A PHENOMENON OF “DUNE TECTONICS”: CURONIAN SPIT, SOUTHEASTERN BALTIC SEA COAST

Localized exposures of clay-rich strata (so called “lagoon marl) have been known to crop out along the lagoon coast of the Curonian Spit from ancient times. Results of several investigations along the Lithuanian segment of the spit indicate that the marl was formed in a freshwater lagoon during the late Holocene Litorina Sea stage. The exposures are 2-3 m high (max. 4-5 m) and reflect a unique geological process described here as “dune tectonics” – a phenomenon of extrusion of lagoon marl from under the sand dunes. Similar phenomenon has been described in central Australia, where Upper Devonian siltstones were deformed due to loading by sand dunes. Along the Curonian Spit this process is possible only due to the interaction of two key factors: presence of lagoon marl horizons below the water table and active movement of massive (>20 m) sand dunes. Based on a study at Parnidis Dune, we present a dynamic model for the formation of marl exposures. Below the water table, the in situ saturated marl (especially organic clay and clayey gyttja) is very plastic and the colossal weight and pressure of the dunes causes its deformation and extrusion at and above sea level. Two scenarios of marl extrusion are proposed: 1) rapid extrusion (thrust mode) due to a catastrophic collapse of sand into the lagoon (sand landslides, massive grainflow, and avalanching), which had been last witnessed in 1922, and 2) slow extrusion of the marl (diapirism) due to gradually increasing load of sand. Such a process has been progressing through time due to episodic eastward movement of large dunes during late Holocene. The most recent manifestation of this process occurred during the winter of 2007-2008, when a 60-m-long, 1.5-m-high exposure was formed along the Curonian Lagoon coast beneath the Parnidis Dune, approximately 50 m north of the old marl outcrop. Geophysical surveys imaged this new segment a few meters beneath the surface in 2005, compared to 8-9 m deep farther from the dune. In the future this unique geological process may slow down or cease altogether as the dune height and the rate of its movement are decreasing due to depletion of sand supply from the seaward side. The main reason for this is that during the past two centuries active dune massifs have been separated from the coastal strip by artificially planted forests along the entire length of the Curonian Spit.