Cenozoic geologic evolution of the lower Colorado River Basin, Northern Patagonia, Argentina

María I Vélez,Gerardo M.E Perillo,Carina Seitz

doi:10.5027/andgeov46n1-3060

Abstract

Climatic changes and eustatic sea levels have been assumed to be the most important controllers of the Colorado River alluvial fan in northern Patagonia. Although the alluvial fan occurs in a region considered tectonically stable, there are pieces of evidence that the Miocene Andean orogeny has reactivated inherited structures, with subsequent geomorphological changes that date back to the Pleistocene. Besides, the clear evidence of neotectonism in the region and their effects on the evolution of this fan, it has not been studied in detail yet. In this study, we map and analyze six sections outcropping in different terraces of the alluvial fan with the primary aim of disentangling the role of tectonism, climate and eustatic changes on the evolution of the alluvial fan. This study is part of a bigger project aimed to understand the origin of the shallow lakes occurring in northern Patagonia. Our results indicate that the alluvial fan of the Colorado River was established in the area around the Middle Pleistocene. Evidence of deformations in Miocene to Pleistocene units indicates significant neotectonism during the Upper Pleistocene. By the Pleistocene-Holocene transition, tectonism produced incision generating a set of terraces. After this time, an important climate change from semiarid to arid favored the calcretization of some terraces. By the Pleistocene-Middle Holocene, the terraces were covered by ancient eolian sediment accumulated during dry conditions. By the Middle Holocene, a broad alluvial fan developed in the region under a warmer and more humid climate generating the Alluvial Colorado River-III deposit at the T3 terrace. In the late Holocene, aggradation process was favored by a high sea level and temperate-arid climate, producing T4 terrace. At the same time, this climate condition favored the local deflation-sedimentation processes that resulted in the deposition of modern eolian deposits (mE) over the T3 terrace. The depressions generated by the deflation were, later on, occupied by shallow lakes when the climate turn more humid. Subsequently, during regressive sea level condition, ca. 2000 years BP, the T4 terrace was partially eroded and the modern alluvial plain formed.

Highlights

The study area is located in the morphostructural Colorado Basin (Fig. 1)
We have focused our study on the geologic processes that controlled the evolution of Colorado River alluvial fan mainly during the PleistoceneHolocene and their relationship with the origin of shallow water lakes existing in the area, which, up to day, still remains unclear
The calcareous blanket and the reddish brown conglomerated will be refered as Calcretized alluvial deposits Level-I (“cACR-I”)

Summary

Introduction

The study area is located in the morphostructural Colorado Basin (Fig. 1). It is a rift basin formed in the Upper Jurassic associated with the initial opening of the South Atlantic. Most of the Colorado basin develops offshore, roughly in an E-W direction (Folguera et al, 2015). The sedimentary basin fill includes sediments up to the Neogene, which have a maximum thickness of 7,000 m (Fryklund et al, 1996; Juan et al, 1996). Despite the location of the Colorado Basin in the extraandean area, a significant influence from the Andean orogeny has been recognized in these distal Upper Miocene and Pliocene synorogenic sequences, associated with subsidence occurring simultaneously with the Andean uplift (Folguera et al, 2015). Folguera and Zarate (2018) suggested that the mechanisms responsible for the deformation of these synorogenic sedimentary sequences are part of the dynamics of

Methods

Results

Discussion

Conclusion