The Geology of the General Bernardo O'Higgins Area, Northwest Antarctic Peninsula1

  1. Jarvis B. Hadley
  1. Martin Halpern

Published Online: 3 APR 2013

DOI: 10.1029/AR006p0177

Geology and Paleontology of the Antarctic

Geology and Paleontology of the Antarctic

How to Cite

Halpern, M. (1965) The Geology of the General Bernardo O'Higgins Area, Northwest Antarctic Peninsula1, in Geology and Paleontology of the Antarctic (ed J. B. Hadley), American Geophysical Union, Washington, D. C.. doi: 10.1029/AR006p0177

Author Information

  1. Geophysical and Polar Research Center, University of Wisconsin, Madison

Publication History

  1. Published Online: 3 APR 2013
  2. Published Print: 1 JAN 1965

ISBN Information

Print ISBN: 9781118655733

Online ISBN: 9781118668528



  • Geochemistry of plutonic rocks;
  • Metamorphic rocks;
  • Northwest Antarctic Peninsula;
  • O'Higgins area;
  • Plutonic intrusive suite;
  • Volcanic and hypabyssal rocks


The O'Higgins area lies in a deformed orogenic or mobile belt. Detailed field mapping, petrographic examination of sedimentary and igneous rocks, potassium-argon dating of key rocks, and paleontologic evidence allow us to assign the name Legoupil Formation to the part of the Cretaceous Period that dates from about 75 to 115 m.y. ago; they also provide details of stratigraphy and tectonic history of the O'Higgins area. This study exemplifies the usefulness of radiogenic dating for establishing age relationships of igneous rocks involved with unfossiliferous mobile belt deposits. The study also stresses the extreme variability of sediment types in mobile belts as the Legoupil Formation is characterized by an abundance of quartz-rich sands and a paucity of graded ‘turbidires.’

The Legoupil Formation's Cretaceous age is revealed by the presence of Platopis sp. (?) and by potassium-argon absolute ages of 116±4 m.y. for a diorite pebble from a pebbly mudstone unit, of 86±7 m.y. for andesite flows inferred to overlie the formation with angular unconformity, and of 74.7±2.8 m.y. for an andesite dike that cuts metamorphosed sediments of the formation. The Legoupil Formation includes argillite, quartz wacke, quartz-feldspathic wacke, quartz-felds-pathic arenite, arkosic arenite, granule and pebble conglomerate, and pebbly mudstone. Local occurrences of phyllite are believed to be the result of dynamic metamorphism of argillaeeous sediments. A sedimentary breccia crops out in several isolated areas, but its stratigraphic relationship to the Legoupil Formation is uncertain. Compositionally, the Legoupil Formation sands are submature. Texturally, the wackes are microbreccias. Localized occurrences of well-sorted arenite, believed to have resulted from winnowing action in shallow water, represents an unusual assemblage. Cretaceous sedimentation occurred within a north-northeast trending geosynclinal belt. Source areas consisted of nearby tectonic lands with acidic plutons and associated metamorphic rocks and possibly other areas farther south and east. The varied nature of the sediments implies an ocean floor irregularly dispersed with troughs and rises; although sedimentation was probably rapid, it was not restricted to a deep-water environment.

Potassium-argon dating of quartz diorite and gabbro plutons yielded dates of 75±8 m.y. and 100±20 m.y., respectively. Although the contact relationships are not visible, the absolute ages imply that the quartz diorite intruded the Legoupil Formation, whereas the gabbro body may have either intruded the formation or acted as a source rock for it, assuming that the gabbro lay exposed to weathering during the deposition of the Legoupil Formation. From absolute ages, major faulting in the area is inferred to be intra-late Cretaceous.

The geochemistry of the plutohic rocks in the Cape Legoupil and Doumer Island areas reveals a definite chemical affinity with the so-called ‘andean Intrusive Suite’ to which others have assigned a late Cretaceous or early Tertiary age. Fossil evidence from the O'Higgins area and the results of nuclear dating prove that this intrusive activity also occurred in early Cretaceous and mid-Eocene time.