COASTAL RELICTS IN THE BALTIC WOODS: GPR IMAGING OF EARLY-STAGE PALEO-SHORELINES AT CAPE KOLKA, LATVIA

Over the past decades, georadar imaging revolutionized coastal stratigraphic research, particularly in paraglacial regions characterized by heterogeneous sediment textures and compositions. This study presents one of the first high-resolution datasets from Kolka Cape (Kolkasrags), a massive Holocene strandplain in western Latvia located at an isostatic fulcrum point. Composed of >30 ridges, it stretches for nearly 30 km and reaches 10 km in its widest section. Along the Riga Gulf shoreline to the east, this complex is truncated by a younger S-N-trending ridge set, with converging longshore currents culminating in a cuspate spit. The forested beach/dune ridges (kangari) attain more than 10 m in relief. Intervening areas range from dry swales to freshwater bogs and lakes (vigas). This study focused on high-relief (>6 m) ridges in the landwardmost section of the strandplain. Separated by a perched wetland from a Late Pleistocene highstand terrace of the Baltic Ice Lake, Ridge 1 has a steep seaward face and a flat top. In contrast, Ridge 2 has a sharp profile with steep flanks, similar to subsequent kangari. Ground-penetrating radar (100-500 MHz GPR) images show attenuation at depth within Ridge 1, which is groundtruthed to be due to high clay content of the lithosome draped by sand only along the seaward slope. Ridge 2 exhibits seaward-dipping clinoforms overlain by hummocky cross-stratification, consistent with aeolian sand recovered in the top 3 m. Iron content in sediment (bulk magnetic susceptibility: 130-560 μSI) suggests prolonged weathering and contributes to both attenuation (red clays) and accentuation (thin heavy-mineral and oxidation horizons) of the electromagnetic signal. Guided by geophysical records, optical dating is underway to help constrain the inception of progradation, and ultimately test a hypothesis of storminess as a key mechanism in the genesis of these mega-ridges.