Glacial sedimentation in the late precambrian bebedouro formation, Bahia, Brazil

Abstract

The possibility that diamictites of the Late Precambrian Bebedouro Formation of northern Bahia, Brazil, are glacial in origin has been based on the areal extent, diversity of the lithology of the stones and the presence of outsize dropstones in rhythmites. More detailed studies on the diamictites show that some of the stones are faceted and their shapes are typical of those developed by glacial transport. Additionally, a small abraded pavement is described and garnets found in the matrix of the diamictite have chattermark trails. Taken in aggregrate, these observations suggest a glacial origin for the Bebedouro Formation. In the study area, the texture of the diamictites range from stone-rich to siltstones containing sporadic stones. The stone-rich diamictites are commonly found in layers, up to a metre in thickness, separated by poorly laminated siltstone. The Formation probably was deposited in a large lake or sea and the layered diamictites are debris flows which were derived from uneven piles of glacial debris deposited on the floor of the lake or sea.