South America

In latest Cretaceous time, northern South America lay within the Palmae palynologic province (Herngreen and Chlonova 1981). Characteristic pollen included species of the genera Psilamonocolpites, Retimonocolpites, Proxapertites, and Spinizonocolpites (the species S. echinatus is said to be identical with pollen of the living palm Nypa fruticans). Muller et al. (1987) presented a comprehensive palynostratigraphy of the Cretaceous through Holocene of northern South America. Their study areas included Colombia, Venezuela, Trinidad, Guyana, Surinam, and Brazil. Much of their information came from published sources, especially from Germeraad et al. (1968). Zone 13 of Muller et al. (Maastrichtian) is defined by the lowest stratigraphic occurrence of Proteacidites dehaani, which becomes extinct at the top of the zone, along with Buttina andreevi, Crassitricolporites brasiliensis, species of Aquilapollenites, and Scollardia (the last two are genera also known in the Maastrichtian of North America). Zone 14 (basal Paleocene) is defined by lowest stratigraphic occurrence of Spinizonocolpites baculatus (which is palm pollen). Hence, a K-T boundary can be recognized by a change in palynofloras, but apparently not a major extinction event. The lowermost Paleocene is characterized by low abundance of fern spores and gymnosperm pollen, and by having first occurrences of pollen of the largely tropical family Bombacaceae and the palm genus Mauritia, both of which are dominant in northern South America at present. Specimens of some of these genera and species are illustrated in Figures 9.1 and 9.2.

Ashraf and Stinnesbeck (1988) described the palynology of a K-T boundary interval near Recife, on the coast of eastern Brazil. Foraminifera were used to determine the age of the section. Ashraf and Stinnesbeck reported that the

Pollen Palmae

Figure 9.1 Fossil pollen of the Palmae palynostratigraphic province (from Herngreen and Chlonova 1981). a - Retidiporites magdalenensis, b - Proteacidites sigalii, c - Buttinia andreevi, d - Echitriporites trianguliformis, e - Foveotricolpites irregularis, f - Aquilapollenites sergipensis, g - Scollardia srivastavae, h - Cupanieidites sp., i - Crassitricolporites brasiliensis. Specimens range from 30 to 60 micrometers in diameter.

Figure 9.1 Fossil pollen of the Palmae palynostratigraphic province (from Herngreen and Chlonova 1981). a - Retidiporites magdalenensis, b - Proteacidites sigalii, c - Buttinia andreevi, d - Echitriporites trianguliformis, e - Foveotricolpites irregularis, f - Aquilapollenites sergipensis, g - Scollardia srivastavae, h - Cupanieidites sp., i - Crassitricolporites brasiliensis. Specimens range from 30 to 60 micrometers in diameter.

Echitriporites Trianguliformis Picture

Figure 9.2 Fossil pollen of the Palmae palynostratigraphic province (from Herngreen and Chlonova 1981). a - Zlivisporis (?Triporoletes) blanensis, b - Gabonisporis vigourouxii, c -Psilastephanocolporites daportae, d - Psilastephanocolporites hoekenae, e - Spinizonocolpites echinatus, f - Psilamonocolpites medius, g - Retimonocolpites sp., h - Proxapertites operculatus, i -Proxapertites cursus. Specimens range from 30 to 70 micrometers in diameter.

Figure 9.2 Fossil pollen of the Palmae palynostratigraphic province (from Herngreen and Chlonova 1981). a - Zlivisporis (?Triporoletes) blanensis, b - Gabonisporis vigourouxii, c -Psilastephanocolporites daportae, d - Psilastephanocolporites hoekenae, e - Spinizonocolpites echinatus, f - Psilamonocolpites medius, g - Retimonocolpites sp., h - Proxapertites operculatus, i -Proxapertites cursus. Specimens range from 30 to 70 micrometers in diameter.

palynoflora is composed primarily of fern spores and made no mention of the pollen species discussed by Muller et al. (1987). There is no palynologic break evident at the position of K-T boundary established by Ashraf and Stinnesbeck. They concluded that the palynoflora shows climate change from subtropical in the Maastrichtian to subtropical to temperate in the Paleocene. Given the absence of characteristic angiosperm pollen, this record is ambiguous at best.

With particular reference to western Venezuela, Pocknall et al. (2001) stated that the transition from the Maastrichtian to the Paleocene in northern South America usually occurs at a change from predominantly marine depositional conditions of the uppermost Cretaceous to continental (fluvial and lacustrine) conditions in the lowermost Paleocene. There may be some uncertainty in the dating of this transitional interval, however, because it is based solely on terrestrially derived spores and pollen. Correlation with the marine fossil record has not been possible. A measured section through the Mito Juan Formation (Maastrichtian) and the Los Cuervos Formation (Paleocene) in the Andes of western Venezuela yielded diverse palynomorph assemblages that include dino-flagellates. Graphic correlation of the Venezuelan dinoflagellate data and a dinoflagellate-biostratigraphic database indicates the presence of three unconformities within this interval. Nonetheless, Pocknall et al. stated that a conformable K-T boundary section might be preserved within a covered interval about 20 m thick, assuming continuous deposition and preservation of sediments. The dinoflagellate taxa present in the measured section indicate that the strata below the covered interval are latest Cretaceous in age and that those just above are earliest Paleocene in age. The data available at present leave the precise position of the K-T boundary unresolved, and associated changes in terrestrial vegetation unclear.

Correlation from the measured section in western Venezuela studied by Pocknall et al. (2001) to their global database containing marine dinoflagellates helps to refine the age ranges of the pollen species that have been used to define the Maastrichtian Proteacidites dehaani and Paleocene Spinizonocolpites baculatus pollen zones in South America (e.g., Muller et al. 1987). The fungal fruiting body Trichopeltinites, which is known to disappear at or about the K-T boundary in sections in North America (see Section 7.2), is also recorded in western Venezuela, although in Venezuela it apparently ranges into the Paleocene. Thus, the most recent data available confirm earlier conclusions, some of which date back to 1968.

Recent fieldwork in Chubut Province, Argentina, has begun to elucidate Late Cretaceous and early Paleocene palynofloras and megafloras, but this work is largely unpublished and stratigraphic resolution is currently at the stage level. In late 2006, fieldwork by one of us (KRJ) in concert with Museo Paleontologico

Egidio Feruglio in Trelew and Pennsylvania State University, located several plant-bearing horizons in the primarily marine Maastrichtian-Paleocene Lefipan Formation in the upper Chubut River valley. We made large collections from several angiosperm-dominated floras and collected ancillary foraminiferal, palynological, and magnetostratigraphic samples in order to acquire age control. Because Maastrichtian or Paleocene megafloras are very poorly known in South America, plant megafossils alone are not sufficient to determine whether these sites are Cretaceous or Paleocene.

In 2004-2006, La Plata University student Ari Iglesias collected several megafloral localities in southern Chubut near the town of Sarmiento. He found several leaf localities at and near a well-known petrified forest in the Salamanca Formation. In this area, this formation contains distinctive Danian marine foraminifera, and additional paleomagnetic data constrains this megaflora to ~62 Ma. The Palacio de los Loros flora is considerably richer than coeval megafloras from North America; a collection of more than 2500 specimens yielded an angiosperm-dominated flora of 36 species (Iglesias et al. 2006, 2007).

Perhaps the most promising area for K-T boundary studies in South America is Colombia. Carlos Jaramillo and his student Felipe De la Parra have recently recovered a 500 m core from the Cesar-Rancheria Basin. Preliminary analysis of 62 samples and more than 14 000 palynomorph specimens suggests a K-T boundary extinction on the order of 60-70% (Jaramillo and De la Parra 2006). This work has not yet been calibrated with paleomagnetics, so the precise age relations are not yet clear. Nonetheless, the proximity to Chicxulub and the apparently high extinction suggest that this region holds much promise.

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