ABSTRACT

Channel-like bodies and wedge forms of sandstone and conglomerate occur within diamictite in the basal proportion of the Mombuca "Tillite" Member of the Gramadinho Formation (Pennsylvanian) in Sao Paulo State, Brazil. In a road-cut near Gramadinho, channel-like bodies include structures 2-4 m high with an upward taper and large clasts locally concentrated at the top, as well as irregular masses of poorly sorted sandstone and conglomerate grading outward into diamictite. Wedge forms taper downward from sandstone beds and attain a maximum height of about 4 m.

Evidence supporting the interpretation of diamictite as tillite is the presence of tapering channel-like structures, considered to be deposits of subglacial streams. The irregular masses with gradational contacts possibly represent englacial or superglacial esker deposits partially diffused during melting of the underlying ice. Wedge forms are interpreted as infillings of open crevasses or fissures in frozen till associated with deposition of the overlying sandstone bed.