| Summary: | 碩士 === 國立臺灣大學 === 地質學系 === 86 === In order to better understand the sedimentary history of late
Quaternary Taipei Basin, this study gathered deep boleholes
data in the basin for faciesand depositional system analysis.
The basin is filled with unconsolidated, interlayers of gravels,
sands, muds and sand/mud interbeds, in which nine lithofacies
were recognized according to the lithological characteristics.
Most of the gravels are thicked-bedded, cobbly in size, poorly
sorted and intercalated by brownish to reddish soil horizons.
Thus, these gravels are believed to be deposited by debris flow
in alluvial fans. Some of the gravels are pebbly in size, fairly
sorted, clast supported and may be interbedded with sands,
indicated these gravels were deposited by high energy river
current in braided plain. Sands and muds are generally
interlayered and they are often less than 20 meters in
thickness. Sands may exhibit a sharp base paved with pebbles, in
which usually fine upward into muds and contained abundant
carbonaceous fragments an few vivianite. Thus, the fining-upward
sequence isdeposited in channel or flood plain. Some muds,
exceeded ten meters and nated, indicating these mud were
deposited by suspension in a fresh-water lake. Neverthelesss,
shells of lagoonal facies also appeared in few parts of sands
and muds, indicating these sedimentswere deposited in a
brackish-water lake. Using laminated
muds in upper Panchiao Formation, gravel beds of alluvial fan in
Chingmei Formation, and distribution of shell fossils in
Sungshan Formation to be a bases of isochronous correlation, and
then combined the results of pollen stratigraphy to set up time
frame. Taipei Basin began to accumulated fluvio-lacustrine
sediments in the middle Pleistocene. In the early stage, the
basin was covered by alluvial fans, fluvial plains and a fresh-
water lake. The Taipei Basin was once dammed by the pyroclastic
flows derived from the north and became a big deep lake, in
which laminated muds were widely accumulated. The depositional
systems then returned to an alluvial fan - fluvial plain -
fresh-water lake setting. Then the basin was filled up by a
huge alluvial fan deposits in the late Pleistocene. In the early
Holocene,marine incursion inundated the basin which became a
brackish-water lake. The lake was then gradually filled up by
the prograding river sediments from the basin margin.
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