The amount of information that can be teased out from a sediment core is pretty extraordinary. So next time you squelch your toes through some squishy mud, spare a thought for where that mud might end up in a few hundred thousand years' time and what story it might have to tell! Going back in time Types of sedimentary rocks There are three basic types of sedimentary rocks: Clastic sedimentary rocks such as sandstone or shale are made of eroded particles of other, older rocks.
Chemical sedimentary rocks such as rock salt or iron ore are made from the precipitation of materials dissolved in water. Organic sedimentary rocks such as coal or some limestones are formed from the accumulation of biological debris. Sedimentology To interpret the history that can be extracted from layers of seafloor mud, you need to know the basic principles of sedimentology—the study of how modern sediments are created, transported, deposited and eventually turn into rock.
Many sediments start their lives on land, but their ultimate resting place is the bottom of the ocean. James St. The sandstone that forms Antelope Canyon, in Arizona, USA is an aeolian deposit, meaning that the sand sediments were transported by wind and deposited to make huge sand dunes that later cemented into rock. The large criss-crossing layers are typical of sand dune deposits.
Pebbles or other sediments that are smooth and rounded have generally been transported quite a long distance. Glaciers can carry sediments of all shapes and sizes. These sediments, called ice rafted debris, are deposited when the glacier melts. Finding fossilised shells in a rock outcrop tells you that the ground you're standing on was probably under the ocean sometime in the past.
A view down the microscope at forams collected from the seafloor near Antarctica, cleaned of the mud in which they were embedded.
A microscope image of a diatom: Chaetoceros affinis. Radiolarians Radiolarians nickname: 'rads' also build elaborate skeletons from silica. A microscope image of a selection of radiolarians, magnified times. Image source: James St. Getting down and dirty: coring the seafloor But hang on.
A piston core barrel is pulled out from the water, with the top of the corer covered in mud. A sediment core, post-sampling. Much of the mud has been carefully dug out at specific intervals down the core. Telling the story of the mud With cores that can be opened up and examined on the ship, one of the first things scientists do is look closely at the colour of the sediments.
Except during periodic storm events, tidal creeks, marshes, and mud flats are low-energy coastal environments where clays and silts are commonly found. This image shows a tidal flat exposed during low tide in Alaska. Sandy beaches are typically associated with passive trailing edge margins, a wide continental shelf, and a sedimentary geologic framework.
These regions often contain barrier structures such as barrier islands, barrier spits, and barrier beaches. Dunes protect inland areas from wind, waves, and storm events. This is how spits are formed: Sediment is carried by longshore drift. When there is a change in the shape of the coastline, deposition occurs. A long thin ridge of material is deposited. This is the spit. A hooked end can form if there is a change in wind direction. Waves cannot get past a spit, therefore the water behind a spit is very sheltered.
The coastline is often eroded inland creating salt marshes. This is less expensive but not always popular with local residents. Happisburgh is an example of a low-lying coastline which has been eroded away very quickly.
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