Time-calibrated balanced-cross sections of the eastern Fuegian Thrust–Fold Belt reveal many complex pro- and retro-vergent structures, rooted at the base of Cretaceous and within Paleocene rocks. These structures involve the unconformity-bounded syntectonic sequences of the Austral foreland basin, and accommodate a minimum shortening of c. 41.8 km. The complex kinematics of the thrust–fold belt are recorded by: (1) propagation of the basal décollement into the foreland, and forward-directed thrusting during the Ypresian; (2) out-of-sequence thrusting in the Lutetian; (3) subsidence and sedimentation from the Late Lutetian to the Oligocene; (4) backthrusting during the Oligocene; and (5) a renewed stage of forward-directed thrusting between the latest Oligocene and the Early Miocene, probably related to accretion below the sole fault in the hinterland. This thrust sequence is interpreted as the result of critical Coulomb wedge behaviour during the first stage of thrust–fold belt expansion, with accretion of new material that led to a taper decrease. The subsequent period of internal deformation corresponds to a subcritical stage, during which backthrusting accommodates significant shortening (c. 15%). After growth and taper increase, the last period of forward thrusting at the wedges front marks the inception of a new critical stage.

Euflabella N. Igen.: Complex Horizontal Spreite Burrows in Upper Cretaceous–Paleogene Shallow-Marine Sandstones of Antarctica and Tierra Del Fuego

Abstract Fine-grained sandstones and siltstones of Late Cretaceous to Eocene age in Antarctica and Tierra del Fuego yield an association of well-known shallow-marine trace fossils. Among them stick out complex spreite burrows, which are formally described as Euflabella n. igen. and subdivided into five ichnospecies with different burrowing programs and occurrences. As shown by concentrations of diatoms, radiolarians, foraminifers, and calcispheres in particular backfill lamellae, the unknown trace makers lived on fresh detritus from the surface as well as the burrowed sediment. In some ichnospecies, vertical sections show that the spreite is three-dimensionally meandering in upward direction and that upper laminae tend to rework the upper backfill of the folds underneath. This could mean a second harvest, after cultivated bacteria had time to ferment refractory sediment components, which the metazoan trace maker had been unable to digest before.

Environmental stress and diagenetic modifications in inoceramids and belemnites from the Upper Cretaceous James Ross Basin, Antarctica

SummaryNew petrographic and isotopic data from inoceramid bivalve shells and belemnite rostra from the lower Campanian and belemnite rostra from the mid-upper Maastrichtian of the Marambio Group, James Ross Basin, Antarctica are presented. Most of the inoceramid data were processed from shell fragments of the large formAntarcticeramus rabotensis (Crame and Luther) at the stratigraphic level marking the extinction of the inoceramids in the James Ross Basin (uppermost early Campanian-basal late Campanian). Standard transmitted light microscopy and cathodoluminescence (CL) studies in thin sections ofA. rabotensis show clear evidence of environmental stress, which is reflected as marked growth breaks in the shell banding of this large inoceramid bivalve. At Redonda Point, CL and the mean oxygen isotopic value (δ18O=-3.11‰ 3 (PDB); n=11; t°=25.4°C) indicate a varied degree of diagenetic modification, but without any evidence of neomorphism along the prismatic microstructures. Early Campanian belemnite rostra are much less diagenetically modified (at the Brandy Bay section; and the Santa Marta section; δ18O=-0.50‰ (PDB); n=5; t°=14.0°C and 3 δ18O=-0.94‰ (PDB); n=21; t°=15.8°C) and are non luminescent 3 except for localized, organic-rich bands. The mean oxygen isotopic value for mid-late Maastrichtian belemnite rostra (at the Seymour Island section; δ18O=-0.11‰ 3 (PDB); n=5; t°=12.5°C) indicates a substantial drop in the sea-water paleotemperature, suggesting a causal relationship between the early extinction of the inoceramid bivalves in high latitudes of the Southern Hemisphere and the falling sea-water temperature.