Late Miocene molluscs from the southwestern Atlantic Ocean (Argentina and Uruguay): a palaeobiogeographic analysis

Abstract

The analysis of the structure and evolutionary history of the Late Miocene marine molluscs along the southwestern Atlantic littoral indicates that these faunas did not give rise to the present assemblages of the Argentinean Province of the same region, the latter being characterised by a typically transitional association (in the sense of Scarabino [(1977) Com. Soc. Malac. Urug. 4, 177–285]) that developed after Miocene times once the cold Malvinas (Falkland) Current was completely operating. The Late Miocene Valdesian and Paranaian Provinces, mainly composed of paratropical warm-water faunas, are defined in this paper. The former includes assemblages recovered from the Puerto Madryn Formation. Its southern limit is placed at 42°S, while in the north its boundary with the Paranaian Province is situated approximately between 37 and 39°S. The Paranaian Province includes assemblages belonging to the Paraná and Camacho Formations. It extends northwards along the Uruguayan and southern Brazilian littoral, but its northern boundary still remains unknown. An incipient thermal gradient is recognised, as the temperature values for the Paranaian Province are slightly higher than those for the Valdesian Province. Taking into account that in the Valdesian Province water temperature was in fact warm, it is concluded that the cold Malvinas (Falkland) Current still had a very low activity in the area during the Late Miocene. The presence of upwelling phenomena is also suggested by the presence of widely distributed turritellid beds in the northern Patagonian region.

Introduction

The main goal of this paper is to discuss the possible links between the Miocene and present molluscan assemblages of the southwestern Atlantic littoral in the light of the biogeographical meaning of the geographic distribution and diversity patterns of the Miocene genera and species in this region. This analysis also allows definition of past molluscan provinces, and provides insight in the changing palaeoclimatic conditions and palaeocurrent patterns.

The modern molluscan provinces recognised along the southwestern Atlantic littoral include, from north to south, the Caribbean, Patagonian and Magellanic Provinces (Valentine, 1973), or, alternatively, the Brazilian (i.e. the southern portion of Valentine’s Caribbean Province), Argentinean and Magellanic Provinces (Scarabino, 1977, Briggs, 1995; Fig. 1). The latitudinal thermal gradient is controlled in the study area by the shallow and cold Malvinas (Falkland) Current (MC) that flows northwards and by the warm Brazilian Current that moves from north to south. Because changing palaeogeographic and palaeocirculation patterns characterised the Southern Hemisphere, we are in need of a comprehensive study of the past molluscan provinces that preceded the present ones.

The Miocene assemblages from southern South America are especially important for addressing this subject because they reflect a southward displacement of the water thermal gradient in comparison with the present one. This would point to the development of warmer conditions in the area than nowadays (Martı́nez, 1989, del Rı́o, 1990), thus being an important clue in understanding the timing of the opening of the Drake Passage and the subsequent development of the MC in the region after the final implantation of the Circumpolar Antarctic Current.

Questions to be solved include: (1) Is there any relationship among the molluscan assemblages recorded from geographically separate ‘Entrerriense’ deposits? (2) Are there any differences supporting the existence of different molluscan provinces during Miocene times? (3) How much do these patterns differ from the present ones along the southwestern Atlantic littoral? (4) Is it possible to consider the Miocene faunas, and, consequently, the Miocene provinces as prefigurations of the present Argentinean and Magellanic ones? (5) Is it possible to get any palaeoclimatic data to identify a changing palaeogeographic pattern through Neogene times?