Secondary Sedimentary Structures

Introduction

All the sedimentary structures discussed in relation to deposition - cross-bedding, graded bedding, ripples, flute casts - form at the time of or immediately after sediment accumulation. Secondary sedimentary structures are fundamentally different: they form sometime after deposition, during sediment burial. Their origin is largely chemical rather than physical, driven by the precipitation of minerals from pore fluids or by chemical replacement of existing grains. Understanding them is essential in sedimentary geology because they can modify the original texture of a rock significantly, and because they carry information about the diagenetic history - the fluid chemistry and pressure-temperature conditions - of the subsurface environment.

Definition

Secondary sedimentary structures form sometime after deposition, during sediment burial. They are largely of chemical origin, formed by precipitation of mineral substances in the pores of semiconsolidated or consolidated sedimentary rock, or by chemical replacement processes. ^[1]

Concretions

Concretions are the most common kind of secondary sedimentary structure. ^[1]

Most concretions are composed of calcite, but concretions composed of dolomite, hematite, siderite, chert, pyrite, and gypsum are also known. They form by precipitation of mineral matter around some kind of nucleus - such as a shell fragment - and gradually build up a globular mass, which may or may not display concentric layering. ^[1]

Shapes of concretions range from spherical to disc-shaped, cone-shaped, and pipe-shaped, and they may range in size from less than 1 cm to as much as 3 m. Concretions are especially common in sandstones and shales but can occur in other sedimentary rocks. ^[1]

The concentric layering, when present, records successive episodes of mineral precipitation outward from the nucleus. The nucleus itself is significant: the pore fluid precipitates cement preferentially around organic matter or around a hard object that disrupts local fluid chemistry. This is why shell fragments, bone fragments, and even twigs can serve as nuclei. The shape of the final concretion reflects the local pore geometry and the isotropy or anisotropy of fluid flow around the nucleus - spherical shapes indicate isotropic flow, while oblate or pipe-shaped forms record preferential flow along a particular direction.

Stylolites

Stylolites are suture-like seams of clay or other insoluble material that commonly occur in limestones as a result of pressure solution. ^[1]

Pressure solution is a diagenetic process in which soluble mineral grains dissolve preferentially at grain contacts where stress concentrations are highest. The dissolved material is carried away by pore fluids, leaving behind an insoluble residue - typically clay minerals and iron oxides - that accumulates as an irregular, interlocking seam. The jagged or sutured geometry of stylolites is a direct consequence of differential dissolution rates across the contact surface. Stylolites are a volumetrically important mechanism of rock compaction: in thick carbonate sequences, stylolites can account for the loss of a significant fraction of the original rock volume.

Sand Crystals and Cone-in-Cone Structures

Less common secondary structures include sand crystals and cone-in-cone structures. Sand crystals are large crystals of calcite, barite, or gypsum that contain sand inclusions. Cone-in-cone structures are nested sets of small concentric cones composed of carbonate minerals. ^[1]

Sand crystals form when large crystite minerals grow within a sand, incorporating the surrounding sand grains as inclusions as the crystal expands. The crystal lattice overprints the host sediment, and the sand grains become trapped inside the growing crystal. Cone-in-cone structures remain incompletely understood but are thought to involve fibrous carbonate mineral growth under directed stress during compaction. Both features are diagnostically secondary: they cannot form during initial deposition.

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