CIG   05423
CENTRO DE INVESTIGACIONES GEOLOGICAS
Unidad Ejecutora - UE
artículos
Título:
The La Tinta pole revisited: Paleomagnetism of the Neoproterozoic Sierras Bayas Group (Argentina) and its implications for Gondwana and Rodinia
Autor/es:
AUGUSTO E. RAPALINI; TRINDADE, RICARDO; POIRÉ, DANIEL
Revista:
PRECAMBRIAN RESEARCH
Editorial:
ELSEVIER SCIENCE BV
Referencias:
Lugar: Amsterdam; Año: 2013 vol. 224 p. 51 - 70
ISSN:
0301-9268
Resumen:
The Late Ediacaran to Cambrian Sierras Bayas Group (Villa Mónica, Cerro Largo, Olavarría and Loma
Negra Formations) and the Cerro Negro Formation, exposed along the Tandilia system in the province of
Buenos Aires (Argentina) were revisited and studied paleomagnetically. Our results supersede those of
Valencio et al. (1980) for the La Tinta Formation (old stratigraphic name of these units). Three hundred
and twenty-eight samples were collected from forty-four sites in gently folded to subhorizontal strata
distributed along the whole stratigraphic succession. Detailed paleomagnetic study comprised systematic
stepwise demagnetization by both AF and thermal methods, the latter being generally the most effective
in isolating the characteristic remanence. Different magnetic components were defined from different
units of the succession. Besides a recent, probably viscous, secondary component (component A), the most
widespread magnetic remanence (component B) is a dual-polarity post-tectonic secondary remanence.
This component, carried by both hematite and magnetite, corresponds to that originally determined by
Valencio et al. (1980) and previously interpreted as primary. This component found in all carbonatic rocks
of Villa Mónica and Loma Negra Formations as well as in several claystones and siltstones of the Olavarría
Formation do not pass conglomerate and regional tilt tests. The mean in situ direction of component B is
Dec: 359.8◦, Inc:
−63.3◦, n: 85 samples, k: 24, 95: 3.2◦ and yields a paleomagnetic pole virtually identical
to the previous one of Valencio and colleagues. It also matches those recently determined from secondary
magnetizations in carbonatic and clastic Ediacaran units exposed in Uruguay.The pole positions suggest
a Late PermianTriassic age as the more likely for the acquisition of component B and reveal the presence
of a widespread remagnetization event that affected very large areas of the Rio de la Plata craton. Despite
this widespread event, some clastic units (claystones, marls) apparently escaped remagnetization. A pretectonic,
dual polarity, mean remanence (Dec: 28.7◦, Inc: 56.1◦, n: 17 samples, k: 15, 95: 9.5◦) was
isolated from the latest EdiacaranEarly Cambrian Cerro Negro Formation (component C). In addition,
the Ediacaran Olavarria Formation recorded another apparently ancient remanence, although no field
test is available. Its direction (component D) is at Dec: 350.9◦, Inc: 47.3◦, n: 13 samples, k: 37, 95:
7.0◦. Siltstones and claystones of the Ediacaran Cerro Largo Formation were carriers of a characteristic
remanence (component E) that shows a better directional grouping after bedding correction, although
the field test is not statistically significant, and yield a mean corrected direction at: Dec: 73.7◦, Inc:
−36.6◦, n: 11 samples, k: 15, 95: 12.1◦. Finally, a purple horizon of marls on top of the Villa Mónica
Formation associated with weathering processes before deposition of the Colombo diamictite, was carrier
of a characteristic remanence that attained a better grouping after bedding correction, but again with
no statistical significance. This direction (component F) was at Dec: 43.4◦, Inc:
−36.3◦, n: 7 samples,
k: 45, 95: 9.1◦. Components CF are interpreted as ancient magnetizations associated either to postdepositional
or early to late diagenesis. Mean geomagnetic poles computed from these components
fall on the apparent polar wander path for the Rio de la Plata craton from around 600 to 520 Ma, in a
correct stratigraphic order and with ages consistent with the most likely ages (or slightly younger) of
the different sampled units. These results confirm the already proposed Ediacaran to Cambrian APWP for
the Rio de la Plata craton, indicating that it remained at intermediate to low latitudes during most of the