Triassic Samples Research Articles

Paleomagnetic evidence for the rotation of the Iberian Peninsula

The results of a paleomagnetic investigation on igneous and sedimentary rocks from Portugal and Spain are presented. The age of the formations investigated varies from Ordovician to Eocene. Apart from geologic studies the Natural Remanent Magnetizations (N.R.M.) were analyzed with the aid of a.c. magnetic field and thermal demagnetization techniques. In the case of three folded formations the characteristic remanent magnetizations could be proved to be acquired before the subsequent folding took place. These formations are the Upper Silurian Almadén volcanics, the Upper Carboniferous-Lower Permian Buçaco Formation of Portugal and the Eocene basalts of the Lisbon region. All three other Upper CarboniferousLower Permian sample groups of Spain yield similar directions of magnetization. Several groups of Paleozoic and Triassic samples revealed only secondary magnetizations. Together with previous results from the Spanish Meseta and the Spanish Pyrenees, the data are compared with results from Africa and from other European countries. The comparison is satisfactory only for the Upper Carboniferous-Lower Permian results: it indicates that the Iberian Peninsula has rotated relative to that part of Europe north of the Alpine fold belts. This rotation has been counterclockwise over approximately 35°. It is argued that a plausible ancient configuration can be realised by rotating the Iberian Peninsula back to its Permian position, while closing the Bay of Biscay at the 2,000-mdepth line. The pivot point of this rotation lies in the western Pyrenees, as previously suggested by Carey (1958).

The rotation of Spain: Palaeomagnetic evidence from the Spanish meseta

The magnetic properties of 39 oriented samples of Triassic siltstones and of 43 oriented samples of Silurian andesites and basalts from the Spanish Meseta were investigated. After a.c. magnetic field demagnetization up to 3,000 Oe, peak value, and after correction for the dip of the geological strata the Triassic samples revealed a characteristic direction of magnetization with an average declination ( D) of 359.5° and an inclination ( I) of 23°. One group of samples of Silurian volcanic rocks from the province of Guadalajara had, after a.c. magnetic field demagnetization and correction for the dip, a characteristic direction of magnetization with an average declination of 159°C and an inclination of 18.5°; another group from the province of Ciudad Real had, after a.c. magnetic field demagnetization and after correction for the dip, a characteristic direction of magnetization with an average declination of 130.5° and an inclination of 22.5°. The palaeomagnetic pole position derived from the Triassic rocks is situated at 63°N 177.5°E, and the pole positions of the first and second group of Silurian samples are situated at 35.5°N 157°W and 21°N 132°W, respectively. The divergency between these results and the already known data for Triassic and Silurian from stable (extra-Alpine) Europe highly supports a counterclockwise rotation of Spain.

The palaeogeographic significance of isotopic age determinations on detrital micas from the triassic of the Stockport-Macclesfield district, Cheshire, England

A trial investigation of the possible geological application of isotopic age determinations to sedimentary studies. Forty-one new potassium-argon age determinations on detrital mica concentrates from the Triassic of the Stockport-Macclesfield district, Cheshire, and two determinations on detrital micas from the Millstone Grit of north Derbyshire are reported. A general discussion of the theoretical problems involved in the interpretation of isotopic results from detrital micas is followed by a detailed analysis of the sedimentation, petrology and subsequent history of the Triassic samples used in the study. It is shown that the isotopic results can be used to supplement stratigraphical and heavy mineral studies and make palaeogeographic interpretation both easier and more certain. In particular, it is concluded that the bulk of the Triassic of the Stockport-Macclesfield district was almost certainly derived from the south, being the deposits of a river that drained the Wessex-Channel Basin via the Worcester Graben. Differences in their isotopic patterns enable a clear distinction to be made between the Bunter and Keuper deposits, and it is suggested that this kind of work may have economic applications.

Frequency of Meteorite Falls throughout the Ages

A DETAILED investigation of Lower Triassic mud and core samples from north-west German oil wells which I have carried out has revealed the presence of cosmic spherules in considerable numbers. Although magnetic and vitreous spherules originate also from various industrial processes, there seems to be little doubt that the spherules referred to represent genuine cosmic matter, as their main components are magnetite and an iron–magnesium silicate (olivine or bronzite). Thus they have the same composition as the Sikhote-Alin spherules extracted from the soil after the fall of that meteorite. Spherules from deep-sea sediments show the same composition.