BIOEROSION ON TESTS OF TYPICAL TEMPERATE-COOL WATER FORAMINIFERA FROM THE CENOZOIC OF PATAGONIA, TIERRA DEL FUEGO ISLAND AND THE ARGENTINEAN CONTINENTAL SHELF

NÁÑEZ, C., LÓPEZ, M.I., MALUMIÁN, N, Y SCARPA, N.

Ushuaia

Simposio; Quinta reunión Argentina de Icnología y Tercera Reunión de Icnología del Mercosur; 2007

Laboratorio de Geología Andina, CADIC-CONICET

The genera “Buccella” and Ammoelphidiella, from the cool-temperate, shallow-water Oligocene-Pliocene foraminiferal assemblages of Patagonia, Tierra del Fuego and the adjacent continental shelf, are frequently bored. A biogenic origin is clear for some holes, whereas in several cases, abrasion or corrosion cannot be disregarded. “Buccella” and Ammoelphidiella share a low trochospiral biconvex test with a partially tuberculate wall surface, being “Buccella” one of the most abundant infaunal taxa of these assemblages. In contrast, borings in Cribrorotalia, one of the most abundant epifaunal genus, are rarely found. Site selectivity is evident: in Ammoelphidiella, the borings are preferentially situated close to the sutures and apparently related to the sutural pores; they are mostly restricted to the spiral side and periphery of the test, and absent on the tuberculate areas of the umbilical side. “Buccella” is usually bored on the sutures of the umbilical side, close to the periphery; some forms exhibit slip-shaped holes close to the sutures on the spiral side, apparently related to the presence of sutural pores.             Borings in the studied material range in size from about 5µm to 40µm. The following boring categories are recognized according to their position in the test: a) holes close to the sutures on the umbilical side, near the periphery; sometimes paired or multiple, at one side or at both sides of the septum, subcircular, irregular or crescent-shaped; b) holes located on the sutures on the spiral side, subcircular, irregular or slit-shaped, the latter might result from the coalescence of subcircular holes; c) holes on or close to the periphery, and d) simple or paired holes on the central part of the chambers, or with an apparently random distribution.             Most of the studied borings are assigned to the ichnogenus Oichnus Bromley 1981 and its species O. simplex Bromley 1981, O. paraboloides Bromley 1981, and O. coronatus Nielsen and Nielsen 2001. Unfinished holes and circular grooves assigned to Fossichnus solus Nielsen, Nielsen and Bromley 2003 are also identified, supporting the cool-temperate distribution previously suggested for this ichnotaxon.