GLACIAL SEDIMENTARY ENVIRONMENTS OF A PALEO-ICE WEDGE LOCALITY, SAO PAULO STATE, BRAZIL

A road cut at KM 111, Highway S.P. 280, western Sao Paulo State, Brazil has exposed a section of late Paleozoic glaciogenic deposits and features. The section, originally described by Rocha-Campos et al (1977) consists of two diamictites, separated by a loessite deposit (wind blown sediments characteristic of a glacial environment). Wedge shaped bodies up to 3 meters in height and made up of a variety of sandstone and diamictite, taper downward from the upper diamictite unit into the lower diamictite. These structures are thought to be ancient ice-wedges developed in frozen ground during the Gondwana glaciation.

The presence of these paleo-frost wedges in a diamictite exposure suggests a unique opportunity to study ancient terrestrial deposits. Terrestrial diamictites (tillites) are rarely preserved in late Paleozoic strata, yet both frost wedges and loess suggest a terrestrial (rather than marine) environment. The goal of this study was to determine if this was a tillite exposure and to interpret the glacial processes that resulted in tillite formation.

In the field, bedding characteristics, sedimentary features and contacts were described. Samples were taken at 10cm intervals and at contacts for analysis at UMM. Stratification of trimmed hand samples, grain size analysis and grain shape observations were utilized to determine sediment origin. Sedimentary analysis indicate that this may be a loessite deposit, however, the presence f stratification in the coarse grained units suggest that this may not be a terrestrial diamictite (tillite) exposure.

Research for this study was funded by grants from the N.S.F.-PAESMEM Program (NSF-HRD 0003111) and the University of Minnesota - Undergraduate Research Opportunities Program (UROP).

Reference: Rocha-Campos, A.C and others, 1977, Alguma Feicoes periglaciais do Subgrupo Itarare (Neopaleozoico), no estado de Sao Paulo. Boletim IG Instituto de Geociencias, USP. Vol 8, p. 55-66.