A series on clay and clay mineralogy – clay crystallography, identification, diagenesis, and deposition.
“… the study of mudstones connects us to most sedimentary rocks,”.
This statement by Paul Potter et al., (2005, Mud and Mudstone) in their introduction nicely encapsulates the importance of mudrocks, or mudstones.
Unconsolidated mud and its lithified cousins (mudstone, shale, argillite etc.) compose 60% to 70% of the global sedimentary record. This includes marine and nonmarine siliciclastic, volcaniclastic, carbonate, and organic-, iron-, and phosphate-bearing lithologies. Mudrocks have also been discovered on Mars.
Historically, sedimentologists have tended to focus on coarse-grained lithologies because they are commonly better exposed, contain an exciting range of sedimentary structures, and provide easily discernible evidence (relatively) of sediment provenance and tectonic setting for sedimentary basin formation. But important information can also be teased from mudrocks:
- As indicators of depositional mechanisms and energy.
- As indicators of sediment rheology (e.g., sediment gravity flows, soft-sediment deformation).
- As indicators of biogenic productivity, recorded for example in the ethology of invertebrate faunas represented by trace fossils.
- As stratigraphic marker beds (stratigraphic correlation, geological mapping).
- As indicators of sea level change.
- For the pelagic and benthic microfossil assemblages that provide stratigraphic ages and paleobiological data.
- As indicators of REDOX conditions, particularly ocean paleo-oxygen levels.
- As kitchens for hydrocarbon generation.
- As mechanically weak rock that focuses fault detachment and slope failure.
- As aquifer confining units and hydrocarbon reservoir traps.
Definitions and terminology
The naming and definition of things (objects, processes, conditions, ideas) ensures familiarity and consistency in science communication (at least theoretically). In geology, as for any science discipline, the real world commonly forces analysts, teachers, and commentators to navigate a veritable jungle of terminologies. For example, geologists deal with rocks (among other things). But even this most basic component of the discipline suffers from disagreements about what constitutes a rock – arguments that commonly centre on the degree of consolidation and induration. Rocks and sediments consisting mostly of mud are another example where subdivision of mudstone categories has given rise to a plethora of disparate names and definitions, depending on lithology, research goals, and to some extent institutional bias.
Mudrocks, perhaps more than any other sedimentary rock type, have suffered from definition fatigue. Some of the names used for mudrocks are listed below – names and descriptors that cross the boundaries of texture (grain size), mineral and chemical composition, internal organization (bedding), and depositional environment. One person’s mudstone may be another’s shale or pelite. The essential problem here is whether we are talking about the same thing?
Nouns (Adjectives): mud (muddy), mudrock, mudstone, shale (shaley), micrite (micritic), marl, lutite, sapropel, ooze, clay (clayey), claystone, argillite (argillaceous), pelite (pelitic), phyllite, slate, wacke, wackestone, chalk, siltstone, radiolarite, diatomite, loam, dirt.
There are many mudstone classification schemes – most were developed post-1930s. Biddle et al, (2025) have provided a comprehensive review and evaluation of various of classification schemes. Two recent schemes (21st C) are worthy of attention: K. Milliken (2014) who approached the problem from the perspective of rock composition and the predictive potential of mudstone classification. Lazar et al., (2015; PDF) focused on textural properties as the root identifiers, augmented by bedding style and compositional modifiers.
The Milliken and Lazar et al., schemes are summarized below.
Some earlier classification schemes
The following diagrams show some of the nomenclatural schemes that express degrees of muddiness in terms of texture (grain size), and mineral composition. Mud is generally defined as a mixture of clay and silt. The silt component is defined by texture – or grain size. Clay on the other hand can be treated as a mineral or a textural property – on the Udden-Wentworth grain size scale it includes all particles ≤ 0.004 mm (4 μm). Textural classifications are commonly shown as ternary diagrams where sand, silt, and clay define the apices. Compositional classifications are expressed as percentages or proportions of minerals as in the Krumbein and Sloss (1956) scheme, or R.H. Dott’s (1964) scheme that distinguishes sandstone, wacke, and mudstone based on the proportion of detrital matrix where matrix is used in a compositional context (this was later modified by Pettijohn, Potter and Siever, (1973). Most of these schemes use either 50% cut-off values to differentiate between sand-clay and silt-clay, or schemes like those of Folk (1968; 1974) that use a 2:1 ratio to distinguish silt from clay.
Mudstone classification schemes have also been proposed using other rock properties. One of the earliest schemes proposed by Twenhofel (1937) used the degree of induration and incipient metamorphism where argillite is the highly indurated equivalent of unconsolidated silt, mud, and clay (the diagram is modified from Twenhofel, 1950, Fig. 41; and Wilkins, 2010, Table 1). A different approach was taken by Fleming (2000) who expressed sand-silt-clay proportions in terms of the hydrodynamic energy required for sediment transport. Fleming’s rationale was that subtle changes in composition from one mudstone layer to the next represented stratigraphic variations in depositional energy, that could be plotted diagrammatically.
K. Milliken’s (2014) scheme
Milliken’s (op cit.) classification scheme for fine-grained sediments (i.e., >50% of particles ≤ 62.5 μm) is based on composition. She makes the fundamental distinction between particles derived from extrabasinal processes (e.g. weathering, volcanic ash) and intrabasinal processes (e.g., fossils, authigenic carbonates, organic matter). The three end-member compositional classes are:
- Terrigenous and volcaniclastic particles (predominantly clay minerals).
- Calcareous biograins and allochems – predominantly intrabasinal.
- Siliceous biograins and allochems – predominantly intrabasinal.
Biograins are presumed to be sourced within the basin – this includes micro- and macrofauna and flora. Allochems includes cements that can form on the sea floor and different stages of burial.
The triad of particle compositions is presented as three basic mudstone categories (labelled as acronyms); the cut-off between extrabasinal and intrabasinal types is 75%:
- TARL (Terrigenous argillaceous mudstone) that are derived primarily from extrabasinal sources.
- CARL (Calcareous argillaceous mudstone) – mainly intrabasinal.
- SARL (Siliceous argillaceous mudstone) – mainly intrabasinal.
Carl and Sarl contain detrital clays in decreasing proportions from the Terrigenous grains apex. Likewise, Tarl contains minor siliceous and /or calcareous components. Compositions having ≤ 10% clay include relatively pure carbonate mud and wackestone, and siliceous ooze or chert. Modifiers in this scheme can include textural information (cf. the scheme by Lazar et al., 2015, noted below), plus additional compositional modifiers such as foraminiferal, glauconitic, or phosphatic.
Milliken’s scheme permits the prediction of cementation trends and concomitant changes in porosity, where allochemical carbonate and silica cements increase towards the C and S apices. Similar trends may also apply to increases in organic matter and kerogen content that accompany increases in planktic microfauna and microflora compared with terrestrially derived organic matter. In this way, Milliken maintains that the classification is a useful predictor of hydrocarbon generation and diagenetic trends
Lazar et al., scheme
The classification by Lazar et al., (2015, op cit.) incorporates textural properties as the fundamental mudstone identifier but qualifies the root name with bedding and compositional characteristics – bedding refers primarily to lamination, and composition to mineralogical, biological and organic content.
Lazar et al. (op cit.) employ the term mudstone for any mixture of clay and silt that is ≥ 50% by volume (or thickness). Instead of the usual sand-silt-clay textural end members, they use coarse, medium, and fine grain size limits for silt to distinguish between Coarse mud and Fine mud end members. Each textural category is augmented by one of 12 types of bedding, or lamination (6 continuous styles are shown in the diagram above; there are also 6 discontinuous styles). The bedding notation is borrowed from Campbell, 1967.
The term lamination signifies the thinnest layer of sediment lacking any other kind of bedding and bound by laminae bounding surfaces – thickness is commonly measured in millimetres. They also recognize lamina sets as repeated laminae having similar character. Beds are bound by bedding surfaces and tend to be thicker and laterally more extensive than laminae or lamina sets. Laminae may be continuous or discontinuous, parallel or nonparallel, with planar, curved, and wavy variations on those characters. The scale for lamination continuity is arbitrary, but usually less than the lateral extent of beds. The attributes of bioturbation can also be added. Based on texture alone, a typical name might be a discontinuous, parallel, wavy, bioturbated mudstone.
Compositional attributes in this scheme focus on mineral components. For example, in the clay-quartz-carbonate triad shown above, each component is treated as the total for quartz, clay, or carbonate. Each class can be refined depending on the actual composition. If the proportions of clay -quartz-carbonate were 2:1:0 respectively, the rock name would include the modifiers argillaceous, siliceous mudstone. If kaolinite was the dominant clay, then the name can be further modified to kaolinitic argillaceous, siliceous mudstone (although specific clay minerals are not likely identified in the field or hand specimen). If a significant component of quartz consists of radiolaria or sponge spicules, then the mudstone name can be modified accordingly. Likewise, if dolomite was the dominant carbonate, then the mudstone name would include the modifier dolomitic.
Including all these characteristics in a single name and adding colour, as in grey-brown, discontinuous, parallel, wavy, bioturbated, argillaceous, siliceous, fine-grained mudstone looks cumbersome and sounds like a mouthful. But it is informative because it tells us all we need to know about this mudstone. However, the name can be shortened if certain characteristics such as bedding style, or composition are repeated through a muddy succession, as in grey-brown, bioturbated, argillaceous mudstone.
The name can also be shortened on a diagrammatic stratigraphic column by incorporating symbols for many of the modifiers – symbols for sedimentary structures, bedding, macro- and microfossils, and bioturbation are commonly included in these representations.
References not linked
R.L. Folk, 1968. Petrology of Sedimentary Rocks. Hemphill’s, Austin Texas.
R.L. Folk, 1974. The Petrology of Sedimentary Rocks. Hemphill’s, Austin Texas.
W.C. Krumbein and L.L. Sloss, 1956. Stratigraphy and Sedimentation. W.H. Freeman & Co. San Francisco.
N.R. O’Brien and R.M. Slatt, 1990. Argillaceous Rock Atlas. Springer Nature
F.J. Pettijohn, P.E. Potter, and R. Siever, 1973. Sand and Sandstone. Springer-Verlag.
W.H. Twenhofel, 1936-1937. Terminology of the fine-grained mechanical sediments. Report of the Committee on Sedimentation for 1936–1937 (National Research Council).
W.H. Twenhofel, 1950. Principles of Sedimentation. 2nd Ed. McGraw-Hill Book Co. New York.
Other posts in this series
Mineralogy of the common clays