Transcript Chapter 2 c Primary Structures e

Primary Structures
The value of
measuring section
Measuring section is the
process of making detailed
notes about each of the
layers/intrusions found in an
outcrop. The process really
forces you to LOOK. We work
up or down section as
appropriate. Note rock/sediment
type, primary and secondary
structures, fossils, note color,
orient surfaces and lineations
with the brunton, etc.
Munsell Rock Color Charts
To note colors, I
use one of
these, but they
are expensive.
In your notes,
just use your
own judgment.
Also note
whether the
sample is wet or
dry
Graded Beds
Flysch: old [Alpine] name for thick sequences of turbidites “laid down in a deep trench
marking an active plate boundary (like a subduction zone).” PM p17
Graded Bedding (2)
Discuss “Younging
direction”.
Cross Beds
Ripples, dunes and deltas in cross section, usually truncated tops
(younging direction) and current direction indicated
Notice the lens in the
photo to give scale.
Use whatever is the
right size and is a
known size, coin, a
pencil, a ruler, your
notebook, etc.
Surface Markings: Load Casts
Sand layer over mud, blobs of sand sink into mud.
Flute Casts
Vortices dig into the unconsolidated sediment. Shallower and
wider downstream.
They curl up when they form, and give the younging direction
Mudcracks
Asymmetrical Ripples
Steep face on downstream side.
Pillow Lavas
Lava erupted under water (MOR, rift valley lakes), or flowed into water (Hawaii)
Unconformities
Angular Unconformity
Nonconformity
Disconformity
Recognizing Unconformities and
erosional surfaces: Channeling
Recognizing Unconformities:
Basal Conglomerate
Recognizing Unconformities:
Age discordance
Givetian
392-385 Mya
Ludlow 423419 Mya
http://www.sciencedirect.com/science/article/pii/S1631071308002800
Recognizing Unconformities:
Soil Horizons – Root Casts
Halokinesis: Salt layer movements
Buoyancy
Salt density about 2200 kg/m3
Sedimentary Rock average density 2500 kg/m3
“When the positive buoyancy [dense sediment over light salt] is sufficient
to upwarp” the sediment above, the salt will rise. PM p 26
http://search.datapages.com/data/open/offer.do?target=%2Fspecpubs%2Fmemoir65%2Fch20%2F0413.htm
Halokinesis: Differential loading
“This may occur when the downward force on the salt layer varies laterally.” PM p.26
Oil traps – Salt Diapir
Diapir, pierces overlying strata
Dome, upward bulge
Igneous Rocks
3 ways of making primary
magma, all basaltic
Plutonic and Volcanic Igneous Structures
Laccolith like a sill , but bows up overlying strata to make a dome
Bowen’s Reaction Series
Molten- VERY Hot
No solids
First mineral to crystallize out
Molten- Not so hot
100% Solid
Fine crystals
Need a microscope
Low silica, HOT, fluid
Course crystals
Easily seen
Intermediate
High silica, warm, viscous
A size comparison of the three
types of volcanoes
A Pahoehoe lava flow
Typical a’a’ flow
Broken, often further from vent
Fluid basalt forms lava tubes
Checking Bowens Reaction Series
Materials extruded from a volcano
• Pyroclastic materials – “Tephra”
Propelled through the Air
Types of pyroclastic debris
• Dust 0.001 mm and Ash < rice sized
• Cinders or Lapilli - pea to walnut-sized material
Particles larger than lapilli
• Bombs - > 64 mm ejected as hot lava
-Surtsey Is. Bombs the size of busses
A nueé ardente on Mt. St. Helens
A volcanic bomb
Bomb is approximately 10 cm long
Some the size of a Bus
Tephra forms Tuff
St. Lucia Anecdote
Source: Gerald & Buff Corsi/Visuals Unlimited, Inc.
Tephra layers fine away from source
Pumice
• Felsic magmas with high water content may
bubble out of a vent as a froth of lava.
• Quickly solidifies into the glassy volcanic rock
known as Pumice.
http://volcanoes.usgs.gov/Products/Pglossary/pumice.html
4700 BC S Oregon
Mt Mazama Eruption and Caldera Collapse
Ngorongoro Crater in Tanzania similar 2 mya
Volcanism on a tectonic
plate moving over a hot spot
Flood Basalts
Hot Spot currently
forming Hawaii
Hey, the plate changed direction !
Flood Basalts
• Fluid basaltic lava extruded
from crustal fractures called
fissures
• e.g., Columbia River Plateau,
• Deccan Traps in India
• Cover huge areas
• Plumes from Mantle
Flood Basalt erupted
from fissures - Snake
River Plain, southern
Idaho
Plume Activity
Lava Plateau Formation
Formation of a volcanic neck
Spanish Peaks and Radiating Dikes (southern CO)
Plutonic igneous activity
• Types of intrusive igneous features
• Dike – a sheetlike injection into a fracture
Discordant - cuts across pre-existing
• Sill – a sheetlike injection into a bedding
plane Concordant - lies parallel to bedding
• Laccolith – A mushroom-shaped concordant
A sill in the Salt River Canyon, AZ
Sill: Sediments above and below sill are baked.
Lava Flow, just baked below.
Why
No
C-C
collisions
Plate tectonics and igneous activity
• Igneous activity along plate margins
• Mid-Ocean Ridges – Basaltic Pillow Lavas
• Great volumes of volcanic rock produced along
oceanic ridges – New ocean floor
– Mechanism of spreading or “rifting”
» Lithosphere pulls apart and thins
» Less pressure results in partial melting in mantle
http://www.archipelago.nu/SKARGARD/ENGELSKA/ICELAND/surtsey.htm
http://volcanoes.usgs.gov/Products/Pglossary/ancientseq.html
Basaltic Pillow Lavas
Plate tectonics and igneous activity
• Igneous activity along Subduction zones
–Descending plate partially melts
–Magma slowly moves upward
–Rising magma can form either
»A Volcanic Island Arc if ocean-ocean plate
collision (Aleutians, Japan, etc.)
»A Continental Volcanic Arc if oceancontinent plate collision (Sierra Nevada)
The Cascades, Washington State
Plate tectonics and magmatism
• Intraplate volcanism
• Associated with plumes of heat in mantle
• Form localized volcanic regions in the
overriding plate called a hot spot
–Produces basaltic magma sources in
oceanic crust (Hawaii)
–Produces granitic magma sources in
continental crust (Yellowstone Park)
–These differences are predicted by a
Crust-Melting model of Granite
generation
Notice the potential for a huge lahar during the eruption of this
big composite volcano (stratovolcano), Mt. Fugiyama.
http://www.mt-fuji.co.jp/Photo/Photo.html