GP31C-01
Tectonic Implications of Widespread Remagnetization of Late Proterozoic Sedimentary Units in Uruguay
A paleomagnetic study was performed on three Neoproterozoic sedimentary successions exposed in Uruguay in order to better define the paleogeographic evolution of the Rio de la Plata craton prior to and during the assembly of Gondwana. These units comprise the latest Ediacaran to Early Cambrian calcareous Cerro Victoria Fm, the late Ediacaran clastic Yerbal Fm and the calcareous Polanco Fm and the Ediacaran clastic Rocha Fm. None of these carry the original remanence and they were affected by remagnetizations. The most widespread remagnetization is shown by the Cerro Victoria, Yerbal and Rocha Formations. A dual-polarity post-folding remanence, carried by hematite, was isolated in all these units. Their corresponding pole positions are virtually identical (CV: 83.4 °S, 312.8 °E, dp: 3.3 °; dm: 4.3 °; Y: 79.5 °S, 295.9 °E, dp: 5.4 °, dm: 7.0 °; R: 77.4 °S, 292.6 °E, dp: 3.5 °, dm: 4.4 °) suggesting the same magnetization age and process. Comparison with the South American path suggests the Late Permian-Early Triassic or the latest Cretaceous-Paleogene as the most likely times for its acquisition. These poles are also identical to the pole of the Late Proterozoic La Tinta Fm., which is interpreted as affected by the same remagnetization, and therefore, invalid for paleogeographic reconstructions. A different magnetic component, also dual polarity and postfolding, was defined at two sites of the Cerro Victoria Fm. and two of the Polanco Fm, possibly carried by magnetite. The mean geomagnetic poles from both units (CVc: 7.8 °N, 343.9 °E, dp: 11.1 °; dm: 15.8 °, P: 5.9 ° N, 325.3 ° E, dp: 13.7 °, dm: 16.9 °) fall on the Middle Cambrian to Early Ordovician segment of the Gondwana APWP, suggesting that age for the remagnetization. This allows us to establish the age of the tectonic event affecting the Neoproterozoic units of the RP craton as Early Cambrian (ca. 525 Ma).
GP31C-02
10 Ma of Igneous Activity in the Transmexican Volcanic Belt: Tectonic and Geomagnetic Implications.
A total of 51 sites with geochronological control were sampled in the central and western segments of the
Transmexican Volcanic Belt (TMVB). Together with other previously published 69 sites from the eastern segment,
they span the spatial and temporal activity of the TMVB. Using now the same reference directions and
methodologies, they are analyzed in order (i) to determine the possible occurrence and significance (spatially and
temporally) of vertical axis crustal block rotations that have been reported in this region; and (ii) to study the
geomagnetic Paleo Secular Variation during the last 10 Ma; to check the previously suggested existence of a
"Pacific Dipole Window" extending to Mexico. Paleomagnetic results, backed by statistical tests performed
according to their geographical distribution (3 structural segments) or according to their ages (Late Miocene,
Pliocene or Quaternary), do not support the notion that large vertical axis block rotations (paleomagnetically
detectable) occurred in this arc after Late Miocene times. They suggest that the TMVB could be considered
paleomagnetically as an unique tectonic domain under a transtensional regime, where its extension component
prevails over its left-lateral component. The mean paleomagnetic directions, obtained in the age ranges 10-5 Ma
and 5-0 Ma, do not differ from their respective reference directions. In both datasets, VGPs have been selected
using quality Fisher's precision parameters and optimum cutoff angles. This results in a circularly symmetrical
data distribution with statistically indistinguishable antipodal normal and reverse polarities. VGP dispersions are
consistent with those from globally distributed observations at Mexican latitudes for the Miocene and the Plio-
Quaternary. An analysis of all the published paleomagnetic data from the TMVB, when combined all together and
selected in the same terms, do not support neither the existence of large crustal block rotations nor the
persistence of a magnetic anomaly in the studied region.
http:www.minas.upm.es/fundacion/jgs/ftproot/2003-
2004/GrupoA/VCRuiz/VCRuiz.pdf
GP31C-03
Surface Reservoir Characterization and Stratigraphic Studies Using Rock Magnetism and EPR in Venezuela: A Review
During the last years, the Paleomagnetic and Rock Magnetic Laboratory at the Simon Bolivar University has incorporated studies of rock magnetism and non conventional techniques in geophysics (as Electronic Paramagnetic Resonance or EPR) to solve diverse problems at the Venezuelan oil industry. Particularly, surface reservoir characterization and EPR-Magnetostratigraphic studies have been performed. At some Venezuelan oil fields (Guafita, La Victoria and Furrial) rock magnetic properties studies (e.g. Magnetic Susceptibility (MS)), extractable organic matter (EOM) and organic matter free radical concentration (OMFRC obtained via EPR) analysis have been applied trying to identified, at shallow levels, the "oil magnetic signature" of subjacent reservoirs. The results obtained in non consolidated samples from the first 1500 m of producers and non producers wells, show the existence of MS, EOM and OMFRC anomalies at shallow levels that are associated with an underlying reservoir and/or oil migration. Authigenic spherical aggregates of submicronic FE-rich magnetic crystals, observed by Scanning Electron Microscopy (SEM), are responsible for the detected MS anomalies. These studies allowed to establish a set of criteria (i.e. EPR, EOM and SEM) in order to link, with a minimum uncertainty, near-surface MS anomalies with the underlying reservoir. We have also explored the application of EPR, combined with petrographic, MS analysis, Qn and S-ratios, to characterize stratigraphic facies and identifying depositional environments at various sections in southwestern Venezuela. The different paramagnetic species that have been identified (e.g. manganese, free radicals and different Fe forms) together with the rock magnetic parameters, seem to give valuable information regarding the lithological characteristics of the studied sections. According to our results, as manganese content is related with the redox conditions, it can be used as a paleoenvironmental change index in stratigraphic columns with large lithological contrasts. For stratigraphic sections that present similar lithologies, the presence and concentration of distinct Fe species could be useful to characterize different depositional environments. The results obtained indicates that this kind of integrated analysis can be used as powerful tool for stratigraphic and environmental studies.
GP31C-04
Magnetic Properties of Drill Cutting from an Oil Well at Golfo San Jorge Basin, Argentina. Their Correlation with Several Conventional Logs
The Golfo San Jorge Basin is one of the most important hydrocarbon-producing basins in Argentina. It is characterized by good source rock, favourable structural complexity and multiple thin tuffaceous sandstones as reservoirs. A study of magnetic properties performed over drill cutting take at different depth levels from an oil well drilled in this basin was carried out. The cutting samples correspond to the upper terms of the Pozo D-129 Formation, main source rock of the basin. Magnetic susceptibility measurements and rock-magnetic studies were performed to define the magnetic mineralogy. Concentration indexes of the different magnetic minerals were determined on the base of isothermal remanent magnetization (IRM) acquisition curves. A correlation analysis between the magnetic properties and conventional well logs (sonic, neutronic, density, induction, resisitivity and spontaneous potential) was performed. Several kinds of correlations were found: significantly positive correlation between magnetic properties and porosities, and significantly negative correlation between the concentration indexes of some magnetic species (titanomagnetite and pirrotite) and the resistivity. The positive correlation between magnetic properties and porosities is especially good with the porosity calculated from the neutronic log. The presence of hydrocarbons in cutting is associated with the frequency dependence of susceptibility factor, showing different relationship with oil and gas.
GP31C-05
Combined Rock Magnetic and Dielectric studies applied to stratigraphic and archeological problems in Venezuela
In the last few years the paleomagnetism research group, at the Universidad Simon Bolivar in Caracas (Venezuela), has undertaken combined studies of rock magnetism (e.g. natural magnetic remanence, magnetic susceptibility, hysteresis parameters etc.) and dielectric properties (maximum current depolarization temperatures and average activation energies) in Cretaceous and Paleogene sedimentary sequences from eastern and western Venezuela. Our main goal has been to find new ways of defining physical markers, in fossil- poor sedimentary rocks, for stratigraphic correlations. Magneto/dielectric characterizations of these rocks have proved also useful identifying lithological discontinuities and paleoenvironmental changes. More recently these two-fold technique have been extended to archeological materials (potsherds) from a series of Venezuelan islands, in order to track down clay sources and find out about different stages of pottery craftsmanship. Magneto/Dielectric characterization of archeological potsherds seems to allow the tracing of their provenance from various mainland prehistoric settlements of distinct Venezuelan amerindian groups. In this paper we present a comprehensive review of this research applied to a contact between two sedimentary formations in eastern Venezuela (Cretaceous Chimana/Querecual) and a number of pottery samples with diverse stylistic features excavated in a single archeological site from Los Roques islands.