Spatial and temporal variations in depositional systems in the Kazusa Group: insights into the origins of deep-water massive sandstones in a Pleistocene forearc basin on the Boso Peninsula, Japan

Takao, Akihiko; Nakamura, Keisuke; Takaoka, Shinichi; Fuse, Masaya; Oda, Yohei; Shimano, Yasushi; Nishida, Naohisa; Ito, Makoto

doi:10.1186/s40645-020-00343-1

Progress in Earth and Planetary Science

Table 1 Lithofacies types and their inferred depositional processes. The outcrop features of each lithofacies type are shown in Fig. S6.

From: Spatial and temporal variations in depositional systems in the Kazusa Group: insights into the origins of deep-water massive sandstones in a Pleistocene forearc basin on the Boso Peninsula, Japan

Type 1 Ungraded sandstones with convolution Thickness: 50 cm–up to more than 250 cm Description: Medium- to coarse-grained sandstones with sharp and flat or locally distinct erosional bases. No distinct sedimentary structures are developed and convolute structures are common. Locally, internal erosion surfaces in association with mudstone clasts are developed, and scattered coarse- to very coarse-grained sands and granules are also locally observed. Interpretation: Rapid fall out of suspended particles from locally erosional high-density turbulent flows, and/or recurrent fall out of suspended particles from sustained high-density gravity flows (Lowe 1982; Kneller and Branney 1995; Talling et al. 2012), in association with bypassing of sedimentation from wakes. Locally developed internal erosion surfaces and mudstone clasts suggest amalgamation of thinner component sandstone beds.
Type 2 Ungraded sandstones with normal grading in the uppermost part Thickness: 50 cm–up to more than 250 cm Description: Major lithofacies features are very similar to those of type 1 except for the development of normally graded sandstones up to 10 cm in the uppermost part. Graded sandstones are medium- to very fine-grained and locally contain parallel- and/or current ripple-laminations and convolute laminations. Interpretation: Rapid fall out of suspended particles from locally erosional high-density turbulent flows, and/or recurrent fall out of suspended particles from sustained high-density gravity flows (Lowe 1982; Kneller and Branney 1995; Talling et al. 2012), in association with fall out of finer grained particles from wakes.
Type 3 Inverse-to-normally graded sandstones with current ripple- and parallel-laminations Thickness: 20–40 cm Description: Inverse-to-normally graded, medium- to very fine-grained sandstones, in association with current ripple- and/or parallel-laminations. The basal and upper surfaces of sandstone beds are sharp and flat, and locally upper contacts are gradational to interbedded sandy siltstones. Interpretation: Invers-to-normal grading indicates waxing and waning of lower density gravity flows, possibly ignited by hyperpycnal flows (Mulder et al. 2003).
Type 4 Alternation of current ripple- and parallel-laminated sandstones Thickness: 20–150 cm Description: Multiple repetition of inverse-to-normally graded, medium- to very fine-grained sandstones in association with current ripple- and parallel-laminations. Structureless and/or normally graded, medium- to very fine-grained sandstones are also locally intercalated. Internal erosion surfaces and/or muddy streaks are also locally intercalated. Interpretation: Alternation of current ripple- and parallel-laminations, in association with internal erosion surfaces and muddy streaks indicate multicycles of waxing and waning gravity flows, possibly induced by hyperpycnal flows and/or breaching of sandy substrates (Mastbergen and Van den Bergen 2003; Mulder et al. 2003)
Type 5 Structureless sandstones Thickness: 3–30 cm Description: Medium- to fine-grained sandstones without any distinct normal and/or inverse grading, and sedimentary structures, except for locally developed convolute laminations. Sharp and flat basal and upper surfaces are common, and some beds have distinct erosional bases and contain mudstone clasts in the basal parts. Interpretation: Rapid sediment fall out of turbulent suspension in locally erosional gravity flows (Talling et al. 2012). Type 6 Normally graded or parallel- and/or current ripple-laminated sandstones Thickness: 3–50 cm Description: Medium- to very fine-grained sandstones with normal grading in local association with parallel- and/or current ripple-laminations. Locally, this type sandstones fine upward to sandy siltstones that commonly contain carbonaceous fragments. Sharp and flat basal surfaces are common with locally developed distinct erosional basal surfaces. Mudstone clasts are included in the middle and upper parts of a single bed. Interpretation: Locally erosive, waning, lower density turbidity currents (Talling et al. 2012). Carbonaceous fragments suggest an influence of hyperpycnal flows (Mulder et al. 2003; Zavala et al. 2006).

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