Primary Petrographic Characteristics of Sedimentary Rocks
Continuation from a Previous blog post: https://steemit.com/geology/@the-geekiest-one/introduction-to-the-elements-of-sedimentary-rocks
As physical sedimentation is produced due to the deposition of fragments of eroded material, the shapes and sizes of the fragments, known as clasts, can tell us much about the environment of deposition. So first we must have some criteria with which to describe (1) clasts, and (2) accumulations of clasts.
Conglomerate.
Sandstone.
- Clasts (sometimes loosely called grains).
i) Grain size- Grain size can be quantified by measuring the average diameter of grains.
Grain size can tell us about the energy of the depositional environment. For example, a coastline which is pounded by large waves is a high energy environment whilst a quiet lagoon is a low energy environment.
The average grain size can change down a river from pebble cobbles and boulders in the high energy source area of a river in mountains to medium and fine sands in the low energy meandering parts of the river close to the low-lying coastal areas.
Also, in deserts, the wind may be able to transport and deposit sand grains, but it will never do this to cobbles and boulders!
Credit: William W. Little, Owner of W.W. Little Geological Consulting; Professor of Geology at BYU-Idaho,
ii) Grain shapes- As clasts are transported they knock against each other and become abraded. Thus, angular grains only survive if the transport distance is short. If the transport distance is great, then the grains become more rounded.
Grains shapes can be divided into very angular, angular, sub-angular, sub-rounded, rounded and well-rounded.
Also, sphericity describes the aspect ratio of grains, with high aspect ratio grains having low sphericity and low aspect ratio grains having high sphericity.
iii) Surface features of grains- Features showing abrasion on the surface of grains can tell us about their mode of transport.
e.g. wind-blown sands (called aeolian sands) impact with each other at high velocities. The have a characteristic frosted surface appearance (with small dents and craters) and very well-rounded grain shapes. They are also well-sorted. The name millet seed sandstone has been applied to sandstones deposited in an aeolian, dune field depositional environment.
iv) The composition of grains- These fall several categories:-
a) Grains eroded from metamorphic or igneous rocks. As you would expect, the minerals that form these rocks form the grains. So quartz grains, feldspar grains, mica flakes are common. Minerals such as olivine, the pyroxenes and amphiboles can also form grains, but because they are subject to chemical breakdown in water, they only survive for very short transport distances. Feldspar and mica last longer, and quartz is the most stable. Quartz grains with unit extinction are undeformed and probably come from an igneous source, whereas, quartz grains with undulose extinction have been deformed and probably come from a metamorphic source. Also, grains can be made of pieces of igneous or metamorphic rock and contain several mineral phases. These grains are called lithic grains. If they recrystallise during metamorphism they can become polycrystalline grains.
b) Grains eroded from metamorphic or igneous rocks. As you would expect, the minerals that form these rocks form the grains. So quartz grains, feldspar grains, mica flakes are common. Minerals such as olivine, the pyroxenes and amphiboles can also form grains, but because they are subject to chemical breakdown in water, they only survive for very short transport distances. Feldspar and mica last longer, and quartz is the most stable. Quartz grains with unit extinction are undeformed and probably come from an igneous source, whereas, quartz grains with undulose extinction have been deformed and probably come from a metamorphic source. Also, grains can be made of pieces of igneous or metamorphic rock and contain several mineral phases. These grains are called lithic grains. If they recrystallise during metamorphism they can become polycrystalline grains.
c) Grains eroded from sedimentary rocks. These grains can obviously include all of the grains types mentioned above.
In addition, material derived from chemical sedimentation can also be eroded into fragments, transported and deposited. So skeletal grains (broken bits or complete fossils usually made of calcium carbonate) and grains showing evidence for chemical precipitation can both be found. Also, bits of organic matter can form grains and these may be fragments of wood, plant cuticles or faecal material.
- Accumulations of grains
Grain size distribution- The grain size classification presumes that most of the grains are very close to the average diameter and the sediment is said to be well sorted. This is not always the case! If there are many grains much smaller and larger than the average diameter, then the sediment is described as poorly sorted.
Sorting can tell us about the mode of transport by which the grains enter a depositional environment. Because large grains are more difficult to transport than small grains, for a given current velocity (be it in water or wind) a certain level of grain size will be deposited or transported. e.g. a windblown sand will be well-sorted whereas sands transported by a flash flood and dumped after the flow ceases may be extremely poorly sorted.
In a poorly sorted accumulation with a bimodal distribution of grain-sizes (i.e. some big grains, some little grains & few intermediate sizes), the largest grains may be surrounded by the smaller grains.
The smaller grains are called the matrix. If the larger grains are separated by large expanses of matrix and are not in contact, then the sediment is matrix supported. If the larger grains are tightly-packed and in contact with one another, then the sediment is said to be clast supported. In sediments which are well-sorted, grains can be tightly packed or loosely packed, features which are referred to as the packing.
If the sediment is divided into thin layers, then it is said to be laminated. If the sediment shows no signs of layering, then it is said to be massive.
- Summary
If sediments are well-sorted and the grains are rounded, then the sediment is said to texturally mature. (textural maturity). If sediments are poorly-sorted and the grains are angular, then the sediment is said to texturally immature.
If the sediment only contains quartz grains (the other minerals having been chemically weathered and destroyed) then the sediment is said to compositionally mature. However, if the sediment contains many different types of grains including feldspars, micas etc., then the sediment is compositionally immature.
Sediments which have been transported for great distances are generally compositionally and texturally mature. Sediments which have been transported for short distances are generally compositionally and texturally immature.
NEXT BLOG POST WILL BE Secondary Petrographic Characteristics of Sedimentary Rocks
http://www.physicalgeography.net/fundamentals/10f.html
http://archives.datapages.com/data/sepm/journals/v38-41/data/040/040002/0695.htm?doi=10.1306%2F74D72018-2B21-11D7-8648000102C1865D
http://www.tulane.edu/~sanelson/eens1110/sedrx.htm