Abstract

Round and oval lapilli occur in bands of fine-grained siliceous sediment of the Haimaraka formation (Precambrian) outcropping in the Mazaruni river valley, Guyana. The lapilli-bearing bands are associated with sandstones and volcanic sandstones exhibiting unmistakable sedimentary features. In structure the lapilli are very similar to those from terrigenous airfall tuff material of volcanic activity in both historic and geologic time that was characterized by eruption of dense clouds of ash accompanied by large amounts of steam. The lapilli in the Haimaraka formation are therefore interpreted as products of redistribution of material derived from ash deposited on land. Deposition of the land-derived material is considered to have occurred near the flank of a geosynclinal trough.

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