42:161 Historical Geology - Midterm

Historical Geology

• Concerned with the chronology of physical and biological events that occurred in the past

• Concerned with the evolution of the Earth and its life, from origins to present day

• A major branch of geology

Fundamental Challenges Faced in Historical Geology

• Time

• Magnitude or Scale

• Complexity of Natural Systems

Fundamental Principles of Historical Geology

• Superposition

• Original Horizontality

• Lateral Continuity

• Uniformitarianism

• Actualism

• Biologic Succession

• Cross-Cutting Relationships

• Included Fragments

• Catastrophism (concept)

Nicolas Steno

• Formulated:

  • Principle of Superposition

  • Principle of Original Horizontality

  • Principle of Original Lateral Continuity

Principle of Superposition

• “In every undeformed sequence of sedimentary rocks, each bed is younger than the one below it and older than the one above it”

Way-Up Criteria

• Ripple marks

• Cross bedding

• Mud cracks

• Graded bedding

• Tool & groove marks

Sedimentary Structure

• A structure in a sedimentary rock, formed either contemporaneously with deposition (primary) or by sedimentary processes subsequent to deposition (secondary)

Principle of Original Horizontality

• “Sediments come to rest under the influence of gravity and are deposited in layers that were nearly horizontal and parallel to the surface on which they were accumulating”

Principle of Original Lateral Continuity

• “Sedimentary strata, as originally deposited, either extend in all directions until they thin to a feather edge or they end abruptly against the edges of the basin or area in which they were laid down”

Catastrophism

• “A concept that explains the physical and biological history of the Earth by a series of sudden, widespread, catastrophic events”

Principle of Uniformitarianism

• “Geologic processes and natural laws now operating to modify the Earth’s crust have acted in the same regular manner and with essentially the same intensity throughout geologic time, and that past geologic events can be explained by phenomena and forces observable today”

• “The present is the key to the past”

James Hutton

• Widely regarded as the father of modern scientific geology

Factors to Consider When Applying The Principle of Uniformitarianism

• Rates of change

• Time

• Cyclicity

Catastrophic Events

• Have a very high preservation potential in the geologic record

Principle of Actualism

• “-The possibility of appreciable differences in the duration and intensity of geologic processes operating in the past”

• Physical and chemical laws are constant, not their rates

  • Recognises fluctuations in energy

Principle of Biologic Succession

• “Fossil organisms (fauna and flora) succeed one another in a definite and recognisable order, each geologic formation having a different total aspect of life from that in the formations above and below it”

• Only rocks formed during the same age (as the fossil group) could contain the same assemblages of fossils

Charles Lyell Described Principles

• Principle of Cross-Cutting Relationships

• Principle of Included Fragments

Principle of Cross-Cutting Relationships

• “A rock, especially an igneous rock, is younger than any other rock across which it cuts. Or a fault must be younger than the rocks it cuts”

Principle of Included Fragments

• “Whenever two rock masses are in contact, the one containing pieces of the other will be the younger of the two”

• Applied to both erosive and intrusive contacts

Erosive Contacts

• Lag deposit on an unconformity surface; sandstone that contains the lag is younger

Intrusive Contacts

• Inclusions in an igneous rock represent older parent rock material, while the igneous rock that contains the inclusions is younger

Geologic Time

• “The period of time dealt with by historical geology or the time extending from the end of the formative period of the Earth as a separate planetary body, to the beginning of written or human history”

Geologic Time Scale

• An arbitrary, tabular arrangement of the divisions and subdivisions of geologic time”

Systems Naming Basis

• (a) Geographic locality

• (b) Rock type

• (c) Tribal name

• (d) Compromise

A System

• A chronostratigraphic division

• Actual rocks formed or deposited during a specific time interval

• “Relative time”

A Period

• A geochronologic division

• Represents increments of absolute time

6 Formal Geochronological Divisions (Decreasing Hierarchy)

• 1) Eon

• 2) Era

• 3) Period

• 4) Epoch

• 5) Age

• 6) Chron

Geochronological Division - Eons

• Precambrian

  • Hadean, Archean, Proterozoic

• Phanerozoic

Geochronological Division - Eras

• Early-Middle-Late Archean

• Proterozoic

• Palaeozoic

• Mesozoic

• Cenozoic

Geochronological Division - Periods

• Cambrian

• Ordovician

• Silurian

• Devonian

• Carboniferous (Mississippian and Neogene)

• Quaternary

Geochronological Division - Epoch

• Paleocene

• Eocene

• Oligocene

• Miocene

• Pliocene

• Pleistocene

• Holocene

Recent Changes To The Geochronological Division

• Addition of the Ediacaran Period

• Deletion of the Tertiary Period

Precambrian Comprises

• 87% of the geologic time scale (4.6 to 0.541)

Origin Of The Earth

• 4.6 billion years ago

Precambrian-Cambrian Boundary

• At 541 mya

Palaeozoic To Mesozoic Boundary (Permian-Triassic)

• At 252 million years ago

Mesozoic To Cenozoic Boundary

• At 66 mya

Tertiary To Quaternary Boundary

• At 2.6 million years ago

Pleistocene To Holocene/Recent

• At 12,000 years BP

Relative Time

• Geologic time determined by the placing of events in a chronological order of occurrence, especially as determined by organic evolution or superposition

Relative Time Scale

• An uncalibrated geologic time scale based on layered rock sequences and the paleontological evidence contained there in, giving the relative order for a succession of events

Key Geological Principles To The Construction Of The Geologic Time Scale

• Principles of

  • Superposition

  • Original Horizontality

  • Original Lateral Continuity

  • Uniformitarianism

  • Biologic Succession

  • Cross-Cutting Relationships

Geologic Uncertainty

• “Not depended on”, “unknown”, “changeable”, “not precisely determined”

Absolute Time

• Geologic time measured in years

• Determined by radiometric dating methods

Types Of Radioactive Decay

• Alpha decay

• Beta decay

• Electron capture decay

Alpha Decay

• Two protons and two neutrons are emitted from the nucleus

  • Resulting in a loss of 2 atomic numbers and 4 atomic mass numbers

    • Decreases atomic number by 2

Beta Decay

• Fast-moving electron is emitted from a neutron in the nucleus

  • Neutron changes into a proton and consequently increasing the atomic number by 1

  • Increases atomic number by 1

Electron Capture Decay

• Proton captures an electron from an electron shell and converts to a neutron

  • Resulting in a loss of 1 atomic number and no change in the atomic mass number

  • Decreases atomic number by 1

One Decay Step Examples

• Rb-87 to Sr-87

• K-40 to Ar-40

Several Decay Step Examples

• U-235 to Pb-207

  • 7 alpha steps and 4 beta steps

• U-238 to Pb-206

  • 8 alpha steps and 6 beta steps

Half Life

• The time it takes for one-half of the atoms of the original unstable parent element to decay to atoms of a new, more stable daughter element

• Constant, regardless of external conditions

Radioactive Decay Occurrence

• Geometric rate rather than a linear rate

Most Effective & Commonly Used Radioactive Isotope Pairs For Dating

• U-Pb

• K-Ar

• Rb-Sr

Dating Methods on Sedimentary Rocks

• Leaching problems

• Age determination measures the age of the parent rock source, not the sedimentary rock age

Dating Methods on Metamorphic Rocks

• “Atomic clock” could be re-set by metamorphism

• Age determination could represent the time of metamorphism and/or time of rock formation

Types of Dating Methods

• Uranium-Lead series

• K-40 to Ar-40

• Rb-87 to Sr-87

• Fission track dating

• Radiocarbon dating

• Tree ring dating

Uranium-Lead Series

• Involves 3 isotopes

  • U-238 (99.28%)

  • U-235 (0.71%)

  • U-234 (0.006%)

K-40 to Ar-40

• Electron capture decay

• Typically used for dating fine grained volcanic rocks via whole rock analysis

• Minerals and rocks that can be dated:

  • Glauconite

  • Muscovite

  • Biotite

  • Hornblende

  • K-feldspar

  • Whole volcanic rocks

Rb-87 to Sr-87

• Single beta emission

• Effective for dating the oldest rocks on Earth as well as meteorites

• Minerals and rocks that can be dated:

  • Muscovite

  • Biotite

  • K-feldspar

  • Glauconite

  • Whole metamorphic or igneous rocks

Fission Track Dating Method

• Age of a sample determined on the basis of the number of fission tracks present and the amount of U that a sample contains

• Most useful for dating samples 40,000 to 1 million years ago

• Important for dating human remains and artifacts

Radiocarbon Dating

• Beta decay

• Uses 3 isotopes of carbon

• Useful in archaeology

Tree Ring Dating

• Useful for dating recent geological events

• Each ring represents 1-year of growth

• Time scale goes back 14,000 years

Fossil

• Remains, traces or imprints of once living organisms preserved in the Earth’s crust since some past geologic or prehistoric time

Paleontology

• “Study of all ancient forms of life, their interaction, and their evolution”

• “Study of life throughout geological time”

Factors That Affect Conditions Favouring Fossil Preservation

• Possession of hard parts

• Quick burials

Places Where Quick Burial Can Occur

• Flood plain deposits

• Lake sedimentation

• Tar pits

• Volcanic ash

Main Types of Preservation/Fossilisation

• Unaltered

• Altered remains

  • (Usually the latter)

Preservation/Fossilisation - No Alteration

• Actual preservation without alteration

• Not common

Preservation/Fossilisation - Altered Remains

• Altered during fossilisation, sometimes referred to as petrification

• Can be classified into groups:

  • Permineralisation

  • Replacement

  • Carbon residues, Destructive distillation

  • Recrystallisation

    • (Classification depends on how much original material size, shape and structure has been preserved)

Preservation/Fossilisation - Altered Remains - Permineralisation

• Original organic material was porous

• Mineral matter carried by percolating solutions may be deposited in voids without altering original material

• Chemical precipitation into pore spaces

• Resulting fossil is heavy and dense

Preservation/Fossilisation - Altered Remains - Replacement

• Under certain conditions hard organic remains are dissolved and replaced by mineral matter of a different type

• Dissolved by ground waters → forms a replica of the original

• Complete destruction of small details

• Can be broken down into two types:

  • Pseudomorphic & Histometabasic

    • Common replacing agents are:

      • Silica (silification)

      • Dolomite (dolomitization)

      • Pyrite (pyritization)

Pseudomorphic Replacement

• Original microstructure has been destroyed

• Only gross shape is preserved

Histometabasic Replacement

• Replacement may be very delicate so that even small structural details are preserved

Preservation/Fossilisation - Altered Remains - Carbon Residues, Destructive Distillation

• Volatile constituents of organic material (H, O, N) are driven off and a carbon residue is left behind

• Solutions or chemical action

• May remove part of the original constituents of a buried organism without adding anything

  • Simplifies the chemical composition

Preservation/Fossilisation - Altered Remains - Recrystallisation

• Hard to distinguish from replacement

  • E.g. Aragonite recrystilises to calcite

Mold

• Impression, or imprint, of an organism or part of an organism in the enclosing sediment

• Impression of the exterior of the original organism

  • Shows shape and surface markings

Casts (Natural)

• Replica of an organic subject

  • Filled with mineral matter

• Will exhibit the same form or ornamentation as the original, but the internal structure is not preserved

Steinkern

• "Rock material consisting of consolidated mud or sediment that filled the hollow interior of a fossil shell or other organic structure”

  • E.g. Shell is dissolved at later date, sediment filling is all that remains, invertebrate shells enclose a hollow space, could be left empty or filled with sediment

Trace Fossils

• Include:

  • Tracks

  • Trails

  • Burrows

  • Borings

• Result of feeding, dwelling, locomotion, resting and/or death

• Preserved in sedimentary rocks

• Can be used to obtain clues about animals characteristics

• Also used in facies analysis and facies modelling

Bioturbation

• The churning and stirring of a sediment by organisms

• Has an Index: 0 (no activity) - 6 (Thorough activity)

• Can describe the diversity, size, and abundance of individual trace fossils

• However, tends to destroy physical sedimentary structures

Ichnology

• Study of trace fossils

• Very useful in environmental analysis

  • Powerful when integrated with sedimentary and stratigraphy

Fossil Uses

• Age

• Correlation

• Paleo-Environmental Analysis

• Record of Life & Evolution

Fossil Uses - Age

• Specific suites of macrofossils and microfossils are characteristic of certain time periods

• Fossil assemblages are more reliable than individual species for determining age

Guide/Index Fossil

• A fossil with a wide geographic distribution but narrow stratigraphic range (i.e. narrow age range)

• Useful in correlating strata and for age determination

• E.g. Scaphites hippocrepis (Cretaceous ammonite)

Fossil Uses - Correlation

• “Demonstration of correspondence in character and in stratigraphic position between geographically separated stratigraphic sections or rock bodies”

• Fossils used in the construction of the geologic time scale

• Based on the Principle of Biologic Succession

Fossil Uses - Paleo-Environmental Analysis

• Widely used to determine:

  • Paleo-climate

  • Paleo-ecology

  • Paleo-geography

  • Paleo-depositional environments

Fossil Uses - Record of Life & Evolution

• Evidence of organic evolution

• Succession of fauna and flora

Taxonomy

• The science of naming, describing and classifying organisms

• Many methods of classification

  • Classification schemes are continually changing

  • Dynamic

Kingdom Bacteria (Monera)

• Superkingdom Prokarya

• Prokaryotic unicellular organisms

• Lack a membrane-nucleus

  • Bacteria and blue-green algae

Prokaryotes

• Organisms that lack membrane-bounded nuclei and other membrane-bounded organelles

Kingdom Protoctista (General)

• Old Protista kingdom

• Superkingdom Eukarya

• Unicellular eukaryotic organisms

  • Possess a true nucleus and well defined chromosomes

Eukaryote

• A cell containing a true nucleus, enclosed within a nuclear membrane, and having well-defined chromosomes and cell organelles

• Occur in 4 kingdoms:

  • Protoctista

  • Fungi

  • Plants

  • Animals

Kingdom Fungi (General)

• Possesses similarities to both animals and plants

• Feed on dead or decaying organic material

• Multicellular eukaryotic organisms

Kingdom Animalia (General)

• Animals

• Devour food for energy

• Multicellular eukaryotic organisms

Kingdom Plantae

• Plants

• Plant-like photosynthesisers

• Multicellular eukaryotic organisms

Kingdom Protoctista Characteristics

• Single celled organisms characterised by the absence of tissues and organs

• Majority are microscopic, a few larger

• Reproduce by fission

• Eukaryotes

• Include Foraminifers and Radiolarians

Foraminifera

• Kingdom Protoctista, Phylum Protozoa, Class Sarcodina

• Range from Cambrian to recent times

• Commonly known as forams

• Secrete chambered shells made of CaCO3

  • Calcareous

• Most are benthic (bottom dwelling) and some are planktonic (floating or swimming)

Radiolarians

• Kingdom Protoctista, Phylum Protozoa, Class Sarcodina

• Range from Cambrian to recent times

  • Mostly found in Mesozoic and Cenozoic

• Mostly planktonic

• Shells made up of SiO2 (silica)

  • Siliceous

Animal Kingdom

• All are multicellular

• Known from the Late Precambrian onwards

• Majority of fossils are found in Phanerozoic rocks

• Informally subdivided into 2 groups:

  • Invertebrates

  • Vertebrates

Invertebrate Animals

• Without backbones

  • E.g. Molluscs, arthropods, and coelenterates