Can sedimentary rock for on land?

Submitted by 24_Elsinore t3_10kfowd in askscience

Hi there. I am not a geologist, and most of my knowledge has come from seminars on soils or reading online sources. So I am here to ask a question of geologists that I am having trouble finding online.

Lithification of marine sediments appears to be the most common process in the creation of sedimentary rock. It's quite easy to find information on this. However, I have found very little information on whether there are terrestrial processes that create sedimentary rock on land. One could assume that the relative absence of information means it doesn't happen, but I don't want to assume that just because I can't easily find the information means it doesn't happen. So, are there any types of sedimentary rock that form in terrestrial environments, and what are the processes? I have read about cements and conglomerates, are those typically the only ones?

Generally it seems that lithification happens in marine environments, the rock is then uplifted into the terrestrial environment, where it then erodes back to the sea. Does lithification generally not happen on land because the accumulation of new sediment is not enough to replace or overtake sediment loss to erosion?

I'd appreciate any pieces of information or sources on this. Thanks!

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CrustalTrudger t1_j5qv300 wrote

> Are there any types of sedimentary rock that form in terrestrial environments, and what are the processes?

Yes. For this, it's better to discuss this in terms of depositional environment instead of the exact rock type as the latter are non-unique. Common terrestrial depositional environments are related to rivers or lakes. The types of rocks deposited by those systems will depend a lot on the system in question. For rivers, it can be quite varied. In rivers that are close to a high relief sediment source (e.g., a mountain range), conglomerates (representing river that were carrying gravels) are common. Moving down the system, grain sizes generally fine, so sandstones to silstones would be common. Most rivers will also have large amount of mudstones/shales associated with them as these represent flood plain (overbank) deposits. Lakes are a little less varied, primarily being represented by shales/mudstones, but you can even get carbonates in lakes as well. As you can see, rock types are non-unique, e.g., you can get mudstones associated with fluvial (river), pluvial (lake), or marine settings. Similarly, deep water clastic systems produce rock types and deposits that look pretty similar to terrestrial fluvial systems, but are deposited offshore. There are some types of deposits that are pretty unique to terrestrial environments, e.g., paleosols, loess, ergs, etc., but in terms of raw rock type, these would still be kind of generic mudstone, siltstones, or sandstones broadly. They would largely be distinguished on the basis of primary features. E.g., Erg deposits that become sandstones will tend to have massive crossbeds, like the Navajo Sandstone.

> Generally it seems that lithification happens in marine environments, the rock is then uplifted into the terrestrial environment, where it then erodes back to the sea. Does lithification generally not happen on land because the accumulation of new sediment is not enough to replace or overtake sediment loss to erosion?

So the focus on lithification vs deposition is kind of misplaced. Regardless of whether we're talking terrestrial vs marine, lithification is not happening at the surface, it's only after burial. Depending on location and the progression of environments, marine deposits could be lithified after being buried by terrestrial deposits (during regressions, i.e., sea level falls) or terrestrial deposits could be lithified after being buried by marine deposits (during transgressions, i.e., sea level rise). Thus, it would be better to move your focus away from lithification in this context.

The main difference between marine and terrestrial environments is accommodation space, i.e., the difference in elevation between the current surface of the Earth in that location and the maximum height to which sediments could be deposited (usually sea level, but not always). Effectively, accommodation space is a hole. For marine environments, there's pretty much always accommodation space. For terrestrial environments, accommodation space is more rare and will be typically localized where there is some process driving subsidence, i.e., a force making a hole. If there's no hole for sediment to fill, it will "bypass", i.e., it will keep moving until it reaches an area where it can fill a hole. This ends up meaning that there are lots of areas in terrestrial environments that are not conducive to sediments depositing. But there definitely are areas where sediments can (and do) deposit in terrestrial environments.

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AStrangerSaysHi t1_j5s2n5u wrote

I don't know why reading this answer triggered a childhood memory for me, but: as a kid, I used to love loess... rocks? stones? collections? deposits? clumps?... and would collect bits of it whenever we visited eastern Georgia on vacations (I'm from the dueling banjos part of Georgia for reference). I remember loving the texture of those little bits.

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d49s00 t1_j5r35og wrote

Just to follow on from the discussion re. accommodation space. The current layout of tectonic plates means that very little accommodation space is being created in the continental realm. So the vast majority of modern rivers are reworking, eroding and transporting sediment into the marine realm.

For accommodation space to be created in the continental realm, the continental part of a tectonic plate needs to break apart, known as a rift basin. The East African rift system is one example we can study today. Go have a look on Google Earth and you’ll see the alluvial fans, fluvial systems, sabkhas forming etc

There have been periods in Earth’s geological history where continental rift basins formed everywhere and vast continental sediments were deposited. This happened throughout the Permo-Triassic when the supercontinent Pangaea broke-up. Across Europe these sediments are known as the Buntsandstein or New Red Sandstone. These sequences are usually followed by marine sediments called the Muschelkalk when the accommodation space fell below sea level.

Over time these sediments were buried to a depth where they were lithified but not beyond the point where they are metamorphosed. Staying in Europe, if it wasn’t for the Alpine orogeny (mountain building event), these sediments would have stayed buried. Instead they have been uplifted and exposed. Once you see one Buntsandstein-Muschelkalk sequence, you’ll start to recognise them everywhere!

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CrustalTrudger t1_j5t7j9q wrote

> For accommodation space to be created in the continental realm, the continental part of a tectonic plate needs to break apart, known as a rift basin.

While it's true that continental rifts are definitely locations where accommodation space is made, it's demonstrably false that these are the only continental environments where accommodation space is made either in the geologic past or in the modern. To start, there are several other tectonic environments where tectonic components of subsidence generate (many times very significant) amounts of accommodation space. The largest by far would be in convergent environments where loads associated with the growth of large mountains and/or negative buoyancy from underthrust lithosphere generates significant subsidence and thus accommodation space. Any region with mountain ranges still experiencing active convergence (e.g., Himalaya, Andes, Greater Caucasus, Taiwan, etc) will largely also be actively generating tectonic subsidence in portions of their respective foreland basins and thus actively generating accommodation space.

Other tectonic environments can also generate subsidence and accommodation space, though they tend to be more localized. For example releasing or transtensional step-overs in strike slip systems tend to also produce large amounts of (very localized) subsidence and thus accommodation space. Ridge Basin in southern California is a classic example of this type of environment (though no longer actively forming accommodation space), but there are many releasing step overs and transtensional bends in large, modern continental strike slip systems (e.g., along the San Andreas, North and East Anatolian, Altyn Tagh, etc.)

Outside of tectonic sources of subsidence, sediment deposition tends to beget more sediment deposition because the deposited mass (1) induces flexure of the lithosphere - generating accommodation space and (2) induces compaction / dewatering of underlying sediments - generating accommodation space. Additionally, in environments with large amounts of organic material incorporated into deposits (e.g., marshes, wetlands, etc.), organic decomposition can lead to large amounts of subsidence. Thus, many coastal environments, especially those adjacent to large deltas (which represent massive amounts of sediment being deposited) experience significant subsidence and accommodation space generation. As an example of this, consider the gulf coast of the US and its rapid subsidence. This is in no small part due to the combined effects of (1) continued subsidence from the mass of the Mississippi delta, (2) subsidence of compaction and organic decomposition, and (3) large scale levee systems keeping the Mississippi along its current course and thereby preventing sediment deposition in the adjacent areas. I.e., in part the elevation of the gulf coast is actively lowering because accommodation space is being generated but virtually no sediment is allowed to fill it through artificial means.

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