Diagenesis:

But first: Outcrop du jour:

The Realm of Diagenesis:

Post depositional alteration of sediments short of metamorphism. Beyond ~300°C and ~1-2 kbar pressure, the changes enter the realm of metamorphism.

Diagenesis typically results in reduction of porosity


Compaction fabrics:


Fabrics can provide clues of temperature and pressure conditions

Cementation

Sometimes considered a form of diagenesis in that it occurs after deposition. Usually occurs via the precipitation of new minerals into pore spaces between clasts.

Diagenetic structures:

Macroscopic features that can obscure original depositional features. Usually take the form of coloration patterns or secondary depositional bodies.

Leisegangen banding: color bands produced in porous sandstone when minerals precipitate marking changing concentration gradients. Can mimic bedding planes or cut across bedding.

Concretions: regular rounded precipitates that form around a nucleus. (right) The nucleus is typically a buried piece of decaying organic matter that alters local groundwater chemistry, facilitating precipitation. In cases like the Carboniferous Mazon Creek Lagerstätte, the nuclei are often the remains of soft-bodied organisms that are preserved inside the concretions.

In other news, concretions can assume bizarre shapes mimicking fossils or artifacts, and cause endless confusion for amateurs.

Nodules: irregular shaped precipitates that do not have an obvious nucleus. Typically precipitate in ancient soil horizons or a sediment layer that has lower porosity and restricts flow. Many types of minerals may form concretions or nodules: calcite, silica, pyrite, etc.

Geodes: Vug (cavity) filled with quartz or calcite. Preexisting cavity fills with euhedral crystals of silica or calcite. Cavity often starts as fenestral fabric resulting from the dissolution of inclusions including fossil.

Diagenetic processes:

Authigenesis: New minerals grow from old recycled chemicals E.G.:


Authigenesis takes two forms:


Diagenetic history

Methods that exploit diagenetic features to illuminate duration and degree of diagenesis including:

Facilitates interpretations regarding large scale tectonic events or feasibility of hydrocarbon recovery.

Examples:

Conodont color index: Conodonts - toothlike phosphatic fossils (Outstanding upper Cambrian through Triassic shallow marine rocks index fossils, but only recently revealed to be from primitive vertebrates!) Carbon content changes color with increasing temperature, enabling us to assess peak temperatures during burial.

Vitrinite reflectance: Measures the metamorphic grade of coal. As coal is exposed to higher temperatures it becomes shinier. Reflected light intensity off the surface therefore gives the maximum diagenetic temperature.

Transformation of clay minerals: indicate maximum temperature

Zeolite facies: Zeolites are hydrous aluminosilicates that characterize the lowest grade metamorphic facies (on the boundaries of the realm of diagenesis). In the absence of carbonates, these form through the alteration of volcanic rocks. They tend to form in narrow temperature ranges. E.G.:


Secondary porosity - pores created upon the dissolution of minerals May or may not be filled in.

Groundwater chemistry: The presence of certain minerals can indicate both pH conditions, oxidizing conditions, E.G.: