Paul M. Frydl
Mobil Oil Canada Ltd., Calgary, Canada

Holocene marine

Approximate reconstruction of southern Boso Peninsula coastline at the time of higher relative sea levels reveals that at least three physiographic sedimentary environments were present. The northern part of the area was occupied by east- and westfacing, broad opened bays, similar to the present Tateyama Bay. The southeastern and western parts of the area were characterized by long, narrow drowned valley-type bays. Finally, stretches of straight coastline framed by beaches and sand dunes, similar to the present coast existed, particularly during periods prior to and after the relative sea level reached its maximum.

Five areas (text-fig. 1), where exposure of Holocene marine sediments is good and which are believed to represent the range of variation within the larger investigation area, were selected and studied in detail.

Sediments in all areas examined conform to, or are part of, the following coarsenin upward sedimentary sequence. Abraded and extensively bored Neogene bedrock, often in the form of a wave-cut bench, is overlain by 10 to 50 cm of angular conglomerate derived from local bedrock, Macrofossils typical of this part of the sequence are Barnea manilensis inornata and Saxidomus purpuratus (boring in the bedrock and conglomerate) and attached forms Spondylus spp., Pretostrea imbricata, Barbatia spp., and Chama spp. The conglomerate grades upward into greyish blue massive silt characterized by shells of Dosinella penicillata, Paphia undulata, and Ostrea denselamellosa in living position. The silt includes silty sand to sandy silt beds, generally increasing in thickness upward in the sequence. In the upper part of the sequence, sand and silty sand become the dominant sediment type and silt and sandy silt are restricted to occa sional intercalations. Characteristic molluscs include Dosinorbis japonicus, Fulvia mutica, Niotha livescens, and Umbonium costatum. The sequence is topped by crossbedded coarse sand, which may be overlain by dune sand.

Tomoe River Area; A nearly continuous exposure exists along the banks of Tomoe River and its tributaries (text-fig. 3). Numa I terrace is preserved in a large part of Tomoe valley; Numa II underlies the village of Daijingu at the mouth of the valley; and Numa III exists as a 500 m wide belt parallel to the coast. Recent sand dunes cover a large part of the coast from the mouth of the Tomoe River to Ito village, and a sand dune correlated with Numa II terrace overlies the Numa I terrace in the vicinity of Inuishi and Matsuoka villages.

The lowest lying sediment is greenish blue clay exposed along the lower reaches of Tomoe River at 3 m above sea level. It contains disarticulated valves of Crassostrea gigas, and its upper surface is extensively burrowed by a network of anastomosing circular burrows about 2 cm in diameter. The greenish clay is overlain by 40 cm of coarse sand containing large amounts of peaty material, occasionally forming small (20 cm thick) peat lenses. The peaty sand is succeeded by 100 cm of silt grading into silty sand toward the top. The lowest 10 cm above the peaty sand contains abundant articulated Ostrea denselamellosa. The upper part of the bed contains Batillaria ssonalis, B. multiformis, and articulated valves of Paphia undulata. At 4,4 m above sea level the grey sandy silt is overlain by a 15 to 20 cm thick layer of brown silty clay extending for at least 400 m laterally. The lower contact is abrupt and sharp, while the upper surface contains burrows, similar to those in the greenish clay, which penetrate to the depth of about 10 cm and are filled with medium grey sand of the overlying bed. Microscopic examination revealed a high percentage (about 70% by volume) of pyrite in the sand size residue. The brown clay layer is overlain by up to 15 m of poorly exposed silty sand and sandy silt alternation, containing Dosinorbis japonicus, Fulvia mutica, Paphia undulata, Macoma incongrua, and Babylonia japonica.

In the central part of the valley, in the vicinity of Nagaoka village, the lowest sediment is 2+ m thick, firm, massive blue-grey silt with Dosinella penicillata in living position. Five to 15 cm thick layers of subangular cobbles and molluscan shells (Chama reflexa, C. lazarus, Serpulorbis imbricatus, Pretostrea imbricata, Spondylus butleri) showing signs of transport occur at 30 to 60 cm intervals throughout the silt bed. A dip of the layers ranges from 0 to 15° away from the valley sides, and the layers can be occasionally traced to lenses of similar composition, up to 1m thick. Fossil content and the character of the sediment below and above these layers does not show any change.

The silt gradually changes to sandy silt and then at about 13m above sea level abruptly to sand with occasional interbeds of silty sand. The sand reaches thickness of 5 m and consists of alternating layers of fine black and coarse light brown sand with horizontal parallel laminations. Fossils occurring in the sand are Ochetoclava kochi, Niotha livescens, Dosinella penicillata, and Dosinorbis japonicus. The parallel laminated sand is overlain by well-sorted, grey coarse sand with low angle cross-laminations. The upper part of this sand is characterized by abundant fossils of Umbonium costatum. The Umboniumrich bed is succeeded by over 1 m of poorly sorted, strongly bioturbated, medium brownish yellow sand, which is overlain by parallel laminated medium sand with occasional shallow straight vertical burrows. The parallel laminated sand is overlain by cross bedded coarse sand reaching up to 1 m in thickness. The cross bedded sand is covered by massive reddish-brown dune sand reaching up to 3 m in thickness.

Bivalve boreholes in the bedrock are found up to 25 m above sea level. A similar sequence is found in the upper part of the valley and tributary valleys, except that the lower silt unit persists up to a higher elevation and the upper sand unit is thinner or absent.

C14 dates indicate that most of the sediments forming the Numa I terrace were deposited between 7,600 yr B.P. and 5,800 yr B.P. The date of 7,690 yr B.P., obtained at 10 to 11 m above sea level, indicates that the lowest clay and peaty sand exposed nearby at 3 m above sea level are substantially older.

Tateyama City Area: The northeastern part of the investigation area is occupied by a low lying coastal plain formed by Numa II and III terraces and fringed along its perimeter by short narrow valleys commonly preserving the Numa I terrace. A series of sand dunes parallel to the coast partly covers the Numa II and III terraces.

Holocene deposits underlying the plain are best exposed along the lower reaches of Heguri and Taki Rivers, and several outcrops are found along the banks of Taki River up to Kokonoe village. Drill hole data, mostly in the area occupied by Tateyama City, provide additional information.

The following sedimentary sequence (text-fig. 4) can be observed in a nearly continuous exposure from Kamehara to Nishigo villages along Heguri River. Neogene siltstone exposed near Kamehara is overlain by angular conglomerate, consisting of bedrock blocks up to 1 m long, but the size of the blocks diminishes quickly within a few tens of meters. The matrix is bluish black, silty clay. An alternation of silty clay and pebble conglomerate beds overlies the basal angular conglomerate. The silty clay is mottled bluish black and completely lacks marine fossils, but it is rich in fine organic material, carbonitized twigs, leaf fragments, etc. It is typically 50 to 70 cm thick. The conglomerate consists of discoid, well-rounded pebbles oriented with their long axes dipping about 10° westward. The pebble size ranges from 1 to 10 cm, with a mean of 4 cm. Bedding is defined by variations in pebble size; bed thickness averages about 20 cm. The conglomerate contains clasts of the underlying clay, ranging in size from a few centimeters to 50 cm. The conglomerate is about 100 cm thick and is overlain by silty clay closely resembling the one at the base of the sequence. The contact between the lower clay and conglomerate is sharp and erosional; the contact between the conglomerate and the upper clay is gradational. The alternation is repeated at least two times, reaching a total thickness of 3 m. The beds dip 5° to the west.

The uppermost clay bed is overlain by a 20 cm thick bed of angular pebble conglomerate, which is overlain by blue-grey marine sediments ranging from silt to fine sand. The silty fine sand reaches a thickness of 3 m and contains abundant fossils of marine molluscs, many of which are in living position. The fossil assemblage is dominated by Dosinella penicillata, Paphia undulata, Ostrea denselamellosa, Saxidomus purpuratus, and Macoma sectior. The sediments appear massive and are probably intensely bioturbated.

The silty sand is abruptly overlain by poorly sorted medium to coarse sand with a silty matrix. The sand, consisting of well-rounded volcanic rock fragments and subangular shell fragments, contains silty clay interclasts and discontinuous clayey seams. It reaches 2 m in thickness and contains the following fossils: Fulvia mutica, Mactra chinensis, Assorinw abbreviatus, Petricolirus aequistriatus, Pinna bicolor, and Novathaca schencki. The coarse sand abruptly grades laterally into blue-grey silty sand near Nishigo village and is overlain by it. Sediment characteristics and molluscari fossils in this silty sand closely resemble the characteristics of the silty sand underlying the coarse sand unit.

Fine silty sand containing characteristic Dosinella penicillata shells in living position crops but along Taki River and also Heguri River downstream of Nishigo Bridge, where it overlies an oyster reef. Near Kokonoe village, in the eastern part of the plain, medium silty sand to sand is present, suggesting an eastward coarsening of the sediment. Near Kokonoe, where the contact between Holocene sediments and bedrock is exposed, the silty sand includes numerous blocks of bored bedrock and rock dwelling shells, such as Chama reflexa and Pretostrea imbricata.

Boring data are available only in the form of simplified well logs recording coarseness of sediment and the presence or absence of fossils. They indicate that the thickness of unconsolidated sediments in the area exceeds 25 m in some places but averages about 15m.

The sediments penetrated by the borings consist of fossiliferous sandy silt and silt interrupted by and partly overlain by medium to coarse sand (text-fig. 5). The fossiliferous sandy silt and silt are probably similar to the Dosinella penicillata silt and silty sand exposed along the Heguri River.

The sand is partly dune sand occurring on the surface and partly marine sand occurring in the form of lenses and westward (seaward) dipping tongues in the marine sandy silt. Conglomerate and coarse sand found at the base of the two deepest boreholes are tentatively interpreted as fluvial deposits similar to those found at the base of the sequence along the Heguri River. Their nonmarine origin is indicated by the absence of fossils, presence of organic matter in the immediately overlying silt, and the inferred proximity of bedrock.

Oyster reef: A. one meter high and at least 200 m long oyster reef is exposed along both banks of Heguri River about 200 m upstream from the confluence of Heguri and Taki Rivers. The reef rests on a flattened bedrock whose top is presently 2.5 m above sea level and which slopes gently to the west (seaward). The bedrock is a soft, bluish grey siltstone. Its surface is extensively bored. Three types of boreholes can be found. The first type is two, three, and more branched circular holes (diameter 15 to 20mm), which penetrate vertically to a depth of 30 cm, where they become horizontal and connect with other vertical holes. The bedrock is bored intensively down to 6 cm. No fossils were found in the borings, but based on the character of the boring, they are assumed to be of crustacean origin. The second is circular borings of the same diameter; but borings that are straight and seldom deeper than 4 cm are also common. In some cases the boring shell Barnea manilensis inornata is still found in the holes. The third is hemispherical borings with wide circular to elliptical openings, diameters ranging from 2 to 10 cm, depths from 10 to 15 cm. They commonly contain both valves of the bivalve Saxidomus purpuratus. Sediment trapped between the valves of these shells silty medium coarse sand.

The oyster reef can be divided into three parts. In the lowest part (20 to 40 cm) oysters are predominantly in vertical living position. Pretostrea imbricata is by far the most common; other molluscs present include Barbatia lima, Acra boucardi, Chlamys nobilis, and Serpulorbis imbricatus. The coral Alveopora verilliana is commonly attached to the shells but other corals are absent. In the central part of the reef, Alveopora disappears, but at least 14 species of coral are present and coral becomes volumetrically as abundant as oyster shells. Several large-1 m in diameter-colonies of Favia speciosa and Favites favosa are present in living position. Fragments of Dosinella penicillata shells occur in the sediment surrounding the corals. The highest part of the reef is characterized by an absence of coral except for a few transported fragments. Oyster shells are mostly horizontal-that is, not in living position. Occasional shells of Dosinella penicillata with both valves present also occur in this part. The matrix in the lower parts of the reef is formed by sandy silt and it changes upwards into silty sand.

Directly above the oyster reef is a 15 cm thick layer, rich in shell fragments, which are mostly Dosinella penicillata. Shell fragments become fewer upwards, and the sediment changes into blue-grey silty sand with Dosinella penicillata in living position.

West Coast Valley Area: The narrow, relatively short valleys incised into the Neogene formations along the west coast and the eastern rim of the coastal plain are filled with marine sediments and often preserve the Numa I terrace. Natural exposures are typically small and few, and the following discussion is largely limited to two valleys: Koyatsu and Shiomi, where exposures are best and to an extent complementary (text fig. 6).

At the lower part of Koyatsu valley a 1 m thick boulder conglomerate bed 1 m above sea level and is overlain by 3 m of medium silty sand. The transition between the conglomerate and sand is gradual. Both beds contain marine molluscan shells; in the conglomerate Chama reflexa, Serpulorbis imbricatus, Barbatia bicolorata and boring Barnea manilensis inornata are common. The sand bed contains Macoma incongrua, Fulvia mutica, and Notochione jedoensis.

In the upper part of the Koyatsu valley, two outcrops reveal a more than 3 m thick coral-rich bed unconformably overlying bored Neogene siltstone. Both outcrops are located adjacent to the side of the valley. The coral bed can be divided into two parts. The lower part contains numerous colonies of Favia speciosa, Favites favosa, and Caulastrea tumida, many of which are in living position. The upper part of the bed consists of blue-grey silty sand, in most places weathered to light brown, containing numerous fragments of platy corals, among which Echinophyllia aspera is predominant.

Bivalve boreholes found in the Neogene siltstone exposed nearby are abundant up to 25 m above sea level, and isolated borings are found up to 27.5 m above sea level.

In the neighboring Shiomi valley, outcrops occur in the central parts of the valley. The thickest bed (from 7.5 to 10 m above sea level) consists of massive silty sand with Panopea japonica, Dosinella penicillata, Clementia papyracea, and Paphia undulata in living position. Fragments of transported coral (Lobophylia japonica, Echinophyllia aspera) are present but rare. In the upper parts of the valley the silty sand bed rests on bluish grey silt with articulated Crassostrea gigas; in the parts of the valley closer to the sea, it is underlain by angular siltstone conglomerate containing the shells of Chama reflexa, Pretostrea imbricata, Ostrea denselamellosa, Niothe jedoensis, Spondylus sp. and Serpulorbis imbricatus. Some Oulastrea crispata is present, but other corals are absent. At 10 m above sea level, the silty sand bed is overlain by sand of the Numa III terrace; the sand bed and terrace are separated by a 20 cm bed of coarse sand and rounded pebbles.

C14 dates obtained from samples from the basal horizon range from 7,840 to 7,230 yr B.P. (TK-337, N-3088) and those from the upper horizon range from 6,720 to 4,010 yr B.P. (TK-329, TH-218, TH-272, TH-219) as listed in tables 3 and 4.

A small outcrop of coral-bearing sediments resembling the upper part of the Koyatsu coral bed is found along a stream north of Kamisanakura village. It contains mainly platy corals embedded in silty sand matrix. Molluscan shells from the top of this bed were dated at 5,500 yr B.P.

Several very small outcrops consisting mainly of transported coral fragments in silty sand matrix are found in the vicinity of Numa village. The outcrop of the type locality of Numa coral bed is presently covered by a pond and is mostly inaccessible.

Chikura Area: On the west coast, the Numa I terrace is particularly well preserved in the valley of Seto River near the town of Chikura. The river cuts through Numa I terrace in the upstream parts of the valley and then through Numa II, III, and IV terraces developed in a nearly 1 km wide coastal zone. Numa II terrace is partly obscured by a wide sand dune, which brings it to level with the Numa I terrace surface.

Dark greyish blue massive silt crops out at 4.9 m above sea level in the downstream part of the valley at Baba (text-fig. 7). Shells of Scapharca inequivalvis, Pinctada margaritifera, and Dosinella pencillata are preserved in living position. The silt reaches a thickness of at least 120 cm and is overlain at 6.1 m above sea level by coarse-to-medium, dark-to-light-grey cross-laminated sand. The 260 cm thick, cross-laminated sand bed is overlain by a 100 cm thick alternation of yellowish brown sand and silty sand. The silty sand beds are about 20 cm thick and have abrupt lower and uneven upper contact surfaces caused by extensive burrows filled with sand of overlying beds. The sandy beds are devoid of fossils, but the silty sand beds contain shells of Solen strictus.

At 9.7 m above sea level, the silty sand and sand alternation is succeeded by over 170 cm of coarse, well-sorted sand with inclined beds of rounded pebble conglomerate about 15 cm thick.

Several small outcrops in the upper and central parts of the valley reveal burrowed silt to sandy silt with poorly preserved molluscan shells resting on silty clay with abundant organic material. The fossiliferous silt is covered by nonfossiliferous medium to coarse sand, reaching up to 4 m in thickness.

C14 dates of samples from this area range from 7,450 to 5,570 yr B.P. (see TH-273, TH-274, TH-275, N-2089, TH-106, TH-277, TH-282, GaK-3067 in table 4).

Maruyama Area: Maruyama and Nuruishi Rivers in the northwestern part of the study area flow through a low coastal plain formed predominantly by the Numa II terrace.

Massive silty clay, similar to the lowest silty clay at Baba, Chikura Area, crops out from 1 to 4 m above sea level along the downstream part of the Maruyama River. Sparse fossils consisting mainly of Dosinella penicillata are found. Along the Nuruishi River, bluish grey massive silt containing carbonized wood fragments crops out at 7 m above sea level. Shells of Cyclina sinensis in living position are common. At 7.75 m above sea level the silt is overlain by a 14 cm thick bed consisting of alternating thin layers of peat and small, rounded gravel and clay. It is succeeded by an 80 cm thick bed of sandy silt coarsening upwards to silty sand. Clay draped ripples are present in the central part of the bed. Further upstream, mottled greyish blue silt with leached out shells is overlain at 14.9 m above sea level by fine sand with a aubparallel, discontinuous, 1 to 2 cm thick laminae of clay. The fine sand bed is 60 cm thick and devoid of fossils.

At a distance of 2 km from the shoreline, both rivers cut through a more than 13m thick alternation of nonmarine clay rich in organic matter and rounded pebble conglomerate beds.

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