Genesis of Paleocene and Lower Eocene shallow-water nodular limestone of South Tibet (China)

Abstract

This paper examines the genesis of the Paleocene to Lower Eocene shallow-water nodular limestones in South Tibet. The negative carbon isotope excursion representative for the Paleocene–Eocene boundary is located in one nodular limestone bed of the Zhepure Shan Formation showing an extraordinary thickness of about 9 m, inspiring the question under which conditions these nodular limestones were formed. Based on field appearance, the shallow-water nodular limestones of Tingri and Gamba can be classified into five nodular limestone categories (Stylonodular Rock I, Nodular Rock I, Nodular Rock II, stylobedded rock and stylomottled rock) and some transitional members (stylobedded rock transitional to Stylobedded Rock II). Clay variations are assumed to be responsible for these various types of nodular limestones within the sediments. Observations of nodular limestones in South Tibet suggest that those sediments were mostly formed due to autochthonous rather than allochthonous processes. Differential diagenesis resulted in an early selective cementation of limestone nodules due to carbonate supply, while the marls were not cemented but provide the carbonate for the nodule cementation. Additionally, cemented and carbonate-rich nodules are resistant to chemical compaction, while the uncemented and clay-rich marl layers are affected by pressure solution processes due to an overburden of sediments. Additionally, a model is presented, illustrating the origin of different nodular limestones described here.