Journal of the Japanese Association for Petroleum Technology

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The uppermost part of delta sequences is composed of overbank deposits, while muddy prodelta deposits form the lowermost part of the sequences. Crevasse splays, levees, bays, or abandoned channles occur commonly in the delta plain of the fluvial-dominated deltas. The delta front and upper prodelta are occupied mainly by distributary mouth bars. Slump blocks derived mainly from mouth bar deposits caused by slope failures may occur in the prodelta. Distal bar and bar front deposits of the distributary mouth bars have a distinct upward-coarsening sequence. Beach facies (dunes, beach ridges, foreshore, shoreface, etc.) or barrier island/lagoon systems are developed in the delta plain to upper prodelta of the wave-dominated deltas. In the tide-dominated deltas, tidal flats and tidal bars occur commonly in the lower delta plain to upper delta front, and in the delta front to upper prodelta, respectively. Bidirectional cross-bedding is often formed in the tidal bar deposits. The most distinct upward-coarsening sequence is observed in the wave-dominated deltas. Tide-dominated deltas do not have an upward-coarsening sequence.
Marine deltas are not only of fluvial-dominated, wave-dominated and tide-dominated types, but also of mixed types. On the other hand, lacustrine deltas are restricted to the fluvial-dominated, wave-dominated, and their mixed type deltas. Nevertheless, the fluvial-dominated and wave-dominated lacustrine delta sequences have sedimentological features similar to those of the fluvial-dominated and wave-dominated marine delta sequences, respectively.
Since marine water has a higher salinity than lake water, montmorillonite and illite occur more commonly as clay minerals in marine delta sediments than in lacustrine delta sediments. Authigenic minerals in lacustrine delta sediments could also be different from those of marine delta sediments because of the different chemical compositions of marine and lake waters.
Furthermore, Gilbert-type deltas which resulted from homopycnal inflow are more commonly formed in lacustrine river-mouths rather than in marine river-mouths. Hyperpycnal or friction-dominated inflow occurs more commonly in lacustrine river-mouths resulting in deposition of turbidites and middle-ground mouth bars. However, large-scale broad mouth bars resulting from the hymopycnal or buoyant inflow are more often formed in marine river-mouths.

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Neogene stratigraphy of the Akita oil fields (s.l.) was first established by OHASHI (in TOYAMA, 1925) in the Oga Peninsula, and later by OMURA (1928, 1930) in and around Akita City, which is an important oil-producing area in the region. The oil-bearing marine strata in the Akita City area belong to the Onnagawa, Funakawa, Tentokuji and Sasaoka Formations, the type locality of the former two is in the Oga Peninsula. The Tentokuji and Sasaoka Formations were designated in the Akita City area and have been correlated with the Kitaura and Wakimoto Formations of the Oga Peninsula. The stratigraphic succession in the Akita City area has long been regarded as the standard succession not only for the Akita oil fields but also for the Neogene sequences along the Japan Sea coastal regions.
Recent studies in magnetostratigraphy and in biostratigraphy based on diatoms, calcareous nannoplankton and planktonic foraminifera indicate that the correlation, so far established between the successions in the Oga Peninsula and Akita City area, is not valid. Based on these studies, the Onnagawa Formation of the Akita City area is correlated with the lower to middle part of the same formation of the Oga Peninsula, and the Funakawa Formation of the Akita City area is correlated with the upper part of the Onnagawa to the basal part of the Funakawa Formation of the Peninsula. The Tentokuji and Sasaoka Formations are correlated with most of the Funakawa Formation of the Peninsula, unlike the usual practice of correlating them with the Kitaura and Wakimoto Formations. It is after the deposition of the Sasaoka Formation that the Early Pleistocene Kitaura and Wakimoto Formations were deposited in a marine basin, which existed as a remnant of the earlier one, to the west of the present coast line of Akita-Noshiro.
The Kitayuri thrust system along the present coast line seems responsible for the difference in the sedimentary environment of the two areas since the time of deposition of the upper part of the Onnagawa Formation in the Oga Peninsula.

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