Ammoelphidiella: a coastal cold current from the Oligocene/Miocene transition of Patagonia

NÁÑEZ, C. Y MALUMIÁN, N.

Neuquén

Congreso; XVIII Congreso Geológico Argentino; 2011

Many of the endemic Austral foraminifera have a complete to partially tuberculate test wall, persistent retral processes and/or complex canal systems. The tuberculation hides structures of systematic importance, which in addition to the high morphological variability of the tests and the relatively small size of some taxa, renders identification particularly difficult. Among the most conspicuous genera are the extinct Antarcticella and Ammoelphidiella, and the extant Cribrorotalia. Antarcticella was originated immediately after the Cretaceous/Paleogene boundary; Cribrorotalia, at the Paleocene/Eocene benthic foraminiferal turnover, whereas the appearance of Ammoelphidiella is apparently associated to the Mi-1 event. Thus, origination of the three genera appear to be related to major global events. Cribrorotalia is recorded since the lower Eocene from the Tierra del Fuego Island, and from the upper Eocene to the upper Miocene in New Zealand; restricted to the high mid latitudes of the Southern Hemisphere, it becomes the most characteristic genus of the shallow paleoenvironments of the Patagonian Cenozoic. Ammoelphidiella was originally described as a monospecific Antarctic genus. Its type species, A. antarctica Conato and Segre, 1974, is considered a fossil guide of the Antarctic Pliocene. The genus is also represented by a new species in the uppermost Oligocene-lowermost Miocene from the Austral Basin (Náñez, 1990; Malumián and Náñez, 1996) up to the Colorado Basin. Another species is known from the lower Miocene of the Victoria Land Basin (Strong and Webb, 2000). Older known records of the genus Ammoelphidiella are due to misidentifications. The distribution of the genus Ammoelphidiella in Patagonia reveals the major penetration of Antarctic waters on the Patagonian Continental Platform in the Oligocene/Miocene transition, recorded mostly by the Martinottiella-Spirosigmoilinella assemblage, a residual agglutinated foraminiferal assemblage with occasional dissolution-resistant calcareous taxa. The wide latitudinal distribution of this assemblage is apparently associated to the Mi-1 event and to a narrowing of the Drake Passage (Malumián and Náñez, 1991; Náñez et al., 2009; Lagabrielle et al., 2009; Malumián and Jannou, 2010). As it was recorded from Antarctica, the tests of Ammoelphidiella exhibit a preferential bioerosion among associated foraminifera (Malumián et al., 2007; Náñez et al., 2007), suggesting that the current has also brought its depredators.