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
- Differential compaction: different grain sizes present in a bed compact to differing degrees. Clays and muds compact more readily than sands and larger clasts. Shales lose about half their thickness upon compaction. This must be taken into account when correlating across various sections.
- Distortion or flattening of grains, especially larger grains (right)
- Pseudomatrix: finer grained rock fragments (such as shales and slates) break down to clays under pressure giving the appearance of a clay matrix
- Pressure solution: under great pressure, grains dissolve at contact points. Dissolved ions may precipitate elsewhere in the rock. Pressure solution is revealed macroscopically as stylolites.
Fabrics can provide clues of temperature and pressure conditions
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.
- Silica cement: composed of SiO2 (right)
- Silica overgrowths: precipitation of dissolved silica on the surface of quartz grains in the same crystallographic orientation as the original grain. Overgrowths can obscure original grain characteristics (rounding).
- Source of silica cement is somewhat enigmatic: current perception of silica solubility in ground water does not account for the observed quantities of silica cement. Recall:
- Amorphous silica has a much higher solubility at low temperatures than crystalline quartz.
- Opaline skeletal clasts and volcanic glass are the most common sources of dissolved silica.
- For any silica cement to form, considerable volumes of groundwater must move through pore space
- For amorphous silica, this must be at shallow elevations
- Crystalline quartz cements form at high temperatures, therefore must at great depth.
- Since circulation of groundwater at depth is very slow, cementation must occur over long intervals. Long time spans and deep burial are required.
Calcite cements: CaCO3
- Much more soluble and more common than silica, especially near the surface
- Cementation is facilitated by the variable effects of changing pH on carbonate solubility.
- Still requires extensive time and groundwater flow to produce the observed amount of calcite cement - estimates require that pore space be flushed > 2700 times. (Don't even think about dolomite.)
- Hematite: Fe2O3
- Goethite: FeO.OH
Source is the weathering of Fe-rich silicates like biotites and pyroxenes.
Dissolution occurs under reducing conditions. Under current Eh/pH conditions, Fe is insoluble in water and quickly precipitates
- Iron oxides can preserve remnant magnetism. Thus, iron oxide cemented sediments can be either useful in magnetostratigraphy as the site of natural remanent magnetism or cause a confounding chemical remanent overprint. (But note: in a given sample the cements are not necessarily precipitated isochronously.)
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.
Authigenesis: New minerals grow from old recycled chemicals E.G.:
- We are already familiar with the formation of clay minerals from feldspars.
- Hematite ( Fe2O3) and pyrite (FeS2) form authigenically, taking iron from other minerals.
Authigenesis takes two forms:
- Recrystallization: We have spoken of this in terms of fossil preservation, but it is actually a common diagenetic process: An existing mineral retains it original material, but crystals grow or are reorganized'
- Amorphous opaline silica recrystallizes to coarser quartz grains
- Aragonite recrystallizes into calcite (right)
- Replacement: New mineral takes over the space once occupied by another. This involves simultaneous dissolution and precipitation.
Common replacement minerals include:
- pyrite (right)
Methods that exploit diagenetic features to illuminate duration and degree of diagenesis including:
- Timing of diagenesis
- depth of burial
- duration of burial
- temperatures - thermal histories
- pressures - barometric histories
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
- >100º C - smectites break down to mixed-layer clays
- ~150º C - kaolinite changes to illite or chlorite
- >200º C - mixed-layer clays change to illite
- >300º C - illites change to chlorite and muscovite
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.:
- Heulandite (right) and analcime: < 100º C
- Laumontite: > 100º C, < 150º C
- Phrenite and plumpellyite: > 150º C
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.:
- Secondary overgrowths of calcite or feldspar require alkaline pH
- Pyrite and siderite form in reducing conditions
- Feldspar overgrowths require the presence of dissolved ions in solution