Daniel A. Valencio

Palaeomagnetism and K-Ar ages of triassic igneous rocks from the Ischigualasto-Ischichuca Basin and Puesto Viejo Formation, Argentina

Abstract The palaeomagnetism of Middle Triassic (224 ± 5 m.y.) igneous rocks from the Ischigualasto-Ischichuca Basin (67°40′W, 30°20′S) was investigated through 86 oriented hand samples from 11 sites. At least one reversal of the geomagnetic field has been found in these rocks. Nine sites yield a palaeomagnetic pole at 239°E, 79°S (α95 = 15°, k = 13). The K-Ar age determinations of five igneous units of the Puesto Viejo Formation give a mean age of 232 ± 4 m.y. (Early Triassic). The palaeomagnetism of six igneous units of the Puesto Viejo Formation (68°W, 35°S) was investigated through 60 oriented samples. These units, two reversed relative to the present magnetic field of the Earth and four normal, yield a pole at 236°E, 76°S (α95 = 18°, k = 14). Data from the Puesto Viejo Formation indicate, for the first time on the basis of palaeomagnetic and radiometric data, that the Illawarra Zone, which defines the end of the Kiaman Magnetic Interval, extends at least down to 232 ± 4 m.y. within the Early Triassic. The palaeomagnetic poles for the igneous rocks of the Ischigualasto-Ischichuca Basin and Puesto Viejo Formation form an “age group” with the South American Triassic palaeomagnetic poles (mean pole position: 239°E, 77°S; α95 = 6.6°, k = 190). The Middle and Upper Permian, Triassic and Middle Jurassic palaeomagnetic poles for South America would define a “time group” reflecting a quasi-static interval (mean pole position: 232°E, 81°S; α95 = 4°, k = 131).

Palaeomagnetism and KAr age of Mesozoic and Cenozoic igneous rocks from Antarctica

Abstract A new analysis of palaeomagnetic data for igneous rocks from Deception Island, 25 de Mayo Island (King George Island) and Cape Spring, are given. K-Ar age determinations indicate that most of the igneous samples from 25 de Mayo Island included in the palaeomagnetic study are of Late Mesozoic/Early Tertiary age. The significance of these palaeomagnetic-radiometric data on the hypothesis of oroclinal bending of the Antarctic Peninsula and on the apparent polar movement of Antarctica is discussed. The positions of palaeomagnetic poles for the Andean igneous complex indicate that there has not been any apparent post-Late Cretaceous/Early Tertiary oroclinal bending in the Antarctic Peninsula from 74°S to 62°S. A comparison of the positions of palaeomagnetic poles for Antarctica and Australia suggests that the direction of apparent polar movement relative to Antarctica reversed after the Miocene.

Palaeomagnetism of Late Palaeozoic and Early Mesozoic rocks of South America

Abstract Further geological evidence and radiometric studies of South American igneous rocks suggest, (i) a Middle Permian age for geomagnetic poles until now quoted as Permo-Triassic or Lower Triassic, and (ii) a Middle-Upper Triassic age for a geomagnetic pole previously quoted either as Middle or Upper Triassic. The palaeomagnetic and radiometric data from igneous rocks of the Quebrada del Pimiento Formation of Argentina, define a period of normal polarity within the Late Palaeozoic Interval of reversed geomagnetic polarity (Kiaman Magnetic Interval); the possibility of using this short normal period for geological correlation is suggested. The Late Palaeozoic geomagnetic poles now available for South America define the polar path relative to this continent fairly well; it is suggested that this polar path is also valid for Africa, South America and Africa having been joined by their Atlantic margins throughout the Late Palaeozoic and Triassic. The discrepancy between the positions of Permian geomagnetic poles of South America and Australia is pointed out. It could represent different polar shifts relative to South America and Australia and could imply that the fragmentation of Gondwana started in Early Permian times with the formation of two super-continents: South America-Africa and Australia-Antarctica-India.

Palaeomagnetism of the Lower Cretaceous Vulcanitas Cerro Colorado Formation of the Sierra de los Cóndores Group province of Córdoba, Argentina

Abstract Measurements are described of the directions of magnetization of 101 samples from six lava flows, one dyke and redbeds from the Lower Cretaceous Sierra de los Condores Group, Argentina. The magnetic remanence of the lowest basaltic flow (Embalse Rio Tercero Formation) was unstable. Primary remanent magnetizations of the other five lava flows, the dyke and redbeds (Vulcanitas Cerro Colorado Formation) were isolated after detailed A.C. and thermal cleaning; all these units have reversed magnetization. On the basis of the mean direction, referred to the bedding, of cleaned remanent magnetization of each unit a virtual geomagnetic pole is computed; the mean of these virtual geomagnetic poles, SAK 2 , is: 14°E 81°S, α 95 = 13°; we think that SAK 2 is a geomagnetic pole. The position of the pole for the Vulcanitas Cerro Colorado Formation (118.5 ± 6 my) is reasonably close to the Serra Geral Formation pole (115–125 my); this supports the interpretation that South America and Africa were separated in Lower Cretaceous time and suggests that the earth magnetic field was, on average, similar to that of a geocentric dipole in the Lower Cretaceous. The reversed remanent magnetization and the K-Ar age (118.5 ± 6 my) of the Vulcanitas Cerro Colorado Formation define the existence of a previously unrecognized period of reversed polarity at the Lower Cretaceous.

The South American palaeomagnetic data and the main episodes of the fragmentation of Gondwana

Abstract The South American palaeomagnetic poles published after the Upper Mantle Conference on Solid Earth Problems held at Buenos Aires in 1970, are summarized. The Late Palaeozoic-Cretaceous section of the South American polar wandering curve is now defined on the basis of twenty palaeomagnetic poles; these poles define five “age groups” at Late Carboniferous, Permo-Carboniferous, Middle Permian, Triassic and Cretaceous times. The comparison of the Late Palaeozoic-Mesozoic sections of the polar wandering curves of South America, Australia and Africa suggests that the former fragmentation of the Gondwana occurred in Late Carboniferous or Permo-Carboniferous times and that the origin of the South Atlantic Ocean took place after the Middle Jurassic (160 m.y.) but before the Early Cretaceous (120 m.y.).

Palaeomagnetism and K-Ar dating of the Carboniferous Andacollo Series (Argentina) and the age of its hydrothermal overprinting

Abstract Palaeomagnetic data from 15 hand samples collected in a sequence of tuffs of Carboniferous age (Andacollo Series) exposed in western Argentina (37°10′S, 70°38′W) are given. These samples show hydrothermal alteration. After AF and thermal cleaning the majority of samples show reversed polarity and yield a palaeomagnetic pole at 73.6°S, 64.7°E (dψ = 6°,dχ = 9°). By comparing the position of this pole with the apparent polar wander curve recognized for the Late Palaeozoic and Mesozoic of South America, quantitative support for a Cretaceous age for the cleaned remanence of these samples is given. K-Ar dating of the Andacollo rocks indicates a mean age of 69 ± 4m.y.; this suggests that the isotopic system of these rocks became closed in Late Cretaceous time. Palaeomagnetic and radiometric data agree indicating a Cretaceous age for the hydrothermal alteration of the Andacollo rocks.

Magnetostratigraphy of a Middle Jurassic red bed sequence from southern Mexico

Abstract Palaeomagnetic results for 178 oriented samples from five sections (combined thickness of 75 m) of the red bed Tecomazuchil Formation, southern Mexico are reported. The formation has been assigned to the Bathonian(?)-Callovian, and the sections studied cover its uppermost portion. Alternating field (a.f.), thermal, combination of a.f. and thermal, and chemical demagnetizations were used to investigate the magnetic stability and vectorial directions of all samples. Most samples showed apparent stable behaviour with dominantly single or two-component magnetization. Reverse and intermediate polarity samples more often showed two- and multi-component remanences. Behaviour during demagnetization and rock-magnetic experiments indicate that the remanences reside in magnetite, haematite and goethite. The magnetostratigraphic results reveal a polarity pattern characterized by frequent polarity changes.