Lecture 15 - Sedimentary rocks and Geologic Time

Lisa Tauxe


You may have noticed that things left outside tend to change; they rust, crack, peel, shrink, wrinkle, and fade, and ultimately fall apart. This happens to rocks exposed to the rain and sun as well. They crack, rust, peel and ultimately fall apart. (I haven't noticed that they shrink, wrinkle, and fade, though). When they fall apart, they become what geologists call regolith. If left alone regolith will become soil. Regolith also readily washes or blows away and the sediment formed by depositing the solid material or precipitating the dissolved material is the topic of today's lecture.


Weathering is the breakdown of rock to form regolith releasing dissolved ions into water. It occurs by mechanical, chemical and biological means. Weathering occurs for the same reason hot soup gets cold, and organic matter rots - the laws of thermodynamics. Things formed at different pressures and temperatures are out of equilibrium with their environment and will slowly come into equilibrium given sufficient time. The time required for rocks to achieve this is dependent on temperature, availability of water and the presence of helpful biological organisms that make a living from the energy released by working the thermodynamic ladder.

Mechanical weathering:

Rocks are inherently fractured. Sedimentary rocks are often layered and igneous rocks have numerous fractures from contracting during cooling. These cracks open up as the rock is brought to the surface, allowing water to get in. Water expands when it freezes, causing the cracks to widen and grow. Also, water absorbed into the rock makes them swell (hydration), enhancing cracking. You can see this kind of weathering very well in the Lagunas as you drive East on Interstate 8.

Rocks also expand and contract as they heat up in the sun and cool down at night. Different minerals expand and contract different amounts in response to heating and cooling causing stresses along mineral boundaries.

Chemical weathering:

Water not only is an agent of mechanical weathering, but also aids and abets what is known as chemical weathering. Rain water is slightly acidic (sometimes a lot more than slightly - as in acid rain. Acid dissolves many minerals; of particular importance in crustal rocks, acid attacks carbonate (a common cement in sedimentary rocks) and feldspar, a principal ingredient of granite, one of the most important types of crustal rock.

In addition to dissolving, some minerals alter from one form to another. For example, iron-bearing minerals literally "rust" in a process known as oxidation whereby oxygen binds with the iron to form iron-oxides (rust). Silicate minerals rich in potassium and aluminum such as potassium feldspar react with water to form clay in a process known as hydrolisis.

Biological weathering:

Living things contribute significantly to the weathering process. Roots and burrowing animals physically disrupt rocks. Plants and bacteria also contribute to chemical weathering of rocks by altering the acidity of the ground water. The ultimate product of biologic agents on rock is soil.

Stabilities of different minerals

Because of their different crystal structures (see lecture and chapter on Mineralogy), different minerals are more or less resistant to weathering. Carbonates will fizz at the drop of a hat (actually at the drop of acidic solutions). Quartz remains when all else is gone (and that is why it is so abundant at the beach!). A quick and dirty guide to mineral stability is to consider how complex the structure is. Minerals made up of individual silica tetrahedra such as olivine weather more readily than framework silicates such as quartz. Minerals bound by loose ionic bonds like halite (table salt), dissolve very rapidly. Also, in igneous minerals, the higher the temperature of formation, the lower the stability at normal Earth conditions. Remember that basalt (silica poor) has a much higher melting temperature than granite (silica rich) and the mafic minerals in basalt alter much more quickly than those in granite during weathering. Within granite itself, feldspar is the first go (since it was the first formed) and quartz, like diamonds, are forever.

The products of weathering: sediments and dissolved material

The results of all this weathering is a pile of loose rock fragments (clastic material) and a lot of dissolved ions: salt, bicarbonate, potassium, etc. The clastics wash (or blow) away, or are carried by glaciers from the source region to some catchment area - a sedimentary basin. While particulate matter gets transported, it weathers further. The particles become smaller, rounder, and less stable minerals continue to dissolve. The dissolved ions are carried in water until they precipitate out, either on land as, for example the spectacular formations in caves known as speleothems, or in the ocean. There plants and animals may use them as the building materials for their houses (shells), or they contribute may precipitate out as salt in evaporite sequences. The ocean is salty because the salt is the last vestiges of the once high and mighty mountains of the world.

Sedimentary Rocks

The term sediment refers to loose particulate material (clay, sand, gravel, etc.). Sediment becomes a sedimentary rock through a process known as lithification. Lithification begins when rocks are buried and become compacted. The pores of the rock then become filled with cement, and certain minerals begin to recrystallize. Sediment is loose material and sedimentary rock holds together when you pick it up.

Sedimentary rocks can be divided into two major categories: clastic and precipitated (or chemical). Since the character of the sedimentary rock is largely controlled by the sedimentary environment, they have a story to tell about the climate, the environment, and even who lived at the time they were laid down. Reading the book of sedimentary rocks is one of the fun things to do in geology.

Clastic sediments

Clastic sediments form from particulate material carried by wind or water. They can form on land by the action of rivers, glaciers and lakes (terrigenous sediments), they can form in the oceans as particulate matter spreads out from the mouths of rivers (deltas), and they can form at the ocean/land interface, otherwise known as the intertidal zone (or beach!).

The major classification of clastic rocks is by grain size ranging from boulders (>256mm) to clay (<.0039mm). (Please study Table 7.5 in your book). The ability of water (or wind) to carry particles depends on its speed. Fast moving water can carry larger clasts. Still water drops its load entirely and the rate that a particle falls through water is a function primarily of its size (smallest particles stay afloat the longest). These principles allow the interpretation of sedimentary structures (such as graded bedding, ripple marks, and various kinds of stratification) to be interpreted in terms of the environment of deposition.

Here are some interesting slides from the web:

Here is an interesting web site..

Chemical Sediments

Chemical (or biochemical) sediments form from precipitation of the dissolved material out of the water. This process is driven by either evaporation, or by biological mediation. When evaporation exceeds the supply of water, the concentration of dissolved material (the salinity) increases. When the water is sufficiently saturated in particular dissolved ions, they tend to crystallized out of the water forming evaporite beds. These are usually rich in salt. This process can also be helped by biological agents, who form calcium carbonate or opaline silica houses or other hard parts. Thus the ocean floor receives a constant rain of particulate matter that is largely formed from the shells of tiny organisms. (Whether or not this material is preserved is a matter of the chemical environment in which it is deposited). Limestones (carbonates) and cherts (siliceous sediments) are made of tiny fossils of marine organisms and is an archive of marine environments and biology through Earth history. Organic sedimentation is a third form of chemical sedimentation. Organic matter such as trees, ferns, (and tree-ferns) may collect in swamps. These may turn into peat which ulitmately transforms into Coal, oil or gas if the conditions are right.

Sedimentary Structures

During and after sedimentation (but before lithification), waves, currents, or even plants and animals disturb it. The latter is a process call bioturbation. Animals and plants leave not only tracks (trace fossils), but bone, teeth, leaves and other remains as fossils. Examine the examples of the following structures in the book and in the links: thick bedding, thin bedding, cross bedding , graded bedding, (see ripple marks (, mud cracks, dinosaur track, copralite, (this is fossizied POOP!),

Sedimentary Facies

Sediments carry with them identifying characteristics, which can be used to interpret the depositional environment. Please check out this excellent link.

What are sediments good for?

The retain the entire history of the Earth - you just need some rules to read the record with.

Rules to do geology by

The history of the Earth is told by the pieces of crust that survive the giant recycling machine of plate tectonics. The problem is that the crust is somewhat like a jigsaw puzzle that has just been dumped out of the box. The pieces may or may not all be there, and they are mixed up all over the place. It takes a clever puzzler to put it back together again. Fortunately, there are some rules that have been devised which help put the puzzle back together again. These are as follows:

Relative and absolute time

Geologists work in two ways. They order the puzzle pieces in terms of relative ages, then they try to put absolute dates on key pieces. Not every rock can be dated in an absolute sense, and the relative sense of ordering cannot possibly give the overall absolute time framework, so the two approaches go hand in hand. First we will talk about setting up the relative framework, then I'll tell you how we put absolute numbers on the geologic ages.

The rules of relative ages:

The following rules may seem obvious to you, and they should be. Nonetheless, they are quite powerful and have served geologists well. Please note that they are not LAWS, but rules-of-thumb and sometimes are not valid.

  • 1)The Principle of Superposition
  • In an undeformed stack of sedimentary rocks, the oldest rocks are on the bottom and the youngest rocks are on the top.

  • 2) The Principle of Original Continuity
  • Sedimentary rocks are deposited in continuous sheets that may have extended some considerable distance. The same rock on the opposite sides of a stream bed can be tentatively identified as the same bed.

  • The Principle of Original Horizontality
  • In general, sedimentary rocks are deposited in nearly horizontal layers. Sedimentary beds which are tilted have been deformed since deposition.

  • The Principle of Unconformity
  • If sedimentary rocks meet at an angle, or are separated by an erosional surface, there is an unconformity, which constitutes a gap in the record. There is a puzzle peice missing. There are four kinds of unconformity: 1) an angular unconformity (shown below) whereby layers meet at an angle, 2) a nonconformity where two different types of rocks meet (for example sediments overlying igneous or metamorphic rocks), 3) a disconformity where an irregular surface indicating erosion occurs between two parallel beds and 4) a paraconformity which is a planar surface between two beds which suggests a period of non-deposition, but no erosion.

  • The principle of inclusion
  • If a rock includes another rock (a xenolith for "strange rock"), the xenolith is older.

  • The principle of cross-cutting relationships
  • If a geologic unit cuts another geologic unit (for example a dike or sill), the one that cross-cuts is younger. This also goes for faults. The fault happened AFTER the sediments were laid down.

  • The principle of Uniformitarianism
  • The physical laws governing the universe operate pretty much business as usual through out geologic history. Processes going on today are similar to those that went on in the past.

  • The principle of fossil succession
  • Fossils show a progressive devlopment through time, and similar fossils indicate similar age rocks the world over.

  • S__T happens
  • Sometimes things go wrong. Meteorites fall from the skies. Ice ages come and go. Catastrophes leave their mark and forever change the face of the Earth.

    Putting it together

    Using the above principles, it is possible to stack up the pieces of the puzzle into an ordered pile. This ordered sequence is known as the Geologic Time Scale:

    Geologic history is broken into time spans with varying durations, from longest to shortest: eons, eras, periods, and epochs. The Phanerozoic eon represents the time during which there were multi-celled plants and animals sufficiently sophisticated to leave hard parts behind (phanerozoic means "apparent life"). The Phanerozoic eon is broken into the Paleozoic, the Mesozoic and Cenozoic eras. These in turn are broken into periods. The end of the Cretaceous period is marked by the demise of the dinosaurs (along with a large fraction of every other living thing) when a large meteorite hit the Earth some 65 million years ago. You should know the geologic time scale to the level of the period. There are many mnemonics to help you remember them. My favorite is a non-sense word: COS DE CARPT-JuCr PEOMP (the PEOMP part are the epochs of the Tertiary period).

    Absolute Ages

    These are put onto the relative time scale by finding suitable horizons that can be dated using radioactive decay (see lecture on rock cycle).

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    Lisa Tauxe