Unravelling the Earth’s past life is a tedious work and requires decent efforts. Geologists, particularly sedimentologists, are concerned with revealing the Earth’s history by just looking at the sediments. They should have a keen eye to look for many things within the small sediments.
Sediments are derived from the parent rock by the processes of weathering and erosion. There is a very thin line of demarcation between weathering and erosion. Weathering is the breakdown of rock material at the place of origin i.e. in situ and erosion is the breakdown of the rock material, transportation of such fragments and finally the deposition. There are three eroding agents: water, wind and glacier.
Sediments can be helpful to geologists as they provide certain clues to build the history of processes which have occured within a region. By gaining knowledge about the past processes, an insight about the future processes can be interpreted.
A well-known geologist named James Hutton has proposed the theory of uniformitarianism. The statement he used in the theory was
“Present is the key to Past”
It means that by looking at the present situations geologists are able to interpret the past processes.
Sediments derived from parent rocks can give clues about provenance, paleoclimate, palaeogeography, transportation distance, agency of transportation and duration of transportation.
Provenance is the study of obtaining information about the source region from the sediment nature. Such studies can provide information from “source to sink”. There are two adopted methods: Petrological methods and Geochemical methods.
Petrological methods include QFL ternary diagrams, heavy mineral assemblages, clay mineral assemblages, reworked fossils, magnetic properties, mineralogical composition of sediments.
Geochemical methods include radioactive dating methods such as Zircon U-Pb dating, Zircon U-Pb and Hf isotope dating, Apatite fission track, Zircon fission track, Zircon-He and U-Pb double dating etc.
Certain deposits are exclusively associated within certain climatic conditions.
- Carbonate and reef deposits are habitual to warm conditions. Such deposits occurred in the tropics.
- Evaporite deposits such as halite, gypsum etc form in hot and arid conditions. Evaporites require excess evaporation along with a restricted influx of seawater.
Instead of having high temperatures at the equator, evaporite deposits are not found there because of high precipitation.
- Coal and oil deposits need accumulation of the organic debris. Such organic debris are under the action of decay and decomposition. For degradation, a warm and humid climate is required.
- Bauxite and Laterite deposits forms due to residual weathering. It requires oxidising conditions.
- Glacial deposits such as tillites or ice caps indicates the glaciation during the past periods.
Thus different types of deposits and sediments represent particular climatic conditions. Thus, geologists are capable of deriving the paleoclimatic conditions.
After deriving paleoclimate now it’s time to reveal palaeogeography. Palaeogeography can be interpreted by either utilizing climatic conditions or by taking help from palaeomagnetism.
By using climatic conditions, an idea about a broad region on the earth can be obtained whereas with the help of palaeomagnetism accurate location with respect to the pole can be obtained.
Palaeomagnetism is the most accurate technique to determine palaeogeography. Certain sediments retain magnetism and the magnetic field direction is assigned along the magnetic field prevailed during the time of formation of rocks.
The magnetic field direction is retained in the magnetic minerals such as magnetite and hematite. Sometimes secondary magnetism is to be removed before looking for primary magnetism.
Mineralogical composition can be helpful in determining the source of sediments but to a limited extent. If the sediments are derived from a homogenous parent rock, it can be identified through a single mineral.
For example: if in a certain area, sand grains or quartz is obtained we can interpret the source can be a nearby quartzite, granite or sandstone.
However, interpreting source area from a mixture of sediments is complicated. In such situations another method of interpreting is to be adopted.
The shape of the mineral represents the transportation distance and duration. Also the shape of the sediment depends upon the original shape of the fragment supplied by weathering.
For example: the quartz of a granite will supply sharp and angular fragments; feldspars will provide grains bounded by parallel cleavage fractures; the micas will give rise to thin, irregularly bounded flakes; zircon and apatite may be liberated as perfectly shaped crystals.
It is normally assumed that the material supplied by weathering is initially angular. Thus by the transportation due to friction, wear and tear occurs within the sediments and the sediments start to become smooth and rounded.
If the minerals are rounded in shape, it indicates that the mineral grain has transported a large distance and has transported for a longer duration before deposition.
If the minerals are angular in shape, it indicates that the mineral grains have transported a short distance and have transported for a short period of time before deposition.
The degree of rounding depends on the grain size. The larger the grain size, the rounder the particle. It also depends on other factors such as the weight of the particle. Heavier particles are more likely to be rounded.
Hardness of a mineral resists the process of rounding. The softer minerals are more susceptible to rounding as compared to harder minerals. Because of this reason, feldspars show a greater degree of rounding as compared to the quartz.
The grain size is also able to reveal the amount of transportation undergone by the detritus. A longer period of shifting to and fro by wind, waves, rivers or glaciers means a greater amount of wear on each particle.
The impact of particles upon each other or upon rock surfaces is more in air than in water and hence aeolian deposits (deposits transported by air) are finer than aqueous deposits when undergone an equivalent amount of transport.
Sorting helps in determining the agency of transportation and the energy with which the transportation has been carried out by the transporting agency. Long drifting to and fro by wind and water tends to separate the sediments according to the mass and surface area.
- Wind is an efficient sorter of sand grains. In desert regions, lighter mica flakes and dust are blown far away and the remaining sands are shifted and re-shifted until mineral uniformity is obtained.
- Transportation through rivers or along shores are effective in producing clean, graded and uniform deposits. The manual mining method such as panning or ground sluicing for obtaining gold or other heavy constituents from sands or gravels take advantage of the sorting effect of river channels.
- Glaciers also help in transporting sediments. However, the deposits transported by glaciers are poorly sorted and thus are a mixture of clay, silt, sand and gravel. However, such a zone forms an impermeable layer thereby holding water.
The deposits can also disclose the depositional environment.
High energy deposits: