THE AGE OF THE PUNTA TORCIDA FORMATION, EOCENE, MAGALLANES BASIN, ARGENTINA: THE GENERA PLANOROTALITES AND IGORINA (PLANKTONIC FORAMINIFERA)

MALUMIÁN, N., NÁÑEZ, C. Y JANNOU, G.

Córdoba

Congreso; XIX Congreso Geológico Argentino; 2014

The Eocene planktonic foraminifera assemblages from the Austral or Magallanes Basin are dominated by small-sized tests and devoid of thermophilic biostratigraphic markers, due to the marginal character and extratropical position of the basin. So that, the biostratigraphy based on planktonic foraminifera mostly relies on small-test taxa. These include microperforate genera such as the serial Chiloguembelina and Jenkinsina and the low trochospiral Tenuitella, (Malumián et al. 2009; Malumián and Jannou 2010); and on the other hand, normal perforate, small-test genera such as Planorotalites and Igorina, recorded from the Agua Fresca Formation, in Chile, and the equivalent Punta Torcida Formation in Argentina. The Agua Fresca Formation (2300m thick, light gray clay shale with large calcareous concretions, and glauconitic horizons in its lower part) was initially assigned by Todd and Kniker (1952) to the Upper Eocene. These authors also virtually proposed a zonation based on species of Elphidium. Charrier & Lahsen (1968, 1969), based on planktonic foraminifera, assigned the formation to the upper Paleocene-lower Eocene. Hromic (1989-1990) recorded Globanomalina australiformis (Jenkins) throughout the formation. The first appearance of this species in high southern latitudes is used to indicate the Paleocene/Eocene boundary, and its last appearance, influenced by latitude, is estimated in 44,2 Ma (Huber and Quillévéré 2005, 2006). In a monographic revision of the genus Acarinina, Berggren et al. (2006) illustrated A. esnaensis (Le Roy) and a questioned A. interposita Subbotina (Plate 9.11, fig. 9-11; 17-19) just from the Agua Fresca Formation, indicating a middle early Eocene age (Zones E5-E6, of Berggren and Pearson 2005), older than 51 Ma according to the LOD (last occurrence datum) of both species. A similar age was assigned to the Punta Torcida Formation (200m thick, dark gray mudstones with thin sandstones, divided into three members) by Malumián and Olivero (2006) and Olivero and Malumián (2008), consistent with the benthic foraminiferal content common to both formations, particularly the co-occurrence of Elphidium aguafrescaense and E. skyringense in both the lower middle part of the Agua Fresca Formation (Todd and Kniker 1952) and throughout the Punta Torcida Formation (Malumián and Jannou 2010). Barbeau et al. (2009) assigned the Punta Torcida Formation to the Middle Eocene based on a maximum depositional age from detrital zircon of 47,32 ±0,77 Ma from a sample located north of Cabo Irigoyen, Atlantic coast of Tierra del Fuego Island, but without formational member identification. The apparent age disagreement promoted a revision and further search of small-test planktonic species in the three members of the Punta Torcida Formation (Malumián et al. 2013). At the formational type locality, Cabo Campo del Medio, Planorotalites pseudoscitulus (Glaessner) was found, relatively abundant in the lower member, and scarce and associated to Igorina convexa (Subbotina) in the middle member; the upper member is barren or with very poorly preserved material. In the upper member, at the Cabo Irigoyen Section, P. pseudoscitulus is missing and replaced by abundant small Igorina convexa (Subbotina). The abundance of P. pseudoscitulus in the lower member and its record in the middle member in the type locality of the Punta Torcida Formation support an age not younger than the Zone E7 (>49Ma, lower Eocene); P. pseudoscitulus also occurs in the Agua Fresca Formation (identified as Planorotalites australiformis (Hromic 1990, fig. 3.5)). According to Olsson et al. (1999), I. convexa has its LOD in the Zone P7 (of Berggren 1995) or E5, mid early Eocene. At the Cabo Irigoyen section, the upper member also contains the small-test and microperforate species Jenkinsina columbiana (Howe) and J. triseriata (Terquem), associated to normal perforate Subbotina patagonica (Todd and Kniker), Acarinina interposita Subbotina, A. mcgowrani Wade and Pearson, Globanomalina australiformis (Jenkins), Globorotaloides eovariabilis Huber and Pearson, G. quadrocameratus Olsson, Pearson and Huber, Globoturborotalita bassriverensis Olsson and Hemleben, and P. wilcoxensis (Cushman and Ponton), all species known in the Zones E5-E7. Concluding remarks: The lower and middle members of the Punta Torcida Formation at the type locality and the upper member at the Cabo Irigoyen Section have an early Eocene age, on the whole equivalent to the Zones E5-E7 interval, 49-52,2 Ma. The zircon age could come from stratigraphically younger beds such as the ?unnamed strata? that overlie the Punta Torcida Formation (fide Olivero and Malumián 2008).