In 2017, a detailed study of the Eastern Gulf of Finland (the Baltic Sea) seafloor was performed to identify and map submerged glacial and postglacial geomorphologic features and collect data pertinent to the understanding of sedimentation in postglacial basins. Two key areas within the Gulf were investigate using a multibeam echosounder, SeaBat 8111 and an EdgeTech 3300-HM acoustic sub-bottom profiling system. High-resolution multibeam bathymetric data (3-m resolution) were used to calculate

more »

... pect, slope, terrain ruggedness and bathymetric position index using ArcGIS Spatial Analyst and the Benthic Terrain Modeler toolbox. These data and resultant thematic maps revealed, for the first time, such features as streamlined till ridges, end-moraine ridges, and De Geer moraines that are being used for the reconstruction of the deglaciation in the Eastern Gulf of Finland. This deglaciation occurred between 13.8 and 13.3 ka BP (Pandivere-Neva stage) and 12.25 ka BP (Salpausselkä I stage). Interpretations of the seismic-reflection profiles and 3D models showing the surfaces of till, and the identification of the Late Pleistocene sediment and modern bottom relief, indicate deep relative water-level fall in the Early Holocene and, most likely, several water-level fluctuations during this time. are located in subsiding areas within the Southern and Southwestern Baltic Sea [1,4-6] and regions of uplift are located within the Northern Baltic Sea [7] . Several key areas of the Eastern Baltic Sea are very important for modelling historic and present tectonic processes (e.g., Eastern Gulf of Finland and Pärnu Bay), which are characterized by very low (from 0 to +3 mm/year) rates of uplift-a near zero rate of recent sea level change. According to a datum obtained by a Kronshtadt gauge measurement, the rate of sea level rise from 1835 to 2005 was 0.7 mm/year [8] . The combination of tectonic processes, total sea bottom coverage by Quaternary deposits, relatively smooth and shallow relief, and widespread Holocene accretion, both above and below the recent sea level, is useful geological and geomorphological information that can be used to reconstruct palaeoenvironmental changes [9] [10] [11] [12] [13] . Between 1986 and 2000, the Eastern Gulf of Finland was mapped by the Russian Research Geological Institute (VSEGEI) [14] , resulting in the publication of a State Geological Survey Map (1:200,000 scale). Approximately 8000 km of seismic reflection sub-bottom profiles (SBP) and more than 6000 sampling sites were used in the mapping effort. The SBP lines were oriented mainly parallel to meridians and spaced approximately 2 km apart. Two acoustic sources were synchronously operated, one with a fundamental frequency of 500 Hz (sparker) and another at a frequency of 7.5 kHz (piezoceramic transmitter). More than 6000 sediment samples including 4000 gravity cores were collected and used for SBP data interpretation. In 2009 and 2011, during joint Russian-Finnish field cruises on board the R/V Aranda of the Finnish Environment Institute, approximately 800 km of 12-kHz pinger sub-bottom profiles were collected by Meridata Ltd. [15] . Recently, these profiles were digitized and analysed using GIS methods. Assessment of regional deglaciation processes and postglacial sedimentation was undertaken to form 3D models compiled of pre-Quaternary relief, consisting of moraines and Late Pleistocene surfaces, from which the thicknesses of till, glacio-lacustrine, and Holocene sediments were calculated [16] . Despite a relatively high confidence in the geology reported to date for the area of our study, there still remain important unsolved problems, specifically those pertaining to the postglacial geological history including (i) the location of end moraines and glaciofluvial deposits in the Gulf of Finland; (ii) the age of deglaciation and rate of glacial front retreat; and (iii) the number of Holocene sea level fluctuations and the amplitude of relative regressions [13, 16] . The best known regional palaeoreconstructions of deglaciation were reported by I. Krasnov (International Map of the Quaternary deposits of Europe, [17], E. Zarrina [18], D. Kvasov [19], A. Raukas [20], D. Subetto [21], and J. Vassiljev [22]. These reconstructions were based on terrestrial investigations where locations of ice-sheet margins within the Gulf of Finland basin are generally shown on maps by dashed lines (inferred) or attributed as unknown [23] . Hypothesized locations of terminal moraines formed during the Pandivere-Neva stage east of Kotlin Island [18] or near the Eastern coast of Narva Bay to Cape Peschany [21] have not been documented as there is no geomorphological or chronological evidence for these glacial forms. The problem regarding Holocene sea level fluctuation, especially the location of regression levels is the subject of much discussion today [9] [10] [11] [12] [13] . Recent advances in marine geological methodology and technology using multiple instruments, such as high-resolution geophysical SBP, multibeam echosounder bathymetry and backscatter, and side-scan sonargraphs have provided data that is useful for the 3D geomorphometric visualization of submerged surfaces, which has significantly improved our knowledge of sea bottom relief and geological structure [24] [25] [26] [27] [28] [29] . In the Eastern (Russian) part of the Gulf of Finland, the first attempts at using these state-of-the-art geological and geophysical methodologies focused on the study of geological hazards, such as submarine landslides, pockmarks [30, 31] , and processes associated with Fe-Mn concretion and regeneration, based on exploration [32] undertaken by VSEGEI between 2012 and 2014. The exploration was part