Magnetostratigraphy and sedimentology of the Lajas Formation at Portada Covunco, Mendoza

KIETZMANN, D.A.; KOHAN MARTÍNEZ, M.; IGLESIA LLANOS, M.P.

Buenos Aires

Simposio; VII Simposio Argentino Jurásico; 2019

Igeba Uba

A systematic paleomagnetic study was carried out in the Lajas Formation (MiddleJurassic), cropping out at the Covunco creek in the central part of the Neuquén basin. TheLajas Formation at the Portada Covunco section consists of a succession of conglomerates,sandstones and mudstones that reaches c. 290 m-thick. On the basis of lithological andtextural features, sedimentary structures, strata geometries, and fossil content, 14 facies weredefined that were grouped in two facies associations within a fluvial-dominated delta system.The age of the Lajas Formation is rather controversial and has been assigned from sequencestratigraphical and palynological data. Thus, from south to north, interpreted ages span theUpper Aalenian?Middle Bathonian through the Callovian. Therefore, magnetostratigraphywas used to better precise the age of the Formation at Portada Covunco. Paleomagnetic dataobtained from 29 sampling sites distributed along c. 290 m-thick section present reliablebehaviors. Rock magnetic studies (thermomagnetic curves, K vs H curves, IRM acquisitioncurves) suggest that the remanence is carried by Ti-poor magnetite. Anisotropy of magneticsusceptibility data show clustering of the Kmax axes produced by the action of paleocurrents.The characteristic remanent magnetization passed a reversal test that proves its primaryorigin. The resultant magnetostratigraphy is characterized by a reverse-dominant with minornormal polarity intervals. When compared with the Bajocian-Bathonian referencegeomagnetic polarity time scale (GTS2016), the best fit is achieved in the Lower throughMiddle Bathonian (Chrons M41 to M39). A paleomagnetic pole was calculated for PortadaCovunco located at 167.1°E, 59.4°S, N =28, A95 = 3.6°, K = 57.2. When compared withanother available paleopole of the same age -170 Ma- derived from dykes exposed at MamilChoique, Patagonia, they both fall almost in the same paleomeridian although the PortadaCovunco pole is located at a slightly lower latitude. Thus, it is interpreted that the studiedsection was not subjected to rotations around vertical axis (R= 4° ± 10°), but compactionproduced inclination shallowing of the magnetic vector (F= 18° ± 11°). Using thepaleomagnetic pole from Mamil Choique as reference, the locality of Portada Covunco wouldhave been located at c. 33°S during the Bathonian, in agreement with paleoclimaticinterpretations derived from palynological data.