Liquefaction and re-liquefaction of sediments induced by uneven loading and glacigenic earthquakes: implications of results from the Latvian Baltic Sea coast

Seven internally-deformed layers were recognised in the shallow marine bay sediments deposited during mid Marine Isotope Stage 2 along the Latvian Baltic Sea coast bluff at Baltmuiža. Detailed sedimentological analyses of these deformed layers indicate that liquefaction was responsible for the development of soft-sediment deformation structures (SSDS) including injection structures, load casts, flame structures and pseudonodules. Traces of both initial liquefaction and re-liquefaction were recognised within at least four of the seven layers with SSDS (two different generations of pseudonodules and injection structures, clastic injection pipes, disrupted load casts). This is a unique finding, as it is difficult to re-liquefy sediment affected by previous liquefaction due to the increased packing of grains and decreased hydration after initial liquefaction. Furthermore, all layers with SSDS are characterised by eroded tops, which indicate that deformation took place when these layers were at the contemporary surface of sedimentation. We suggest that the initial liquefaction of fluidised coastal sediments was caused by various triggers (uneven loading, glacigenic earthquake), while the re-liquefaction could be related to a single trigger of mid-Late Weichselian glacigenic earthquakes at the front of the advancing last Fennoscandian Ice Sheet. Our results show that despite compaction after liquefaction, sediment re-liquefaction caused by an allogenic trigger is still possible under favourable conditions, i.e., sediment anisotropy, cyclic loading, and preserved high water saturation. Our findings are also the first to demonstrate the sedimentological imprints of re-liquefaction.