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| Choja Formation | |
|---|---|
| Name | Choja Formation |
| Type | Geological formation |
| Period | Late Cretaceous |
| Primary lithology | Sandstone, mudstone |
| Other lithology | Siltstone, conglomerate |
| Named for | Choja Hills |
| Named by | Kazuo Takahashi |
| Year named | 1963 |
| Region | Central Honshu |
| Country | Japan |
| Subunits | Unconformable upper member |
| Underlies | Kakegawa Group |
| Overlies | Nagaoka Group |
| Thickness | 150–420 m |
| Extent | Niigata Prefecture basin |
Choja Formation
The Choja Formation is a Late Cretaceous terrestrial sedimentary succession exposed in central Honshu, Japan. It is notable for its fluvial sandstones, overbank mudstones, and occasional conglomerates that preserve vertebrate and plant fossils, and it has been the focus of stratigraphic, paleontological, and resource-oriented studies since the mid-20th century.
The Choja succession comprises predominantly medium- to coarse-grained Sandstone, interbedded with fine-grained Mudstone and Siltstone, and locally capped by pebble conglomerates derived from erosional pulses. Bedding is typically massive to planar cross-bedded in channel sandstones, with paleosol horizons and root traces in the overbank mudstones. Detrital composition reflects recycling of older Cretaceous and Paleozoic source rocks, with heavy mineral assemblages including zircon and tourmaline, and framework grains dominated by feldspar and quartz. Diagenetic features include calcite cementation and authigenic clay replacement consistent with shallow burial and episodic groundwater flow influenced by regional tectonics linked to the Izumo Orogeny and nearby uplift of the Japanese Alps.
The Choja Formation is positioned stratigraphically within the Late Cretaceous sequence of central Honshu, commonly assigned a Campanian–Maastrichtian age based on associated index fossils and detrital zircon U–Pb geochronology. It rests unconformably atop the Nagaoka Group and is overlain by the Kakegawa Group or directly by Neogene volcanic and sedimentary units in some localities. Biostratigraphic correlations use vertebrate assemblages that compare to contemporaneous faunas from the Nemegt Formation and Wulagan Formation, while palynological zones and magnetostratigraphy have been applied to refine the chronology and correlate to the global Late Cretaceous timescale.
Sedimentology and facies analysis indicate a predominantly fluvial depositional system with sinuous channel belts, crevasse-splay deposits, and floodplain lakes. Channel sandstones show evidence for point-bar accretion and chute-channel geometries, whereas overbank mudstones contain root traces and paleosol development indicative of seasonally wet floodplain vegetation. Intervals of conglomerate and coarse-grained sandstone mark high-energy braided reaches or proximal alluvial fan inputs related to episodic tectonic uplift and climate-driven discharge events. Comparisons have been made to modern analogues in the Yellow River catchment and paleoenvironmental reconstructions for the Izanagi Plate margin during the Late Cretaceous.
The Choja Formation has yielded a diverse vertebrate and plant assemblage, including fragmentary dinosaur remains, turtles, crocodilian osteoderms, freshwater fish, and palynomorphs. Dinosaur material reported from collected localities has been compared to taxa described from the Nemegt Formation, Mongolia, and Santonian–Maastrichtian assemblages of East Asia, although most specimens remain indeterminate at genus level. Plant fossils include leaves and wood fragments referable to Ginkgoales, Cycadales, and various angiosperm groups that contribute to paleoclimatic interpretations. Microfossil and palynological suites permit correlations with marine-derived dinoflagellate and pollen zones used in studies by researchers affiliated with the University of Tokyo and the National Museum of Nature and Science (Tokyo).
Exposures of the Choja Formation are concentrated in the Niigata and northern Nagano Prefectures within central Honshu, occupying intermontane basins and fluvial palaeovalleys formed along the forearc and backarc domains. The type locality was described from the Choja Hills section near the town of Jōetsu, designated in the original description by Kazuo Takahashi in 1963 and subsequently revised in regional geological maps by the Geological Survey of Japan. Lateral facies changes and thickness variations reflect syndepositional tectonics related to the convergence of the Pacific Plate and Philippine Sea Plate during the Late Cretaceous.
Reservoir-quality sandstones within the Choja Formation have been investigated for groundwater and potential hydrocarbon reservoir analogues; however, hydrocarbon potential is limited by thermal maturity and structural complexity. The formation’s sandstone and conglomerate units have local value as construction aggregate and crushed stone, and clay-rich paleosol horizons have been evaluated for ceramic raw material by companies in the Niigata Prefecture industrial sector. Geotechnical studies for regional infrastructure projects have used Choja lithologies to assess slope stability and foundation conditions for rail and highway corridors administered by the East Japan Railway Company and regional governments.
The Choja Formation was proposed by Kazuo Takahashi in 1963 following systematic mapping campaigns undertaken by the Geological Survey of Japan and academic teams from the University of Tokyo and Tohoku University. Subsequent work through the 1970s–2000s expanded lithostratigraphic descriptions, palynology, and vertebrate paleontology, with notable contributions from fieldworkers affiliated with the National Museum of Nature and Science (Tokyo), Hokkaido University, and international collaborators from institutions such as the American Museum of Natural History and the Natural History Museum, London. Ongoing efforts focus on refining chronostratigraphy using detrital zircon geochronology and improving paleoenvironmental models through multidisciplinary approaches.
Category:Geologic formations of Japan