Geology of the Cerro Negro Formation (Cretaceous) in Livingston Island: contributions to its geochronology and paleontological content.

Abstract

Byers Peninsula is located on the westernmost part of Livingston Island in the South Shetland Islands. Geologically this peninsula is recognized by the marine and non-marine sedimentary sequences and the Cretaceous-Tertiary volcanism, belonging to the uppermost levels of the Byers Group, named Cerro Negro Formation. The magmatic sequences are divided in two main groups, pyroclastic rocks, divided into five units in the area. The second group of lavas and intrusive rocks placed at Don Carlos Hill, Usnea Plug, Chester Cone, Sealer Hill and Negro Hill. The sedimentary rocks are green clastic rocks (conglomerates, sandstones to mudstones) with rhythmic, convolute, thin bedded and crossed structures. The Cerro Negro Formation is related to a fluvial environment with a rain regime, high energy, decreasing in time, the development of lakes and shallow lagoons suitable for a good preservation of the macro and microflora. At the new fosiliferous localities, Rotch Dome, Lomada de las Plantas (Plants Hill), Don Carlos Hill and 904, have been recognized: Araucarites sp., Archangelskya furcata Herbst, Cladophlebis sp., Dictyozamites sp., Elatocladus sp, Phyllopteroides sp., Pseudoctenis sp., Ptyllophyllum menendezii Cantrill, Sphenopteris sp., Eocyathea remesaliae Césari, Sergioa austrina Césari and Taeniopteris sp. Finally some new radiometric ages are presented for the pyroclastics, intrusions and lavas of the Cerro Negro Formation. Unit 1 was dated 125 ± 7 Ma, unit 2 (Don Carlos ignimbrites) 110 ± 30 (errocron) Ma the andesites of the Usnea Plug 92 ± 5 Ma. Two radiometric data were coincident in 63 ± 2 Ma for the Sealer eruptions. Finally, the youngest age Byers Peninsula, was obtained on the basalts from Don Carlos Hill (32 ± 1 Ma).