Many modern epicontinental seas were dry land
before their marine flooding by the mid-Holocene
glacioeustatic sea-level rise, whereas the Baltic Sea Basin
was covered by a huge postglacial lake. This change from a
postglacial lake to the present-day semi-enclosed brackishwater
sea is studied here in sediment cores and acoustic profiles
from the Baltic Sea major sub-basins, based on novel
datasets combined with information extracted from earlier
publications. In shallow areas (<50m water depth), the base
of the brackish-water mud is erosional and covered by a
patchy, thin, transgressive silt-sand sheet resulting from decreased
sediment supply, winnowing and the redistribution
of material from local coarse-grained deposits during transgression.
This erosional marine flooding surface becomes
sharp and possibly erosional in deep areas (>50m water
depth), where it may be locally less clearly expressed due to
reworking and bioturbation. Both in the shallow and deep
areas, the brackish-water mud is strongly enriched in organic
matter compared to underlying sediments. Bioturbation type
changes at the flooding surface in response to the increased
sedimentary organic content, but no firm-ground ichnofacies
were developed because of low erosion. It is concluded that
the base of the brackish-water mud is a robust allostratigraphic
bounding surface that is identifiable by the lithologic examination
of cores over the Baltic Sea. The surface is a distinct
reflector in seismic-acoustic profiles, which facilitates mapping
and basin-wide stratigraphic subdivision. Detailed geochronologic
studies are required to confirm if sediments immediately
overlying the erosional flooding surface in shallow
areas are younger than the basal part of the brackish-water
mud in deep areas that is predicted to be time-equivalent to
the erosion.