Page 151 - Petrology of Sedimentary Rocks

PETROLOGY OF MUDROCKS

Mudrock (word coined by Ingram) is a general term used herein to cover those
terrigenous rocks that contain more than 50 percent silt and/or clay. The most obvious
division of mudrocks is on the basis of texture and structure:

Grain si ze Indurated, Indurated,
of mud fraction Soft non-fissile fissile

over 2/3 si I t silt si I tstone si I t-shale

subequal si I t mud mudstone mud-shale
and clay

over 2/3 clay clay cl aystone clay-shale

(if the rocks contain over IO percent sand, its median diameter should be prefixed to
the above terms, e.g., very fine sandy siltstone, medium sandy clay-shale, etc. Soft yet
thinly banded clays can be termed “laminated clay” where the term “clay-shale” would
be undesirable because of the implied hardness. The writer has often used “silty clay-
shale” or “si I ty claystone” for rocks containing 5 to 33 percent silt, reserving the
unmodified term “claystone” for those with over 95 percent clay). See Picard 1971
J.S.P., for another scheme.

Mudrocks have long been ignored by petrographers, therefore not much is known
about them. Partially this is due to a defeatist complex, caused by the very mistaken
impression that they are too fine grained for anything useful to be seen with the
microscope. When examined in thin section, however, they reveal a surprising diversity
of texture, structure, and mineralogy.

Mineral Composition. If the rock contains more than a small percentage of silt, it
can usually be related to one of the terrigenous rock clans. Orthoquartzite-type
mudrocks have silt grains consisting almost entirely of quartz (or chert). Phyllarenite-
type mudrocks are rich in fine-grained micas and may have some discrete metamorphic
rock fragments and quartz silt showing composite or undulose extinction. Arkosi c
mudrocks contain much potash feldspar.

The nature of the clay mineral present often allows hints as to source-area
lithology, climate, or environment of deposition (see section of this syllabus on clay
mineralogy). The clay mineralogy should always be checked by X-Ray; the petrographic
microscope can usually identify the dominant clay mineral, but those minerals present
in small proportions will be missed. On the other hand, the microscope is superior in
picking out clay minerals that are present in small patches but make up little in overall
proportions of the rock (e.g. authigenic chlorite or kaolinite patches).

Well-sorted (mature) siltstones of course have the same cements as the corre-
spondi ng sandstones: carbonates, quartz, iron oxides, gypsum, etc. But thin sections of
clay shales often show pores and irregular openings filled with authigenic quartz,
carbonates, barite, gypsum, and other minerals; some of these have grown within the
yielding clay by physical displacement of the mud.

145