Date of Award
Master of Science
John L. Isbell
Julie A. Bowles
Fernando F. Vesely
Anisotropy of Magnetic Susceptibility, Brazil, Diamictite, Late Paleozoic Ice Age, Paraná Basin
The term ‘diamictite’ is used as a lithologic descriptive term without assigning a particular origin to a rock unit as either glacial deposits (till), proglacial, glacially influenced deposits (resulting from meltwater plumes and ice rafted debris), or mass transport deposits (glacial or non-glacial related). While in some cases, it is possible to delineate between the origins of diamictites, in other instances, weathering and lack of exposures make it difficult to determine. In general, the occurrence of diamictites within the Gondwana succession has been traditionally used to indicate the occurrence of subglacial deposition despite the potential occurrence of other depositional modes. Thus, the extent of glaciation during the Late Paleozoic Ice Age is interpreted to be much greater than it actually was. . One area of interest in Gondwana where interpretation of these deposits is problematic, and hence has resulted in problems determining ice extent, is the Paraná Basin in Brazil. The ability to better differentiate subglacial processes from proglacial, subaqueous mass transport, glaciomarine/glaciolacustrine rainout, and/or ice rafting, in addition to determining glacier flow or mass transport directions, will allow researchers studying these deposits to more accurately reconstruct the environments timing and extent of glaciation during the LPIA.
In sedimentary fabrics, anisotropy of magnetic susceptibility (AMS) is a geophysical method, which depicts the preferred orientation of magnetic particles during the final stages of transport and/or synsedimentary deformation. The technique is used to determine the preferred orientation of the constituent grains, therefore a useful indicator to help determine the mode of deposition, direction of sediment transport, and the nature of stress and strain during deformation. In August of 2016, samples were collected from deposits assigned to the Itararé Group, which outcrop along the southern and eastern margins of the Paraná Basin, Brazil. 19 fabrics were analyzed from seven different locations (Alfredo Wagner, Aurora, Cachoeira do Sul, Campo do Tenente, Ibaré, Porto Amazonas, and São Gabriel), stretching across the states of Rio Grande do Sul, Santa Catarina, and Paraná. While in most cases, AMS allowed us to delineate between the origins of diamictites, in other cases, it proved to be more difficult. In most cases, AMS measurements were beneficial in determining the direction of sediment transport. Our findings are consistent with past studies in which AMS was used to infer a variety of glaciogenic deposits, but also acts as a study case for the different types of fabrics that may develop as a result of Newtonian vs non-Newtonian sediment gravity flows.
While flow directions along the southern margin of the basin are consistent with the inferred N/NW ice movement into the basin, some of the flow directions along the eastern margin are not, revealing deviations in topography. Flow directions obtained from mass transport deposits in the area stretching from Campo do Tenente to Porto Amazonas (an area in which different stratigraphic levels of glaciation are exposed) tend to show uniform flow to the south. This observation is consistent with other AMS studies of similar deposits within the area, suggesting the existence of a southward paleoslope, which strongly influenced subaqueous deposition throughout the extent of the Itararé Group.
Amato, James Anthony, "Using Ams to Help Interpret Glaciogenic Deposits of the Late Paleozoic Ice Age in the Paraná Basin, Brazil" (2017). Theses and Dissertations. 1575.