Glauconites Formed in the High-energy Shallow-Marine Environment of the Late Mesoproterozoic: Case Study from Tieling Formation at Jixian Section in Tianjin, North China

Abstract

Glauconite is a kind of hydrous-layered aluminium silicate mineral, and this kind of mineral is frequently used as a facies-indicative mineral. Numerous studies on glauconites formation indicate that modern glauconites are mainly formed in the deeper sedimentary environment, which tends to slow down the sedimentation rates and can be further used as a discerning mark for the condensed section in sequence-stratigraphic divisions. Glauconites developed in limestones of the stromatolitic bioherm in the second member of the Mesoproterozoic Tieling Formation at the Jixian section in Tianjin City of North China, are mainly rich in the boundary between stromatolites and the leiolites as a fashion of colloids, which reflect the features of the autochthonous glauconite that is formed in high-energy shallow sedimentary environment. Also, the high content of K 2O (>8%) shows that these glauconites can be grouped with the high-evolved glauconites. Peculiarities of these autochthonous glauconites, i.e., the special forming sedimentary environment that is marked by the high-energy shallow water, the particular occurrence form as a fashion of colloids in the boundary between stromatolites and the leiolites, the older forming times that is in the late Mesoproterozoic, indicate that these glauconites are strongly different from the modern glauconites. Sedimentologically, glauconites from the Mesoproterozoic Tieling Formation at the Jixian section reflect that the normally high-energy shallow water is in the condition of the slightly reduced state resulted from the low-oxygen levels in the air of the late Mesoproterozoic. Obviously, glauconites in the stromatolitic limestones of the Mesoproterozoic Tieling Formation cannot be used as a discerning mark of the condensed section in sequence stratigraphy, and cannot be grouped into the product formed in the long term hiatus. Therefore, glauconites in the Tieling Formation are the representative of the Precambrian, which indirectly reflect the diversity of the glauconitic forming environment in the long evolving geological history.