Structure and origin of the Vaivara Sinimäed hill range, Northeast Estonia

The distribution, structure and origin of the hill range of Vaivara Sinimäed and bedrock folds are discussed. Many narrow (150–500 m), east- to northeast-trending folds with 1–10 km long axes are found in the studied area. Anticlines resulting from diapiric processes are prevailing. Cambrian clay-, silt- or sandstones are cropping out in the centre of the anticlines. Terrigenous outcrop zones are surrounded by Middle Ordovician carbonate rocks, which enabled us to use low-resistivity anomalies for tracing the distribution of the proven anticlines. The Vaivara Sinimäed, a 3.3 km long and 200–300 m wide range of three elongated hills, rise 20–50 m above the surrounding land. The tops of Tornimägi, Põrguhauamägi and Pargimägi hills are 69, 83 and 85 m a.s.l., respectively. Two saddles between the hills are on the level of 50–55 m a.s.l. Uplift of Middle Ordovician carbonate rocks at Pargimägi Hill is mostly due to the thickening of Cambrian claystone. Its core, and most likely also the cores of Tornimägi and Põrguhauamägi hills, consist of squeezed-out and folded sedimentary bedrock, diapirs, which are probably of glaciostatic origin. The dominant glaciotectonic feature is a glacial erratic. The surrounding bedrock and cores of the hills are covered with a thin blanket of Quaternary deposits: till, glaciofluvial gravel and sand, glaciolacustrine silt and clay. The Vaivara Sinimäed as a whole represent a diapir, modified by glaciers.