U-Pb zircon constraints on the age of the Cretaceous Mata Amarilla Formation, Southern Patagonia, Argentina: its relationship with the evolution of the Austral Basin

Augusto N Varela,Daniel G Poiré,Simone Hoffmann,Javier N Gelfo,Axel Gerdes,Francisco J Goin,Thomas Martin

doi:10.5027/andgeov39n3-a01

Abstract

Despite the abundant fossil content of the Mata Amarilla Formation (Southern Patagonia, Santa Cruz Province, Argentina), its age has always generated a considerable number of questions and debates. The chronological data provided by invertebrates, dinosaurs, fish, turtles, plesiosaurs and fossil flora are contradictory. In this work, twenty U-Pb spot analyses by laser ablation were carried out on the outer parts of the zoned zircon crystals from a tuff layer of the middle section of the Mata Amarilla Formation, yielding a U-Pb concordia age of 96.23±0.71 Ma, which corresponds to the middle Cenomanian. The deposition of the lower section of the Mata Amarilla Formation marks the onset of the foreland stage of the Austral Basin (also known as Magallanes Basin); this transition is characterized by the west-east shift of the depositional systems, which is consistent with the progradation of the Cretaceous fold-and-thrust belt. Thus, the onset of the foreland stage could have occurred between the upper Albian and lower Cenomanian, as the underlying Piedra Clavada Formation is lower Albian in age. On comparing the data obtained with information from the Ultima Esperanza Province in Chile, it can be suggested that the initiation of the closure of the Rocas Verdes Marginal Basin occurred simultaneously.

Highlights

The study area is located in the southwest of the Santa Cruz Province, Argentina, near the locality of Tres Lagos (Fig. 1).The Mata Amarilla Formation is a key unit in the development of the Austral Basin, as it marks the transition between the thermal subsidence stage and the foreland stage (Varela, 2009, 2011)
Twenty U-Pb spot analyses by laser ablation were carried out on the outer parts of the zoned zircon crystals from a tuff layer of the middle section of the Mata Amarilla Formation, yielding a U-Pb concordia age of 96.23±0.71 Ma, which corresponds to the middle Cenomanian
The deposition of the lower section of the Mata Amarilla Formation marks the onset of the foreland stage of the Austral Basin; this transition is characterized by the west-east shift of the depositional systems, which is consistent with the progradation of the Cretaceous fold-and-thrust belt

Summary

Introduction

The study area is located in the southwest of the Santa Cruz Province, Argentina, near the locality of Tres Lagos (Fig. 1).The Mata Amarilla Formation is a key unit in the development of the Austral Basin, as it marks the transition between the thermal subsidence stage and the foreland stage (Varela, 2009, 2011). The type section is located on the southern margin of the Río Shehuen or Chalía, approximately 23 km east of the locality of Tres Lagos, in the area surrounding the Estancia Mata Amarilla, known as Estancia La Soriana, (locality 3, Fig. 3). It overlies the Piedra Clavada Formation, which is lower Albian in age, with transitional contact and it is unconformably covered by the Campanian sediments of the La Anita Formation (Varela and Poiré, 2008; Varela, 2009, 2011) (Fig. 4). This formation (see Fig. 4) was deposited during the early Upper Cretaceous, extending from the Cenomanian to the Santonian (Poiré et al, 20071; Varela and Poiré 2008; Varela, 2009)

Methods

Discussion

Conclusion