•Rocks are the solid physical record of past environments. Our classification of rocks is based on the processes that produced them.
•Igneous (chilled from a molten state) and metamorphic (recrystallized by intense heat &/or pressure) do not contain fossils.
•In contrast, sedimentary rocks (those made by fragments of previously existing rocks transported and redeposited) often contain fossils.
•Sedimentary structures (such mud cracks, raindrop marks, ripple marks, crossbeds, and the like), and other features such as the size, sorting, and roundness of clasts, record the environments on Earth's surface (where living things live and die) at the time the rocks formed.
•Because sedimentary rocks form by deposition of particles that were being transported, they naturally form layers (strata).
Fossils are contained in rocks, and therefore in order to understand dinosaurs one has to understand
how rocks came to be and what information they contain. Rocks are our key to understanding
environments of the past; how those environments (including position of the continents and
composition of the atmosphere!) change over time; and to uncovering time itself.
Naturally occurring cohesive solids comprised of one or more minerals or mineraloids
Are a record of the environment in which they formed
Are generated in one of three primary manners (basis of rock classification):
Formed by the cooling of molten material
Classified by whether they cooled on the surface (volcanic) or while still underground (plutonic) and
Because initial conditions are hundreds or more degrees C, no fossils will be found
Detrital (also called "siliciclastic" and "clastic") sedimentary rocks:
sediment is grains of various sizes weathered from previously existing rock, cemented
together by minerals in the ground water
Very commonly produced in terrestrial and near-shore environments
By far the most common in which dinosaur fossils are found
Basic detrital sedimentary cycle: Uplift to erosion to transport to
deposition to lithification (typically cementation):
A region experiences uplift, pushing up once-buried rocks and exposing them to the surface
This source rock (or more likely, rocks) experiences erosion: weathered away and broken
up by wind, rain, water, plant roots, gravity, etc.
The broken fragments (sediment) are transported by water, wind, glacial ice, etc.
As sediment is transported from the host rock, it undergoes changes
As distance increases, roundness increases (edges get worn way)
As distance increases, sorting increases (different sized particles get winnowed out)
As distance increased, maturity increases (softer and more easily dissolvable minerals breakdown,
leaving only clay and silica (aka sand and silt) in the end)
Sediment is deposited at some location (stream bed, banks of a river, desert,
delta, lake bed, ocean, etc.)
These locations (deserts, flood plains, rivers, lakes, swamps,
coastlines, continental shelves, etc.) are called depositional environments
The particular environment of deposition will leave different types of sedimentary structures: see below
Sediment is lithified (turned to rock): sometimes simply by compression, but more
often by cementation:
Ground water percolates between the grains of sediment
Dissolved minerals in the ground water precipitate out, glueing (cementing) the grains together
The major types of detrital rocks are based on their sediment size, shape, and mixture:
Breccia: big angular chunks mixed in with
smaller sediment; deposited very close to the source rock (and thus not rounded or sorted)
large rounded chunks surrounded by smaller sediment; deposited further from source than breccia, commonly form in
channels of rivers
Sandstone: formed by relatively
well-sorted, well-rounded particles; deposited in many environments (deserts, beaches, river beds, nearshore marine, etc.)
Various sorts of mudstones: very well sorted with
very small particles; deposited in quiet water (lakes, floodplains, off shore, lagoons, etc.)
The Rock Cycle: any rock can be
transformed to any other major class of rock, because rocks are classified by the process in which they are formed. So
if you melt an igneous, metamorphic, or sedimentary rock, and it cools down, you form a new igneous rock; if you recrystallize
an ingneous, metamorphic, or sedimentary rock, you form a new metamorphic rock; and if you erode an igneous, metamorphic, or
sedimentary rock and deposit the sediment from it, you form a new sedimentary rock.
Because sedimentary rocks form where animals and plants lived and died, these are the rocks in which fossils are
common. One of the main categories of information sedimentary rock contain is the paleoenvironment (the conditions
that existed when that rock was formed). The different environments
of deposition represent different paleoenvironments. Some of the clues to discover
The roundness, sorting, and maturity of sediment in a detrital sedimentary rock indicates the relative distance to the source rock
(i.e., breccias form right near their source, mudstones at a great distance)
The energy of the environment (how fast the water or wind was moving) is reflected in different sized
particles of sediment (and, since fossils are buried by the sediment, different types of fossils):
Quiet water (lagoons, lakes, deep ocean, etc): very fine grained sediments (mudstones, fine-grained limestone, etc.),
preserve small details, but unlikely to contain fossils of large animals (which would not be buried before decay sets in)
Faster moving water, wind, etc.: deposit large amounts of sediment (esp. sand) quickly, more likely to bury large objects
(such as large dinosaur bodies)
Environment of deposition often indicated by sedimentary structures: traces left in the
sediment by various processes before lithification. Some common sedimentary structures include:
Mud cracks: indicate
mud that was wet, then dried, then buried by later sediment
Ripple marks: indicate
moving wind or water (current in stream; wave action along shore; etc.)
Raindrop marks: indicates
a wet surface (but exposed to air) that was rained on, then buried
Coal beds: indicates abundant
plant life that was buried faster than it could decay
Putting the sedimentary structure and rock type (lithology) evidence together allows you to reconstruct the
paleoenvironment. For example, this set of dinosaur tracks
are found associated with the impressions of halite (rock salt), indicating that the dinosaur was walking in an
Of course, another main bit of information that sedimentary rocks contain are fossils, the subject of
the next lecture.