Concept of Sediment Maturity

Sediment maturity refers to the degree of which a terrigenous clastic sedimentary rock has changed when compared to the starting material/s it has been derived from. Analysing the maturity of a sample allows us to determine kinetic energy during transport, transport history and depositional conditions. Maturity is measured by the two fundamental properties of a sedimentary rock; its texture and composition.

Textural maturity of a sediment is an expression of how well the clasts within the unit are sorted and rounded, and also the amount of matrix present. The road from a clayey, poorly sorted angular sediment to a well sorted, rounded sediment with little to no matrix always follows three sequential steps. In order of occurrence, these steps are; (1) removal of clay, requiring least energy; (2) attainment of good sorting in the non-clay portion, requiring more energy; and (3) rounding of the grains, requiring most energy (Folk, R.L. 1951). The fact that these steps do not all occur simultaneously, but rather at much later times, is the basis of the textural classification scheme. Textural maturity ranges from immature to supermature.

An immature sediment contains considerable amounts of clay and fine micas, with clasts being poorly sorted angular grains. Submature sediments contain little to no clay, but the clasts still remain poorly sorted and angular. It is not until the mature stage until the sorting of the sediment becomes more prevalent, but still are angular. At the supermature stage, everything desirable for a terrigenous clastic sediment rock is attained; well sorted, rounded clasts lithified within a non existing matrix. Each of these stages represent an increased energy level within the area of deposition/lithification.

Folk proposes the following figures to determine the precise boundaries for each stage of maturity. Stage I passes into stage II when the sediment contains less than 5 percent detrital clay. Stage II passes into stage III when the sediment clasts has a spread of 1.2 phi units between the 10th and 90th percentile in regards to sediment sorting. Stage III passes into stage IV when the grains attain an average roundness of >0.50, using Krumbeins (1941) visual comparison scale.

As minerals are transported and re-worked, they are constantly being abraded and dissolved. Which minerals break down first is determined by the molecular structure of each mineral, and is covered by Bowen’s reaction series table (1928). Bowen shows that olivine and calcium rich plagioclase are unstable at the Earth’s surface and are susceptible to the elements, with amphibole and pyroxene following the trends. On the other end of the reaction series is quartz, the most stable of all elements due to its tightly packing molecular structure. Composition maturity is based on the fact that not all minerals break down at the same rate, and compares the ratio of stable and non-stable minerals in a sample to determine maturity. An example of a compositionally immature sediment is one with a relatively high percentage of olivine and/or calcium rich feldspar (non-stable minerals) within a sample when compred to the high stable quartz and potassium-feldspar. It is an immature sediment as it hasn’t had enough time for the non-stable minerals to break down and leave only the resistant minerals.

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(Bowen’s reaction series)

Analysing a sediments maturity allows us to determine the environment in which it was deposited based on the energy required to reach each stage of maturity. The less clayey, rounded and well sorted sediment is much more mature in respect to a clayey, angular poorly sorted sediment as it has had more energy being input into the system, allowing the sediment to be reworked progressively. Finding large traces of unstable minerals in your sediment indicates compositional immaturity, as the unstable minerals have yet to degrade and removed from the sediment. A very compositionally mature sediment should contain very high percentages of quartz. This indicates that all other elements have been eroded, only to leave quartz, the most stable element.

References

Allaby, A., Allaby, M. Textural Maturity. (1999). Retrieverd from http://www.encyclopedia.com/doc/1O13-  texturalmaturity.html.

Folk, R. L. (1951). Stages of Textural Maturity in Sedimentary Rocks. Journal of Sediment Petrology, 21(3), 127-130.

Nicholas, G. 2007. Sedimentology and Stratigraphy second edition. West Sussex, UK: Wiley-Blackwell.