Andrés Pardo

Middle Miocene closure of the Central American Seaway.

Early closing between oceans The Central American Seaway, which once separated the Panama Arc from South America, may have closed 10 million years earlier than is believed. Montes et al. report that certain minerals of Panamanian provenance began to appear in South America during the Middle Miocene, 15 to 13 million years ago (see the Perspective by Hoorn and Flantua). The presence of the minerals indicates that rivers were flowing from the Panama Arc into the shallow marine basins of northern South America. One interpretation of this finding is that large-scale ocean flow between the Atlantic and Pacific had ended by then. If this is true, then many models of paleo-ocean circulation and biotic exchange between the Americas need to be reconsidered. Science, this issue p. 226; see also p. 186 The ocean gateway that once separated South America from North America disappeared longer ago than was thought. [Also see Perspective by Hoorn and Flantua] Uranium-lead geochronology in detrital zircons and provenance analyses in eight boreholes and two surface stratigraphic sections in the northern Andes provide insight into the time of closure of the Central American Seaway. The timing of this closure has been correlated with Plio-Pleistocene global oceanographic, atmospheric, and biotic events. We found that a uniquely Panamanian Eocene detrital zircon fingerprint is pronounced in middle Miocene fluvial and shallow marine strata cropping out in the northern Andes but is absent in underlying lower Miocene and Oligocene strata. We contend that this fingerprint demonstrates a fluvial connection, and therefore the absence of an intervening seaway, between the Panama arc and South America in middle Miocene times; the Central American Seaway had vanished by that time.

THE BIOGEOGRAPHY OF THE ARAUCARIAN DISPERSED POLLEN CYCLUSPHAERA

The pollen genus Cyclusphaera was first described from the Campanian of Peru. The distinctive pollen is spherical to ovoid and disc shaped, with two large openings, resembling pores, opposite each other on the pollen grain. We emend the description of Cyclusphaera scabrata from the Cenozoic of northern South America, based on new data from transmission electron and scanning electron microscopy. The worldwide spatial and temporal distributions of Cyclusphaera are examined. This taxon originated in midlatitudes in the Southern Hemisphere (Gondwana) and subsequently expanded its distribution to higher and lower latitudes, reaching a maximum latitudinal and longitudinal distribution by the middle Cretaceous. Following the Cretaceous, Cyclusphaera was extirpated from middle and high latitudes during the late Cretaceous and Paleogene while increasing its frequency in the tropics during the Cenozoic in both Africa and South America. By the Oligocene, Cyclusphaera was a common element of palynofloras in Colombia and Venezuela. During the Neogene, the distribution in tropical South America became restricted, with the youngest record occurring in the late Miocene of western Colombia. The overall biogeographic pattern of Cyclusphaera is similar to that of Podocarpaceae, one of the most persistent gymnosperm families in the tropics. The extinction of Cyclusphaera could be related to the increase in aridity and expansion of savanna habitats in the northern tropics of South America during the Neogene.

LOW-LATITUDE MIOCENE CALCAREOUS AND SILICEOUS MICROFOSSIL BIOSTRATIGRAPHY FROM NW SOUTH AMERICA: LADRILLEROS-JUANCHACO SECTION, COLOMBIAN PACIFIC

Abstract. The Neogene sedimentary basins of the Colombian Pacific coast were formed in forearc basins associated with the uplift of the Andes. In this area, some marine sequences remain unexplored and are marked by a lack of biostratigraphic information. Additionally, available high-resolution biostratigraphic and biochronologic data from this region are limited almost exclusively to open sea research conducted by the Integrated Ocean Drilling Programs (IODP). The Ladrilleros-Juanchaco section is a continuous and well-exposed marine terrigenous sedimentary sequence with abundant Miocene microfossils. Biostratigraphic and biochronologic studies with calcareous nannofossils, planktonic foraminifera and diatoms carried out there revealed the presence of seventeen astronomically calibrated and standard calcareous microfossil biohorizons as well as two tropical diatom biozones. Such information enabled the establishment of a chronologic framework between the Base of Praeorbulina glomerosa and Catinaster coalitus Base, corresponding to 16.27 and 10.79 Ma, respectively (Burdigalian-Tortonian). The proposed biozonation favoured the observation of a high correlation between low-latitude planktonic foraminifera and calcareous nannofossil bioevents whereas diatom biozones and calcareous nannofossil biohorizons are marked by variations since the lowest occurrence of diatoms around 12.2 Ma. Furthermore, by means of interpolating age between the Sphenolithus heteromorphus Top and Fohsella praefohsi Base bioevents, the fact that the Reticulofenestra pseudoumbilicus (>7 µm) Base common took place around 13.6 Ma is interpreted and correlated with other sites, therefore revealing that it can be considered a good bioevent for the base of the Serravallian in these tropical areas.

LOW-LATITUDE MIOCENE CALCAREOUS AND SILICEOUS MICROFOSSIL BIOSTRATIGRAPHY FROM NW OF SOUTH AMERICA: LADRILLEROS-JUANCHACO SECTION, COLOMBIAN PACIFIC --- Uncorrected Proofs doi:10.5710/AMGH.11.08.2016.2978

Abstract. Fragmentary anuran remains in the Ameghino Collection in the Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” were recently considered as belonging in Ceratophryidae (the clade of the South American horned frogs) and coming from middle—late Oligocene sediments exposed south of Lake Colhue Huapi, Chubut, Argentina. However, both the taxonomic placement and the geographic and stratigraphic origin attributed to this material are problematic. A new study of these remains show they possess several features that allow their attribution to the extant genus Ceratophrys Wied-Neuwied. The available information on the collection locality of this material is dubious —namely Gran Barranca, where rocks range from the middle Eocene to early Miocene in age. Additionally, details of the history of this collection and the specimen itself suggest that its provenance should better be considered unknown.The Neogene sedimentary basins from the Colombian Pacific coasts were formed in forearc basins associated with the uplift of the Andes. Some marine sequences in this area remain unexplored and lack of biostratigraphic information. In addition, the available high-resolution biostratigraphic and biochronologic data from this region are limited almost exclusively to research in the open sea by the Integrated Ocean Drilling Programs (IODP). Ladrilleros-Juanchaco section is a continuous and well-exposed marine terrigenous sedimentary sequence with abundant Miocene microfossils. In this section, we carried out biostratigraphic and biochronologic studies through calcareous nannofossils, planktonic foraminifera, and diatoms identifying seventeen astronomically calibrated and standard calcareous microfossil biohorizons and two tropical diatom biozones. This information allowed the establishment of a chronologic framework between the Base of Praeorbulina glomerosa and Catinaster coalitus Base, corresponding to 16.27 and 10.79 Ma, respectively (Burdigalian–Tortonian). Low-latitude planktonic foraminifera and calcareous nannofossil bioevents show a high correlation, whereas variations between diatom biozones and calcareous nannofossil biohorizons were observed since the lowest occurrence of diatoms at ~12.2 Ma. Furthermore, using an interpolated age between the Sphenolithus heteromorphus Top and Fohsella praefohsi Base bioevents, it is interpreted that the Reticulofenestra pseudoumbilicus (>7μm) Base common took place around 13.6 Ma, which is correlated with other sites revealing that it can be considered as a good bioevent for the base of the Serravallian in these tropical areas.

Stratigraphic Transect of Northwestern Colombia: a Key to Understanding the Origin of the Panamanian Isthmus

We present new data on the stratigraphy of the Miocene in northwestern Colombia, in the Pacific Basin. The sedimentological and biostratigraphic study, based on an analysis of calcareous nannofossils, foraminifera, palynomorphs, and diatoms, has enabled a new framework to be constructed that allows the closing process of the Isthmus of Panama to be monitored and palaeoceanographic implications to be made.