I. LECTURE OBJECTIVE

• To acquaint you with how sedimentary rocks may be used to reconstruct past environments.
• Additional resources: 6 web links

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II. INTRODUCTION

• The processes responsible for transporting and depositing sediments leave clues behind, allowing geologists to interpret ancient environments of millions of years ago.
• Geologists can interpret the ancient environments of large regions by assembling these clues from similar aged rocks.
• The environments dinosaurs lived in can be interpreted by examining:
• the type of sedimentary rock,
• sedimentary structures,
• regional changes in the characteristics and distribution of sedimentary rocks,
• other fossils contained within the host rock, etc.

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III. CLUES TO THE PAST

A. Sedimentary Structures- form as a result of physical and/or biological processes during deposition of sediments.

1. strata or beds: distinct layers of rock or sediment separated by bedding planes. Individual beds are characterized by distinctive differences in composition, color, grain size, and other features.  Most sedimentary rocks reveal bedding planes caused by temporal changes in: the energy of the transporting agent, sediment source, sediment type, size, etc. Some bedding planes develop as surfaces of non-deposition (an interruption of sediment supply) or may be erosional surfaces.

a. graded bedding- exhibiting a upward decrease in grain size within a bed.
• turbidites
• streams
• graded bedding image

 
b. varves- finely bedded, often alternating between carbon rich and carbon poor
• A varve consists of two layers. One layer is a thick and light-colored layer of silt and fine sand which forms in the spring and summer and the other layer is a thin, dark-colored layer of clay which forms in the fall and winter.
• Varves form in lakes but not in seawater. Salt and electrolytes in seawater cause a more uniform particle mixture, preventing the formation of varves.
• Varves are more commonly formed in lakes from cool temperate to glacial lakes.
• varve image

 
c. cross-bedding: beds oriented at an angle to the surface they were deposited upon.
• Inclined downward in the direction of flow.
• Formed in aqueous environments under the influence of currents (stream and oceanic environments) and in eolian environments by atmospheric movement of sand, silt, and clay.
• Caused by the migration of ripples or dunes.
• streams
• sand dunes
• image with cross-bedding
 
2. Examples of sedimentary structures:


a. ripple marks: ridge-like linear or curved features formed by wind or water.
• gentle upstream and steep downstream slope
• can indicate current direction in lake, stream, and oceanic environments
 
b. mud cracks: form in clay rich sediment that has dried-out
• indicates periodic drying
• flood plains
• lake shores, etc.

B. FOSSILS

• Fossils found in sediment include:
• the tracks and trails of animals
• microscopic fossils of plants (spores and pollen)
• microscopic fossils of marine animal and plant organisms (diatoms, coccolithophores, radiolarians, foraminifera, ostracods, dinoflagellates, etc.)
• invertebrates (sponges, corals, echinoids, pelecypods, brachiopods, etc.)
• vertebrates (fish, amphibians, reptiles, and mammals)
• plants
• What characteristics of fossils make them useful for interpreting past environments:
• they have a distinct geographic range
• their distribution was influenced by:
• water or atmospheric temperature
• climate variability
• elevation
• oceanic salinity, nutrients, oxygen content, turbidity, etc.
• and more
• Examples will be shown in class of the use of fossils in interpreting paleoenvironment, including:
• lake pollen records are used to infer vegetation and climate changes
• leaves as indicators of climate and elevation
• oceanic microfossils used to infer:
• sea surface temperature
• shifting oceanic boundaries
• extent of sea ice
• communication between oceans (e.g. Central American Landbridge)

C. TYPES OF SEDIMENTARY ENVIRONMENTS

• Charts of continental, transitional, and marine sedimentary environments:   Cited are rock type, composition, color, grain size, grain shape, sorting, inorganic sedimentary structures, organic or biogenic sedimentary structures, and fossils. Charts include continental, transitional, and marine sedimentary environments.

 

D. SEDIMENTARY ENVIRONMENTS MOST OFTEN FAVORING DINOSAUR SKELETON PRESERVATION

1. FLUVIAL ENVIRONMENTS
• Sedimentary rocks formed in fluvial or river environments produced the greatest number of dinosaur remains.
• Sedimentary rocks of this environment are usually sandstones or conglomerates.
• Fluvial systems rapidly buried and preserved dinosaurs caught and killed by flooding waters.
  
  
2. LACUSTRINE ENVIRONMENTS
• Lacustrine or lake environments include sedimentary rocks formed in lakes or around their margins.
• Sediments deposited in this environment are finer grained, typically clay and silt which is later converted to claystone and siltstone.
• Most dinosaur remains found in lacustrine environments were deposited around lake margins where they were killed by predators or mired in mud.
  
  
3. EOLIAN ENVIRONMENTS
• Eolian environments include those where wind is the major agent of sediment deposition.
• Eolian sedimentary rocks are usually fine grained sandstones and siltstones.
• Dinosaur remains are seldom found from eolian derived sedimentary rocks; however, dinosaur tracks are occasional occurrences.
  
  
4. DELTAIC ENVIRONMENTS
• Deltas are large bodies of sediment deposited where streams or rivers empty into lakes or the sea. Sediment accumulates as the energy of the transporting stream decreases where it joins the lake or ocean, dropping its load of sediment.
• Sediment accumulates rapidly on deltas and many dinosaur remains are found in deltaic sediments.
• Deltaic sediments are usually fine grained and converted into shales and siltstones.