programme tuesday, july, 31 - Université de Caen Basse Normandie

Tidalites 2012, 8th International Conference on tidal environments, Caen, France – Abstract book
LATE QUATERNARY STRATIGRAPHIC EVOLUTION OF BAEKSU OPEN-COAST
TIDAL FLAT, WEST COAST OF KOREA: REGIONAL UNCONFORMITY-BOUNDED
TWO TIDAL DEPOSITS
Taesoo CHANG, Jincheol KIM, Dong-Geun YOO
KOREA INSTITUTE OF GEOSCIENCE & MINERAL RESOURCES, #124, Gwahang-No, Yuseong-Gu,
305-350, Daejeon, Korea, tschang@kigam.re.kr
Along the eastern margin of the Yellow Sea, numerous macrotidal flats are extensively
developed. In the last two decades, lithostratigraphic studies of these tidal deposits have been
based on sedimentological anlayses of vibrocores up to ~6 m long, and hence stratigraphically
the tidal deposits were restricted mostly to the Holocene in age due mainly to lack of deep cores.
Another reason for that was likely a general dearth of dateable materials and credibility of age
dates prior to the Holocene. One of the important findings from the Korean tidalite researches in
the past was the presence of the semi-consolidated, yellow oxidized layers overlain by Holocene
tidal deposits with a stark erosion boundary defining them. Under the situation without any reliable
age constraints for that layers, the pre-Holocene strata have been controversial.
In a recent year, however, advanced dating technologies, i.e., OSL and 14C AMS, have
enabled to ensure the age constraints of older tidal deposits dating back to late Pleistocene, and
to understand the stratigraphic packages in response to local changes in sea level. Previous
works showed that some tidal deposits at the Korean west sea coasts can be grouped into at
least two unconformity-bounded sequences, which formed in response to late Quaternary
sea-level fluctuations (Lim and Park, 2003; Choi and Dalrymple, 2004; Choi and Kim, 2006).
However, some age dates determined by AMS, particularly ages around 40,000~50,000 yr BP,
are still questionable, the ages leading to discrepancy between the curve reconstructed and the
elevation of the deposits. In order to elucidate the chronostratigraphic problems, an OSL dating
method was applied to the pre-Holocene tidal deposits recovered from the Baeksu tidal flat,
Korea. In addition, the relationship between the seismic reflection boundary and the lithologic
surface based on the analysis of seismic profiles and deep- drilled cores will be addressed.
Sediment cores from an open-coast, Baeksu macrotidal flat, Korea, contain about 45 m
thick two tidal deposits stratigraphically separated by a yellow, semi-consolidated mud layer and a
gravel layer. Two tidal units, a Holocene unit and an underlying late Pleistocene unit, have
recurred, as previously reported in the other tidal basin. In the course of core examination, the
Baeksu open-coast tidal deposits can be grouped into distinct 6 facies associations, shoreface
sands, sand flat, gravelly channel lags, paleosoil horizons, mudflat/salt marshes, and fluvial
gravel lags with muds in descending order. The Baeksu tidal flats comprise at least two
sequences which formed in response to fluctuations in local sea-level during the late Quaternary.
The oxidized layer characterized by semi-consolidated, yellowish sediments separates them,
producing a regional unconformity. Such an unconformity-bounding surfaces correlate well with a
prominent mid-reflector, observed on seismic profiles. Consequently, the layer associated with
the striking reflector may signify the emergence of the Yellow Sea shelf during the pre-Holocene
lowstand. Each sequence consists of lower fluvial deposits and overlying tidal deposits. However,
intertidal sands are commonly missing in lower sequence. The presence of two tidal deposits
warrants that two cycles of major sea-level fluctuations are recorded, the similarity between
Holocene and late Pleistocene tidal deposits indicating macrotidal regime recurred in the region.
The extensive shallow shelf and relatively stable tectonic setting of the Yellow Sea appears to
promote the repetition of tidal sedimentation in the Baeksu open-coast tidal flat.
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