PART 1. MAJOR KINDS OF ROCKS
Before we look at any dinosaurs, we need to be able to identify rocks and understand how they form. You will need to refer back to these notes for several of the later labs. As traditionally classified, there are three basic kinds of rocks:
Sedimentary rocks
Rocks formed from deposition of materials ("sediment"), usually by water (lakes, seas, rivers), but sometimes by wind (deserts). The depositional environment can be characterized by it’s levels of oxygenation and energy. A high-energy environment is one in which the water is fast-moving and agitated, able to carry particles of large grain sizes; typically this occurs in shallow lakes & seas and in rivers, as well as deserts. Conversely, a low-energy environment is one in which the water can be very deep and quiet; it can only carry the finest grain sizes (if any!). The oxygen levels in the depositional environment correlate roughly with the energy levels (high-energy = high oxygen); high-energy water is well-agitated and contains much oxygen, whereas low-oxygen water is usually low-energy (either deep, or stagnant, as in some ponds and lakes).
Clastic sediments
Consist of fragments (clasts) of pre-existing rocks, which have been transported and deposited by physical processes (e.g. sandstone, mudstone).
The grainsize depends almost entirely on the water energy (high-energy => large grainsize). Dinosaur bones are most likely to be found in clastic sediments.
|
Typical grainsize |
Rock name |
Depositional environment |
|
> 2mm |
Conglomerate |
Alluvial fan, rivers |
|
1/16 - 2 mm |
Sandstone |
Rivers, beaches, deserts, deltas, shallow sea/lake |
|
1/256 - 1/16 mm |
Siltstone |
Floodplain, mudflats, deltas, intermediate-depth sea/lake |
|
< 1/256 mm |
Claystone |
Deep sea/lakes |
Chemical sediments
Formed by chemical precipitation from solution (e.g. rock-salt). Chemical deposits are very import to the Earth system.
Limestone (CaCO3) in particular, is the largest reservoir of carbon in the Earth System as we will see in the Carbon cycle lab..
Biogenic, biochemical and organic sediments
Formed by animals' life processes and accumulations of dead bodies (e.g. limestones, cherts, coal). These are always also chemical deposits.
Limestones form in water (lakes or seas) where there is no clastic input (e.g. far from any rivers) or in soils in semiarid regions. If formed in the oceans, they typically contain remains of invertebrates. Dinosaurs are not often found in limestone because they are terrestrial creatures, but when they are they are often very well preserved (e.g. Compsognathus and Archaeopteryx at Solnhofen).
Biogenic material, specifically, organic carbon compounds, are the second largest reservoir of carbon in the Earth system after limestone.
Lithification
After deposition sediments are consolidated into a rock mass by burial and lithification ("making into rock" - e.g. squeezing out pore water and cementing the grains in place). Sedimentary rocks are typically bedded or stratified; beds can vary greatly in thickness.
The three principal properties used in identifying sedimentary rocks are texture, composition and fabric. The texture refers to particle sizes and shapes, and is related both to the condition which prevailed when the sediment was deposited and to the source of the sediment. For example, sandstones consist of fairly coarse grains (you can feel their roughness with your fingers) and are indicative of high-energy environments such as dunes, rivers and beaches. Composition refers to the mineralogy of the particles making up the rock, and of the cement. It is determined by the composition of the sediment source of the sediments, and can be altered by the physical conditions at the site of deposition, as well as during transport. Sedimentary rock fabric, which tells us the most about the environment at the time of deposition, includes such things as the style of layering, burrows, ripple marks and mudcracks.
Station 1
There are several examples of sedimentary rocks: clastic rocks, limestone, coal, and halite (rock-salt). Dinosaur bones are only likely to be found in the clastic rocks so be sure you can distinguish these from the other sediments.
Make brief notes on each hand sample noting color, texture (grain size - whether the grains are visible or not, and how big they are), and any other features that will aid in recognizing these rocks again!
Station 2
This is a slab of ripple-marked sandstone.
1. Where do you think it was deposited?
2. What was its level of energy and oxygenation?
3. What kind of fossils, if any, might you expect to find in this rock?
Igneous rocks
Rocks that are formed from hot (typically >900°C), molten material (magma) that flows up from the deeper part of the crust or the mantle.
Igneous activity is a major source of carbon dioxide to the atmosphere (and thus volcanoes are an important part of the carbon cycle) (Lab 4).
Classification
Igneous rocks are classified on both their grainsize and chemical composition. The grainsize is a function of how rapidly the magma cooled: fine grainsize indicates rapid cooling (i.e. the individual crystals did not have time to grow) whereas large grains indicate slow cooling. Rapid cooling occurs at the Earth’s surface: the rocks are referred to as extrusive or volcanic. Slow cooling occurs within the Earth’s crust; this magma never made it to the surface, hence the rocks are referred to as intrusive.
|
No quartz |
0-20% Quartz |
High Quartz (>20%) |
|
|
Fine-grained |
Basalt |
Andesite |
Rhyolite |
|
Coarse-grained |
Diabase/dolerite/gabbro |
Diorite |
Granite |
Physical form of the igneous rock
Examples of intrusions include sills (horizontal layers of rock) (e.g. the Palisade sill), dikes (vertical intrusions) and plutons (large masses). Extrusives include lava, pumice, and ash from volcanoes. (Note: lava refers only to a particular form of extrusive igneous rocks.)
Station 3
Look at the sample of pumice with all its vesicles.
1. What do you think the vesicles were originally filled with when this rock was
extruded ?
2. Would you expect to find fossils in igneous rocks? Explain your answer.
Station 4
Compare the granite, gabbro, and basalt, making note of the grain size differences and overall color.
Metamorphic rocks
These are rocks formed by the profound alteration of other rocks by the physical effects of heat and pressure, and the chemical effects of contained fluids. The pre-existing rocks may have been sedimentary, igneous, or other metamorphic rocks. Depending upon the amount of heat and pressure affecting the rock, a variety of new metamorphic rocks may be formed. Typically, heating a rock will cause the mineral grains to recrystallize; the higher the temperature, the larger the grains. Subjecting a rock to great pressures will cause laminations to form; the thickness of the laminae will depend on the grainsize of the rock. Usually a rock subjected to great pressure will also be subjected to high temperatures (e.g. during mountain-building events); exceptions are metamorphism occurring at subduction zones where cold rock descends into the mantle. However a rock can be heated without being subjected to pressure, e.g. when magma is intruded into rock.
Examples of metamorphic rocks and their starting rocks are: limestone becoming marble; and shale becoming slate, then schist, and finally gneiss.
Station 5
Look at the shale-gneiss series.
1. What are shales composed of?
2. Under what type of conditions are shales deposited?
3. What is the order of increasing temperature and pressure of these rocks?
4. How can you tell this?
PART 2. FOSSILS
What is a fossil? The word is derived from the Latin fossilis, which means "dug up". For many years any object that was dug out of the ground was considered to be a fossil. By the late eighteenth century, the term had become reserved for prehistoric remains that were buried by natural processes and, subsequently, permanently preserved. Thus, a fossil is simply the remains or traces of a prehistoric organism usually more than 10,000 years old.
Note that almost all evidence of living things is destroyed and does not get fossilized. Only rarely are the conditions present such that living material arrives, more or less intact, into an environment in which it can be preserved.
While organisms or traces of organisms are only very, very rarely preserved, fossils themselves are very, very common. This is because of the vast amounts of time represented by preserved rocks, and because places which preserve rock are the very same ones which tend to preserve organisms. Sedimentary sinks (such as ocean basins or giant lakes) are an obvious example. Everything that does not end up in sedimentary sinks is eventually recycled and lost forever to fossilization.
Styles of fossil preservation
Fossils fall into three classes:
1. Body fossils: The actual remains, or impression of the remains, of a part of a (dead) organism. (e.g. bones, shells, teeth)
2. Trace fossils: The traces of the organism’s activity (made while it was alive). (e.g. worm burrows, dinosaur footprints, coprolites (fossil feces))
3. Molecular fossils: A chemical constitute that is detached from the organism which made it. (e.g. oil, amino acids)
Body fossils
Soft parts
Compressions
Formed by loss of volume, leaving a flattened organism. Decay degrades the organism's tissues leaving a fossil residue of the soft parts & flattened arrangements of the hard parts. Such fossils range in composition from nearly pure carbon to complex waxes and even proteins. Compressions are often associated with low-energy low-oxygen environments, except in the case of plants.
Impressions
Formed when organic matter in a rock, such as a leaf, is removed, leaving an impression of the leaf on the rock.
Replacement
The original material is replaced by another mineral at a very fine scale. It is very rare but occurs with muscle tissue in some low-energy low-oxygen situations.
Hard parts are much more commonly preserved than soft tissues.
Original form
The original unaltered material (i.e. shell or bone). Virtually unaltered fossils can be found in recent rocks, but are progressively more rare in older rocks. This type of preservation is unknown for dinosaurs.
Permineralization
The process by which voids in the hard parts of organisms are filled with minerals. This is a common form of preservation for vertebrates, including most dinosaur bones, and much petrified wood where the wood cell cavities are filled with silica.
Replacement
The process whereby hard parts in their entirety are replaced by another material. The replacement usually occurs by encasement, dissolution and then re-cementation of the hard part. Sometimes replacement occurs in a stepwise fashion via numerous stages of permineralization. In rare cases replacement can record fine structural details. An example is some kinds of petrified wood in which even the woody organic matter is replaced by silica.
Molds and casts (as body fossils)
Molds are impressions of a fossil's surface. Once the organism dies, sediment buries the individual; in time the sediment lithifies. If a fossil shell (body fossil) in rock is exposed to ground water or other corrosive fluids the shell material will tend to dissolve. All the original shell material may dissolve away so that only a cavity remains where the shell was - the walls of which are a mold of the fossil. An external mold is an impression of the outside of the organism. An internal mold is an impression of the inside of an organism, i.e. the material which filled the inside of the shell before the shell dissolved. Later, dissolved substances or sediment may fill the cavity, forming a cast of the original shell (a type of replacement). Note that a mold is the original impression, and a cast is what fills the mold.
NB Generally a microscope is needed to distinguish between permineralization & replacement.
Trace fossils
Footprints - Molds and casts (as trace fossils):
Footprints of animals in sediment may harden as a mold. When this track layer becomes buried by more sediment, the infilling of the track is called the cast. An example of this are the dinosaur tracks in Triassic and Jurassic sediments of eastern North America.
Gastroliths
Smooth, rounded pebbles found in the rib cages of some dinosaurs. These stones probably aided the dinosaur's digestion just as gravel in the gizzards of chickens helps them crush grain.
Burrows
The tunnels or filled tunnels of burrowing organisms in originally soft material such as mud. Such fossils are often exceedingly common.
Borings
of worms and mollusks (i.e. clams) which are formed in hard substrates (e.g. rock, wood). Sometimes petrified wood contains borings.
Coprolites
Fossil excreta; gives a clue as to the diet of ancient animals. These lumpy fossils are often found together with the fish and tetrapods from which they came. Coprolites are preserved via the same mechanisms as body fossils (see above).
Eggs and eggshell
Usually found at or very close to nesting sites of dinosaurs. Preserved by the same mechanisms as body fossils.
Artifacts
Tools, weapons, or any purposefully made item made by our hominid ancestors or our contemporaries. Found in many parts of the world. The first stone artifacts were crude and may be difficult to recognize as such. More recent artifacts, such as arrow heads or Coke™ pop tops, are more distinct in shape and are often polished. If they are less than 10,000 years old they are usually not considered fossils, and if they are very recent they are usually considered garbage.
Environments of body fossil preservation
Surface preservation:
A limited number of fossils are preserved for up to 100,000 years at the Earth's surface and are not buried. Environments in which fossils have been preserved at the surface are:
a) tar pits: e.g. Quaternary age mammals in the La Brea tar pits (Los Angeles, US)
b) permafrost and ice: e.g. in Siberia and Alaska, 25,000 year old fossil mammoths have been found in the frozen ground and eaten.
c) caves: e.g. mummified ground sloths in caves in Arizona.
d) amber (the hardened resin of ancient trees): e.g. insects preserved in amber.
Preservation by burial:
Most fossils are preserved in sedimentary basins (regions where substantial thicknesses of sediment accumulate, as a result of long-term burial). Basins are created by plate tectonic processes, and form in extensional and compressional settings. An example of extensional basins are the Triassic-Jurassic rift sediments of eastern North America, formed when the Atlantic started to open.
Organisms may get buried in one of several different environments in basins, e.g.:
High-energy, high-oxygen environments, e.g. rivers, sand dunes, and beaches. These are characterized by rapid sedimentation and coarse grain sizes (sandstones & conglomerates). However, the material that is deposited is often reworked numerous times so that organisms’ remains tend to be destroyed. The best chance for preservation is if an organism is rapidly buried and the sediment which it is in remains undisturbed; such conditions may result from a storm or a flood. Occasionally, whole organisms can be preserved, but this is quite rare. If the organisms are large (e.g. big dinosaurs), trace fossils (e.g. footprints) may be preserved. This is the most common environment in which to find isolated dinosaur bones.
Low-energy, high-oxygen environments e.g. soils, flood plains, shallow lakes, & most marine regions. Grain sizes are smaller than in high-energy environments (e.g. mudstones, siltstones, fine-grained sandstones; also limestones). The obstacle facing preservation in these environments is that the animal or plants are usually consumed before they have a chance of being buried. Non-consumables, such as bones, wood, and shells, tend to be preserved here if they get buried quickly enough. This is the most common environment for complete skeletons. This environment can also preserve spectacularly detailed trace fossils.
Low-energy, low-oxygen environments e.g. deep lakes, deep ocean basins, coal swamps & tar seeps. Sediments such as clays, mudstones & shales are formed in these environments. This environment tends to preserve whole organisms, often including some soft tissue, sometimes even on the molecular level. Whole organisms are often preserved in a very thin flattened layer. Whole organism preservation is important to paleontology since it can provide rare details of the soft parts of an organism. However, preservation in this environment is quite rare because the environment of preservation is, by its nature, inimical to most life.
Questions
I. Examine the fossils we have displayed (you do not need to sketch them).
1. State whether they are trace or body fossils.
2. What type of preservation is represented (e.g. permineralization/replacement, original material; cast/mold, etc.)?
3. Under what overall conditions do you think these fossils were preserved, and why: e.g. burial under high-energy, high oxygen conditions?
II. Examine the footprints of Early Jurassic dinosaurs in the hallway on the 5th floor.
1. State whether they are trace or body fossils.
2. Under what overall conditions do you think they were preserved, and why: e.g. high-energy, high oxygen conditions?
3. What environment were they probably deposited in? (e.g. on land, in
rivers, lakes, beaches, shallow or deep sea - make an educated guess!)
4. Present a scenario for the production and preservation of these footprints.
NEXT WEEK (Lab 2):