362 Proc. Japan Acad., 57, Ser. B (1981) [Vol. 57(B),

71. Early Cretaceous Beachrock from the Miyako Group, Northeast Japan

By Tetsuro HANAI and Tatsuo On Geological Institute, Faculty of Science, University of Tokyo

(Communicated by Teiichi KoBAYASHi, M. J. A., Dec. 12, 1981)

Introduction. Numerous modern examples of beachrock are known dominantly from tropical and subtropical areas, but fossil analogues have rarely been reported. This is firstly because a beach is a site of erosion and, therefore, beachrock, if formed, is rarely

preserved, and secondly because the diagnostic features of beachrock are difficult to detect in a sequence of ancient sediments. The purpose of the present study is to report on the discovery of an Early Creta-ceous beachrock from the Miyako Group, Northeast Japan. The occur-rence of beachrock provides physico-chemical evidence that a tropical or subtropical climate possibly dominated at that time and that the ancient intertidal zone was located in that area (Donaldson and Ricketts, 1979). We thank Prof. Raymond Siever for reading the manuscript.

Description of the Miyakoan beachrock. Megascopic observa-tion. The rock in question is found in the upper Aptian Tanohata Formation of the Miyako Group, on the northern coast of Hiraiga village, Tanohata-mura, Iwate Prefecture. The group is composed of near-shore sediments dominated by calcareous sandstone and cal-carenite, and contains abundant reef building organisms. It consists of four formations, each of them representing one cycle of sedimen-tation (Hanai, Obata and Hayami, 1968). The Tanohata Formation consists of the lower high-energy sediments and the upper low-energy sediments of the second lowest cycle of the Miyako Group. The beach-rock occurs at the boundary between these two types of sediments

(Figs. 1, 2). It is a 130 cm thick pebble conglomerate, strongly in-durated and almost free from recent weathering. The conglomerate is distinguished from other conglomerates because it lacks matrix and the lithic fragments are cemented directly by beige calcite. The lithic component is poorly sorted; grains range from sand to cobble size, and granules and pebbles are the most common size class. The

grain composition is nearly oligomictic; chest and siliceous shale are the most abundant, accompanied by smaller amounts of sandstone,

volcanic rock, and bivalve fragments. Grains are well-rounded, mostly elongate, and arranged subparallel to the bedding plane. The

No. 10] Early Cretaceous Beachrock from the Miyako Group 363

beachrock cement is beige calcite, whose individual crystals are in-visible to the naked eye. Voids other than those containing beige calcite are filled either with silt or with large transparent crystals of calcite. The beachrock lies on a cobble conglomerate with a sharp and undulating boundary and is overlain by siltstone. The cobble con-

glomerate has a fine sand matrix, intercalated with slitstone contain-ing many wood fragments and a few ostreids. The upper surface of the beachrock is undulating and cuts across bedding planes. Joints

extend in two mutually perpendicular directions, splitting the beach-rock into several large blocks ; the joints are filled with the overlying slltstone.

Several kinds of encrusting organisms in growth positions partly cover the uneven upper surface and the lateral side of the block. An ostreid Pycnodonte sp. is the most abundant, and the others include Amphidonte (Amphidonte) subhaliotoidea, Spondylus (Spondylus) decoratus, serpulids and solitary corals. Observation reveals some

Figs.

of

1-2. 1:

beachrock

Local columnar section at the beachrock locality. 2:

blocks and encrusted upper surface of block 1.

Disposition

364 T. HANAI and T. On [Vol. 57(B),

Fig. 3. 1: Probable microstalactic and microstalagmitic cement (arrows), over- lain by microcrystalline calcite cement with pelletoids. Crossed nicols.

(Scale bar is 1 mm) 2: Microcrystalline calcite cement with pelloids (a), overlain by isopachous bladed rim cement (b). Remaining space was filled

with silt (c). (Scale bar is 1 mm) 3: Encrusting organisms on the upper surface of beachrock. Pycnodonte sp. (a), Amphidonte (Amphidonte) sub- haliotoidea (b), serpuldid tubes.

No. 10] Early Cretaceous Beachrock from the Miyako Group 365

tendencies for an order to successive settlements by encrusting organisms. Large individuals (10-15 cm) of A. (A.) subhaliotoidea occur throughout on the upper surface of a block, while the distribu-tion of small individuals (1-1.5 cm) of A. (A.) subhaiiotoidea and numerous Pycnodonte sp. is restricted to the northwestern peripheral area of the upper surface. The large individuals settled directly on the block or on a few pre-existing shells of small ostreids and are overlain by numbers of small ostreids. These facts suggest that the large individuals of A. (A.) subhaliotoidea and a small number of

small ostreids were settled in early stages, followed by the encrusta-tion of numerous small ostreids, in a limited area of distribution.

Microscopic observation. Although the original crystal fabric may have, to some extent, disappeared during diagenesis and the

cements are identified as low-Mg calcite, three types of beachrock cement and two types of post-beachrock pore filling can be distin-

guished. They are described in order of formation as follows (Fig. 3). 1) Probable microstalactic and microstalagmitic cements. These

types of cement show preferential asymmetric development on the tops and the bottoms of large sand grains. Composed of a mosaic of sparry calcite, the individual crystals are elongate and arranged normal to the grain surface. Faint lamination is often present. The asymmetric cement of this, type was probably caused by gravity. The cement probably originated by alternate wetting and drying of the sediments in the inter- to supratidal zone (Purser 1969, p. 221). A similar cement was reported from modern beachrock in the Persian Gulf by Taylor and Tiling (1969), in Bermuda by Muller (1971), from Pleistocene-Holocene carbonate in Israel by Buchbinder and Friedman

(1980), and from the Bathonian carbonate in France by Purser (1969) and Purser and Lobreau (1972). The microstalactic and -stalagmitic cement is overlain by microcrystalline calcite cement.

2) Microcrystalline calcite cement. Mieraerystalline calcite cement refers in this paper to aggregate of calcite grains 2-8 jM in diameter. It is subtranslucent and frequently pelleted. Pelloids are round or ovoid in thin section and are 20-200 pM in diameter. Grains of the constituting crystals are finer (2-4pM) than those of the sur-rounding microcrystalline calcite and give them a darker appearance. The microcrystalline calcite cement often shows a geopetal texture in voids : Lower parts of the voids are occupied by microcrystalline calcite, and upper parts by overlying sparry calcite or silt. This fact suggests that at least some of this calcite may be mechanically infiltered. 3) Bladed rim cement. This cement grew on the grains or on the previously formed microcrystalline calcite cement and coated them

366 T.BANAI and T. OJi [Vol. 57(B),

with a continuous rim. The constituent sparry crystals are elongate,

probably recrystallized from acicular crystals, and arranged approxi-mately perpendicular to the substratum on which they grew. The cement is characterized by numerous tiny inclusions. The thickness of the cement is almost uniform in one sample, but varies from one sample to another.

4) Silt. Silt fills the remaining spaces of the previously cemented voids close to the surface of the conglomerate. Mechanical infiltration of silt occurred during the deposition of overlying silt after the synsedimentary cementation. The silt includes small amounts of very fine sand, some clay, carbonaceous matter and authigenic pyrite.

5) Sparry calcite cement. Where the infiltration of silt into voids was not complete, clear blocky sparry calcite fills the remaining spaces, occupying the upper position with respect to silt. The geopetal texture indicates that the sparry calcite cement was precipitated after the infiltration of silt. The cement is transparent and forms a mosaic of crystals, coarser grained toward center of the void, without any

preferred orientation. Discussion. The occurrence of ancient beachrock is difficult to demonstrate. Only a few authors have tried to enumerate criteria for its identification. Yet procedures for identification seem to have emerged gradually. Donaldson and Ricketts (1979) classified diag-nostic features of beachrock into megascopic and microscopic charac-teristics. Most studies of modern breachrock have concentrated onn the long-standing controversy on the origin of beachrock cement. This has led to the accumulation of detailed knowledge of the minerals and microfabrics of the carbonate cement, which, however, may be easily altered or obliterated by later diagenesis in ancient beachrock. Thus, for the identification of ancient beachrock, information on megascopic features constitutes reliable criteria that are particularly important. Criteria for the identification of the beachrock may be classified

into evidences of syn- and post-lithification. Evidence for formation before or during lithification are mostly microscopic. The original textures of various kinds of cement are still preserved in the recry-stallized carbonate cement of the Miyakoan beachrock. The probable microstalactic and microstalagmitic cements suggest cementation in the vadose environment. Precipitation of microcrystalline calcite cement, which may correspond to the modern micritic beachrock cement, precedes that of bladed rim cement, which may be comparable to the fibrous aragonite or high-Mg calcite rim of modern beachrock . Evidences for formation after lithification are based mos~ly on

No. 10] Early Cretaceous Beachrock from the Miyako Group 367

megascopic observations ; those observations give evidences for syn-sedimentary lithification of the conglomerate in question. They are the joints and eroded surfaces of the conglomerate, fragmentation of the conglomerate into blocks, and encrusting organisms on both upper and lateral surfaces of the conglomerate blocks. The slightly dis-

placed mode of occurrence of the conglomerate layer suggests an intermediate stage of formation of a flat-boulder conglomerate.

A source of information on the environment before lithification is the presence of ostreids in the underlying siltstone and sandstone, and on the environment just after the lithification of the conglomerate is the growth of encrusting organisms, e.g. ostreids, Spondylus, ser-

pulids and solitary corals. The occurrence of these organisms indi-cates an environment within or even below the intertidal zone. There is no independent evidence of general sea-level change. Therefore it is quite likely that this conglomerate lithified under intertidal condi-tions and, if it was exposed to air during a sea-level lowering, the duration of exposure was relatively short in time. After or during the burial of Beachrock under the overlying siltstone, the remaining voids were filled with silt and thereafter with sparry calcite.

References

Buchbinder, L. G., and Friedman, G. M. (1980) : Vadose, Phreatic, and Marine Diagenesis of Pleistocene-Holocene Carbonates in a Borehole : Mediterranean

coast of Israel. J. Sediment. Petrol., 50, 395-407. Donaldson, J. A., and Ricketts, B. D. (1979) : Beachrock in Proterozoic dolostone of the Belcher Islands, Northwest Territories, Canada. ibid., X4'9, 1287-1294.

Hanai, T., Obata, I., and Hayami, I. (1968) : Notes on the Cretaceous Miyako Group. Mem. Natl. Sci. Mus., .1, 20-28, pls. 1-4 (in Japanese with English summary). Muller, G. (1971) : "Gravitational" cement: an indicator for the vadose zone of the subaerial diagenetic environment. Carbonate Cements (ed. 0. P. Bricker),

The Johns Hopkins University, Studies in Geology, no. 19, pp. 301-302. Purser, B. H. (1969) : Synsedimentary marine lithification of Middle Jurassic

limestone in the Paris Basin. Sedimentology, 12, 205-230. Purser, B. H., and Lobreau, J. P. (1972) : Structures sedimentaires et diagene- tiques precoces Bans les calcaires bathoniens de la Bourgogne. Bull. Bur. Rech.

Geol. Miniere, 2e ser., sect. 4, 2-1972, 19-47, pls. 1-4. Taylor, J. C. M., and Illing, L. V. (1969) : Holocene intertidal calcium carbonate cementation, Qatar, Persian Gulf. Sedimentology, 12, 69-107, pls. l-6.