Goals:
Understanding the generation of sedimentary rocks and their utility as markers of past surface conditions

The iconic backdrop of Boulder, Colorado, The Flatirons!
Artist Jerry McElroy, in a famous modern painting, humorously suggested that they were constructed in 1931.

by Jerry McElroy

Of course, sorry Jerry!  No.
These are sedimentary rocks!
The rocks that comprise the Flatirons  were deposited by rivers and streams that flowed off the Ancestral Rocky Mountains, around 290-295 million years ago.
How do we know this?– There are many details, but the gist of the matter is that these rocks look “for all the world” like material that we find in fast moving rivers near mountain fronts.  They contain layers of silt/sand, and layers of cobbles (conglomerate), and patterns of bedding (e.g. cross-beds) that show flowing water.  They contain the mineral material characteristic of an eroding mountain range (high K-feldspar content and angular cobbles that have not experienced lengthy transport).  And, finally, in places they contain fossils indicative of this time period, and are also sandwiched between well-established rock layers that are older and younger than their proposed age of 290-295my.

Ryan Franz, Anton Krupicka and Stefan Griebel climb the Third Flatiron
photo by Eric Lee, Boulder Weekly Magazine

OK, Let’s move onto sedimentary rocks and their formation.

 

Lithification—Sediments to Sedimentary rocks

Sedimentary rocks are rocks made of lithified sediment. Sediments are grains of rocks, minerals, or mineraloids deposited on the surface of the earth. Reflect on the rock cycle for an indication of the relationships between the rocks that erode to become sediments and sedimentary rocks. For sediment to become sedimentary rock, it usually undergoes burial, compaction, and cementation.

Clastic sedimentary rocks are the result of weathering and erosion of source rocks, which turns them into pieces—clasts—of rocks and minerals. Once they become pieces, these clasts are free to move away from their source rock and they usually do. They are most often transported by water and deposited as layers of sediment.
Clastic sedimentary rocks– also sometimes called “detrital” from the word detritus, meaning worn down and broken material.

The burial stage of lithification involves the deposition of more sediment layers top of those that had been deposited earlier. In a sedimentary basin where sediment is being deposited, it is common for subsidence (lowering) of the basin to be taking place, either because the crust and lithosphere beneath it are subsiding into the mantle to some extent, or because the surrounding uplands are undergoing uplift relative to the basin, or both. This allows thousands of feet of burial, in some cases tens of thousands of feet of burial, to occur.

As sediments are buried, the weight of overlying material exerts pressure, causing compaction of the sediments. The pressure, known as lithostatic pressure, “squeezes” the sediments from all sides into a smaller volume. Lithostatic pressure packs the sediment grains closer together and reduces the porosity – space between the sediment grains.

Some chemical sedimentary rocks are rock as soon as the sediments have been deposited by crystallization of minerals from substances dissolved in water, at the earth’s surface. Examples include rock salt and other evaporite deposits. These sediments of salt crystals and other minerals form sedimentary rock without having to undergo burial and compaction.

During burial and compaction, sediments will undergo some amount of cementation. Cementation refers to the growth of new minerals between the sediment grains. These new minerals bind the sediment grains together. One form of cementation is growth of quartz rims on the surfaces of pre-existing quartz grains in the sediment. This new mineral growth is a result of water in the pore spaces that dissolves and precipiates quartz. A second common cementing mineral is hematite, a red or rust-colored iron oxide mineral, which precipitates onto the sediment grains from a combination of dissolved iron and oxygen from water in the pore spaces. A third common cementing mineral is calcite, which also precipitates from ions dissolved in the water in the pore spaces during lithification. Although there are other cementing minerals, quartz, hematite, and calcite are common cementing minerals that grow between or on the surfaces of the original sedimentary grains.

Generalized steps from source to sedimentary rock:
weathering >> erosion >> transport >> deposition >> burial >> compaction >> cementation >> sedimentary rock

Minerals and Sedimentary Rocks

Any type of rock containing any type of mineral will undergo weathering and erosion at the earth’s surface. However, some minerals are more stable than others in earth’s surface environments and are more likely to be found in sedimentary rocks.

Grains of clastic sediment, which are called clasts are winnowed and modified during the weathering-to-deposition process. Weathering of minerals will gradually eliminate the physically weaker and chemically more reactive minerals, increasing the relative abundance of more resistant minerals. Quartz tends to become increasingly abundant during the process, due to its common occurrence in the source rocks combined with its hardness and lack of cleavage, which makes it resistant to breaking down physically. Quartz is not easily dissolved or chemically altered, so it is resistant to breakdown by chemical reactions as well. That is why beach sand is often more rich in quartz than any other mineral. Sediments rich in quartz are considered mineralogically “mature” because they have been subjected to a longer interval of physical and chemical modification during the erosion-to-deposition process. Mature sediments are deposited farther from their source rocks in space and/or time than immature sediments.

Feldspars are the most common type of mineral in earth’s crust and are also abundant in many clastic sediments and sedimentary rocks. Although feldspar is a fairly hard mineral, it does cleave (split apart) and is chemically reactive, especially in the presence of water. The most abundant product of chemical reaction of feldspar and water is clay minerals. During the erosion-to-deposition process, clastic sediments lose feldspar and gain a larger proportion of clay.

Like the FLATIRONS, pictured above… certain SED RKS contain a lot of feldspar, and are called ARKOSIC or ARKOSE.
This is a little odd, given the above paragraph (re-read it!)–
Because feldspar (K-feldspar, in particular) breaks down rather quickly into clay minerals, during normal sedimentary transport.
So, a sedimentary rock that contains considerable feldspar is one that has eroded a region rich in feldspar (typically igneous or metamorphic highland/mountain) but has not experienced much distance of transport!
Arkosic sediments are therefore markers of the erosion of nearby mountain ranges.

 

Other minerals such as amphiboles, micas, and carbonates are relatively soft and chemically reactive and tend to be scarce or absent as sediment grains in mature clastic sediments, although calcite may be present in clastic sedimentary rocks as a secondary, cementing mineral that grew during lithification.

Minerals in chemical sedimentary rocks precipitate from water and usually remain in place or are not transported far before lithification. Such mineral sediments are subjected to little, if any, erosion and transportation. Therefore, the minerals in chemical sedimentary rocks are not winnowed during the weathering-to-deposition process as are the minerals in clastic sedimentary rocks. In some cases, during the formation of chemical sediments, the minerals may change as a result of chemical reactions. For example, dolostone is a chemical sedimentary rock that forms in certain coastal environments by alteration of precipitated calcite to dolomite.

Besides minerals, mineraloid solids occur in some chemical sedimentary rocks. For example, the carbonaceous material in coal is an organic mineraloid rather than a mineral. Another example, opal, is a chemical sedimentary rock that does not have a fully developed crystal lattice and therefore is a mineraloid.

Sediments

Sandstone is one of the common types of sedimentary rocks that form from sediments. There are many other types. Sediments may include:

• fragments of other rocks (detritus) that often have been worn down into small pieces (clasts), such as sand, silt, or clay.
• organic materials, or the remains of once-living organisms.
• chemical precipitates, which are materials that get left behind after the water evaporates from a solution.

Rocks at the surface undergo mechanical and chemical weathering. These physical and chemical processes break rock into smaller pieces.

• Physical weathering simply breaks the rocks apart.
• Chemical weathering dissolves the less stable minerals.

These original elements of the minerals end up in solution and new minerals may form.

Sediments are removed and transported by water, wind, ice, or gravity in a process called erosion (Figure 2).

Streams carry huge amounts of sediment (Figure 3). The more energy the water has, the larger the particle it can carry. A rushing river on a steep slope might be able to carry boulders. As this stream slows down, it no longer has the energy to carry large sediments and will drop them. A slower moving stream will only carry smaller particles.

Sediments are deposited on beaches and deserts, at the bottom of oceans, and in lakes, ponds, rivers, marshes, and swamps. Avalanches drop large piles of sediment. Glaciers leave large piles of sediments, too. Wind can only transport sand and smaller particles. The type of sediment that is deposited will determine the type of sedimentary rock that can form. Different colors of sedimentary rock are determined by the environment where they are deposited. Red rocks form where oxygen is present. Darker sediments form when the environment is oxygen poor.

Sedimentary Rock Formation

Accumulated sediments harden into rock by lithification, as illustrated in Figure 4. Two important steps are needed for sediments to lithify.

1. Sediments are squeezed together by the weight of overlying sediments on top of them. This is called compaction. Cemented, non-organic sediments become clastic rocks. If organic material is included, they are bioclastic rocks.
2. Fluids fill in the spaces between the loose particles of sediment and crystallize to create a rock by cementation.

The sediment size in clastic sedimentary rocks varies greatly (see table 1).

Table 1.  DETRITAL SED ROCK–  particle sizes and features
Rock	Sediment Size	Other Features
Conglomerate	Large	Rounded
Breccia	Large	Angular
Sandstone	Sand-sized
Siltstone	Silt-sized, smaller than sand
Shale	Clay-sized, smallest

When sediments settle out of calmer water, they form horizontal layers. One layer is deposited first, and another layer is deposited on top of it. So each layer is younger than the layer beneath it. When the sediments harden, the layers are preserved. Sedimentary rocks formed by the crystallization of chemical precipitates are called chemical sedimentary rocks. As discussed in the “Earth’s Minerals” chapter, dissolved ions in fluids precipitate out of the fluid and settle out, just like the halite in Figure 5.

Biochemical sedimentary rocks form in the ocean or a salt lake. Living creatures remove ions, such as calcium, magnesium, and potassium, from the water to make shells or soft tissue. When the organism dies, it sinks to the ocean floor to become a biochemical sediment, which may then become compacted and cemented into solid rock (Figure 6).

Sedimentary Rock Textures

In clastic sediments the sedimentary texture includes the grain size, rounding, and sorting of the grains, all of which are related to what happened to the sediment during the weathering-to-deposition process.Because the processes that lead to the formation of chemical sedimentary rocks do not involve the weather-to-deposition process, there is no widely agreed-upon texture scheme that applies to chemical sedimentary rocks.

Clastic Textures

Clastic sedimentary textures are described in terms of the size of the sediment grains, how round they are, and how well they are sorted.

Grain Characteristics

The diameter or width of a clastic sediment grain determines its grain size. Specific ranges of grain size have specific names.

• Gravel is an overall name for large sediment grain size, which includes boulder, cobble, and pebble.
• Sand includes sediment grains ranging in size from 2mm to 0.625 mm.
• Silt is the name of a sediment grain that range in size from 0.625 mm to 0.0039 mm.
• Mud is the smallest grain size and is also known as clay. It is important to distinguish the grain size called clay from the mineral called clay. Clay sized grains are too small to see individually without the aid of a microscope.

Rounding

Clastic sediment grains can be round, angular, or in-between (subangular or subrounded). Breccia is a clastic sedimentary rock that by definition consists largely of angular grains of pebble size or larger. Conglomerate, another sedimentary rock, consists largely of rounded grains of pebble size or larger. The schematic diagram below shows classes of rounding, from the left: angular, subangular, subrounded, rounded. Not shown are very angular and well-rounded, which are less common.

Sorting

The extent to which all the grains are the same size is known as sorting. If all the grains are the same size, they are well sorted. Some sandstones are well sorted, and some are not. Most conglomerates are poorly sorted, and consist of a mixture of grain sizes ranging from sand to pebble. The schematic diagrams below represent poorly-sorted, moderately-sorted, and well-sorted sediments.

Other Aspects of Texture

Other aspects of clastic sedimentary texture include the packing of the grains, the porosity of the rock, and the hardness of the rock as a whole.

The packing of the grains applies only to poorly sorted sediments in which the finer clasts form a matrix around the coarser clasts. If the large grains are touching each other, the packing is said to be clast-supported. If the coarse grains are separate and not touching each other, with the finer-grained sediment between them, the packing is said to be matrix-supported.

The porosity of a rock or sediment is the amount of empty space between the grains of sediment.

The hardness of the rock refers to how easily it breaks apart. Shale is harder than claystone, for example.

How to Identify Sedimentary Rocks

Clastic Textures

The common names of clastic sedimentary rocks—conglomerate, sandstone, siltstone, and shale—are based solely on grain size. However, there are more precise names within these broader categories that are based on clastic sediment features other than grain size. Breccia is a clastic sedimentary rock distinguished from conglomerate by the angularity of its clasts.

A clastic sedimentary rock simply made of sand-sized grains qualifies as sandstone, regardless of what minerals the grains are made of. More complete names for various sandstones are based on their mineral composition. Some sandstones are made almost entirely of quartz. Sandstone made virtually entirely of quartz is called arenite. Other sandstones have lots of feldspar mixed with quartz. Such sandstones are called arkose. Other sandstones are a mixture of feldspar, quartz, clay, and small fragments of “lithics,” dark-appearing minerals and rock fragments, which represent mineralogically immature sediments. Such sandstones are technically known as lithic wackes, although geologists often call them by their old name, graywackes, and the rocks are sometimes informally described as “dirty sandstones.”

In using the sedimentary rock classification table that accompanies this section, you will see that the clastic sedimentary rocks are classified on the basis of grain size. Sandstones are rocks made of sand-size grains. Shale is layered sedimentary rock made of fine mud-size grains too small to see with the naked eye.

Chemical Textures

The key to naming chemical sedimentary rocks is the minerals from which they are made. For example, all limestones consist mostly of the mineral calcite. Rock salt is made of salt minerals such as halite. Gypsum rock is made of the mineral gypsum. Chalcedony is made of microcrystalline quartz, quartz grains so tiny that they cannot be distinguished even with a standard optical microscope.

Chemical sedimentary rocks like limestone are susceptible to later modification by dissolution– and this is something that rock climbers appreciate, as it can created little holes and pockets for fingers or hands!

Table 2 shows some common types of sedimentary rocks.

Table 2. Common Sedimentary Rocks
Picture	Rock Name	Type of Sedimentary Rock
	Conglomerate	Clastic (fragments of non-organic sediments)
	Breccia	Clastic
	Sandstone	Clastic
	Siltstone	Clastic
	Shale	Clastic
	Rock Salt	Chemical precipitate
	Gypsum	Chemical precipitate
	Dolostone	Chemical precipitate
	Limestone	Biochemical
	Coal	Biochemical

Sedimentary Rock Classification

Clastic Sedimentary Rocks
Grain Size	Rounding	Sorting	Rock Name
gravel coarse (> 2 mm)	angular	poorly	breccia
	rounded	poorly	conglomerate
sand medium (0.0625–2 mm)	variable	variable	sandstone
	rounded to subangular	well to moderately	quartz arenite— >90% quartz
	subrounded to angular	well to moderately	arkose— >25% feldspar
	subangular to angular	moderately to poorly	graywacke—high % lithics, clay matrix
silt fine (0.004–0.0625 mm)	variable	well	siltstone
clay extra fine (< 0.004 mm)	not visible	well	shale—breaks along bedding planes
			mudstone—lacks bedding planes
			claystone—pure mineral clay, very soft
Chemical & Biochemical Sedimentary Rocks
Mineral	Distinguishing Characteristics	Rock Name	Common Depositional Environment
calcite	softer than glass, reacts to HCl	limestone	warm shallow ocean
calcite	visible fossils, softer than glass, reacts to HCl	fossiliferous limestone (biochemical)	warm shallow ocean
dolomite	white to pink, softer than glass, reacts to HCl only when powdered	dolostone	saline lagoon
calcite	gray or brown, porous, reacts to HCl	travertine	evaporated spring or cave water
calcite	white, powdery, reacts to HCl	chalk	warm shallow ocean
microscopic quartz	conchoidal fracture	chert	deep ocean floor
microscopic quartz	red, conchoidal fracture	jasper	deep ocean floor
microscopic quartz	black, conchoidal fracture	flint	nodules in limestone
microscopic quartz	translucent, layered, conchoidal fracture	agate	nodules in volcanic rocks
microscopic quartz	white, powdery	diatomite	lakes in volcanic environments
carbon	black, soft, low density	coal (biochemical)	heavily vegetated swamp
halite	tastes like salt, cubic crystals and cleavage	rock salt	evaporating water body
gypsum	softer than fingernail	gypsum rock	evaporating water body