Stratigraphic Correlation between the Boreholes

3.4 Reconstruction of geological profile across the Shanchiao fault

In recent years, the Central Geological Survey of Taiwan has carried out a number of drillings in the Taipei Basin in order to better understand its subsurface geology and engineering environment (e.g. Lin et al., 1999). Among them three boreholes in the Wuku area, SCF-1, 2, and WK-1, have been selected for the present analysis due to their optimal locations associated with the Shanchiao fault (Fig. 3.2A) and data quality. All three wells are situated in flat marshy lowland in the Wuku area, western edge of the basin. The WK-1 reaches the deepest known depth of the basin late Quaternary basin deposits at 679 m. Many radiocarbon dates have been acquired (Lin et al., 1999) which are crucial for a successful stratigraphic correlation. At SCF-1 and SCF-2 boreholes, the Sungshan Formation of the uppermost layers was studied by Huang et al. (2007) whose classification of strata serves as a basis for this study. The main stratigraphic system of the whole basin deposits is adapted from Teng et al.

(1999), as discussed above. Through stratigraphic analysis of the three boreholes by incorporating the growth faulting scheme, we reconstruct the Wuku geological profile to reveal the present configuration of fault zone stratigraphy and structure in central part of the Shanchiao Fault.

3.4.1 Stratigraphic correlation between the boreholes

First, we carried out analysis and correlation of stratigraphic units between three boreholes of SCF-1, SCF-2 and WK-1 based on lithology and radiocarbon dates. We reprocessed all available raw radiocarbon dates which were previously published (Lin et al., 1999, Huang et al., 2007) by calibrating these radiocarbon dates to calendar years before present (cal. yr BP) using the model curve of Fairbanks et al. (2005).

Table 3.1 shows details of the results. Note that a few calibrated radiocarbon dates reveal a reversed stratigraphic order (i.e. older ages on top of younger ones), implying that they might be reworked samples. Thermoluminescence ages acquired in WK-1 (Lin et al., 1999) are summarized in Table 3.2. We thus reconstructed a detailed correlation of lithostratigraphic units of the three boreholes (Fig. 3.3) and the unit descriptions are listed in Table 3.3. Comparing the strata in three boreholes (SCF-1, SCF-2 and WK-1), one finds that the thickness of all the four formations increases dramatically toward the east. Hereafter we describe how we correlated the lithofacies units between these three boreholes (Fig. 3.3).

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Fig. 3-2. (A) Map of the Wuku area, showing locations of the three boreholes used in this study, the Shanchiao Fault trace (dashed lines), and geomorphic features. (B) Interpreted Wuku geological profile. We interpret the Shanchiao fault as a combination of a main fault between SCF-2 and WK-1 to the east and the branch fault between the SCF-1 and SCF-2 to the west. Note that there is no vertical exaggeration beneath the sea level, and 20 times vertical exaggeration for surface topography.

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For the uppermost Sungshan Formation at SCF-1 and SCF-2, Huang et al. (2007) divided this formation into three units as C1, C2, and C3. We find that such division also applies to the Sungshan Formation at WK-1 (Fig. 3.3 and Tab. 3.3). The C1 unit comprises mainly sandy layers with mollusk shells at bottom, whose onset of deposition is around 8400 years B.P. (8.4 ka) and is of alluvial facies (Huang et al., 2007). At SCF-1 this unit extends from land surface to depth of 14.5 m (where the shells occurs) and is filled with fine to medium sands with occasional thin silts at 2-3 m and 8 m depths. At SCF-2 this unit is composed of fine sands with one thin mud and silt layers, and ends at the shell lag of 22.3 m depth. Collectively, we find that the C1 unit shows a prograde depositional character in both SCF-1 and SCF-2. At WK-1 the shell layer is found at 33.6 m depth with age ~ 8.4 ka, which lies under interlayers of fine to medium sands and muddy-silts; these sediments of floodplain facies in a prograding fashion (Teng et al., 2000b) are therefore considered to be correlative to the C1 unit.

Underneath the C1 unit, the C2 unit which was deposited from ~9 to 8.4 ka is predominantly clayey and rich in peat and charcoal as defined at SCF-1 and SCF-2 (Huang et al., 2007). The upper part of C2 at SCF-1 is filled with muddy silts and the lower part with mud to depth 21.3 m. At SCF-2 the C2 unit consists of mud in the upper section and silts near the bottom at 32.4 m depth. At WK-1 a thick muddy-silt layer of estuarine environment (Teng et al., 2000b) extending to 51.1 m deep is found corresponding to the C2 unit with analogous sediments. In fact, the radiocarbon ages also support this correlation of stratigraphy. The sediments of the C2 unit are distinctively finer-grained than those above (C1 unit) or beneath (C3 unit), making it much easier to recognize. Abundant detrital charcoal chips and mollusk shells are present in C2 unit at all three boreholes.

The C3 unit, the lowermost member in the Sungshan Formation, is made up of sandy layers with alternating thin charcoal-rich clay layers older than 9 ka at SCF-1 and SCF-2. At SCF-1, the C3 unit consists of silts and fine sands of 13.9 m thickness, and their accumulation started at ~ 10 ka. At SCF-2 the C3 unit is 27.5 m thick composed of interbedded fine sands and silts, and began sedimentation ~12 ka.

Between 51.1 m and 110.8 m depth at WK-1, alternating charcoal-rich layers of fine-medium sands and mud is found equivalent to the C3 unit described above, albeit of greater thickness and longer time span of deposition ranging from ~23 to ~9 ka.

The C3 sediments in WK-1 exhibit an upward transition from alluvial fan to distal floodplain facies in a retrograde stacking pattern, without a break of sedimentation between C3 and underlying Jingmei Formation of C4 unit (Teng et al., 2000b). By contrast, there appears a substantial hiatus at the same stratigraphic boundary between

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C3 and C4 units at SCF-1 and SCF-2, according to the radiocarbon dates.

The Jingmei Formation (lateritic clast-supported conglomerate layer, C4 unit in Table 3.3) also appears to increase its thickness towards the east. At borehole SCF-1, it rests unconformably above weathered Tertiary basement rock and is 9.9 m thick. At SCF-2 it is 33.1 m thick (between 59.9 m to 93 m), and at WK-1 it is 30 m (between 110.8 m to 140.8 m), both lying on the Wuku Formation. The Jingmei Formation, product from the diversion of the Tahan River into the Taipei Basin, was interpreted to be formed during 23 and 25 ka (Teng et al., 2004a). The only radiocarbon date of 38-40 ka in the lower part of the Jingmei Formation at SCF-2 is presumably acquired from reworked material.

The remaining lower part of SCF-2 of about 71 m thick (93 - 164 m), below the Jingmei Formation, is composed of silts in combination of sands, with three conglomeratic units, and is defined as the upper part of the Wuku Formation (C5 – C12 units), which lies unconformably upon the folded Miocene basement rocks. At borehole WK-1, below the Jingmei Formation, two late Quaternary formations lying unconformably above the Tertiary basement rocks can be observed: (a) the Wuku Formation lies in between 140.8 to 301 m (about 140 m thick) and (b) the underlying Banchiao Formation extends from 301 to 679 m (378 m thick). The uppermost Wuku Formation was dated to be far older than 50 ka, implying a large gap of sedimentation before the onset of depositing the overlying Jingmei conglomerates. The oldest dated age for the Wuku Formation is about 200 ka at WK-1 borehole. The Banchiao Formation constitutes the lower half of the WK-1 borehole. Existing thermoluminescence ages imply that the age for the Banchiao Formation is no younger than 150 ka (Table 3.2) and might be no older than 400 ka (Wei et al., 1998;

Teng et al., 2001).

All three boreholes penetrated to the Tertiary basement rocks which are known exposed surrounding the Taipei Basin as the rugged foothills. Basement rocks reached at SCF-1 and 2 are probably the late-Miocene Nanchuang Formation, with weathered top basement rock in SCF-1. Rocks retrieved from WK-1 in 679 m to 741 m belongs to the early Miocene Taliao Formation, which is thrust over the Nanchuang Formation at 750-760 m depth by a shear zone containing fault breccias (Lin, 2005). Note the dramatic increase of basement depth between SCF-2 and WK-1 boreholes compared to a relatively mild one between SCF-1 and SCF-2 boreholes (Fig. 3.2B), implying the location of the main Shanchiao Fault between the SCF-2 and WK-1. We will discuss later in more details.

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Through lithologic correlation in aid of radiocarbon ages, the Sungshan and Jingmei formations are present across the entire profile, while the Wuku Formation is only found at SCF-2 and WK-1, and the Banchiao Formation is restricted to WK-1.

The Sungshan Formation (and units within), the Jingmei Formation, and the Wuku Formation all thicken toward the east, and the stratigraphic horizons deepen eastward.

Sedimentation rate appears to vary both temporally and spatially. We found two major hiatus: one between the deposition of Wuku and Jingmei sediments, and the other one between deposition of Jingmei and Sungshan sediments at SCF-1 and 2.

Sediments within the Sungshan and Jingmei formations are quite similar between the boreholes, indicating the sediment source was generally the same and was presumably basin wide, as from the Tahan - Danshui river system (sediment transporting direction normal to the profile and rather homogeneous). Tributaries drained from the Linkou Tableland to the east didn’t seem to assert significant contribution to the deposits since no alluvial-fan conglomerates are found beyond deposition of the Jingmei gravels in these boreholes, suggesting little local sediment transport.

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Fig. 3-3. Stratigraphic correlation between boreholes (SCF-1, SCF-2, and WK-1) of the Wuku profile. Location of the boreholes is indicated in Fig. 3-2. Four major geological formations consist of the late Quaternary deposits of the Taipei basin:

from top to bottom, the Sungshan, Jingmei, Wuku, and Banchiao Formations.

Each formation has been divided into several stratigraphic units. See details in the text. Please note different depth scales between 0-200 m and 200-750 m, and omitted section between 450 m and 650 m.

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Table 3-1: Radiocarbon age data of the three boreholes along the Wuku Profile.

a Raw radiocarbon ages reported in Huang et al., 2007, and in Central Geological Survey report Lin et al., 1999, are labeled with (1) and (2), respectively.

b Calibration with Fairbanks 0107 calibration curve (Fairbanks et al., 2005).

Borehole Data

(1) 34060±600 38824-40038 87.52 Jingmei

(1) >50000 N/A 102.20 Wuku

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Table 3-2: Thermal luminescence (TL) ages. Data from Lin et al., 1999

Borehole TL age (ka B.P.) Depth (m) Unit

WK-1 80-120 155.9 Wuku

83-113 177 Wuku

73-99 234.2 Wuku

164-264 251 Wuku

169-227 328 Banchiao

192-288 351.5 Banchiao

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Table 3-3: Lithofacies of the Wuku Profile.

Thickness (m)

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lamina

T3 Two conglomeratic layers in sands

- - 49.8 Wuku c

T2 Varve with rare thin silt/sand

- - 21 Banchiao c

T1 Non-stratified clay containing vivianite concretes,

cross-laminated sand at bottom section

- - 29 Banchiao c

C13 Silt-clay with thin conglomerates

- - 59.3 Banchiao

C14 Conglomerate with rare thin sand/silt

- - 268.7 Banchiao

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