Prof. Stephen A. Nelson
|Mudrocks are fine grained sedimentary rocks consisting of mostly silt and clay size
fragments. They are sometimes called argillites. Because
of their small grain size, they are difficult to study, even with the petrographic
microscope. But, they are important rocks because they are the most abundant
sedimentary rocks, making up over 65% all sedimentary rocks, are likely the source rocks
for petroleum and natural gas, and are sometimes valuable ore deposits. In addition,
the mudrocks are the protoliths (precursor rock) for
aluminous metamorphic rocks, often called pelitic metamorphic
|Grain Size||Description||Fissile Rock||Nonfissile Rock|
|>2/3 silt||Abundant silt sized grains visible with a hand lens||Silt-shale||Siltstone|
|>1/3, <2/3 silt||Feels gritty when chewed||Mud-shale||Mudstone|
|>2/3 clay||Feels smooth when chewed||Clay-shale||Claystone|
Elements of texture that can be observed in mudrocks include the shapes of the grains, the fissility or lack of fissility, and laminations.
Clay minerals are the most abundant minerals in mudstones, making up over 60% of all mudstones. Other minerals like quartz, feldspar, carbonate minerals, organic compounds (not really minerals), sulfides, and hematite also occur.
|Environments of Deposition
Mudrocks represent texturally and mineralogically mature sediments deposited in low energy environments. The fine grain size of these sediments means that that the sediment can be suspended for long times in relatively quiet, low energy currents. This results in deposition on the abyssal plains of the oceans, at the distal ends of deltas, in quiet lakes and swamps, and as wind blown dust. Large deposits of wind blown dust are called loess, and consist of mostly silt-sized fragments. While the small fragments can be transported into lakes and oceans easily by streams, it is likely that large quantities of these fragments could also reach the oceans by wind transport.
|Return to Geology 212 Home Page|