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.

Rocks:

• 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):
  • Igneous
    • Formed by the cooling of molten material
    • Classified by whether they cooled on the surface (volcanic) or while still underground (plutonic) and by composition
    • Because initial conditions are hundreds or more degrees C, no fossils will be found
  • Metamorphic
    • Formed by recrystallization of previously existing rocks due to intense heat and/or pressure
      • Rocks are "baked" and/or "squashed", but not melted (or they would become igneous...)
    • Classified by the direction and intensity of pressure or degree of heat, and the resulting changes from the original rock to the new type
    • Any fossils previously existing in the rocks will likely be obliterated by metamorphism
  • Sedimentary
    • Formed by accumulation and lithification of bits of previously existing rock and/or organic matter
      • Bits of previous rock and/or organic matter are called sediment
      • Because sedimentary rocks form by deposition, they naturally create horizontal beds called strata (singular stratum)
      • Major divisions of sedimentary rock reflect the type of sediment:
        • Biogenic sedimentary rocks: sediment made of solid bits of organic material (whole or broken up) that gets deposited
          • Coal: buried remains of plant life
          • Many types of limestone are made primarily of the shells of once-living things
            • Made of calcium carbonate
            • Typically form in salty water (and thus marine or brackish lakes)
            • Chalk is a type of limestone formed by microscopic algal plates, common in the warm shallow seas of the later part of the Age of Dinosaurs
            • Most of the carbonate mud in the shallow marine environment (and the white beaches of places like the Bahamas and the Caribbean) is the broken down remains of the internal skeleton of marine algae
        • Chemical sedimentary rocks: sediment is in the form of dissolved bits (ions) that precipitate out of water:
          • Some limestones form from crystals of calcium carbonate that precipitate directly out of warm salty water, rather than being biogenic
          • Evaporites (rock salt, gypsum, and related rocks): form when salty water evaporates leaving a residue of salt
        • 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 elements
            • 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)
            • Conglomerate: 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 igneous, 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 paleoenvironments:

• 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.)
  • Crossbeds, basically ripple marks in cross-section, formed by currents (wind or water) pushing sand off the back of a ripple and onto the front (indicate streams, small dunes, etc.)
  • Trough crossbeds, cross-section of big sand dunes where shifting winds produced complex movement of sand (deserts, beaches)
  • 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, a set of dinosaur tracks found associated with the impressions of halite (rock salt), indicates that the dinosaur was walking in an arid environment.

Let's consider some of the main environments in which dinosaur fossils can be found:

• Rivers form the deposits where most dinosaur fossils can be found. (They didn't live in the rivers, but the streams produced the deposits that buried the corpses.) The stream channels might contain conglomerates (if fast enough to roll around pebbles) and sand deposits: the sand will have unidirectional current ripples and crossbeds. When the rivers overflow their banks they deposit mudstones in their floodplains. Rivers migrate over time, burying the floodplains and soils that were there before with newer channel deposits, then covering old areas with new floodplain deposits. Collectively, the sediments generated by rivers, their floodplains, and small lakes are the alluvial plain.
• Deserts are characterized by trough crossbedded sandstones. Desert deposits don't have many fossils (since fewer animals and plants live there), but they are an excellent way to bury the ones that die there.
• Swamps are low-energy environments with a lot of plant growth. As a consequence their sediments are interbedded mudstones and coals.
• Lakes are great depositional environments, as stuff flows into the lakes but generally not out of them. Since lakes are generally slower moving than rivers (duh!!) they do not carry sand; it is silt and clay (i.e., mud) which is the main sediment types. There are two major sub-environments in them:
  • Lake margins: the edges/shorelines of the lake has slightly higher energy, and so has deposits mostly of siltstone. The action of waves produces bidirectional wave ripples. This environment (which gets periodically wetted and dried) is a great one for mudcracks and raindrop marks to form.
  • Lake interiors: the energy is very very low in the bottoms of the middle of lakes, so fine muds such as clays are the only important sediment type. With low energy the sediment is not being moved around much (in fact, there may be almost no oxygen since the water isn't mixing around.) So lake interiors are mudstones with fine layers called laminations.
• Marine Coastal Deposits: Dinosaur fossils are sometimes wind up in coastal deposits. Some coastlines have carbonate sedimentation, and others have detrital ones. Shorelines have similar sedimentary structures as lake margins, and lagoons have similar sedimentary structures as lake interiors. Deeper marine deposits occasionally have dinosaur deposits from corpses that washed off-shore to drop to the sea floor.

Of course, another main bit of information that sedimentary rocks contain are fossils, the subject of the next lecture.

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Last modified: 27 June 2024