Abstract
Larger foraminifera Orbitolina-bearing limestone pebbles were discovered in the conglomerate of the lowermost Lower Yezo Group in the Nakagawa area, northern Hokkaido, Japan. This is the first discovery of Orbitolina in this area. The conglomerate that includes the limestone pebbles is interpreted as a sediment gravity flow deposit into a deep-water basin. The limestone pebbles consist of either Orbitolina grainstone or oolitic-peloidal packstone/grainstone. The former is rich in Orbitolina tests which are typical and common in the mid-Cretaceous carbonate platform sediments of the Tethys-Atlantic region. The Orbitolina specimen identified belongs to the Orbitolina (Mesorbitolina) parva -O. (M.) texana lineage (Schroeder, 1975), which ranges from the Upper Aptian to the Lower Albian. The obtained Orbitolina data in addition to previous biostratigraphic studies suggest the Upper Aptian for the lowermost part of the Lower Yezo Group.
During Late Aptian, the Orbitolina-bearing carbonate in the Nakagawa area was presumably deposited in a tropical shallow marine environment and then emplaced by sediment gravity flow into a deep-water basin. These geohistorical events in the Lower Yezo Group of the Nakagawa area are similar to those recorded in the Yubari Mountains, about 300 km south of Nakagawa, indicating the presence of these events across a wide area.
