Wednesday, November 2, 2016

Siliciclastic Sedimentary Rocks

Sedimentary rocks are the most interesting rocks in my opinion. I am a sedimentary geologist/petrologist by training meaning that I like learning about both the processes that affect sediments, the sediments themselves, and the sedimentary rocks those sediments become. I find that it is the most amazing thing to be able to look at a sedimentary rocks and glean information out of it that will indicate the depositional environment in which it formed. With that information, a picture develops as to what the landscape looked like, what the ecosystem looked like, and how long ago the sediment was deposited. As we look at sedimentary rocks, keep the question: 'How did it come to be that way?' in your mind. It will enable you to be able to see the data necessary to interpret the depositional environment. In this class, since it is an introductory class, we will be only making very basic interpretations. If you go on in geology, you will take classes that will allow you to make much more in depth interpretations and do the analyses necessary to make those interpretations.

The majority of sedimentary rocks fall into one of two end-member groups:
  1. siliciclastics: rocks composed primarily of silicate minerals such as conglomerates, sandstones, siltstones, claystones, mudstones, and shales
  2. carbonates: rocks composed primarily of carbonate minerals such as calcite and aragonite (a polymorph of calcite meaning that they both have the same chemical formula but different crystal structures) such as bioclastic limestones and oolitic limestones
There can be some mixing of the two types of sediment to produce mixed siliciclastic-carbonate rocks but for the most part, and for this class, they will be considered separate. There are other groups of sedimentary rocks such as the organic sedimentary rocks, those rocks composed entirely of organic material such as peat, lignite, and coal, as well as chert, which belongs to no group.

The siliciclastic sedimentary rocks are those rocks composed of silicate minerals that were weathered out of preexisting rock, transported by wind, water, glaciers or gravity to the location in which they were deposited. These sediment are referred to as clastic sediments. The locations that accumulate sediments are called basins, they are depressions in the crust where sediment is deposited and doesn't erode away. Some basins are on land such as the Permian Basin in West Texas and the intermontane valleys of the mountainous west. The largest and ultimate basin is the ocean.

The transported clastic sediments were then buried over time, compacted, and cemented together (lithified) to form sedimentary rocks. All siliciclastic rocks have a clastic texture and all have a detrital origin. Remember that a clastic texture is a rock texture in which the particles that make up the rock were once loose sediment that was later compacted and cemented together (lithified) to form a rigid framework. A detrital origin means that the particles that comprise the rock were weathered out of a source rock located outside of the depositional basin, transported by water, wind, glaciers or gravity to the depositional basin where the sediments were deposited.

Siliciclastics are named based on the dominate grain size present in the rock as well as it's mineralogy. For example:
  1. a quartz sandstone is composed of sand-sized quartz grains and quartz comprises at least 90% of the rock.
  2. a conglomerate contains primarily gravel-sized grains of quartz, rock fragments, some feldspar, etc.
Mineral composition adds to the name to help distinguish one particular rock from another such as a hematitic quartz sandstone from an arkose. Remember that all siliciclastics have a detrital origin and all detrital rocks have a clastic texture.

Remember our definition of rock texture which refers to the size, shape, and arrangement of grains in a rock. Rock texture can tell us a significant amount about the conditions under which a sediment was deposited. Siliciclastic sedimentary rocks are named based on grain size. Refer to the table below.

This table shows the relationship between grain size, sediment name, and rock name.
In addition to grain size, we also look at grain shape. The grain shape we are most concerned with is the roundness, that is, how common corners and edges are on the grain. Sorting is another important textural parameter to examine. Some transporters of sediment such as wind and water are very good at sorting sediments. Gravity and glaciers, not so much. Refer to the figure below for a visual comparitor for sorting and roundness.

A visual comparitor for sorting and roundness of sediment grains.

Lets look at some examples of the rocks from your boxes:

Quartz sandstone: contains at least 90% quartz, frequently the sediment is well-rounded and well-sorted. In order to have quartz concentrated to this level, the sediment must have been well-weathered and well-traveled. Therefore, quartz sandstones are considered to be mineralogically and texturally mature.

Here is a quartz sandstone in hand sample. We can see that the color is very uniform. It appears to be composed of only one mineral. When we see a sandstone that appears to be only one color (one mineral) it is safe to bet that that mineral is quartz.

This is a quartz sandstone under the microscope. All of the round grains are quartz sand grains. There is a bit of dust, probably clay minerals or iron oxides on the grains but by and large, it is primarily composed of quartz. This photo is courtesy of Suvrat Kher and his blog titled Rapid Uplift.

Here is a photomicrograph of another quartz sandstone. This time, the upper polarizer has been inserted into the microscope. This allow geologists to identify minerals as they will change color when the stage is turned. Quartz turn from white to gray to black as the stage is turned. The quartz grains are the rounded grains in the photo. The extra bit of quartz outside the dust rim is the quartz cement. The cement is what holds the rock together. This photo is courtesy of Suvrat Kher and his blog titled Rapid Uplift.
Quartz sandstones are often interpreted to be beach deposits. Beaches along passive continental margins (coastlines without a plate boundary nearby), tend to have beaches composed of quartz sand. Often times the beach is part of a barrier island such as Santa Rosa Island, FL as shown in the photo below.

Quartz sand at Gulf Coast National Seashore on Santa Rosa Island between Pensacola Beach and Navarre Beach in Florida.

Hematitic Sandstone: is a sandstone that contains enough hematite to stain the rock red. Typically these rocks are predominated by quartz grains.

Hematitic sandstone in hand sample. The scale bar indicates 1 cm.
The above hematitic sandstone in thin section. It was photographed using plane polarized light. Most of the grains here are quartz. They all have a rim of hematite. The scale bar represents 1 mm. 
This photomicrograph is taken of the same spot as show in the photomicrograph above but using crossed polarized light. It is a technique that allows for mineral identification. Quartz will appear in colors ranging from white to gray to black. The scale bar represents 1 mm

Arkose: a sandstone with at least 25% feldspar. The typical source rock for an arkose is granite or gneiss, rocks with significant amounts of feldspar. This rocks is considered texturally and mineralogically immature since the grains have not be subjected to much weathering. The sands below illustrate what the sediment that makes up an arkose may have looked like.

River sand from Yosemite National Park. The source rock was a 
granodiorite which is very similar to a granite except that granites
have more potassium feldspar (k-spar)The minerals present in this 
sand include: quartz, plagioclase, and biotite. If this sand became a 
sandstone, it would be called an arkose. The width of this photo is
10 mm. Photo courtesy of Sandatlas.
This beach sand is from the Canadian Arctic along the  shore of the 
Coronation Gulf, Nanauut. The minerals in this sand include:  
quartz, potassium feldspar (k-spar), andhornblende. The rock that 
would result from the lithification of this sand is an arkose. The width of the photo is 10 mm. Photo courtesy of Sandatlas.

This image depicts the alluvial fan in Rocky Mountain National Park. The 2013 floods moved a significant amount of sediment down the canyon and onto the alluvial fan blocking/covering roads in the process.

The West Alluvial Fan Parking lot is now covered under feet of alluvium brought out of the mountains in 2013 due to the unprecedented amount of rain the area received.

Wide view of the West Alluvial Fan farther down hill on the fan.

Arkose "flat irons" of the Fountain Formation at Roxborough State Park near Denver, CO.

A notched weathered in to some of the exposed Fountain Formation at Roxborough State Park, CO.

Close up of the Fountain Formation rock at Roxborough State Park. the pink potassium feldspar crystals are easy to see in this photo. The larger grains are approximately 1 cm in size. 

In thin section, the quartz, feldspar, and other igneous minerals are easily identified. Additionally, any weathering products such as hematite will coat the grains. The rounding and sorting of the grains is also easier to see than in hand sample.

This view gives a good impression of the range of sizes of sand grains. Some are angular, some are more rounded. The degree of rounding should depend on how much time they have spent being transported in the river system. Clear grains are mostly quartz, cloudy grains are feldspar. Dark iron oxide material forms a thin coating on the grains and makes up part of the matrix or cement, giving the red-brown or purplish colour to the rock. Plane polarized light, field of view is 7 mm across.

Between crossed polars we see the great variety of different types of material making up the grains. There is quartz (e.g. large grain at left), potassium feldspar with "tartan" twinning (top right) and fragments of various rock types including quartzite, sheared quartzite, and Lewisian Gneiss. Some of these rocks occur nearby, such as the gneiss. Others, like the quartzites, do not, and must have been brought down the rivers from much further away. Crossed polars, field of view is 7 mm across.

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