Unexpected events are not exceptional in geological processes, and could be detected even in well-ordered sedimentary systems. Deposition of sandstone beds intercalated in reef limestones is a typical example of an occurrence of disorderly events in a highly ordered sedimentary process.
In this study we have considered mathematical properties of the van del Pol's equation in which sea-level fluctuations and metabolic activities of reef-building organisms are included as indispensable parametric factors controlling limestone deposition. Analytical results infer that the unexpected intercalation of sandstone beds in reef limestones has been a consequence of nonlinear reciprocal behavior between geological and biological agencies.

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Our purpose is to clarify Late Oligocene to Early Miocene paleoceanographic changes in the back-arc region of the Kuril Arc. We have only a few of reports on sedimentological and organic geochemical considerations as the Paleogene formations around the Sea of Okhotsk, which prompted us to research.
Kitami area, our research field, is located on the back-arc region of the Kuril Arc (Fig. 1), and has Tatsukobu-Tsubetsu biosiliceous deposit whose thickness attains more than 2500m. Based on biostratigraphic and rediometric ages, Tatsukobu-Tsubetsu biosiliceous deposit appears to be accumulated during the period ranging from Late Oligocene to earliest Early Miocene. This biosiliceous deposit begins with a basal sandstone including glauconite, which rests unconformably upon the Wakamatsuzawa Formation including silicified woods and roots. According to lithostratigraphic correlations, andesitic volcaniclastic bed of the upper part and acidic tuff bed of the middle part of the Tatsukobu Formation in the Kitami area are correlated to biotite-rich acidic tuff bed and hornblende-rich welded tuff bed in the Ponki-Ashoro area. Radiometric ages of acidic tuff bed and welded tuff bed in the Ponki-Ashoro area indicate 23.8Ma (fission track age) and 27.4Ma (K-Ar age).
To clarify the marine primary production of Tatsukobu-Tsubetsu biosiliceous deposit, we measured organic carbon congtents, C-H-N-O compositions of kerogen (insoluble organic matter) and Sulphur contents. Atomic composition analysis of kerogen concentrates is to examine the presence of terrigenous organic carbon. Plotted in a “van Krevelen Diagram” (i.e., H/C vs. O/O), estimated marine organic carbon (EMOC) and estimated non -marine organic carbon (ENOC) were calculated.
Based on stratigraphic changes in the amount of EMOC and ENOC, Tatsukobu biosiliceous deposit was divided into three stages.
In Depth 1335m-1450m and Depth 0m-350m (Fig. 5), stratigraphic variations in the amount of EMOC are parallel to those of ENOC. However, increase in EMOC are associated blosely with decrease in ENOC within Depth 350m-1335m. EMOC/ENOC ratios indicate that organic matter within Depth 350m-1335m included higher contents of marine-origin organic materials than those within other horizon. According to sedimentary petrological investigations, this deposit also contains larger volume of non-siliceous rocks in Depth 350m-1335m than in other horizon. In the past ocean, we conclude that amount of nutrients and dissolved silica components derived from land were parallel to those of ENOC. If this had happened at the back-arc region of the Kuril Arc during Late Oligocene to earliest Early Miocene, nutrients of deep water-origin contributed more to marine primary production than those of land-origin at the time of Depth 350m-1335m. This presumption suggests that the proto form of the Kuril Basin served as a vessel of nutrient-rich and silica-poor deep water at about 27.4Ma (Fig. 2).

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The present study reports on the method for estimating the water depth from wave-formed ripple marks, using data obtained from field and laboratory. Ripples having crestlines approximately parallel to each other and to wave crestlines are classified into four types based on the relationship between ripple spasing and orbital diameter and on the ripple morphology-orbital vortex ripples (OVR), suborbital vortex ripple (SVR), anorbital vortex ripples, and anorbital planar ripples (APR). OVR form under short-period waves in the limited-fetch coast (lakes and bays) or in lagoon inside reef. The other three are observed in the exposed coast where long period waves prevail.
The water depth of formation of OVR can be calculated using the method for orbital ripples (e.g., Komar, 1974). As to SVR, the following relationship is established: λ/wT=8h/L, where λ is the ripple spacing, w is the sediment fall velocity, T is the wave period, L is the wave length. Based on this equation, the combinations of T and h for given λ and w (which is convertible into sediment grain size) can be obtained. Also the T-h relation can be obtained from AVR, because the following relation is found for AVR: λ/wT=6.6h/L. These results enable us to estimate the ancient water depth from ripple marks preserved in strata. Because APR may not be preserved in deposit, we are unable to use APR as an indicator showing the ancient water depth.

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Paleo-wave conditions were estimated from wave ripples in the shallow marine sediments of the Pleistocene Osaka Group. Using new method by Takeda (1992), we can subdivide ancient ripples into four types, based on the relationship between orbital diameters and ripple spacings. Four types of wave ripples are discriminated in the group. Wave ripples in Neyagawa City are “anorbital vortex ripples” and “anorbital planar ripples”. Waves of 0.7-1.5m high in wave height and 2-4sec in wave period are calculated from those ripples. Wave ripples in Toyonaka City are “suborbital vortex ripples”, and 0.2-0.4m in wave height and 0.5-1sec in wave period are obtained. Wave ripples in Izumi City are “orbital vortex ripples”, and 0.1-0.9m in wave height, 2-4sec in wave period are calculated. Those wave conditions are similar to modern waves in the Sennan region, southern part of Osaka Bay. The minimum lengths of fetches for each reconstructed wave are also estimated.

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高分解能地層探査機による久慈川沖の陸棚上を調査した結果, 第三系を不整合に覆う2つの堆積ユニットが確認された. ユニット間を境する面は, 下位のユニットの内部構造を切っていることから浸食面と考えられ, 最終氷期後の海進に伴って形成されたラビーンメント面である.
これらのユニットは下位がユニットKa, 上位がユニットKbで, ユニットKaは海水準低下期から上昇期に堆積した河川から沿岸の堆積物, ユニットKbは海水準上昇期から高海水準安定期に堆積した陸棚堆積物と考えられる. ユニットKbの層厚分布は比高0-5m, 1-3km離れて, 等深線に対して北側に15-25°開いた方向をもつリッジとトラフの形状を示しており, 海進期に形成されたサンドリッジと結論付けられる.
調査地域にみられる海進期の堆積層は, 時間面と斜交するラビーンメント面を境に2つに分けられ, ラビーンメント面と海進面は明瞭に区別される.

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中～巨大巨礫の入り込んだ河成礫層を瀬戸層群で観察した. 瀬戸層群は河湖成の上部中新統～鮮新統である. 小論では, 巨礫の入り込んだ河成礫層と庄内川における現世の巨礫のみられる河成堆積物を記載し, 瀬戸層群の巨礫の入り込んだ河成礫層の堆積環境を考察した.
瀬戸層群にみられる巨礫の入り込んだ河成礫層は, 淘汰の良い礫と砂基質に円～亜円の花崗岩質の巨礫が入り込んだもので, 斜交あるいは平行層理を示し, 上方細粒化が認められる. 同様な特徴が庄内川において川が山麓線と交差する地点での礫質な河川州堆積物にも認められた.
庄内川における礫質な河川堆積物との比較によって, 瀬戸層群に見られる巨礫の入り込んだ礫層は, 礫州へ周囲の風化した基盤岩中から残留巨礫が崩落して入り込んでできたものと考えられる. この礫層の特徴的な堆積構造としてはオブスタクルマークがある. この礫層が形成された場は, 堆積盆地の縁辺部であり, そこでは山麓線が形成されていたとみられる.

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