Syngenetic rapid growth of ellipsoidal silica concretions with bitumen cores

Hidekazu Yoshida,Masayo Minami,Seiji Kadowaki,Ryusei Kuma,Richard Metcalfe,Sin-Iti Sirono,Nagayoshi Katsuta,Shoji Nishimoto,Natsuko Takagi,Hitoshi Hasegawa

doi:10.1038/s41598-021-83651-w

Abstract

Isolated silica concretions in calcareous sediments have unique shapes and distinct sharp boundaries and are considered to form by diagenesis of biogenic siliceous grains. However, the details and rates of syngenetic formation of these spherical concretions are still not fully clear. Here we present a model for concretion growth by diffusion, with chemical buffering involving decomposition of organic matter leading to a pH change in the pore-water and preservation of residual bitumen cores in the concretions. The model is compatible with some pervasive silica precipitation. Based on the observed elemental distributions, C, N, S, bulk carbon isotope and carbon preference index (CPI) measurements of the silica-enriched concretions, bitumen cores and surrounding calcareous rocks, the rate of diffusive concretion growth during early diagenesis is shown using a diffusion-growth diagram. This approach reveals that ellipsoidal SiO2 concretions with a diameter of a few cm formed rapidly and the precipitated silica preserved the bitumen cores. Our work provides a generalized chemical buffering model involving organic matter that can explain the rapid syngenetic growth of other types of silica accumulation in calcareous sediments.

Highlights

Anaerobes, have been shown to play an important role in concretion formation[21,22,23,24,25]
Raiswell and Fisher[19] suggested that such features are readily explained by a pervasive model of concretionary growth and this model is widely accepted. They note that these features can be explained by a diffusive growth model and that it can be difficult to distinguish the two models from observations made on concretions
In this paper we present detailed analyses of silica concretions from the Eocene Green River Formation in Utah, U.S.A. in order to shed light on their nature and rates of the processes involved in their formation

Summary

Introduction

Anaerobes, have been shown to play an important role in concretion formation[21,22,23,24,25]. Bojanowski and C larkson[26] proposed a diffusive model driven by chemical gradients to explain siderite concretions developed around central organic remains. Such concretions can be considered analogous to silica concretions developed around organic-rich cores. Silica concretions are mainly found in the upper part of the formation in the Indian Canyon section, western Uinta Basin (Fig. 1a). Ellipsoidal silica concretions with bitumen cores are observed in the carbonate-rich fine sedimentary rocks and occur mainly in the fluctuating shallow-lake stage. The bitumen cores of all concretions are brownish to dark coloured and ellipsoidal in shape with longer axial dimensions of 1 –2 cm in the centre (Figs. 1b-1–b-3, Supplementary Fig. S1)

Methods

Results

Discussion

Conclusion