Primary Structures

Primary StructuresThe value of measuring sectionMeasuring 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 BedsRipples, 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 CastsVortices dig into the unconsolidated sediment. Shallower and wider downstream.

MudcracksThey curl up when they form, and give the younging direction

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

http://www.sciencedirect.com/science/article/pii/S1631071308002800

Ludlow 423-419 Mya

Givetian392-385 Mya

Unc ~30My

Recognizing Unconformities:Soil Horizons – Root Casts

Halokinesis: Salt layer movementsBuoyancy

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 DiapirDiapir, pierces overlying strataDome, upward bulge

Igneous Rocks3 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 SeriesMolten- VERY HotNo solids

Molten- Not so hot

100% Solid

First mineral to crystallize out

Fine crystalsNeed a microscope

Course crystalsEasily seen

Low silica, HOT, fluid High silica, warm, viscousIntermediate

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 AirTypes 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

Source: Gerald & Buff Corsi/Visuals Unlimited, Inc.

St. Lucia Anecdote

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

Mt Mazama Eruption and Caldera Collapse4700 BC S Oregon

Ngorongoro Crater in Tanzania similar 2 mya

Volcanism on a tectonic plate moving over a hot spot

Hey, the plate changed direction !Hey, the plate changed direction !

Flood Basalts

Hot Spot currently forming Hawaii

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, southernIdaho

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.

WhyNoC-Ccollisions

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 LavasBasaltic 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 ocean-continent 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

http://www.mt-fuji.co.jp/Photo/Photo.html

Notice the potential for a huge lahar during the eruption of this big composite volcano (stratovolcano), Mt. Fugiyama.