Analise exoscopica de grãos de quartzo e seu interesse para a geomorfologia (área de Soure e Salvaterra, Ilha de Marajó, Pará).

Abstract

The coast of Pará has been the object of important research on depositional environments, but the authors who refer to the exoscopic analysis of quartz grains as environmental indicators are rare. The present work is an initial contribution to the subject, taking as example sands from beaches and dunes of Soure and Salvaterra on Marajó Island (13 sites). The fractions obtained by particle size analysis (4 to 5 mesh sieves) were observed and selected under binocular magnification for further examination under the Scanning Electron Microscope (SEM) and constitution of reference dossiers, as well as comparison with fractions from other coastal areas of northeastern Pará. The morphometric study of about 130 grains in the SEM allowed us to detail their degree of roundness, angularity, and the texture of their surfaces. There are two main stocks of grains, as to shape: spherical grains dominate in the medium fractions of beaches and dunes (rounded to rounded angles), use concordant with the depositional system. The surfaces show mechanical actions of use by water (shiny, polished grains) and wind (shocks, conchoidal fractures, matted texture ...), but also significant chemical actions (weathering marks, dissolution), evidencing different intensity of geomorphological use in time and space. Further detailed research with specific granulometric fractions should be carried out in 2005/2006 in order to obtain new quantitative data, constitute a database, and better interpret the results obtained on this difficult issue.