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HISTORICAL GEOLOGY LECTURE, PAGE



I. Introduction
A. Geology

- the study of the Earth


1. Physical Geology

- study of the Earth's materials, such as minerals and rocks, and the various physical and chemical changes that occur on its surface and in its interior


2. Historical Geology

- history of the planet and its life forms from its origin to the present


B. The Birth of Modern Geology
1. James Hutton (1726 - 1797)

- Scottish gentleman farmer and geologist; the "Father of Geology"

- formulated concept of "Uniformitarianism"
a. Uniformitarianism

- "the present is the key to the past"

- the Earth is shaped by daily, mundane processes

- the Earth is very old

- believed that "great catastrophes" have only minor influence
2. Charles Lyell (1797 - 1875)

- English Geologist, wrote Principles of Geology (the first volume appeared in 1830)

- his influential popularization of Hutton's principles influenced generations of geologists
3. More recent studies use the concept of Actualism
a. Actualism

- apply studies of modern processes to ancient rocks

- the processes that now shape the Earth were similar in the geologic past, although the rate of change may vary

- recognizes that "catastrophes" can have powerful influence on the Earth


4. Geologic Time Scale

- the Earth is 4.6 billion years old

- the subdivisions of the time scale are based primarily on the predominant life forms living during specific times
II. Minerals and Rocks
A. Mineral

- naturally occurring, inorganic, homogeneous, crystalline solid; more than 90% of rock-forming minerals are silicates (contain silicon, oxygen and one or more metals)


1. Chemical Composition of Minerals
a. Elements

- fundamental components, cannot be broken down to simpler substances by ordinary chemical processes

- there are approximately 94 naturally-occurring elements

- the 8 most common are Oxygen (O), Silicon (Si), Aluminum (Al), Iron (Fe), Calcium (Ca), Sodium (Na), Potassium (K) and Magnesium (Mg); comprise 98% of the Earth's Crust


b. Atoms

- fundamental units of elements


Nucleus - positively charged center of mass; includes protons (with mass and a positive charge) and neutrons (with mass and a neutral charge)
Electrons - with no mass and a negative charge; the number and orientation of electrons determines chemical behavior
c. Chemical Reactions

- filling of the outer shells of electrons


Ions = charged atoms; atoms with too few or too many electrons; includes cations (positively charged) and anions (with negative electrical charges)
2. Physical Properties of Minerals

- use color, streak, hardness, crystal form, cleavage, fracture, luster, specific gravity, magnetism, chemical reactivity, radioactivity, fluorescence, etc. to identify minerals

- see lab manual
3. Mineral Classification
a. Based on dominant anion present in the Mineral

- including silicates, oxides, sulfides, halides, phosphates, carbonates, native elements and hydroxides


b. Silicate Minerals

- the most abundant chemical group constitute (about 90% of the Earth's crust)

- Silicate Bonding with four oxygen for each silicon; bond directions require a tetrahedral arrangement of the atoms; the tetrahedra may be isolated or form single chains, double chains, sheets, or frameworks
- Important Silicate Minerals Include:
Feldspar - a framework silicate found in almost all rock types; constitutes about 50% of the Earth's crust; includes potassium feldspar and plagioclase feldspar series
Quartz - a very pure framework silicate; common in continental rocks but rare in oceanic and mantle rocks
c2. Nonsilicate Rock-Forming Minerals
Carbonates - with one carbon and three oxygen atoms; Exs. = calcite, dolomite
Sulfates - with one sulfur and four oxygen; Ex. = gypsum
Sulfides - sulfur combines with some other element (not oxygen); Ex. = pyrite, galena
Halides - one or more metals combine with one or more halogen elements (fluorine, chlorine, iodine, bromine); Exs. = halite, fluorite
Oxides - one or more metals combine with oxygen; Exs. = magnetite, hematite, corundum
Phosphates - one or more metals combine with phosphate group (1 phosphorous and 4 oxygen atoms; PO4); Ex. = apatite
Native Elements - mineral consists of a single element; gold, silver, copper, sulfur, graphite, diamond
B. Rocks

- naturally-formed, solid materials composed of one or more minerals or mineraloids


C. Igneous Rocks

- rocks that solidify from molten material (magma)


1. Melting of magmas

- is due to radioactivity, movement of rock masses into high temperature zones, transfer heat upward from deep crust or mantle


2. Magma Composition

- ultimate magma composition is especially influenced by type of parent magma (which is primarily a product of where the magma is formed); mafic magmas (rich in iron and magnesium) are characteristic of oceanic crust, felsic magmas (rich in silica) are characteristic of continental crust

- magmas change composition primarily through fractionation (remove crystals from magma chamber which alters magma composition)
3. Emplacement of magmas is due to lithostatic ("rock") pressure, pressure due to increased gas volume, tectonism ("mountain-building"), and stoping (magma surrounds and engulfs crystals or rocks)
4. Intrusive (Plutonic) Igneous rocks

- rocks that solidify beneath the earth's surface

- magma cools slowly and rocks form large crystals

- Examples = peridotite, gabbro, diorite, and granite


5. Extrusive ("Volcanic") Igneous Rocks

- solidify at or near the Earth's surface

- forms from lava (magma flows onto Earth's surface) or tephra/pyroclastic material (magma is blown onto Earth's surface)

- Examples = basalt, andesite, rhyolite, tuff, agglomerate


D. Metamorphic rocks

- rocks formed from pre-existing rocks by solid state transformation in response to change in the physical or chemical environment


1. Contact Metamorphism

- occurs in country rock bordering igneous intrusion

- typically created under relatively low pressure, high temperature

- types of rocks include marble, quartzite, hornfels and some ore deposits


2. Regional metamorphism

- occurs on a regional scale due to orogenies (mountain-building events) triggered by plate tectonics

- sequence of rocks formed under greater temperature and pressure includes mudstone -> slate -> phyllite -> schist -> gneiss
E. Sedimentary Rocks

- rocks formed from consolidation of loose sediment, formed by chemical precipitation, or rocks consisting of secretions or remains of plants and animals

- most important rocks for interpreting Earth history
1. Sedimentary rock is product of :
a. Provenance

- pre-existing rocks from which sediment forms and effects of weathering on sedimentary composition

b. Process

- what happens during transportation, deposition and after deposition


2. Transport of Sediment
a. Agents of transport

- wind, water, ice, gravity


b. Physical Transport
b1. Physical Load

- sedimentary particles carried by current

- forms Clastic or Detrital Rocks (conglomerates, sandstones, siltstones, mudstones)
b2. Dissolved (Chemical) Load

- dissolved ions carried by water; deposited when chemical or physical changes concentrates solutes and cause them to precipitate

- form Chemical Rocks such as Carbonates (limestone, dolomite) and Evaporites (halite, gypsum)
3. Sedimentary texture

- size, shape and arrangement of grains


a. Sedimentary Grain Size
a1. Wentworth Scale

- describes size of sedimentary particles

- includes clay, silt, sand and larger size particles

- size of particles transported depends on type of transporting agent and energy of transport


b. Sorting

- range of particle sizes in a sediment


Well sorted- particles of similar size\
Poorly sorted - wide variety of grain sizes
- Sorting depends on type of sedimentary transport: Gravity and glaciers = poorly sorted; Water = well sorted; Air = most selective sorting
c. Shape
c1. Roundness (Angularity)

- degree of curvature of the corners of the particles;

- depends on type of parent material and degree of sediment transport
c2. Sphericity

- degree to which a particle approximates the shape of a sphere

- typically depends upon the degree of sediment transport
4. Sedimentary Structures

- multigrained features formed during or after accumulation of sediment and before lithification


a. Primary Sedimentary Structures

- form as sediment is being deposited


a1. Stratification

- accumulation of sedimentary particles in horizontal layers


a2. Massive beds

- thick and uniform; form under constant conditions or where bedding is destroyed by organisms or dissolution


a3. Graded bedding

- grain size increases or decreases from base to top due to changing current velocities


a4. Cross-Bedding

- layers are inclined

- cross-bedding is used to determine ancient current (paleocurrent) direction
b. Secondary (Postdepositional) Sedimentary Structures

- form after sediment deposition, usually due to groundwater interaction with buried sediments and rocks


b1. Nodule

- irregular, round, flat structure formed by filling voids in sediment


b2. Geodes

- hollow, subspherical structures; form around water-filled pocket by crystals growing inward


b3. Concretions

- mineral segregations that replace or force aside the surrounding sediment


5. Lithification

- transformation of sediment into sedimentary rocks


a. Rocks are composed of:
a1. Particles

- clastic rocks are often made of quartz; also feldspar, rock particles

- Limestone particles include fossils (most important), pellets (invertebrate feces), oolites (sand-size concentrically-ringed carbonate particles) and intraclasts (carbonate mud "rip-ups")
a2. Matrix

- fine-grained material deposited with particles

- clay in clastic rocks

- carbonate mud (micrite) in limestones


a3. Cement

- transported in solution by groundwater and precipitates between grain particles after they are deposited

- often calcite (termed "sparite" in limestones) or silica (especially quartz)

a4. Pores

- void space between sedimentary particles
b. Diagenesis

- physical, chemical and biologic changes that occur after deposition and before metamorphism

- includes effects of compaction, cementation and recrystallization

III. The Sedimentary Archives


A. Sedimentary Environments

- portion of the earth's surface with distinctive physical, chemical and biological characteristics


1. Facies

- body of sediment or rocks with distinctive characteristics


2. Environmental Analysis
Determination of Ancient Sedimentary Environments Utilizes:
a. Physical Criteria

- including shape of the deposit, rock type (lithology), textures (grain characteristics), sedimentary structures, fining/coarsening upward in sedimentary grain size


b. Geochemical criteria

- especially isotopic ratios of elements


c. Biological criteria

- fossil content


d. Walther's Law

- the vertical sequence of rocks may reflect the horizontal succession of environments/facies


Transgression - relative rise in sea level
Regression - relative drop in sea level
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THE FOLLOWING ARE COMMON NON-MARINE (TERRESTRIAL) SEDIMENTARY ENVIRONMENTS:
B. Soils

- largely product of biological weathering; with rock debris and humus (= decaying organic matter)


1. Factors that Influence Soil Formation
a. Type of Parent Rock

- influence the type of materials present, amount of fractures, and permeability (ability of fluids to flow through the system)

- granite often produces sand-rich soils; basalts often produce clay-rich soils
b. Time

- as time progresses and with more weathering soils often become more alike


c. Climate

- affects precipitation and vegetation, which greatly influences soil characteristics


2. Ancient Soils (Paleosols) and Environments:
a. Wetland Paleosols

- often gray, organic-rich


b. Tropical Paleosols

- often with aluminum-rich bauxites or red laterites


c. Arid paleosols

- often red; with shrink-swell clays and mudcracks; often with duricrusts (mineral layers that accumulate at the soil's upper surface due to capillary action during evaporation, and consist of silica, carbonates or iron)


C. Lacustrine (Lake) Environments

- landlocked body of water occupying some kind of basin due to faulting, crustal warping, or glaciation

- controlled by water circulation, salinity and temperature (climate factors), biological factors and provenance ("source") of sediments
1. Clastic Lake Deposits

- often exhibit a circular morphology, with sand and gravel outside (near the shore) and mud in middle of lake facies

- lakes typically produce cyclical, often repetitive sedimentary units
2. Carbonate Lake Deposits

- are most typical of subtropical to tropical climates

- precipitate limestones, dolomites and often contain stromatolites (carbonate algal structures)
3. Playas

- broad shallow depressions in desert regions which may be covered by thin sheet of water

- often with evaporite deposits such as gypsum and halite
D. Fluvial Environments
Rivers - major transporting and depositional agents for continental sediments
1. Meandering Rivers

- with high sinuosity

- these are perennial rivers (they typically flow year-round), with substantial base flow (base flow is the water contributed by groundwater)

- often produce fining-upward facies with nonmarine fossils; there is often considerable sandstone in the channel, with deposits of shale (and paleosols) on the floodplain


2. Braided Streams

- interlaced network of low sinuousity channels

- braided streams are due to seasonal "flashy" discharge; often form in temperate mountains, arid regions and areas with monsoon-influenced climates

- often create a broad, sheet-like morphology; they are usually coarse-grained and typically with poorly-sorted sediments; fossils are rare


E. Semiarid Areas (Steppes) and Deserts
1. Desert

- areas with less than 25cm rainfall per year

- most deserts are situated at 25° - 30° latitude (due to global circulation patterns in the subtropics producing high-pressure, low-moisture conditions)

- often with Interior Drainage (where rivers drain into central desert depressions rather than flowing to the sea)


2. Semiarid Area (Steppe)

- interior continental areas with 25-50 cm rainfall; grassy vegetation


3. Desert Facies Include:
a. Wadis/Arroyos

- "dry washes"; often with braided stream-like features


b. Alluvial Fans

- cones radiate downslope from the point where streams emerge from rocky highlands

- like braided stream deposits but wedge-shaped and with debris flows
c. Sand Dunes

- hill of sand deposited by wind (eolian deposits)

- sand dunes consist of well-sorted coarse silt/fine sand; they have a steep slip face and gently-dipping windward face

- sand dunes typically form thick, crossbedded quartz sandstones


F. Glaciers
Glacier - system of flowing ice that originates on land through the accumulation and recrystallization of snow
1. Continental glaciers

- Continental glaciers are large, thick, continental- or subcontinental-size glaciers that have been very important during the Earth's "Ice Ages"

- continental glaciers have both depositional facies (form tillites, composed of glacial deposits termed till) and erosional facies (often form unconformity surfaces and glacial striations due to the weight of the ice eroding the landscape)

- continental glaciers are often associated with a combination of fluvial, lacustrine, eolian and shallow marine environments


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THE FOLLOWING ARE COMMON MARINE SEDIMENTARY ENVIRONMENTS:


G. Deltas

- depositional body of sand, silt and clay formed where a river discharges into a body of standing water

- usually cone-shaped, coarsens upward in grain size, and with cyclothems (repetitive sedimentary sequences alternating from marine- to non-marine deposition)
H. Barrier - Backbarrier Complexes
1. Barrier islands

- elongate islands built by large waves

- barrier islands are composed of sand, gravel, and/or shell debris

- barrier islands are separated from the mainland by lagoons or bays


2. Backbarrier complex

- depositional environments situated between a barrier island and the mainland

- examples include bays and lagoons
3. Sedimentology of Barrier Facies
a. "Surf-side" of Barriers

- beaches are often sandy; they are typically well sorted, cross-bedded and are quartz-rich

- seaward from beaches, facies shift to silt and clay
b. Sheltered Sides of Barriers (Backbarrier Complexes)

- wave action is typically insignificant in lagoons and bays

- chief influences on backbarrier sediments are tides, organisms and climate

- usually organic-rich muds are the prevalent sediment type


I. Tidal-influenced Environments

- sea marginal areas subject to effects of tidal fluctuations (tides are due to Moon-Sun gravitation on oceans)

- lithology often consists of oolites (sand-size grains of limestone), stromatolites (algal-built structures), skeletal debris, coal, and evaporites

- sedimentary structures often consist of repetitive fining-upward sequences of sand, silt and clay


J. Organic Reefs

- solid but porous limestone structure standing above the surrounding seafloor and constructed by living organisms (often with skeletal material, especially corals)

- with a wide variety of morphologies, from isolated Patch Reefs to continuous Barrier Reefs
K. Marine Shelves
1. Types
a. Continental Shelves

- submerged, relatively flat, continental margins (that represent the "true" edges of the continents)

- shelf edge averages approximately 130 meters water depth
b. Epeiric (Epicontinental) Platforms

- broad, shallow sea over continental area

- not common now but was important during periods of major rise in sea level (Example = Cretaceous Period)
2. Terrigenous shelves

- with land-derived sediments

- sand/gravel nearshore, silt and clay facies offshore

- fining upward in grain size (transgressive sequence) or coarsening upward (regressive sequence)


3. Carbonate Shelves

- form in tropical/subtropical environments

- create thick limestone sequences
L. Continental Slopes

- slope seaward of continental shelf

- lower limit 500-5000m

- often with turbidites (graded beds deposited when dense, sediment-charged turbidity currents slow down); carve submarine canyons and deposit deep sea fans


M. Pelagic Deposits

- deep marine deposits accumulated due to vertical sedimentation

- usually consist of Oozes (with microscopic silica or carbonate shells derived from planktonic organisms) or clay-rich facies

IV. Stratigraphy

- study of rock layers (strata)
Lithostratigraphy and Biostratigraphy have been the major ways in which Relative Geologic Time (sequencing geologic events) has been established
A. Lithostratigraphy (Physical Stratigraphy)

- defines rock units on the basis of their physical features (i.e. lithologic features)


1. Stratigraphic Laws
a. Superposition

- in series of undisturbed strata, the oldest bed is on the bottom


b. Original Horizontality

- sedimentary units are originally deposited in horizontal layers

- if the layers are not horizontal, the rocks have been deformed
c. Cross-cutting Relationships

- a unit that cuts across another unit is younger than the unit it cuts across


d. Inclusions

- a rock included within another unit is older than that unit


2. Formal Rock Units

- in decreasing order, includes Supergroup - Group - Subgroup - Formation - Member - Bed

- are separated by Contacts (the boundaries between different rock units)
a. Formation

- formations are the basic mapping units in stratigraphy

- a formation must have mappability (typically mapping is done using air photos) and lithologic constancy (rocks should be of similar type in a given formation)

- Formation names are designated by local geographic names and are capitalized


b. Member

- subdivision of a formation

- often only have local significance
c. Beds

- smallest rock-stratigraphic unit

- beds are informal units, and therefore their names are usually not capitalized

- beds may have economic significance (Coal bed, oil sand, etc.) or used in mapping ("key beds" or "marker beds" are used for correlation of strata)


d. Groups

- assemblage of 2 or more successive formations

- formations lumped together to form groups are related by lithology (rock type) or by position with reference to unconformities
3. Defining Formal Rock Units
a. Conformities

- contacts between rocks which exhibit continuous depositional histories

- contacts are typically defined at the boundaries between differing lithologies or textures
b. Unconformities

- unconformities are gaps in the rock record due to erosion or nondeposition; unconformities often form contacts between groups or formations


b1. Angular Unconformity

- surface separating tilted or folded strata from overlying undisturbed strata


b2. Disconformity

- unconformity between essentially parallel strata


b3. Paraconformity

- unrecognizable in outcrop without the use of fossils, absolute dating, etc.


b4. Nonconformity

- erosion surface between sedimentary and igneous/metamorphic rocks


4. Correlation

- matching stratigraphic sections of the same age


B. Biostratigraphy ("Stratigraphic Paleontology")
1. Biostratigraphic Unit

- body of rocks delimited from adjacent rocks by their fossil content

- fossils are often used for Correlation
a. Biozone

- basic unit of biostratigraphic classification

- based on the distribution of Index Fossils (fossils characteristic of key formations; should have short time span, wide geographic range, independent as possible of facies, abundant, rapidly changing and with distinctive morphology)
b. Range Zones

- plot stratigraphic range of fossil(s)


Taxon Range Zone – represents the total horizontal and vertical range of a taxon
Concurrent range zone - overlapping ranges of specified taxa
- Taxon and Concurrent Range Zones are the major types of biozones
2. Major Fossils used in Biostratigraphy

- best are pelagic [planktonic (floating) or nektonic (swimming)] forms

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