Biotic and environmental changes across the Cretaceous/Paleogene boundary in Patagonia

CASADÍO, S.; GRIFFIN, M.; PARRAS, A.; CONCHEYRO, A.; FELDMANN, R.; GASPARINI, Z.; PARMA, S. G.

San Luis

Simposio; 10º Reunión Argentina de Sedimentología, Simposio límite K/T de Argentina; 2004

Asociación Argentina de Sedimentología

The late Maastrichtian molluscan fauna of the Neuquén Basin includes species of characteristically austral genera (e.g. Neilo, Australoneilo, Lahillia, Austrotrigonia and Pacitrigonia, supporting the northward extension, into Northern Patagonia, of the range of some elements of the Weddellian Province during the late Maastrichtian (Zone CC26; Perch-Nielsen, 1985). Concheyro and Villa (1996) recorded the presence of calcareous nannofossil taxa typical of high latitudes and suggested that this could be related to a northward expansion of the Falkland Plateau Bioprovince, as defined by Wind (1979), which would have reached north to 36ºS during Chron 30N. Similarly, Gasparini et al. (2003) pointed out that the northern Patagonian Maastrichtian elasmosaurids are closely related to those of the same age from the Antarctic Peninsula and New Zealand, they supporting the hypothesis of a southern Gondwana distribution for some pelagic reptiles (Novas et al., 2002). This contradicts Olivero et al. (1990), who believed that the Northern Patagonian Masssif acted as a paleogeographic barrier restricting the connection between the Chubut River area and the area north of the Colorado River. Displacement of high latitude species towards mid latitudes is probably related to a long-term cooling of surface and/or deep marine water over the Campanian - Maastrichtian interval, as indicated by numerous oxygen isotopic studies (Barrera and Savin, 1999 and references therein). The δ18O records of planktonic and benthonic foraminifera from most locations indicate that surface and intermediate waters became cooler during the period spanning 75 to 65.4 Ma, particularly at high latitudes. This episode seems to have terminated rapidly at 65.4 Ma, as both intermediate and surface waters warmed globally by about 2ºC (Barrera and Savin, 1999). Li and Keller (1998) pointed out that the water paleotemperatures obtained at ODP (Ocean Drilling Program) Site 690 indicate cooling of surface water from about 11ºC at 70 Ma to about 6ºC at 67 Ma. Climate changes during the last 2 m.y. of the Maastrichtian indicate a relatively cool mid latitude South Atlantic, interrupted by a short-term warming of 2-3ºC in intermediate water temperatures at the base of Chron 29R (Li and Keller, 1999). At the beginning of Chron 29R and associated to species of clear austral affinities, appear for the first time in the Neuquén Basin some endemic taxa such as the oldest representatives of the oysters Cubitostrea and Turkostrea, together with newly arrived species of Arca, Atrina, Amphidonte, Gyrostrea, Veniella, Plicatula and Camptonectes, all of them with Tethyan affinities. This is probably related to a global increase of about 3-4ºC (Barrera and Savin, 1999) in surface and intermediate water temperature at 65.5 Ma., and also to the sharpest increase of the relative sea level recorded in the Neuquén Basin during the Cretaceous. Other invertebrate groups appear also as from Chron 29R, i.e., stomopneustids (Gomphechinus; Parma and Casadío, in press) and decapod crustaceans (species of Lobonotus and Xanthilites; Feldmann et al., 1995). From the early Danian onward the molluscan fauna of the Neuquén Basin was dominated by warm-water taxa. These show clear affinities with other of similar age recorded in northern Brazil, the Caribbean and northern Africa. Among these taxa we can mention several species of nautiloids (Casadío et al. 1999). This is coincident with the record of a shallow water scleractinian coral reef (Baron-Szabo et al., 2003), and of two species of crabs with Tethyan affinities, i.e., Thaumastoplax rocaensis Feldmann, Casadío, Chirino-Galvez and Aguirre-Urreta, 1995 and Costacopluma australis Feldmann, Casadío, Chirino-Galvez and Aguirre-Urreta, 1995, and several taxa of echinoids, i.e., species of Micropsis and Paraster (Parma and Casadío, in press). Analysis of the geographic distribution of the molluscs suggest that the modifications observed in the marine fauna across the Maastrichtian-Danian boundary in northern Patagonia were the result of important environmental changes, related mainly to a drop of the relative sea level during the Danian, which in turn probably modified oceanic circulation, water temperature, oxygenation and salinity, contributing all towards the observed faunal changes (Casadío, 1994). The long-term cooling of surface and/or deep marine waters over the Campanian-Maastrichtian interval could have facilitated a northward dispersion of the Weddellian fauna into northern Patagonia. Near the end of the Cretaceous, the warm climate event that took place during Chron 29R favored immigration of species arriving from lower latitudes. The arrival of these species coincided with the onset of mainly carbonatic sedimentation in the northern area of the Neuquén Basin. Such an increase in water temperature, in addition to the descent of relative sea level after the K/P boundary would have caused an increase in salinity that becomes more clearly evident towards the top of the Roca Formation (Zone NP4), in the central area of the basin. Such a salinity change probably was the cause of the significant decrease in the diversity of the fauna in the area during that time