Abstract

The Neogene benthic foraminiferal ecostratigraphy of offshore Pelotas Basin has not yet been researched. The benthic foraminifera were examined in twenty-nine ditch cuttings samples from Well 1-SCS-3B, present water depth 200 m. These samples ranged from the Lower Miocene Catapsydrax dissimilis planktonic foraminiferal Zone to the Pliocene Globigerinoides trilobus fistulosus Zone. Only ditch cuttings were available, which allowed us to present just broad paleoenvironmental trends within the studied section. Benthic foraminifera indicate deposition mostly between outer neritic to upper bathyal paleodepths. A peak in uvigerinid relative abundance marks a possible flooding surface or condensed section with a high nutrient flux, low diversity (marked by the Shannon Function H), and sluggish circulation within the Globorotalia mayeri Zone. This peak is bracketed by high abundances of Cibicidoides, which indicate stronger current action than within the condensed section. Biostratigraphic hiatuses within these Cibicidoides-rich intervals are concluded not to indicate subaerial exposure, but to reflect current action sufficiently strong to induce either non-deposition or seafloor scouring. A second possible flooding surface occurs at the top of the section, indicated by a decline in Cibicidoides associated with upticks in Sphaeroidina and Bolivina and a dip in H. The top of this younger maximum flood was not encountered, probably occurring above the studied section. That the timing of these floods differs from those recorded elsewhere in the Pelotas Basin implies that they were tectonically induced, perhaps as distal effects associated with the interaction between the South American and Nazca tectonic plates. Future studies with better quality core samples will add detail to this picture; such studies can incorporate eustatic sea-level changes into the model presented.

Highlights

  • The Pelotas Basin is situated in south-easternmost Brazil and southeast Uruguay

  • The benthic foraminifera were examined in twenty-nine ditch cuttings samples from Well 1-SCS-3B, present water depth 200 m

  • A total of 7942 benthic foraminiferal specimens were recovered from the samples

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Summary

Introduction

The Pelotas Basin is situated in south-easternmost Brazil and southeast Uruguay. Zoogeographically, its offshore portion currently straddles the boundarySubmitted on: 25/April/2020 Approved on: 27/July/2020 Associate Editor: Felipe Toledo Editor: Rubens M. The Pelotas Basin is situated in south-easternmost Brazil and southeast Uruguay. Its offshore portion currently straddles the boundary. Submitted on: 25/April/2020 Approved on: 27/July/2020 Associate Editor: Felipe Toledo Editor: Rubens M. Lopes between the West Indian Province and the North Patagonian Subprovince of the Argentine Province at ~30°S (see Boltovskoy et al, 1980, Marengo, 2015). During the Cenozoic, Uruguayan deposition was mostly continental. Sprechmann (1978), in an examination of the paleontology of a series of water wells drilled on the Uruguayan coastal plane, found marine fossils in the Chuy 364 borehole 53°26’W), drilled in the southern portion of the Pelotas Basin. His study ushered in a series of examinations of the Pelotas Basin marine

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