Sedimentary Rocks

A Basic Introduction

Unless otherwise noted the artwork and photographs in this slide show are original and © by Burt Carter. Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin. Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities. Other copyrighted material is used under the fair use clause of the copyright law of the United States. WtoW globe image by Tatiana Baeva, to whom I am grateful.

Sedimentary Rock – A rock formed of material weathered from a pre-existing rock (the “parent” or “source” rock), eroded and transported to a different place by one or more transport mechanisms, deposited, and then lithified. (Note: even sediment that has undergone all of these things except lithification are usually treated as rocks. The loose sand of Providence Canyon, GA, for example, is

treated as a rock stratum even though it is by no means lithified. It is called the “Providence Sand”.) Most of the “rocks” in the southeastern Coastal Plains are not lithified.

The Providence Sand in its “type section” at Providence Canyon, GA. The uppermost red layer is a weathered residuum of the younger Clayton Formation

From the definition you can see that there are a series of steps that must happen in order to form a sedimentary rock. Those steps must, in fact, happen in a specific order. They are: 1) Weathering of the source rock. (We have covered this in some detail previously.)

2) Erosion of the weathered material. (Remember that until this happens we do not consider the

weathered material sediment. Instead we call it “regolith” or “soil” as appropriate. As soon as it is moved we begin to consider it sediment.)

3) Transport of the sediment. (The system that originally moves the material also begins the transport stage and so these are often considered together. Remember though that there may be a complex history of repetitive erosion and transport by several systems before the sediment arrives at its final resting place. The next few slides elaborate on the erosion and transport steps in the process.)

4) Deposition of the sediment. (This happens, of course at some place where no process is able to remove it to yet another location. Were it to move again it could not undergo the last step in the temporary location. This will be covered in a separate slide show.)

5) Lithification. (A set of processes exist that take loose sediment (if it is loose to begin with) and harden it to rock. These will be discussed individually later in this slideshow.)

EROSION and TRANSPORT Consider Stone Mountain, GA. The igneous rock of this stock is weathering in the hot Georgia Piedmont both chemically (hydrolysis and oxidation) and mechanically (exfoliation, abrasion, impacts, Frost wedging, tree root wedging … ) Imagine what will happen when the loose, exfoliated slab shown by the arrow slides off the steep side of the mountain.

N ~1000 feet

At the first budge, the rock is a large gravel particle, moving under the influence of gravity alone. This type of movement – gravity-driven movement down a slope – is called mass wasting and we will return to it later in the term. Mass wasting will only be a viable transport mechanism until the rock gets to the bottom (red arrow). Beyond that the slope is not steep enough. But this rock will begin its travels via mass wasting.

When the falling rock slams into the rocks already at the bottom of the hill it and they will be broken into smaller pieces (including sand and finer material) by the impact. Even though the large gravel will not be moveable without further weathering, the finer pieces will. Suppose a strong wind is blowing southeastward (blue arrow) when the rock hits and breaks. The smaller broken pieces may be lifted and blown for some distance downwind by this, continuing the transport step via another transport system (red squiggle). Of course this can also happen later, after the sand has sat in place for a while.

Or imagine that the wind never blows hard enough to lift some particles. The sediment is still above the level of the lakes that you see, and as Mr. Bill always says, “mister gravity always wins.” Mud, sand, and even fine gravel can be washed down even fairly gentle hills by sheetwash during and after a hard rain. We will come back to sheetwash later in the term. The point is that the movement of material toward the lakes around the mountain is inevitable. The lakes, of course, are man made. As the story continues we have to back up in time to a point before the dam was built that formed the lakes. Then Stone Mountain Creek and its tributaries flowed freely through their valleys. Once sediment is in a stream its transport becomes much easier.

TRANSPORT (cont)

1

2

3

Stone Mountain Creek flows a few miles eastward and joins the Yellow River (location 1). The Yellow flows southward and joins the Alcovy River between Jackson and Monticello to form the Ocmulgee River (loc. 2), the Ocmulgee flows southward for a long way, then makes an eastward bend. Soon it is joined by the Oconee River near Hazlehurst (loc. 3) to form the Altamaha. All of these are lowland streams with generally low gradients, so the sediment may spend centuries on sand bars along the streams. The ultimate goal of the streams, however, is the Altamaha Delta, just north of Saint Simons Island (loc. 4) and the river system has the potential ultimately to deliver the sediment there as well.

4

TRANSPORT (cont)

When our sediment from Stone Mountain reaches the delta it is not necessarily done. The finer material is likely to remain suspended in the sea water and move out onto the continental shelf or beyond. In addition, waves create a longshore transport system that moves sediment away from the mouths/deltas of streams and parallel to the shore. The barrier islands of the Mid-Atlantic and Southeastern U.S. are all made of sand transported this way. Storms and tidal currents may also move sediment offshore and onto the continental shelf at any point. Some of that sediment may cross the shelf, go down the slope and end up on the continental rise. The shelf becomes very narrow at Miami, FL and any sediment brought down the longshore system that far is funneled to the continental rise anyway.

TRANSPORT (cont)

Part of the point of this story is to introduce to you the idea of an environment of deposition or a sedimentary environment. We have talked about hypothetical sedimentary particles being eroded and transported by these various systems: mass wasting, wind, streams, rivers, deltas, the longshore transport system, systems that move sediment onto the shelf, or across it to the continental rise. (Only the very finest of detrital sediment makes it far onto the abyssal plains). But, of course, the sediment doesn’t necessarily, or even usually, go directly from source to rise in one step. It is deposited at various points along the way, potentially many, many times, and if conditions are right it can stay in one of those places permanently, become lithified, and eventually be studied as a sedimentary rock. The next slide will elaborate on deposition. For now we are still interested in transport. The main point of the story so far is to have you realize that the potential for transport of material is enormous. When we are talking about these systems in detail later in the term we will introduce the notion of base level and specifically for the present point, base level of erosion. For virtually every system that base level is sea level. Anything above sea level is fair game for erosion. Not a square inch of land is exempt. Sedimentary material can be moved by gravity, wind, water, glacial ice, or any combination of them from any point above sea level. Transport begins then and can continue through a complex series of systems before the sediment arrives at its final resting place. That resting place may not even be at or just below sea level. Notice that all these systems operate because of gravity, and that gravity will move things to the lowest possible point unless something interferes. The lowest place to which sediment can move depends upon the type of continental margin that occurs near base level. In the Atlantic that is the continental rise or abyssal plain. In the Pacific it is the bottom of a trench. Not all sediment gets there – something usually interferes – but a lot of it does.

TRANSPORT (cont)

Everything in green here can be considered potential source rock.

Everything in blue can be considered a likely environment of deposition (including the Great Lakes).

Well over 90% of all sedimentary rocks ever studied were deposited in the environments indicated by the light red here. At this point all the rocks we see at the surface and most we see only in boreholes have been studied, so it’s safe to say that virtually every sedimentary rock that was ever formed was deposited here. We do find rocks made of sediment deposited by mass wasting, by wind, by glaciers, by streams (particularly lowland rivers like the Altamaha), environments well above base level. Similarly, there are sediments and sedimentary rocks from the abyssal plains. Both groups, however, are pretty rare. Almost all sedimentary rocks are from coastal (deltas, tidal areas, barriers), continental shelf, or continental rise environments.

Decreasing Probability of Preservation

Decreasing Probability of Preservation

DEPOSITION

The crux of the matter is base level. Expect erosion above it and deposition below it. There are exceptions to bot of those, but they are unusual.

DEPOSITION (Cont.)

Gulf of Mexico

Florida

~1 mile

You should get the idea from mention of the “environment of deposition” (or “depositional environment”) or “sedimentary environment” that the notion of an environment is important in understanding deposition. For a sedimentary particle to move, the system doing the moving must have enough oomph – enough energy – to move it. If the system’s energy becomes lower it may not be able to carry the sediment it was carrying before, and it will be forced to deposit some or all of it. Thus the energy of the transporting medium is an important part of the depositional environment. However, that is never the only thing that influences how the sediment is deposited, or other characteristics of the deposit, or the organisms that inhabit the environment.

This is the delta of the Suwannee River in Florida. Within a narrow coastal zone the one-way flow energy of the river ceases (its slope drops to zero), the banks that once constrained the flow disappear, the purely fresh water of the river system changes to brackish as it mixes with the sea water, and finally , not far offshore, the water is purely salt water. The delta accumulates because of the immediate lowering of energy at the river’s mouth, but the other factors mean that the sediment accumulates in different patterns in the river valley, on the delta, and on the shelf. The change from current to wave and tide energies play a role in how the sediment behaves. The different chemistries play a role. And all these things also control what kinds of plants and animals can survive. The plants and animals influence how the sediment behaves. It is all very complex and so the sediment reacts in very complex ways. The point is that we can eventually look at the sediment and make very good interpretations of what kind of environment allowed it to settle. We study the sediment, the fossils in it, the relationships to other environments, and so on. We will come back a little later and see the basics of how to do all this.

LITHIFICATION – 1) Compaction.

There are three principle ways in which loose sediment can become hardened or lithified. Or that sediment becomes hardened as it forms. Each works best upon certain types of rocks and some don’t work at all, or not very well, for some kinds of rocks. The first type to understand is lithification by compaction. This only works in sediment with a very high content of clay or similarly fine material, and the more fine sediment, the better. It works because the arrangement of charges on crystal faces is not homogenous, and this is particularly true of clay minerals. Some parts of a crystal face are slightly positive, some are slightly negative, or in the case of clay minerals, an entire face may be positive or negative. In large particles this is true too, but in those large sediments the grain size means that gravity and other forces for outweigh the electrical differences. In very small grains, however, the cohesion provided by different charges can hold the rock together. The compaction of mudstones can occur in a couple of ways. Initially the sediments may be up to 90% water. Removal of the water, perhaps only as the grains settle more closely, perhaps by evaporation (below) or some other process brings the grains close enough to begin to cohere and harden. You’ve all presumably seen this in drying mud. The resulting “mudchips” are surprisingly hard. I’ve used the one in the left photo for over three decades to illustrate the process. “Mudcracks” are the cracks between the mudchips in the right-hand photo. Once the water content gets low enough the mud can no longer contract homogenously and so the cracks allow smaller parts of the layer to contract. At this point the edges begin to curl upward. We’ll come back to this because there are not many environments where mud can get wet and then dry out, over and over again. Mudcracks and mudchips are therefore good indicators of those environments.

LITHIFICATION – 2) Cementation.

Most kinds of sediments, and most particularly gravels, sands, and limestones, are lithified by having their grains (pebbles, sand grains, fossils) cemented together by material precipitated in the spaces between them. Geologists make thin sections of rocks by polishing a face flat and gluing it to a microscope slide, then cutting off most of the chip and grinding it very thin – 3 microns, usually. This allows light to pass through most minerals and makes them easy to identify. In this thin section photomicrograph (x55) the dominant mineral is quartz. You can see the outlines of what were originally quartz sand grains (at A, for example) because each one has a thin rind of hematite (B) on its surface. (In color you’d see the reddish color of it.)

From Pettijohn, Potter and Siever, 1972

Beyond that rind, the mineral is again quartz, but that quartz was not part of the original sand grains (C). Had it been, the hematite rind would have been on the outside of that. That quartz was precipitated between the loose grains by groundwater with dissolved SiO4

-4 in it, perhaps from weathering of feldspar or other aluminosilicates. But the story gets even more convoluted. Those grains, and the cement around them, were weathered and eroded from the sandstone and moved somewhere else and deposited. Those grains were cemented by yet another round of quartz cement with a different, finer texture, which you see as the speckled material between the grains. As the cement crystals grow within the intergranular pores they lock the grains together, just as artificial cement locks the gravel together in concrete. This lithifies the rock.

A B

C

D

LITHIFICATION – 3) Crystallization From Water and Recrystallization. This is a photograph of rock salt. If you look carefully you should be able to see the boundaries between numerous crystals. The mineral, of course, is halite. It forms in only one way – precipitation from an evaporating water rich in Na and Cl ions. Seawater works, of course (if it can be evaporated), and so does desert lake water. As the crystals grow larger and touch each other they become locked together in much the same way that crystals become locked in an igneous rock. In fact, this should remind you of phaneritic texture in, say, a granite. The crystals do not appear to be cubic for the same reason that quartz crystals do not appear to be hexagonal in granite. The proper arrangement of atoms is there, but the crystals grew into whatever space was availably, not freely into whatever form they “wished”. This lithifies the rock as it is forming, just like in igneous rocks.

Something similar happens in other rocks as well, and limestones in particular. The modestly high solubility of the calcite (or aragonite) means that when fresh water with Ca and CO3 in it passes through the pores the individual grains can increase in size, taking on a texture very much like this. That process, which also lithifies the rock, occurs some time after deposition of the loose sediment. The rock does not lithify as it forms (though the appearance is the same as if it had), but later. This is called recrystallization.

One Final Point We have talked about sedimentary rocks as if deposition and lithification are the end of the story, but, of course, on a dynamic Earth it never is the end of the story. One of three things may eventually happen to a sedimentary rock. 1) We saw an example of one possibility two slides back. In the photograph there we saw evidence that a sand from some source rock had accumulated and become lithified by coarse cement. The resulting sandstone was then itself weathered and eroded to provide sand grains for a second sandstone. In other words, any sedimentary rock encountered above base level is subject to being the source rock for younger sediments.

2) We alluded to another possibility when we explored the origin of magma. At the Peru-Chile Trench a slab of oceanic crust is being forced beneath the edge of South America. But the basalt of the plate itself is not the only thing going down the chute. It is carrying with it sediments and sedimentary rocks from the seafloor, the trench itself (which gets a lot of sediment from the mountains), and even the edge of the continent beneath the trench. It also carries a lot of water and the ions dissolved in it. Thus sediments and sedimentary rocks can serve as the parent material for igneous rocks.

3) We have not yet gotten to the third possibility, but we soon will. The Himalaya has formed, and is still forming, as a result of the force of India pushing against the southern edge of Asia. The two landmasses are on separate tectonic plates and there is a convergent boundary between them. Before they came into contact there was a trench here, and a volcanic arc, and there was a lot of sediment in that trench. There was also abyssal plain with sediment on it, and a continental shelf on India with a lot on it. You can bet that the heat and pressure that caused these mountains to be uplifted also altered those sedimentary rocks and their low temperature/pressure minerals. Thus sediments and sedimentary rocks can serve as the parent material (protoliths) for metamorphic rocks.