Jacobus P. Le Roux

Repeated mass strandings of Miocene marine mammals from Atacama Region of Chile point to sudden death at sea

Marine mammal mass strandings have occurred for millions of years, but their origins defy singular explanations. Beyond human causes, mass strandings have been attributed to herding behaviour, large-scale oceanographic fronts and harmful algal blooms (HABs). Because algal toxins cause organ failure in marine mammals, HABs are the most common mass stranding agent with broad geographical and widespread taxonomic impact. Toxin-mediated mortalities in marine food webs have the potential to occur over geological timescales, but direct evidence for their antiquity has been lacking. Here, we describe an unusually dense accumulation of fossil marine vertebrates from Cerro Ballena, a Late Miocene locality in Atacama Region of Chile, preserving over 40 skeletons of rorqual whales, sperm whales, seals, aquatic sloths, walrus-whales and predatory bony fish. Marine mammal skeletons are distributed in four discrete horizons at the site, representing a recurring accumulation mechanism. Taphonomic analysis points to strong spatial focusing with a rapid death mechanism at sea, before being buried on a barrier-protected supratidal flat. In modern settings, HABs are the only known natural cause for such repeated, multispecies accumulations. This proposed agent suggests that upwelling zones elsewhere in the world should preserve fossil marine vertebrate accumulations in similar modes and densities.

Seasonal sediment transport pathways in Lirquen Harbor, Chile, as inferred from grain-size trends

Bottom sediment samples were collected in June and December 1997 at 166 stations on a rectangular grid in a small port (Lirquen Harbor, BioBio Region, Central Southern Chile) facing siltation problems with a view to determining local seasonal sediment transport pathways through the application of three different methodologies based on grain-size trends. Measured winds and near-bottom currents together with hindcast wave allowed to establishing the influence of the prevailing meteorological and hydrodynamic conditions on local sediment circulation. The results suggest that transport of deep and shallow water sediments occur rarely under high-energy conditions. The current data suggest that tidal asymmetry would be an important agent of sediment transport in Lirquen Harbor. The sediment transport pathways inferred from grain-size trends are compared with the measured water circulation of the study area and sediment dispersal patterns on aerial photographs. The results yielded by the grain-size trend methodologies of Gao-Collins (1992) and Le Roux (1994b) indicate a possible seasonal variation in transport pathways agreeing with the prevailing meteo-hydrodynamic conditions, whereas the McLaren-Bowles (1985) approach does not show a significant difference. It is suggested that the latter methodology may represent transport on a spatial macroscale as opposed to the mesoscale patterns yielded by the other two techniques. Based on the results of the three grain-size trend methodologies, the observed current and wind data, the hindcast wave data and aerial photographs a conceptual model of net annual sediment transport is proposed for Lirquen Harbour.

Sedimentary processes on a Gilbert-type delta in Lake Llanquihue, southern Chile

A small Gilbert-type delta on the southern shore of Lake Llanquihue, southern Chile, was studied over a period of two months. Bottom samples were taken by scuba diving along a 10 x 10 m grid to determine the distribution of sedimentary facies. The wind direction, wave conditions, underwater current directions and the orientation of sedimentary structures were recorded on a daily basis. Observations made during fair-weather conditions indicate that neither waves nor currents have any significant effect on the bottom sediments, and that the only transport is produced by bivalves causing small avalanches on the steep delta slope. This probably prevents the oversteepening and large-scale slope collapse typical of many Gilbert deltas. Observations recorded from shore during one major storm, show an excellent agreement with wave theory and empirical predictions, indicating that storm waves can affect the whole delta front and slope. These waves break about 35 m from shore where there is a clear transition from clast-supported gravel on the inner delta front to matrix-supported gravel and gravelly sand on the outer front. The storm waves are able to transport cobbles up to 40 cm in diameter. During these events, strong lakeward-directed bottom currents enhanced by the effluent plume of the river, transport some of these clasts to the edge of the delta front, where they avalanche down to the foot of the delta. During the waning stages of the storm and shortly thereafter, dense, sediment-laden bottom currents discharged from the river mouth carry plant material down the delta slope and over the pro-delta, burying the cobbles just deposited. A single cycle of delta progradation should produce two coarsening-upward cycles, which might be confused in the rock record with two distinct phases of delta progradation.

Estacionalidad de la erosión y el transporte eólico de partículas en el desierto costero de Atacama, Chile (23°S)

Resumen es: La costa del Desierto de Atacama se caracteriza por la ocurrencia de vientos fuertes del sur y suroeste. El clima arido, la geomorfologia plana y las car...

Stratigraphic Implications of Latest Middle Miocene to Earliest Late Miocene Diatoms in the Navidad Formation at Lo Abarca, Central Chile (33° 30'S)

Neogene marine strata crop out at various localities along the Chilean coastline (see Encinas et al., 2008 and references therein). The best studied of these successions occurs in the Navidad area, between San Antonio and Punta Topocalma (33°30’34°30’S) (figure 1). Named and described by Darwin (1846) as the Navidad Formation, the geology and paleontology of this unit was subsequently studied by many other authors (e.g., Philippi, 1887; Brüggen, 1950; García, 1968; Tavera, 1979) and has been considered the stratigraphic reference for the marine Neogene of Chile (e.g., Cecioni, 1980; De Vries and Frassinetti, 2003). However, its age, stratigraphy and depositional environment were a matter of debate for several decades (e.g., Brüggen, 1950; García, 1968; Tavera, 1979; Encinas et al., 2006). Recent foraminiferal and sedimentologic studies indicate a late Miocene to early Pliocene age and a deep-marine setting for this unit (Finger et al., 2007; Encinas et al., 2008). Yet, the older latest Oligocene to middle Miocene age proposed for the molluscan fauna of this unit (De Vries and Frassinetti, 2003) still constitutes an unresolved problem (see below). North of the Navidad area, between Valparaíso and San Antonio (33°00’?33°30’S), Neogene successions have been traditionally referred to as the “capas de Lo Abarca” (Covacevich and Frassinetti, 1990) (figure 1). These strata have been considered equivalent to the Navidad Formation by some authors (Brüggen, 1950; Fuenzalida and Varela 1964; Encinas et al., 2006) and as a different unit by others (Martínez-Pardo and Parada, 1968; Covacevich and Frassinetti 1990). The main argument for distinguishing the successions is based on their different molluscan faunas (Covacevich and Frassinetti, 1990). Nevertheless, Encinas et al. (2006) considered both successions as corrrelative and included the Lo Abarca strata in the Navidad Formation because their diatoms and foraminifera indicate similar ages and both units show similar facies. Essential to the age constraint of the Lo Abarca succession and its correlation with the Navidad Formation is the diatom assemblage found at Lo Abarca section that we document in this note.

Depositional environment of Stelloglyphus llicoensis isp. nov.: a new radial trace fossil from the Neogene Ranquil Formation, south-central Chile

Stelloglyphus llicoensis isp. nov. is a large radial, discoidal to ellipsoidal trace fossil with a central shaft and single to bifurcating branches radiating from different levels. A 30 m thick measured section of the Ranquil For- mation at Punta Litre contains an associated trace fossil assemblage including Zoophycos, Chondrites, Phycosiphon, Nereites missouriensis, Lockeia siliquaria, Psammichnites(?), Parataenidium, Ophiomorpha, and Rhizocorallium, some of which reworked the Stelloglyphus traces. The sedimentology, together with micro- and macrofossils and the associated trace fossil assemblage, suggest that the succession was deposited in an outer continental shelf to slope environment in subtropical to tropical waters.

Mesozoic sedimentation on an isolated platform at the eastern entrance to the Strait of Magellan, Tierra del Fuego (Chile)

El Bloque area Magallanes esta situado en el oceano Atlantico, dentro de la actual plataforma continental Argentina. Se ubica en la desembocadura oriental del estrecho de Magallanes y consiste de dos cordones de altos estructurales, de orientacion noroeste, que constituyen un horst, parcialmente separado por un graben central. En esta estructura se han encontrado y producido hidrocarburos desde el ano 1994 desde la Formacion Springhill, la cual se deposito durante el Valanginiano-Barremiano. Esta formacion representa una transgresion marina regional con tres subciclos de transgresion-regresion (parasecuencias) separados por superficies de inundacion marina. Las dos parasecuencias basales estan relacionadas con variaciones menores del nivel del mar o por actividad de fallas, mientras que el subciclo superior refleja una gran transgresion, que se extiende en casi toda la cuenca Austral. Las asociaciones de facies depositacionales identificadas incluyen supramareales, intermareales, playa barrera y ambientes de frente de playa, las cuales guardan una relacion clara con la paleotopografia del bloque. De esta manera, las playas-barrera se depositaron en torno a areas topograficamente elevadas, mientras que las facies supramareales e intermareales fueron depositadas hacia las zonas mas elevadas, detras de las playas-barrera. Las facies de frente de playa fueron depositadas hacia la zona baja de estas areas elevadas. Las planicies intermareales tambien se depositaron en embahiamientos internos dentro del graben central. Finalmente, al progresar la transgresion marina, el sistema depositacional evoluciono hacia una plataforma marina extensa y abierta donde se depositaron las arcillolitas de la Formacion Estratos con Favrella.