Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.063 millimetres (0.0025 in) with individual grains too small to be distinguished without a microscope. With increased pressure over time, the platy clay minerals may become aligned, with the appearance of fissility or parallel layering. This finely bedded material that splits readily into thin layers is called shale, as distinct from mudstone. The lack of fissility or layering in mudstone may be due to either original texture or the disruption of layering by burrowing organisms in the sediment prior to lithification. Mud rocks such as mudstone and shale account for some 65% of all sedimentary rocks. Mudstone looks like hardened clay and, depending upon the circumstances under which it was formed, it may show cracks or fissures, like a sun-baked clay deposit.
Mudstone can be separated into these categories:
- Siltstone — more than half of the composition is silt-sized particles.
- Claystone — more than half of the composition is clay-sized particles.
- Mudstone — hardened mud; a mix of silt and clay sized particles. Mudstone can include:
In the Dunham classification (Dunham, 1962) system of limestones, a mudstone is defined as a mud-supported carbonate rock that contains less than 10% grains. Most recently, this definition has been clarified as a matrix-supported carbonate-dominated rock composed of more than 90% carbonate mud (<63 μm) component.
The identification of carbonate mudstone
A recent study by Lokier and Al Junaibi (2016) has highlighted that the most common problems encountered when describing a mudstone is to incorrectly estimate the volume of 'grains' in the sample - in consequence, misidentifying mudstone as wackestone and vice versa. The original Dunham classification (1962) defined the matrix as clay and fine-silt size sediment <20 μm in diameter. This definition was redefined by Embry & Klovan (1971) to a grain size of less than or equal to 30 μm. Wright (1992) proposed a further increase to the upper limit for the matrix size in order to bring it into line with the upper limit for silt (63 μm).
Mudstone mineralogy on Mars
On December 13, 2016, NASA reported further evidence supporting habitability on the planet Mars as the Curiosity rover climbed higher, studying younger layers, on Mount Sharp. Also reported, the very soluble element boron was detected for the first time on Mars. In June 2018, NASA reported that Curiosity had detected kerogen and other complex organic compounds from mudstone rocks approximately 3.5 billion years old.
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