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| Author(s): |
Lena Simone Barata Souza
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Instituto de Geociências (IG/BT) |
| Defense date: | 2010-12-16 |
| Examining board members: |
Dilce de Fátima Rossetti;
Francisco Hilário Rego Bezerra;
Vagner Roberto Elis;
Norberto Morales;
Claudio Riccomini
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| Advisor: | Dilce de Fátima Rossetti; Renato Luiz Prado |
| Abstract | |
There has been an increasing volume of investigation suggestive of tectonic deformation in the development of Quaternary depositional systems in Marajó island. This is revealed by a combination of sedimentological and stratigraphical information with morphostructural analysis. However, subsurface studies aiming to demonstrate the presence of tectonic structures in this area remain to be presented. The present study had the main goal of verifying if tectonic structures suggested in surface in eastern Marajó island are expressed in shallow subsurface, thus allowing to discuss their effect in the Neogene and Quaternary sedimentation, as well as their trigger mechanism. This was approached with a study integrating geomorphologic, geophysical and geological information. Geomorphological information, based on cartographic data and Landsat images, aimed analysis of drainage basins, identification of drainage anomalies and extraction and analysis of morphostructural lineaments. Geophysical data consisted of vertical electric sounding (VES) acquired along this study, added to the reprocessing of data available in the literature. This study was complemented with acquisition of ground penetrating radar (GPR) sections. SEV data collected along this study were combined with reprocessed data derived from available literature. The geophysical study includes also acquisition of ground penetration radar (GPR) sections. The analysis of subsurface data was complemented with geological studies of based on natural exposures along coastal cliffs, road cuts and quarries, as well as cores. Three drainage basins were recognized in the study area, corresponding to the Arari, Camará and Paracauari rivers. All these basins show evidence of morphostructural anomalies, mostly including: rapid changes in drainage pattern within a same basin, ranging from trellis, rectangular, multibasin to subparallel; straight channels commonly connected at right angles; meandering channels that change to straight channels; local meandering; and highly asymmetric basins (asymmetry index > 60). Drainage anomalies consisting of frequent changes in river courses forming straight angles and isolated and compressed meanders are also recorded in the paleodrainage preserved in the study area. Two morphostructural compartments were recognized with basin morphostructural lineament density: compartment I, situated in the center-eastern part of the study area, where lineament density is high and with two preferential directions to NWSE and NE-SW; and compartment II, corresponding to the remaining of the area, where lineament density is low to moderate, and with a NE-SW main trend. The study focusing on 132 VESs revealed that the eastern and southern portions of the study area are extremely resistive (values up to 13.000 \'ômega\'m). This is related to the occurrence of lateritic paleosol near the surface, as well as ferrified strata immediately underlying this horizon, which were formed in association with the unconformity at the top of the Miocene strata of the Barreiras Formation. In contrast, only low resistivity values (< 90 \'ômega\'m) were recorded in the remaining of the study area, being related to conductive deposits represented by muddy, heterolithic and, less commonly, sandy lithologies corresponding to the Post-Barreiras Sediments of late Quaternary and Holocene ages. These data are consistent with analysis of GPR sections, which revealed reflectors with high frequency and lateral continuity near the surface associated with the lateritic paleosol. Taking this into account, two stratigraphic units were recognized in the eastern portion of the study area, with the lower one related to the Barreiras Formation and the upper one corresponding to the Post-Barreiras Sediments. In constrast, the eastern and northern portions of the study area displayed absence of reflectors, which was associated with the presence of dominantly muddy Quaternary sediments bearing intrastratal brackish and/or saline waters. Several characteristics support the presence of tectonic structures in the study area. This was suggested by the abundance of drainage anomalies (modern and ancient) and morphostructural lineaments with directions that coincide with the orientation of main tectonic faults recorded in Amazonia. The presence of faults was also indicated by sharp contacts between the Barreiras Formation and the Post Barreiras Sediments, as recorded in the VES sections. This proposal is consistent with the analysis of surface data that shows concordance of the places with sharp lithological contacts with morphostructural lineaments. Likewise, the GPR sections reveal an abundance of reflectors with vertical offsets that are only justified taking into account tectonic faults. Additionally, the basal portions of several GPR sections displayed undulatory reflectors, which were related to wide syncline and anticline folds. The origin of these tectonic structures is discussed considering three potential mechanisms: Andean tectonics, gravitational tectonics related to sediment loading in the Amazon submarine cone, and marginal rifting. The hypothesis of relating the tectonic structures recorded in this work with the dynamic evolution of the Marajo Graben System seems the most likely. The folds and faults recorded in the Barreiras Formation and Post-Barreiras Sediments might be explained considering strike slip deformation, with association of transpression and transtension during and after the Miocene. (AU) | |