Yezo super Gr

Sorachi-Yezo Belt (Yezo Main Zone)

Synonym:

The Yezo Group is characterized by thick terrigenous clastic sequences ranging from middle Early Cretaceous (Barremian) to Late Cretaceous (Campanian or locally to Paleocene), and is also known to yield many fossils (especially ammonoids and inoceramids) along with well-preserved sedimentological and paleo-environmental records (e.g. Ando 2003; Takashima et al. 2004). It has been classically treated as a supergroup divided into Lower Yezo Gr, Middle Yezo Gr, Upper Yezo Gr and Hakobuchi Gr (e.g. Ando 2003), although some researchers regard the sequence as a single group (e.g. Takashima et al. 2004).

Yield many fossils (especially ammonoids and inoceramids).

The lower part of the Yezo Group conformably overlies the Sorachi Group, and its base is defined as the onset of terrigenous clastic supply such as siliciclastic sandstone or black mudstone deposited upon the volcanogenic upper Sorachi deposits (Kanie et al. 1981; Kito 1987). Based on this stratigraphic relation, the Yezo Group is regarded to be a residual forearc basin (Kiminami et al. 1985a. B: Niida & Kito 1986) after the classification by Dickinson & Secly (1979), generated upon oceanic crust connected to a continental margin.

Although there are significant variations in local depositional environments and influences related to sea-fevel change, the Yezo Group shows a general shallowing-upwards sequence presumably reflecting infill of the basin.

From the upper parts of the lower Yezo Group to the basal parts of the middle Yezo Group (Aptian-Albian), there are several kinds of characteristic deposits suggestive of regional tectonic events. In the middle and northern areas of the basin, deep-sea deposits containing slide blocks of shallow-water, Orbitolina limestones occur (Sano 1995; Iba & Sano 2007; Iba et al. 2011). In the middle-north areas of the basin, detrital serpentinites occur as blocks in stumps (Nagata et al. 1987) or clasts in debris-flow conglomerate and sandstone associated with calcareous bioclasts and ooids (Yoshida et al. 2003, 2010), suggesting exhumation and uplift of mantle materials within the basin. In the southern areas, submarine or locally fluvial unconformity can be recognized at several localities, where the middle Yezo Group overlies metabasites and meta-sediments of the Kamuikotan Zone rocks.