Mudstone is generally a sedimentary rock. It is fine-grained in nature and occurrence. The stone is mud hardened with components including clay and silts that constitute very fine particles. A mudstone usually will soften in water, with the related clay stones and siltstones remaining hardened. The grain size of a mudstone is up to 0.0625 mm (0.0025 in). The individual grains are too small and cannot be distinguished without the use of a microscope. Increased pressure on the mudstone over time aligns the platy clay minerals making features of parallel layering. The finely bedded material splitting into the thin layers is the shale. Lack of layering in mudstone could be because of original texture or to the regular disruption of layering done by burrowing organisms. Mud rocks, for instance mudstone comprise of 65% of all the underground sedimentary rocks.
Mudstone resembles hardened clay. Depending on circumstances of formation, it might show fissures or cracks like that of a sun-baked clay deposit. The mudstones have three main categories including siltstone (more than half of the mudstone composition is silt-sized), clay stone (more than half of the mudstone composition is particles of clay) and Mudstone (hardened mud that results from mixing of clay and silt sized particles) (Harvey & Robert, 1996).
Heidelberg (2005) studied the Karak mudstone. He wrote that chemical and mineral composition of mudstone, inter-bedded in an Eocene sequence. The main domination is by R-1 ordered illite-smectite. It holds a 20 to 30% component of the compound. Minor phases of the mudstone include chlorite, kaolinite, muscovite, potash feldspar, plagioclase, quartz, pyrite and dolomite. The present illite-smectite could have been probably the original smectite or the expandable illite-smectite that underwent gradual conversion to make illite-smectite a low-temperature (ca. 100°C) closed-system. He says that sedimentary basinal environment at a sea depth of ca. 5 km. Al3+ and K+ were necessary for the reaction and conversion.
It is evident that mudstones exist within different environments. Heidelberg (2005) argued that, mudstones form permanently or washed away together with other sediments to large water bodies. In this case, he argued that deposition takes place in various means and seasoning begins where the debris including sediments of sand and mud combine forming a think and hard substance named the mudstone. The environment surrounding resulted from potash feldspar breakdown. A 5 km thick pile Burial of sediments produced these observed structures. The chemistry suggested in Heidelberg presentation suggests that the formation of mudstone happens through a weathering process involving the reaction of acidic source rocks. This process is dependent on other factors that slow or fasten the reactions. He explained that the formation of mudstones is prominent in huge water body masses because of the transportation of debris in an easy and continuous manner in the water bodies and depositions in strategic places where mudstones form.
The distinct patterns of stratification in Lower Liassic mudstones result from the regular shell beds occurrence and concretionary horizons. The mudstones formation took place in four depositional environments including a storm shallow marine system, a hemi pelagic system, a marine system influenced by changes in climate at astronomical frequencies and a pro-deltaic transition marine system. The tectonic setting was important in determining the sequence of the facies, whereas sea-level changes did not have any influence.
The process of mudstone as per Harvey and Robert (1996) happens in a simple but stage process. Being fine-grained, clay easily flows into the sea and lakes through water debris transportation. Here, it accumulates with sand, silt and calcareous organisms forming typical sequences of siltstones, limestone sandstone that later lay depositions and harden with mud to form mudstones. The mudstones harden by day because of weathering process that happen in the best environment enhancing these processes. They also agree to the fact that some depositions are not possible because of the continuous flow. The successive depositions make the mudstone harder following continuous debris accumulation.
The environment of deposition that leads to mudstone formation occurs differently in different regions of formation. Moon (2002) said that regarding deposition, mudstones formation is adjacent to the region of formation. He looked into Formation of Pohang Basin. It has accumulated some armored mudstone balls occurring in a chaotic conglomerate bed. This deposited from the flow of debris on a steep slope. Sparsely armored or unarmored mudstone balls are also found in the same bed. The basin also has isolated armored mudstone occurring in a homogeneous mudstone bed.
The formation of armored mudstone balls was likely through some retrogressive submarine channel failures and successive rolling over pebble and sand along the floor of the submarine channel. This happened prior to the later incorporation of the debris flow. Their occurrence suggests also that there must have been a large-scale slumping on the upper slope. This stipulates some of the deposition procedures leading to mudstone formation. The basis of argument is that the debris accumulates the basin leading to a formation of thick structure later hardening to the mudstones. Several factors affect the deposition including the flow of debris and the amount deposited. Contents of the debris are also a factor since they determine the kind of mudstone formed. This later depends on the time span in the weathering process as well as the intensity.
Armored mud balls are common in many modern environments for instance fluvial and alluvial mud balls. This is because of the debris depositions commonly featuring sand and silts in the water basin. However, only a few of the mudstones survive deposition and preserved in an ancient sequence resembling a “fossil”. This is because of the external forces of weathering and slide that dismantle the mudstones in the process of formation reducing their size. Therefore, their happening in ancient deposits implies characteristic preservative and depositional conditions also within a characteristic depositional environment. They show how conducive and environment has been in facilitating the formation of the mudstones.
Some mudstone balls are common in the Hunghae Formation of the Pohang Basin. The formation relatively consists of alternating fine to coarse sandstone and homogeneous mudstone and lying in beds with intercalated wedged like conglomerate bodies. Sedimentary environments and conditions are reconstructed from bioturbation and primary sedimentary structures characteristics. Flood and soils deposits in the coastal-plain region determine mudstone characteristics. This is through a rapid sediment reworking and deposition in near shore areas. Most of the sediments transfer by storm and slope deposits.