The Cretaceous-Paleogene agglutinated foraminifera from the Magallanes or Austral Basin, southernmost South America

MALUMIÁN, N. Y NÁÑEZ, C.

Zaragoza

Workshop; Ninth International Workshop on Agglutinated Foraminifera; 2012

The Austral or Magallanes retroarc basin possesses an almost complete Cretaceous-Paleogene marine clastic sequence, paradigmatic in the middle- high latitudes of the Southern Hemisphere, where agglutinated foraminifera (AF) are common to dominant from the Aptian up to the lower Miocene. The distribution of these AF reflects and at the same time defines the main tectonosedimentary regions: the active Andean margin comprises mostly flysch type cosmopolitan assemblages, while towards the stable platforms mixed calcareous and agglutinated assemblages occur, and on the stable cratonic edge, impoverished assemblages of AF with fairly endemic forms are found. The most outstanding features and relevant taxa of these assemblages are indicated. In the Hauterivian-Barremian, despite the good to exceptional preservation of calcareous foraminifera, the AF are very scarce to absent, probably due to a selective attack of bacteria on their organic cement in the dominant dysaerobic environments (Loeblich and Tappan 1989), and consequently, only one species, Sculptobaculites goodlandensis (Cushman and Alexander), is locally common. In the Albian?Cenomanian, with the abrupt onset of oxygenate and open oceanic waters conditions, mixed assemblages appear towards more stable environments (Springhill platform) with well-known cosmopolitan species of the Marssonella Assemblage, such as Spiroplectinata annectens, S. complanata, Tritaxia gaultina, Marssonella oxycona, and the abundant and conspicuous Dorothia mordojovichi Cañón and Ernst, one of the few apparent endemic species. In marginal environments, the Ammobaculites Assemblage contains species common to the Great Artesian Basin, Australia, such as Bigenerina cf. pitmani (Crespin), Spiroplectammina egelli Crespin, Textularia wilgunyaensis Crespin, and Trochammina delicatula Crespin, (Malumián and Náñez 1990, 2002). A proxy indicator of Turonian age is given by Spiroplectina ona (Malumián and Masiuk) in mixed assemblages in Tierra del Fuego, also widespread in the adjacent Malvinas Basin. In the upper Santonian-lower Campanian, the Uvigerinammina jankoi Assemblage, a well developed flysch type assemblage, is recognized in the southern Patagonian Andes; while in the Fuegian platform, Textularia juliana Malumián and Masiuk is the typical agglutinated species within mixed assemblages. In the regressive facies, of very shallow and probably marshy environments, the so called Textularia-Spiroplectammina Assemblage, with high dominance of enfeeble biserial tests, is widespread nearly all over the basin in the Santa Cruz Province. Its appearance is recorded in the Mata Amarilla Formation, outcropping in the Cerro Indice locality, of late Coniacian age according to its Gauthiericeras content (Riccardi 2002). The youngest record of this assemblage is in the subsurface, overlying beds with Hoplitoplacenticeras plasticum, considered of late Campanian age (Malumián and Náñez 1990). The Assemblage of Cribrostomoides ex gr. H. rugosus is recognized and extended from the Andean foothills to the Atlantic coast overlying the Spiroplectammina-Textularia Assemblage, and differs from the latter by the dominance of involute forms. During the Late Campanian- Maastrichtian, the AF are dominant in both the Southern Patagonian Andes and the Fuegian Andes. In the former, in shallow facies, the first AF of alveolar wall in the basin appears between Hoplitoplacenticeras (upper Campanian) and Maorites (Maastrichtian) horizons (Náñez and Malumián 2000). In the Fuegian Andes, the finely stratified black shales include horizons with monospecific assemblages of Rzehakina lata, being this genus well represented up to the lower Eocene (Caramés and Malumián 2006). The first extensive Atlantic transgression on the Patagonian platform occurs in the Maastrichtian and locally is represented by different agglutinated assemblages. In the northern Patagonia, low diversity assemblages of deltaic type are recorded in the Lefipán Formation. In the Magallanes Basin, Ammobaculites type assemblages are distributed on the Springhill platform, while flysch type assemblages are recorded in the Policarpo Formation in Tierra del Fuego Island, dominated by Thalmannammina cf. turbinata (Brady), Spiroplectammina spectabilis, Recurvoides sp., Gerochammina conversa (Grzybowski), and Gaudryina healyi Finlay (Olivero et al. 2002, Náñez and Malumián 2008, Malumián and Náñez 2011). The last species is a typical Austral one, originally described from New Zealand and also recorded from Antarctica. The first appearance of S. spectabilis is a proxy for the base of the Maastrichtian, in assemblages where other age indicators are absent. This species is absolutely dominant in the Andean Fuegian organic-rich upper Paleocene, which also contains contrasting and localized horizons with exclusively well preserved calcareous foraminifera (Caramés and Malumián 2002). The absence of Caudammina, so abundant elsewhere, is an apparent negative general characteristic of the Campanian and Maastrichtian from the Magallanes Basin. In the only record of earliest Eocene from the Magallanes Basin, a new genus with remaneicid test gross morphology but with calcareous cemented perforate wall, manifests within a mixed assemblage the accentuated endemism resulting after the Paleocene/Eocene turnover (Malumián et al. 2012). In the Fuegian Oligocene-Miocene the abrupt appearance of deep-water facies is represented by assemblages of cosmopolitan AF under the calcite compensation depth. In the Patagonian platform, in the earliest Miocene, the existence of a current of Antarctic corrosive water is evident by the northward dispersion of the agglutinated Spirosigmoilinella-Martinottiella Assemblage, which occasionally includes dissolution resistant calcareous foraminifera of Antarctic origin such as Antarcticella antarctica and Ammoelphidiella.