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RESEARCH PRODUCT

Morphology, internal structure and genesis of the Burtnieks drumlin field, Northern Vidzeme, Latvia

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Abstract The Burtnieks drumlin field is one of the largest in the Baltic countries. It occupies a broad and uneven bedrock macrodepression. The uneven distribution of drumlins is related to the relief of the bedrock surface. Needle-shaped drumlinoids occur in the proximal part of the drumlin field, and they gradually merge into typical drumlin forms downglacier. This morphologic change reflects the differences in their genesis as suggested by internal structure. The cores of drumlins consist mainly of glaciotectonically deformed Pleistocene sediment and, in some places, also weakly cemented, dislocated Middle Devonian sandstone. All of them are folded into linear anticlines trending parallel to the crests of drumlins. These anticlines contain the remains of preceding subglacial deformation structures. Superimposed imbricate overthrusts cover the flanks of drumlins. Drumlinoids contain diapiric folds in their cores, trending parallel to their axes. The Burtnieks drumlin field was formed during the last activation of the North Vidzeme tongue and three main phases may be distinguished in the development of its glaciotectonic structures. Two alternative genetic hypotheses are proposed for the formation of this drumlin field, both involving multiphase glaciotectonic deformation.