**4. Results**

The Okinawa Trough is a long, N-S trending that connects Japan and Taiwan (**Figure 1**). It is a back-arc basin formed by extension within the continental lithosphere landwards of the Ryukyu trench-arc system [13, 14]. The initial opening of the northern and middle part of the current Okinawa Trough occurred during the Miocene, while it delayed until the Pleistocene in the southern part of the Trough [15, 16]. It is also believed that this back-arc basin may be suited even westwards before Miocene, controlling the emplacement and the tectonic setting

**Figure 2.** Bathymetric map with location of seismic lines. Red lines indicate the locations of the selected seismic profiles. White circles along the red lines indicate the locations of the shot points. Blue square A indicates the location of the

64 Seismic and Sequence Stratigraphy and Integrated Stratigraphy - New Insights and Contributions

The Taiwan is located between the large marginal seas of the East China Sea and the South China Sea and includes a young orogenic belt, formed by Late Miocene collisional events in the SE Eurasian continent (**Figure 1b** and **c**) [17, 18]. In addition to metamorphic basement rocks, the body of the Taiwan orogenic belt is mainly composed of metamorphic basement rocks and deformed Tertiary rift basins, including Paleogene syn-rift and Neogene post-rift sedimentary sequences [19]. Since the Taiwan orogenic belt is moving southward, different evolutionary stages of the orogenic belt

of the East China Sea Shelf Basins [3, 5].

exploration wells.

## **4.1. Line X**

We interpret the seismic data and depict stratigraphic boundaries, mainly on the basis of the concepts carried out by Vail and Mitchum [34] and Mitchum et al. [35]. Line X runs NE-SW, extending approximately 30 km and perpendicularly to the coastline of Taiwan (**Figure 2**).

**Figure 3.** Selected reflection seismic profile along Line X. See **Figure 2** for the location.

The most of line X is located on the continental shelf, and the central and northeastern part of the Line X have the bathymetry deeper than 200 m (**Figures 2** and **3**). A drilling site at the NE part of the survey line X, providing geological information and chronostratigraphic controls in study area [8, 9].

A seismic boundary namely U1 at about 1 second Two-Way Travel Times (twt) has been observed (**Figure 3**, marked as a green line), extending in the whole profile. In the middlesouth part of the profile X (SP 2500-4000; **Figure 4a**), the reflectors beneath the U1 show very apparent termination against the U1, of which we interpreted as erosional truncation. In the northeastern part of the profile (SP 6500-8000; **Figure 4b**), the termination relationship between the U1 and the underlying reflectors shows erosional truncation as well. Collectively, we suggest that U1 is an unconformable surface. At the southwestern part of the profile, the unconformity U1 is located around 0.85 s (twt) (**Figure 3**), while the unconformity U1 becomes deeper to around 1.2 s (twt) at the north-eastern end of the profile, showing a gently descending trend toward north-east.

The characteristics of the seismic reflectors above the unconformity U1 are not always consistent along the Line X. In the central-south and northernmost part of the profile (**Figure 3**,

**Figure 4.** Selected seismic sections showing the termination feature of the U1. See **Figure 3** for locations.

**Figure 3.** Selected reflection seismic profile along Line X. See **Figure 2** for the location.

66 Seismic and Sequence Stratigraphy and Integrated Stratigraphy - New Insights and Contributions

SP 500-5000; 7000-8500), it is dominated by layered reflectors. However, in the middle-north part of the profile (**Figure 3**, SP 5000-7000), it shows strong seafloor reverberation above 1 s (twt) and an area of reflection-free below 1 s (twt). It seems that the strong seafloor reverberation has intervened between layered reflectors to both southern and northern ends of the seismic profile. We interpret that this reflection feature is plausible to be the seismic response of very hard and solid seafloor material, probably crystallization or extrusive rocks.

#### **4.2. Line Y**

Line Y is located near offshore northern Taiwan, and runs NW-SE, parallel to the coastline of the Taiwan (**Figure 2**). It extends around 40 km along dip of the continental shelf, showing even no drastic change in bathymetry (**Figures 2** and **4**). There are some gentle bathymetric reliefs associated with dominant fault structures (indicated by write triangles in **Figure 5**; SP 4300-4500, 5900-6100, 8300-8600).

In the north-western part of the seismic profile Y (SP 400-1700; **Figure 6a**), the truncated reflectors are tilted, suggesting the occurrence of tectonic and/or erosional events. In addition, the

**Figure 5.** Selected reflection seismic profile along Line Y. See **Figure 2** for the location.

Seismic Stratigraphic Features of the Late Miocene-Present Unconformities and Related Seismic Units… http://dx.doi.org/10.5772/intechopen.70819 69

SP 500-5000; 7000-8500), it is dominated by layered reflectors. However, in the middle-north part of the profile (**Figure 3**, SP 5000-7000), it shows strong seafloor reverberation above 1 s (twt) and an area of reflection-free below 1 s (twt). It seems that the strong seafloor reverberation has intervened between layered reflectors to both southern and northern ends of the seismic profile. We interpret that this reflection feature is plausible to be the seismic response

Line Y is located near offshore northern Taiwan, and runs NW-SE, parallel to the coastline of the Taiwan (**Figure 2**). It extends around 40 km along dip of the continental shelf, showing even no drastic change in bathymetry (**Figures 2** and **4**). There are some gentle bathymetric reliefs associated with dominant fault structures (indicated by write triangles in **Figure 5**; SP

In the north-western part of the seismic profile Y (SP 400-1700; **Figure 6a**), the truncated reflectors are tilted, suggesting the occurrence of tectonic and/or erosional events. In addition, the

**Figure 5.** Selected reflection seismic profile along Line Y. See **Figure 2** for the location.

of very hard and solid seafloor material, probably crystallization or extrusive rocks.

68 Seismic and Sequence Stratigraphy and Integrated Stratigraphy - New Insights and Contributions

**4.2. Line Y**

4300-4500, 5900-6100, 8300-8600).

**Figure 6.** Selected seismic sections showing the termination feature of the U1 and U2. See **Figure 5** for locations.

U1 in the seismic profile Y also serves as a top surface of the dominating tilted fault blocks. In this way, the U1 is of regional importance, and the distribution of the U1 reflects the local tilting of the fault blocks.

We also identified another seismic boundary U2 in the seismic profile Y (**Figure 5**, marked as purple lines), although it often coexists with seafloor multiple signals (**Figure 5**, SP 1700-3000 and 5000-6000). In the middle-south part of the profile (SP 3500-5000; **Figure 6b**), the U2 is identified at around 0.3-0.4 s (twt), serving as a termination surface of underlying updipping reflectors. On the basis of on the parallelism of terminated reflectors [35], we interpret an erosional truncation relationship between the U2 and terminated reflectors. In the middle-north part of the profile (SP 6000-8700; **Figure 7**), the parallelism of terminated reflectors and the feature of erosional truncation become much obvious. We interpret the U2 as an unconformity that may mark a tectonic event as the unconformity U1.

We note that both the unconformities U1 and U2 do crop out at seafloor in the seismic profile Y (**Figure 6**). The unconformity U1 crops out at the crest of a local structural high, along with a fault-block bounding fault (**Figure 7**, SP 8200-8500). The U2 crops out at southeasternmost of the profile (**Figure 7**, SP 12000). These may be the result of the interaction between the degree of fault block rotation and local sediment discharge. In addition to the U1 and U2 in the seismic profile Y, we observe a strong and continuous reflector shown between the clear features of the U1 and U2 (dashed line in **Figure 7**). Such reflector may also indicate the existence of the minor and local events, probably the halts of the fault block rotations.

**Figure 7.** Selected seismic sections showing the termination feature of the U1 and U2. See **Figure 5** for locations.

#### **4.3. Seismic units**

On the basis of the U1 and U2, three seismic units are determined accordingly: the SU I, SU II, and SU III, which indicate the sedimentary sequence beneath the U1, the sedimentary sequence between the U1 and U2, and the sedimentary sequence above U2, respectively (**Figures 3** and **5**). In the seismic profile Y, we note that the thickness of the SU II varies along the profile, mainly depending on the relief of the underlying fault structures (**Figure 5**), while the thickness of the SU III change slightly and diminish toward the southeast end of the seismic profile Y (**Figure 5**, SP 10000-12000). The thickness variation of the SU II and SU III may indicate the change in subsidence patterns and rates, probably reflecting the change in controlling factor of subsidence [36, 37, 38].
