Table 6 Correlation of the sand layers among the core samples

T1

It was defined as the Towada-a event (Table 3), see the discussion in the body text. It continues throughout the core samples (Fig. 3), just below the paddy field soil. It is also consistent that two more tephra events were found underlying it, interbedded with peaty mud, except in H13, H07, and H06, where there was only one, probably due to erosion

S1

It only appears in the core sample H02 (the thickest sedimentary record). Therefore, it is difficult to identify a lateral continuity, essentially due to its thickness (Table 1, Fig. 3)

S2

It was traced among H02, H01, and H07 due to its relative stratigraphic position and lithological characteristics

S3

It only appears in the core sample H02 (the thickest sedimentary record). Therefore, it is difficult to identify a lateral continuity, essentially due to its thickness

S4

It was traceable among H02, H01, H07, H08, and H03 (Fig. 3). Such correlation was made on the grain size, color, and relative stratigraphic position

S5

It was traced throughout all the core samples, representing the sand with the furthest extent. It is the second thickest sand layer, after S8, in the sedimentary sequence. Its position above the peaty-mud layer and T4 (marking horizon), upward fining, and bottom erosive contact allowed its lateral correlation. The dating result obtained from the peaty-mud layer underlying S5 between Ha8 and H04 supported the correlation (see Sect. 5 in the body text)

T4

It is a consistent ash layer, defined as a white clay with homogeneous thickness among the sand layers. Due to its consistency, lateral continuity, and lithological characteristics, it was used as a marker horizon to define the upper boundary of S6 and the lower boundary of S5, considering the peaty mud interbedding between the sand and tephra layers (Table 1, Fig. 3)

S6

It is located below the peaty mud underlying T4; it was found in the core samples H02, H03 and H04. Such correlation was made based on the characteristic lenticular stratification, silt composition, and the thickness of the layers (Table 1)

S7

It was traceable among H02, H07, H08, H03, H09, H10, and H04 (Fig. 3). Grain size, the bottom transitional or erosional contact, and the relative stratigraphic position led to its lateral correlation

S8

It was traceable throughout all the core samples between H02 and H06 and represented, in general, the thickest sand layer. Its thickness, upward fining, and relative stratigraphic position allowed its lateral correlation

S9

It was traceable among the core samples H02, H07, H08, H03, H10, and H04. The two thin laminas interbedded with the thin peaty mud led to the correlation between H02 and H03. The stratigraphic position allowed the correlation among the other core samples. The dating result obtained from the peaty-mud layer underlying S9 and overlying S10 in Ha8 and H04 supported the correlation (see Sect. 5 in the body text)

S10

It was traceable among the core samples H02, H07, H08, H03, H10, and H04. The stratigraphic position, overlying To-Cu and the peaty mud, led to its lateral correlation. The dating result obtained from the peaty-mud layer underlying S9 and overlying S10 in Ha8 and H04 supported the correlation (see Sect. 5 in the body text)

T6

It was defined as the Towada-Cu event (Table 3), see the discussion in the body text. It continues throughout all the core samples at the end of the sequence. In all the cases, it was found as a thick lapilli layer