Sedimentary significance of the autochthonous glauconite in stromatolitic limestones of the Mesoproterozoic Tieling Formation in Jixian, Tianjin, North China

Glauconites are widespread in the stromatolite limestones of the upper part of the Mesoproterozoic Tieling Formation in Jixian, Tianjin, North China. Field investigation shows that glauconites are mainly distributed in the micritic limestone between stromatolite columns as thin film centering on the outer edge of stromatolite sheath. Microscopically, they commonly occur as irregular colloids which are indicative of autochthonous typical feature. The EPMA data indicates they are moderately matured in nature. The fact that the glauconites occur in stromatolite limestones deposited in the high-energy subtidal environment suggests that they were not necessarily formed on the substrate with a low sedimentation rate or even a sedimentation hiatus; this is quite different from the case of modern glauconites. Therefore, such medium-mature autochthonous glauconite in stromatolitic limestones is formed under special sedimentary background. Its unique features of glauconite presented in this study provide an alternative case different from the common sense of glauconite origin.