Lower Sorachi Gr
Period:
Jurassic
Age Interval:
Kimmeridgian - Berriasian?
Province:
Hokkaido
Type Locality and Naming
Sorachi-Yezo Belt (Yezo Main Zone)
Synonym:
Lithology and Thickness
The lower part consists almost entirely of pillow and massive basalt and their volcaniclastic rocks (=the basaltic section of the Horokanai Ophiolite).
Lithology Pattern:
Lava
Relationships and Distribution
Lower contact
Horokanai Ophiolite Fm (conformable?)
Upper contact
Upper Sorachi Gr (conformable)
Regional extent
Sorachi-Yezo Belt (Yezo Main Zone)
GeoJSON
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Fossils
No data.
Age
Age Span:
Beginning stage:
Kimmeridgian
Fraction up in beginning stage:
0
Beginning date (Ma):
154.78
Ending stage:
Berriasian
Fraction up in the ending stage:
0
Ending date (Ma):
143.10
Depositional setting
These basalts are characterized by non-vesicular and aphyric tholeiites, associated with picrite in places. Basalts show chemical compositions similar to N-MORB, and were therefore regarded as an obducted or trapped ocean floor (Ishizuka 1981; Kiminami et al. 1985a, B; Niida 1992). Nagahashi & Mivashita (2002) however pointed to the more extensive melting involved in the production of the lower Sorachi basalts as compared to typical N-MORB, and correlated them with oceanic plateau basalts (OPB).
Ichiyama et al. (2012, 2014) showed that picrites of the Sorachi Group originated from extremely high-temperature mantle plume, which they attributed to an oceanic plateau, whereas Takashima et al. (2002) considered the depleted nature of the lower Sorachi basalts to have resulted from back-arc basin spreading along the Eurasian continental margin.
Ichiyama et al. (2012, 2014) showed that picrites of the Sorachi Group originated from extremely high-temperature mantle plume, which they attributed to an oceanic plateau, whereas Takashima et al. (2002) considered the depleted nature of the lower Sorachi basalts to have resulted from back-arc basin spreading along the Eurasian continental margin.
Depositional pattern:
Additional Information
Compiler:
Hayato Ueda. 2g Hokkaido. in Moreno, T., WALLIS, S., Kojima, T. & Gibbons, W. (Eds) 2016. The Geology of Japan. Geological Society. London. 201-220.