2.1 Forearc basin fills: the Miura and Kazusa groups
The central part of the Honshu arc is located near a trench–trench–trench-type triple junction (Fig. 1a) where the Philippine Sea Plate is being subducted beneath the North American Plate and the Pacific Plate is being subducted beneath the North American and Philippine Sea plates (McKenzie and Morgan, 1969). Deformation of the Neogene deposits exposed on the Miura and Boso peninsulas records compressional convergence owing to the interaction of the Philippine Sea and Eurasian plates near the triple junction after the Miocene (Ogawa et al., 1989). In this tectonic setting, the Hayama–Mineoka uplift belt consists of an ophiolitic complex that acted as a trench–slope break (Takahashi, 2008) that created space for the deposition of the forearc basin fill (Fig. 1b). The forearc basin fills on the Miura and Boso peninsulas are called the Miura (middle Miocene–Pliocene) and Kazusa (Pliocene–middle Pleistocene) groups, which are approximately 2000 and 3000 m thick, respectively (Suzuki et al., 1995). These groups consist of shallow- and deep-marine deposits (Mitsunashi and Kikuchi, 1982; Ito and Katsura, 1992), and they contain numerous marker tephra beds and abundant microfossils that facilitate lateral stratigraphic correlations and reliable determinations of depositional age. Moreover, the fact that the tephra beds act as stratigraphic markers and can be traced laterally has allowed sedimentologists to recognize spatial variations in the depositional sequences (Ito, 1992, 1995), the sandstone beds in submarine fans (Hirayama and Nakajima, 1977), and large-scale mass-transport deposits (Utsunomiya, 2018; Utsunomiya et al., 2018; Utsunomiya and Yamamoto, 2019). Previous studies on stratigraphic correlations between the Boso and Miura peninsulas (Ida et al., 1956; Mitsunashi, 1973; Mitsunashi and Kikuchi, 1982; Suzuki et al., 1995) have been based mainly on similarities of the macroscopic and microscopic characteristics of the marker tephra beds. A combination of microfossil biostratigraphy and the geochemical compositions of volcanic glass in the tephra beds allowed significant improvements in correlating the stratigraphic units of Pleistocene sequences around Tokyo Bay, such as on the Miura, Boso, and Choshi peninsulas (marine deposits) and in the Tama hills (terrestrial and shallow marine deposits) (Fujioka et al., 2003; Fujioka and Kameo, 2004; Takahashi et al., 2005; Tamura et al., 2010; Mizuno and Naya, 2011; Suzuki and Murata, 2011; Tamura et al., 2019; Utsunomiya et al., 2019). The stratigraphy established for the Miura and Boso peninsulas has played a significant role in interpretations of seismic profiles (Chiba Prefecture, 2004; Asao and Ito, 2011) and deep-borehole sequences around Tokyo Bay (Suzuki and Horiuchi, 2002; Yanagisawa et al., 2006; Chiyonobu et al., 2007; Naya et al., 2013). Moreover, the tephra beds of the Kazusa Group recently recognized at depth (> 1000 m) around Tokyo Bay (Tamura et al., 2010; Utsunomiya et al., 2020) have allowed the integration of seismic-scale structures with the high-resolution stratigraphy established from outcrop-based studies.
The boundary between the Kazusa and Miura groups is called the Kurotaki unconformity, with the lower Kazusa Group lapping onto the underlying Miura Group as the depocenter of the Kazusa Group migrated northward (Mitsunashi and Yamauchi, 1988). On the Boso Peninsula, the unconformity between the Kazusa and Miura groups is distinctly angular, representing a temporal gap from ca. 3 to 2.4 Ma (Kameo and Sekine, 2013), but on the Miura Peninsula there is no significant angular relationship between the two groups (Koike, 1951; Akamine et al., 1956). No significant temporal gap corresponding to the Kurotaki unconformity has been identified in the Inubo Group, exposed on the Choshi Peninsula, based on biostratigraphy and tephrostratigraphy (Sakai, 1990; Tamura and Yamazaki, 2010; Tamura et al., 2014). Utsunomiya et al. (2017) demonstrated that the uppermost parts of the Miura Group and the lowermost parts of the Kazusa Group on the Miura Peninsula are time-equivalent deposits, based on the lateral tracing of tephra beds. Gradual variations in the rate of sedimentation indicated that the Kazusa Group of the Miura Peninsula was deposited conformably on the Miura Group during 3.2–2.4 Ma, based on calcareous nannofossil biostratigraphy and magnetostratigraphy. The stratigraphic flamework is therefore being reconstructed with a focus on Pliocene stratigraphy as the key interval.
2.2 Miura Peninsula
The geologic map of the northern Miura Peninsula in Fig. 2a has been modified from Eto (1986a), Eto et al. (1998) and Utsunomiya et al. (2017) based on the distribution of tephra beds shown by Inagaki et al. (2007), Utsunomiya et al. (2017), and this paper. The Miura and Kazusa groups generally dip 5°–30°NE, but the Miura Group and the lowermost parts of the Kazusa Group have been deformed by a NE–SW trending syncline. The Miura Group consists of the Miocene–Pliocene Zushi and the Pliocene Ikego formations, in ascending stratigraphic order. A basal conglomerate called the Tagoegawa sandstone and conglomerate Member (15–50 m in thickness) is overlain by mudstone-dominated alternating sandstones and mudstones (1000–1500 m in thickness) of the Zushi Formation (Eto et al., 1998). The lower part of the Ikego Formation consists of the Takatoriyama volcaniclastic Member (0–210 m in thickness) and the Jimmuji volcaniclastic and mudstone Member (0–60 m in thickness), and the main upper part of the Ikego Formation consists of alternating beds of sandstone and tuffaceous sandy mudstone 150–400 m in thickness (Eto et al., 1998). In detail, the Takatoriyama Member consists of massive and cross-bedded tuffaceous sandstones and conglomerates that have been interpreted Fig. 3
as canyon-fill deposits, given the convex-down shape inferred from their geographic distribution (Soh et al., 1991). The Jimmuji Member consists of mass-transport deposits (Fig. 3b; Eto, 1993; Yokohama Defense Facilities Administration Bureau, 1993) that contain chaotic deposits with blocks of sandstone and mudstone, likely generated by submarine landslides of both the canyon wall and the canyon fill (Taira et al., 1993). The upper Ikego Formation consists mainly of alternating beds of sandstone and tuffaceous sandy mudstone, which cover an irregular erosional surface of a slide block of sandy mudstone in the Jimmuji Member (Fig. 3; Eto, 1993; Yokohama Defense Facilities Administration Bureau, 1993; Utsunomiya et al., 2017). The lowermost part of the Kazusa Group, the Urago Formation, consists mainly of tuffaceous medium- to coarse-grained sandstones and tuffaceous muddy sandstones (Utsunomiya and Majima, 2012). Trough- and tabular-cross bedding is common in the tuffaceous sandstones, and this has been attributed to the migration of dunes under the influence of bottom currents that varied in direction between northward and eastward (Utsunomiya et al., 2015).
The paleobathymetry of the upper Ikego Formation and the Urago Formation has been estimated at between 500 and 2000 m based on benthic foraminiferal assemblages (Eto et al., 1987) and between 400 and 600 m based on molluscan asemblages (Utsunomiya and Majima, 2012). The upper part of the Ikego Formation has been assigned to the CN12a subzone of Okada and Bukry (1980) by Okada (1993) and to the RN12 Zone of Kamikuri et al. (2004) by Suzuki and Kanie (2012).
Although the top of the Ikego Formation has been interpreted as the westward extension of the Kurotaki unconformity (Mitsunashi, 1973; Eto, 1986b; Eto et al., 1998), the bedding planes of the Ikego Formation are almost parallel to those of the overlying Kazusa Group (Koike, 1951; Akamine et al., 1959). Utsunomiya et al. (2017) demonstrated that the Urago Formation of the Kazusa Group conformably overlies the Ikego Formation, based on tephra bed correlation, then identified the following stratigraphic markers in ascending order (Fig. 4): top (3.21 Ma) of the Mammoth subchronozone (C2An.2r); base (3.13 Ma) and top (3.05 Ma) of the Kaena subchronozone (C2An.1r); last appearance of Discoaster tamalis (MIS G7–G8, 2.76 Ma; Kameo and Okada, 2016); the widespread tephra bed KGP (ca. 2.5 Ma); and the last appearance of Discoaster pentaradiatus (MIS 95, 2.41 Ma; Kameo and Okada, 2016). The KGP tephra bed occurs within the lowermost Matuyama chronozone (Tamura et al., 2010; Ueki et al., 2013), which suggests the Gauss–Matuyama boundary (2.61 Ma) is in the middle of the Urago Formation.
2.3 Boso Peninsula
The geologic maps of the western (Fig. 2b) and eastern (Fig. 2c) Boso Peninsula are from Nakajima and Watanabe (2005) and this study, respectively. Although the Miura Group on the Boso Peninsula is often called the Awa Group (Nakajima et al., 1981; Nakajima and Watanabe, 2005), for convenience we use the name Miura Group following Suzuki et al. (1995) in order to identify time equivalent strata on both peninsulas. The Miura Group on the western Boso Peninsula generally has an E–W trending strike, and it has been deformed by ENE–WSW trending folds (the Seri Syncline and Terao Anticline: Fig. 2b). On the central Boso Peninsula, the fold axes trend ESE–WNW, and the folds are called the Kiyosumi syncline and Kiyosumi anticline. The Miura Group on the Boso Peninsula consists of the Fukawa, Kanigawa, Kinone, Amatsu, Kiyosumi, and Anno formations in ascending stratigraphic order (Nakajima et al., 1981), among which the Kiyosumi and Anno formations were the subject of our study.
The Kiyosumi Formation consists mainly of alternating beds of hemipelagic mudstone and sandstone, and it varies in thickness from 76 to 196 m on the western side (Nakajima, 2005) to ~ 800 m in the center of the peninsula (Tokuhashi and Ishihara, 2008). The Kiyosumi Formation represents sand-dominated submarine fan deposits with a feeder system from north to south (Tokuhashi, 1989). The spatial–temporal distribution of turbidite deposits shows the development of syn-sedimentary E–W trending folds, which resulted in the strata being thicker along the synclinal axis (Tokuhashi, 1976a, b; Tokuhashi, 1989). The paleobathymetry of the Kiyosumi Formation has been estimated to be 1000–1800 m (Hatta and Tokuhashi, 1984), based on benthic foraminiferal assemblages.
The Anno Formation conformably overlies the Kiyosumi Formation and is unconformably overlain by the Kazusa Group across the Kurotaki unconformity. The thickness of the Anno Formation is 125–386 m in the western part of the Boso Peninsula (Nakajima, 2005). Nakajima (1973) numbered the marker tephra beds in the Anno Formation in ascending stratigraphic order with numbers prefixed by “An” (Nakajima et al., 1981; Natural History Museum and Institute, Chiba, 1995, 1996; Nakajima, 2005). In the western part of the peninsula, the Anno Formation generally shows an upward-coarsening succession from mudstone, sandy mudstone, and muddy sandstone to sandstone (Nakajima, 2005). The Anno Formation represents submarine fan deposits (Nakajima et al., 1981; Ishihara and Tokuhashi, 2005) or a slope base system (Saito and Ito, 2002). Ishihara and Tokuhashi (2005) recorded the lateral distribution and successive changes in the turbidite facies and paleocurrent directions. They identified six sedimentary facies associations: channel fill deposits of pebbly sandstone and conglomerates; natural levee deposits of alternating thin turbidites and hemipelagic mudstones; thick-bedded lobe deposits; marginal deposits of alternating thin turbidites and hemipelagic mudstones; marginal deposits of alternating hemipelagic mudstones and tephra beds; and extensive slump deposits. The upper part of the Anno Formation consists of marginal deposits of alternating hemipelagic mudstones and tephra beds that coarsen upward, suggesting a shallower depositional environment or the winnowing of fine-grained sediment owing to bottom currents (Ishihara and Tokuhashi, 2005). There are many mass-transport deposits in the Anno Formation, of which “Ta slump” below An22 and “Sak slump” below An157 can be traced Fig. 5
laterally (Nakajima, 2005). There is a hiatus due to submarine landslide(s) and residual mass-transport deposits called “Sak slump” between An155 and An157 (Fig. 5). Sak slump ranges in thickness from 0.2 to 5 m, and it caused the erosion of 8–15 m-thickness of underlying strata, including tephra beds An155-2–An156-4 (Nakajima, 2005). The paleobathymetry of the Anno Formation has been estimated to be 1200–2800 m (Hatta and Tokuhashi, 1984), based on benthic foraminiferal assemblages.
The depositional ages of the Kiyosumi and Anno formations have been estimated using calcareous nannofossil biostratigraphy (Kameo et al., 2010; Kameo and Sekine, 2013), fission-track analyses of the tephra beds (Kasuya, 1987; Tokuhashi et al., 2000), and magnetostratigraphy (Niitsuma, 1976; Haneda and Okada, 2019, 2022). The Kiyosumi Formation includes C3n.4n and C3n.3n normal polarity subchronozones, and the top of this formation belongs to the C3n.2n normal polarity subchronozone (Niitsuma, 1976; Takahashi, 2008). Haneda and Okada (2019) recognized the period in the Anno Formation that extends from the top of the Nunivak normal polarity subchronozone (4.49 Ma) to the top of the Mammoth reverse polarity subchronozone (3.21 Ma). The Sak slump is intercalated within the Mammoth subchronozone, and it may have caused the erosion of strata over a period of approximately 90 kyr (Haneda and Okada, 2019).
The Kazusa Group on the Boso Peninsula is made up of a shallow- to deep-marine succession that is ~ 3 km thick (e.g., Ito and Katsura, 1992; Suzuki et al., 1995) in the middle of the peninsula (Nakajima and Watanabe, 2005; Utsunomiya and Ooi, 2019). The Kazusa Group is divided into 13 formations based on lithofacies and stratigraphic positions. The lower and middle parts of the Kazusa Group consist of deep-sea (submarine fan and basin plain), upper slope, and outer shelf deposits, whereas the upper parts of the Kazusa Group comprise shallow marine deposits (Ito and Katsura, 1992). The Kazusa Group laps onto the Miura Group, and this resulted in the base of the Kazusa Group younging toward the central part of the peninsula. The basal sandstone and conglomerate succession called the Kurotaki Formation is thought to be contemporaneous with the laterally adjacent turbidite successions of the lower part of the Kazusa Group on the eastern and western sides of the peninsula (Kawabe et al., 1981; Ito et al., 1992; Nakajima and Watanabe, 2005). The paleoslope dips southeastward in the lower part of the Kazusa Group and northeastward in the middle and upper parts, as inferred from paleocurrent analyses of the turbidites (Hirayama and Nakajima, 1977; Tokuhashi, 1992).
2.4 Widespread tephra beds used for inter- and intra-basinal correlation
There are many widespread vitric ash beds intercalated in the Miura Group and the lower parts of the Kazusa Group on the Boso Peninsula, and these tephra beds are also distributed around the Tokai, Kinki, and Chubu districts in the western Japanese islands. In the Miura Group, tephra beds An51, An53, An77, An85, An112, An129, and An130 are regarded as widespread tephra beds named Trb1-Ya4, Sk-Ya5, Ksg-An77, Znp-Ohta, Ymp-SF8.3, Hgs-An129, and Sr-Ity, respectively (Kurokawa and Higuchi, 2004; Satoguchi et al., 2005; Tamura et al., 2008; Tamura and Yamazaki, 2010; Satoguchi and Nagahashi, 2012). In the lower parts of the Kazusa Group, KW2, KH1, MY, KB, HS C, Kd44, Kd39, Kd25, and Kd24 are regarded as widespread tephra beds named Fup-KW2, Obr-Bnd1, OM1-OKIII, Bnd2-O1, Tmg-R4, Kd44-Nk, Ho-Kd39, Eb-Fukuda, Om-SK110, and Srt-SK100, respectively (Nagahashi et al., 2000; Suzuki and Nakayama, 2004; Tamura et al., 2019). Moreover, some thick (several meters) crystal-rich tephra beds with a “salt and pepper” appearance have been used for intra-basinal correlation. For example, Am78, Ky21, and Ky26 on the Boso Peninsula have been correlated with Ok, Hk, and Nt on the Miura Peninsula (Urabe et al., 1990; Urabe, 1992; Suzuki et al., 1995).