THE SYN-VOLCANIC NAAJAAT LAKE, PALEOCENE OF WEST GREENLAND

The Naajaat lake in the Nuussuaq Basin on Disko and Nuussuaq formed in a geological setting between cratonic crystalline Precambrian rocks o verlain by Cretaceous sediments and an actively forming Paleocene volc anic province. The lacustrine deposits, shales as well as hyaloclastit e breccias, accumulated in low-lying areas inundated by fresh water an d sealed off from marine transgressions by a broad subaerial volcanic terrain. Foreset-bedded hyaloclastite breccias demonstrate water depth s of up to 450 m, and the area of the lake was 2500 km(2) at its maxim um extent. The lake probably existed for less than 0.5 million years. The lake received clay and silt from two provenance areas. Quartz cont ents of more than 25% in the majority of the sediment samples indicate that large amounts of material were continuously supplied to the lake from the crystalline terrain, whereas the volcanic terrain supplied s mectite and mixed-layer minerals to the lake, High kaolinite contents stem from the crystalline or both provenance areas, The shales are cha racterized by high TOC (up to 11%), lack of pyrite, presence of terres trial spores and pollen and lack of marine dinoflagellates. The lacust rine sediments rest on an erosional unconformity and its correlative c onformity. The unconformity developed during the latest Cretaceous and Early Paleocene. Five stages are recognized in the geological develop ment of the lake. Stages 1-4 are characterized by accumulation of hyal oclastite breccias, rise in lake level, and eventual transgression of subaerial terrains. The rises in lake level were caused by stemming of fluvial run-off behind the aggrading volcanic pile. Stage 5 correspon ds to cessation of volcanic activity, a stable lake level, and prograd ation of elastic sediments, resulting in infilling of the lake. During the lacustrine transgression only sediment in suspension was transpor ted into the central parts of the lake. (C) 1998 Elsevier Science B.V, All rights reserved.