Mudrocks


Ithaca, NY
Mudrocks are rocks formed primarily from silt to clay-sized clasts. Fine-grained quartz and feldspars and various amounts of carbonates, sulfides, iron oxides, heavy minerals, and organic carbon can be present as minor components.

Whereas the composition and provenance of conglomerate framework clasts can be assessed in hand sample, and sandstones can readily be assessed in thin-section, if not in hand samples, mudrocks, because of their extreme fine grain size resist easy interpretation. Even a mass spectrometer only gives us information on bulk composition. Alas, we would really like to know more because:


As Dr. Kaufman says,


Recovery of Cretaceous turtle from Aguja Fm. Big Bend National Park

"Give them love, since they surely deserve it."

But alas, they are difficult to love:

But mudrocks are beloved of:


Mudrock Classification:

There is no standard classification for mudrock and shale. This is because:

  • There is a great diversity of clay minerals
  • The rocks containing them are understudied because of their intractability.

    Nevertheless, the following major rock types are generally recognized:


    Shale and Sandstone, Purslane Fm. near Hancock, MD
    Shale: Mudrocks can be completely structureless. Sometimes, however, they display fissility: thin lamination and a tendency to break along parallel laminations cause by the alignment of platy clasts. Any highly fissile mudrock is termed shale (right).

    Mudrock composition:

    The same components used to classify sandstones are present in mudrocks:

    The difference is in the proportion of clay minerals, which make up >75% of volume. We therefore focus on them.

    Clay minerals are phylosilicates - sheet silicates, whose fundamental components are:

    From these layered components, along with ions or molecules included in interlayers, clay minerals are formed.
    These combine to make "open faced" and "close faced" sandwiches of clay layers between which various cations can be sandwiched. Principal groups of clay minerals are defined by their structure. The four primary groups of clay minerals are kandite, illite, smectite, and chlorite.

    1:1 clay:


    2:1 clays:

    Utility of clay minerals:

    Clay minerals as indicators of source material:

    Clay minerals as indicators of weathering:

    Caveat: You would think from the last two observations that clay mineral composition would be a good indicator of depositional environment character and sediment provenance, but alas, no: Clay minerals tend continuously to equilibrate with their current environments, so chemical alteration is ongoing. Thus, any chemical record of depositional environment tends to get erased.


    Green River Fm. lacustrine sediments and fossil from Stephen Hui Museum
    Energy of depositional environment: One thing we can tell, assuming that clay minerals were deposited as clasts (and weren't secondary digenesis products), is that Stoke's Law dictates that the depositional environment had very low kinetic energy, or else the clasts would never have settled. Usually:

    Special fine-grained siliciclastic rock and sediment types:

    Glauconite: ((K,Na)(Fe3+,Al,Mg)2(Si,Al)4O10(OH)2) An iron-rich clay mostly found in sand-sized aggregates (usually by replacement of fecal pellets). This mineral only occurs in open marine conditions in aerated water. The big thing: It forms very slowly (~1000 years) at the sediment/water interface. Thus, it forms during episodes of sediment starvation or major transgression. The really big thing: Glauconite is the only clay mineral that is chemically stable. Thus, it the ONLY clay mineral that faithfully records its depositional environment - shallow, sediment-starved marine environments.


    Bentonite
    Bentonite: Mostly montmorillonites and other smectites. Commonly form from chemical alteration of volcanic ash. Very high electrical conductivity. Very high water expansion. Many agricultural industrial uses, including construction and drilling.

    Because bentonites result from one-time widespread depositional events (ashfalls) they facilitate stratigraphic correlation. Rarely, they preserve sufficient radiometric signal to allow numerical dating.


    Rock flour on Matanuska Glacier
    Rock Flour: Glaciers relentlessly grind their bed load against the bedrock, reducing it to clay sized lithic fragments called rock flour. These are not clay minerals. Could be anything, but are commonly ground up feldspars and quartz.


    Loess near North Pole, AK
    Loess: What happens to silt-clay sized glacial sediment? It gets picked up by wind and water. Wind-blown silt particles typically settles out within a few hundred km of the glacier, accumulating in thick deposits. Loess deposits from the last glaciation contribute to some of the world's best agricultural soils.

    Examples of loess dominated regions:


    Marl: A rock that only a paleontologist would love. A mixture of siliciclastic and carbonate materials (Usually 10-50% carbonate) that can form in many marine environments, but are commonly deep-water. (For biological and hydrodynamic reasons carbonates and clastics tend not to mix in shallow marine environments.)

    Oozes: Extremely fine-grained rocks with a very large component of mcirofossils, chiefly:


    Stenopterygius with soft-tissue preservation in Early Jurassic Posidonienschiefer
    Organic-rich mudrocks: (A.k.a. black shales or euxinic shales) These can be silt or clay dominated, but always have > 0.5% total organic carbon content (TOC). These form petroleum source rocks and oil shales during burial and graphite beds at greater depth. Organic material is commonly preserved under two conditions: Note: In today's world, only the Black Sea (from whose Greek name Euxeinos Pontos we get the term euxinic) has conditions favorable to the formation of black shales. It is not clear that it is a good model for the depositional environment of ancient black shales, however.