Folded calcite cracks in noncalcareous shales: a window into shale diagenesis and hydrothermal influence

Abstract

ABSTRACT Shale diagenesis is not well understood, and cracks in shale contain important information about diagenetic conditions. The way these cracks open reveals physical changes in the sediment, and the infilling minerals provide insight into the chemical conditions of the formation water. Typically, the authigenic minerals filling the folded cracks are consistent with the chemical composition of the host rocks. For example, folded calcite cracks are found in limestone. This paper, however, focuses on a set of folded calcite cracks in noncalcareous black shales. The goal is to improve our understanding of shale diagenesis by deciphering the origins of these cracks. The cracks are sinuously to ptygmatically folded in a vertical view and weakly sinuous on the bedding plane. They are filled with calcite, bitumen, pyrite, or a combination of them. Evidence of bioturbation and low redox-sensitive trace-element ratios suggest suboxic to oxic depositional conditions of the shale. The cracks were likely opened by gas expansion in unconsolidated mud. The main mineral filling the cracks, calcite, was sourced from hydrothermal fluid that passed through the underlying dolomite. Hydrothermal influences are indicated by the presence of bornite and microcrystalline pyrite in the cracks, as well as Fe and Mn enrichment in the host sediments. Hydrothermal activity can also explain the presence of buddingtonite, an ammonium feldspar in the shale. The results of this study suggest that folded cracks filled by minerals, gradually narrowing towards the top, and lacking internal detrital matrix are likely formed during early diagenesis. The inconsistency between the chemical compositions of the infilling material and the host sediment requires further exploration to identify the source. Hydrothermal fluid passing through the underlying dolomite may be the source of folded calcite cracks in noncalcareous sedimentary rocks. These cracks resemble molar-tooth structures (MTS), which are sinuous cracks filled with microcrystalline calcite mostly found in Precambrian limestone and calcareous shales. If these cracks are indeed MTS, they may be an exception to the age and host-rock lithology constraints of MTS.