The Matsuzaki Lowland is a valley bottom plain situated in the lower reach of the Naka River which flows from the southern part of the central Izu Peninsula.
To clarify the sedimentary environment and its change during the Holocene in the Matsuzaki Lowland, all core boring was carried out to take systematic samples. The environmental changes of the Matsuzaki Lowland region was reconstructed on the basis of the observation of the sediments, C-14 dates, analysis of volcanic ash and palaeontological analyses of molluscs, foraminifers, ostracods and diatoms from the cored samples.
The environmental changes and palaeogeographical evolution during the Holocene in the Matsuzaki Lowland are summarized in the following.
1. ca. 10, 000-9, 000 years ago
At this stage, the sea level was remarkably lower than the present and the sea had not yet invaded into the present Matsuzaki Lowland and the fluvial deposits were accumulated in the lower reach of the Naka River.
2. ca. 9, 000-8, 000 years ago
Judging from the appearance of the shallow marine mollusca such as
Grassostrea gigas and
Batillaria zonalis, a tidal flat was formed in the lowland by the Holocene transgression.
3. Ca. 8, 000-7, 000 years ago
As the sea level rose, the sea water invaded into the inner part of the present Matsuzaki Lowland to form a drowned valley. In the outer part of the drowned valley,
Teora lata, Ringicula doliaris and
Putilla matusimana etc. wihch live in the subtidal environment dominated in the molluscan assemblages, and the benthic foraminifera such as
Elphidium advenum and Pseudononion japonicum which live on the sandy bottom in the middle to the outer part of a bay were more common than
Ammonia beccarii. On the other hand, the shallow marine mollusca and
Ammonia beccarii were abundant in the inner part of the drowned valley. Judging from these results, the outer part of the drowned valley became deeper than the former period.
4. ca. 7, 000-6, 000 years ago
According to the abundance of the subtidal mollusca both in the outer and the inner parts of the drowned valley, a whole area of the drowned valley was under a subtidal environment. In this period, the ratio of the planktonic foraminifera increased and the subtropical benthic foraminifera such as
Bulimina cf.
fisiensis and
Trimosina orientalis appeared even in this drowned valley. This indicates that the influence of the open sea water became stronger than before.
5. ca. 6, 000-5, 000 years ago
The inner part of the drowned valley began to be filled with the fluvial deposits. In the outer part of the drowned valley, the mollusca such as
Nitidotellina minuta and
Semelangulus tokubeii which live on the sandy bottom appeared. At this stage, a sand bank began to be formed by the accumulation of sand and gravel on the sea floor of the bay mouth.
6. ca. 5, 000-3, 000 years ago
Judging from the appearance of the shallow marine mollusca such as
Crassostrea gigas and
Batillaria zonalis, the inner part of the drowned valley changed into a tidal flat again. The ratio of the planktonic foraminifera rapidly decreased and
Ammonia beccarii forma A which lives under the low salinity condition increased in the inner part of the drowned valley. These represent that the influence of the open sea water was reduced. On the other hand, the development of a sand bank continued in the outer part of the drowned valley.
7. ca. 3, 000 years ago
The inner part of the drowned valley was changed into a swampy area by the construction of the sand bank on the bay mouth. The upper limit of the marine deposits in the Matsuzaki Lowland is identified at 5m below the present sea level by the fossil analysis. Its age is estimated at about 4, 000 years ago. This indicates that the western coast of the Izu Peninsula has been subsiding during the late Holocene.
View full abstract