Eighty-seven oriented hand specimens of redbeds from the La Quinta Formation (Triassic-Jurassic), collected at four localities in the Venezuelan Andes, were studied. The results, after thermal demagnetization at 500°C, are moderately consistent at two of the four localities with normal and reverse polarization present. The average vectors are closer before correction for present bed attitude than after, a fact which suggests that the natural remanence of these rocks is of secondary origin. In some individual samples both normal and reversed components are present, which indicates that their magnetism was acquired during an interval that spanned at least one reversal of the earth’s field. Comparison of the vectors erased on demagnetization and the residual vectors at the four localities suggests that the magnetism was acquired during the folding, and that its orientation is not related simply to either original or present bed attitude. Measured inclinations conform approximately with that of the present dipole field, but declinations are significantly northeast of it. If the declinations are valid, either a clockwise rotation of South America (for which there is no independent paleomagnetic evidence) or a tectonic rotation within the Venezuelan Andes would be implied.

Two samples of dacitic tuff (believed on tenuous grounds to be of Early Triassic age) were collected near the base of the La Quinta section at the type locality and yield results which are consistent with each other but which are distinct from those of the redbeds; the results from the dacitic tuff probably reflect the only primary magnetization measured in this study. After 150 oersted alternating field demagnetization and correction for bed attitude, their vector mean is D = 13, I = –18, K = 106, and α95 = 25. South coordinates are 68.5°S., 108.5°W.

First Page Preview

First page PDF preview
You do not currently have access to this article.