Sedimentology and biostratigraphy of the upper Triassic carbonates from the Shiriya cape, North Kitakami Belt, Japan

Abstract

Relics of the Panthalassa Ocean occur today in accretionary complexes and terranes on the entire Circum-Pacific region. Among them, remains of shallow-water carbonate systems provide valuable information about the ecology and environmental conditions that prevailed in this immense ocean. In this regard, the Shiriya Cape of the North Kitakami Belt (Aomori Prefecture, Honshu Island, Japan) is an essential place to study. Characterized by the presence of numerous limestone deposits embedded in typical accretionary mélange, including pluri-kilometric massive slabs and metric bedded deposits, the cape is indeed a key vestige of carbonate deposition in the Panthalassa. All the limestone outcrop form the Shiriya Cape were sampled and studied in detail for the first time. Despite a general poor preservation, macroscopic sedimentary features such as burrows and large megalodontids patches as well as four microfacies were identified and are presented in detail in this work. On the basis of foraminifers and conodonts biostratigraphy, the age of the limestone deposits is defined as Norian. Similarities with analogous and synchronous systems from Panthalassan and Tethyan realms are extensively discussed. The closest analog to the Shiriya limestone appears to be the Panthalassan Dalnegorsk limestone (Russian Far East), both regarding microfacies and depositional setting. Sedimentary successions similar to Lofer-cycles, well known in Tethyan synchronous systems, were also discovered in the Shiriya Cape. Along with the large limestone slabs, countless limestone-bearing conglomerates also crop out on the Shiriya Cape. Their study and comparison with similar deposits in Japan highlighted their importance in the understanding of the dismantling of mid-oceanic carbonates. The observations and comparison with well-known carbonate systems permitted to establish a hypothetical depositional model of the Shiriya limestone corresponding to a Norian isolated carbonate system developed on an immerged volcanic seamount.