Geology Field Guide

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Geology Field Guide

• By Carolyn Langdon
• and
• Istana Thumser

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Our High Schools Mesozoic Basin
Battlefield Haymarket, VA Osbourn Park
Manassas, VA
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Produced by Frank Gregorio (OPHS)
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Produced by Frank Gregorio (OPHS)
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Produced by Frank Gregorio (OPHS)
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Produced by Frank Gregorio (OPHS)
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Produced by Frank Gregorio (OPHS)
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Produced by Frank Gregorio (OPHS)
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General Piedmont and Mesozoic Basin Description

• The Piedmont region of Virginia was exposed after
  Gondwana collided with North America in the
  Paleozoic 310 million years ago creating the
  Alleghanian Orogeny.

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Piedmont

• During the opening of the Atlantic ocean (DCM)
  that started approximately 230 million years ago,
  sediments from the eroding mountains started
  collecting in the geographic region now known as
  the Piedmont.

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Transgression and Deposition

http//csmres.jmu.edu/geollab/Fichter/Wilson/Stage
C.html
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Piedmont in Detail

• Alleghanian Orogeny cooked rock
• folded, twisted faulted
• Igneous and sedimentary rock turned to
  metamorphic rock from immense pressure
• Faults formed from broken rock
• Landscape wore down to almost sea level
• Deep red clayey soils present
• Indicates chemical weathering high in FeOx

http//www.nps.gov/archive/mana/pphtml/subenvironm
entalfactors13.html
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Faults

• Reverse and strike-slip faults can also be found
  in this area, running in the northeast southwest
  direction
• Created by tension between N. America Africas
  separation

Piedmont
Fault line
C. M. Bailey, College of William Mary
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Mesozoic Basin

• Low basins in the Piedmont where sediments
  collect in grabens between faults are Triassic
  and Jurassic in age.
• The largest basin in Virginia is the Culpeper
  Basin.

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Culpeper BasinNewark Supergroup

• Borders eastern front of Appalachian Mountains
  from Culpeper, VA into MD
• Low, gentle rolling to flat topography
• Sediment filled grabens half grabens
• Stage O of Wilson Cycle
• Formed by rifting separation of Africa from N.
  Am
• Aligned in NE-SW orientation
• Sedimentary and igneous rocks (Jurassic
  Triassic)
• Basalt flows
• Most common rock type diabase as dikes sills

http//www.nps.gov/archive/mana/pphtml/subenvironm
entalfactors13.html
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Day 2 Haymarket, Manassas, Quarry
Day 1 USGS Great Falls
                 WPWestern Piedmont
                  Mz Mesozoic Basins
Early Paleozoic meta- sedimentary and igneous
rocks associated with the suture zone between
Blue Ridge (Laurentian) rocks and terrains of the
Piedmont.
Triassic sedimentary rocks deposited in half
grabens grabens during rifting that produced
the Atlantic Ocean.
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Pre-Lesson Visit USGS
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Rock Garden Scavenger Hunt

• Examples of rock units in Piedmont
• Show sedimentary, structural, mineralogical and
  weathering features

http//www.usgs.gov/visitors/rocks.html
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Great Falls Western Piedmont Virginia and
Maryland

• Major attraction Potomac River
• Formed by uplift of Piedmont Plateau
  Appalachian Highlands
• Slope of land increased streams deepened their
  valleys coalesced into the Potomac
• Great Falls formed during Ice Age
• Occupies broad valley
• Thick metamorphosed sandstone layers
• River flows into Mather Gorge below falls
• Excavated 2 MYA (Pleistocene glaciation)

www.cr.nps.gov/history/online_books/grfa/sec4.htm
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Guided Inquiry Incomplete Trail Log

• Walking tour with notable geological features
  seen from park trails
• Virginia and Maryland sides
• Describe and discuss rock features
• Use hand lens to identify minerals
• Include geologic time chart

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Rocks of Mather Gorge Formation(Late
Proterozoic and Precambrian)
Buried metamorphosed by heat/pressure Precambria
n rock!!

• Mica Schist
• Metagraywacke
• Amphibolite intruded 550-600 mya (zircon)
• Granite intruded 425 mya
• Lamprophyre mostly mica 360 mya
• Pegmatite
• Granodiorite (jd)470 mya

http//www.cr.nps.gov/history/online_books/grfa/se
c5.htm
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Geologic Time Chart
http//www.cr.nps.gov/history/online_books/grfa/se
c5.htm
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Metamorphosed Sed/Ig Rocks

• Deformed during collisions of continental plates
• Resulted in Alleghanian Orogeny
• Sedimentary and Igneous rocks metamorphosed to
  schist and gneiss (CZms) migmatite (CZmm)
• Tightly folded layered w/ foliation planes of
  mica
• Contain closely-spaced fractures

http//geology.er.usgs.gov/eespteam/Greatfalls/IND
EX.HTML
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Mather Gorge Formation

• Made of metamorphosed sedimentary rocks
• 600 mya (Precambrian)
• Originally deposited as sand, mud, silt in
  Iapetus Ocean (before Atlantic)
• Eroded from an ultramafic igneous rock
• Biotite, muscovite, staurolite, kyanite garnet
  crystals grew as rocks cooled

http//geology.er.usgs.gov/eespteam/Greatfalls/IND
EX.HTML
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After Granite Intrusions

• Crust rose overlying rocks eroded
• Rock temps declined rocks fractured
• Molten material formed lamprophyre dykes
• Gorge cut along several faults (Triassic)
• Contain siliceous white quartz veins some native
  gold
• Have same orientation as Triassic age faults
  close by (200mya)

http//www.cr.nps.gov/history/online_books/grfa/se
c5.htm
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Current Landscape

• Results from erosion of Potomac River
• River follows cracks faults in Mather Gorge
• Terraces (level surfaces) cut into bedrock by
  river remnants of flood plains
• 6 recognized terrace levels
• Down-cutting created islands, islets, oxbows,
  potholes and more
• Exposures of Piedmont bedrock

http//geology.er.usgs.gov/eespteam/Greatfalls/IND
EX.HTML
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Metagraywacke and Schist (folded beds )
http//geology.er.usgs.gov/eespteam/Greatfalls/IND
EX.HTML
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Amphibole (dark) plagioclase (light) crystals
in amphibolite 
Vein quartz intrudes all Mather Gorge Formation
http//geology.er.usgs.gov/eespteam/Greatfalls/IND
EX.HTML
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usgs/great_falls/sec7.htmLast Updated
01-Mar-2005
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Haymarket

• Extrusive igneous rock layered with sedimentary
  rock layers
• Vesicular basalt from Jurassic indicates tectonic
  activity
• Aphanitic rock shows it cooled very quickly

Randy Korntev, WUStL
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Manassas Battlefield ParkPart of Culpeper Basin

• Composed of sedimentary rock matrix (i.e.,
  siltstone sandstone)
• Igneous diabase forms intrusive dikes sills
• Eastern park soil from parent siltstone
• Western park soil from diabase
• Progressed up through siltstone during upheaval
  of Alleghanian Orogeny

http//www.nps.gov/archive/mana/pphtml/subenvironm
entalfactors13.html
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Soils

• 79 from siltstone
• Strongly acidic, well-drained silt loam
• 19 from diabase
• Loamy, clay minerals, fragipan layers limit
  permeability
• 2 alluvial (carried/deposited by stream action)

http//www.nps.gov/archive/mana/pphtml/subenvironm
entalfactors13.html
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Siltstone and Diabase Collision

• Heat of emerging diabase melted siltstone
• Created narrow metamorphic rock bands
• Contained epidote, pyroxene, garnet, cordierite
  minerals

http//www.nps.gov/archive/mana/pphtml/subenvironm
entalfactors13.html
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Gainesville Quarry

• Intrusive igneous rock in dikes throughout the
  existing sedimentary rock
• Medium grain size and dark color indicates
  diabase and shallow underground cooling

www.nr.gov.nl.ca/minesen/geosurvey/education/feat
ures/intrusive/
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Post Lesson Inquiry Activity

• Discovering The Wilson Cycle

By Dr. Lynn Fichter
http//csmres.jmu.edu/geollab/fichter/Wilson/platb
nd2.html