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Alegret, Ortiz & Kaminski (eds.), 2012. Ninth International Workshop on Agglutinated Foraminifera, Abstract Volume
The Cretaceous-Paleogene agglutinated foraminifera from the Magallanes or
Austral Basin, southernmost South America
Norberto MALUMIÁN 1 and Carolina NÁÑEZ 1
1 CONICET and Servicio Geológico Minero Argentino, Benjamín Lavaisse 1194, (C1107BJD) Buenos
Aires, Argentina.
e-mail: n.malumian@yahoo.com
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
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