Structural Geology Pp TX

7/28/2019 Structural Geology Pp TX

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Primary structures

- structures that are acquired during the

genesis of rock.

Non-tectonic structures

- locally driven or small scale features not

immediately due to tectonic interaction.


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Primary sedimentary structure

-a syngenetic sedimentary structure determined

by the conditions of deposition, developed before

lithification of the rock in which it is found. It includes

bedding, bedding-plane markings, and deformation

structures produced by slumping or unequal loading.


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Bedding

- primary layering in a sedimentary rock,

formed during deposition, manifested by changes in

texture, color and composition; may be emphasized

by the presence of parting.


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BEDDING


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CROSS - Bedding

also known as cross-stratification, is sedimentary structures of

inclined layers within a bed


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COMPACTION

- squeezing unlithified sedimentary inresponse to pressure exerted by the weight ofoverlying layers.


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Overturned beds

beds that have been rotated past vertical in an

Earth-surface frame of reference; as aconsequence, facing is down.


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Phyllite

Parting

- the tendency of sedimentary layers to split or

fracture along planes parallel to bedding; partingmay be due to weak bonds between beds ofdifference composition, or may be due to apreference for bed-parallel orientation of clay and

fissility.


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Stratum/Strata

- are distinctive layers or beds, of sediments orsedimentary rocks deposited consecutively, or withinterruption by unconformities, atop other rocks.


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Stratigraphic facing

also known as younging direction, the direction toyounger strata, or, in other words, the direction tothe depositional top of beds.


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The Use of Bedding in Structural Analysis

Depositional environment

-The setting in which the sediment was originally deposited.

Stratigraphic facing or younging direction

-The direction in which in a sequence are progressively younger.

Current direction

-The direction in which fluid was flowing during depositions


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Facing Indicators


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Grading

the gradual reduction of the land to a level surface.

Turbidity flow

a flow of dense, muddy water moving down a slopedue to a turbidity current.

Turbidite

the sediments or rocks that formed as a result of aturbidity flow


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Graded Bedding in a turbidite sequences (Boumasequence)


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Bouma Sequence


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Graded Bedding


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SURFACE MARKINGS

Animal tracks

Clast imbrications

Flute casts

Mudcracks

Raindrop impression

Ripple marks

Traction lineation

Worm burrows


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DISRUPTED BEDDING Sand volcano

also known as sand blow

a cone of sand formed by the ejection of sandonto a surface from a central point.

erupts mass quantities of sand instead of lava


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Load cast

form due to density contrast combined with aplastic substratum


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CLASTIC DIKE a seam of sedimentary material that fills a

crack in and cuts across sedimentary strata orother rock types

form passively by water, wind, and gravity

are commonly vertical or near-vertical


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SALT STRUCTURES

Most often found in passive margin basins.

Halokinesis = deformation of halite by flowing, dueto gravity.

Usually occurs due to:

(i) density inversion

(ii) differential loading

(iii)slope at base of salt layer


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Salt is non-porous and has low compressibility.Halite deposits typically have a density of about2200 kg/m3, whereas clastic sediments may startoff less dense (when they are first deposited) but

then get compacted and dewater, typicallyending up at about 2500 kg/m3. This frequentlyleads to a density inversion, and the salt may riseupwards, often as a diapir. Its ascent stops at the

level of neutral buoyancy, typically about 450 to1500 m below the surface.


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Salt Structure: Halokinesis

Salt is a sedimentary rock that forms by the precipitation ofevaporite minerals (halite, gypsum, anhydrite, calciumsulfates) from saline water.

Salt differs from other sedimentary rocks in that it is muchweaker and, is able to flow like a viscous fluid underconditions in which other sedimentary rocks behave in abrittle fashion. In some case, deformation of salt is due totectonic faulting or folding, because salt is so weak, it may

deform solely in response to gravity, and thereby causedeformation of surrounding sedimentary rock.


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Stages in the Formation of Salt

Structures


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ThePrincipal Types

ofUnconformities


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Disconformity

beds of rock sequence above and below theunconformity are parallel to one another, butthere is a measurable age difference between thetwo sequences. Its surface represents a period ofnondeposition and erosion.


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Angular Unconformity

strata below the unconformity have a differentattitude than strata above the unconformity. Bedsbelow the unconformity are truncated at theunconformity, while beds above the unconformityroughly parallel the unconformity surface.


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Nonconformity used for unconformities which strata were

deposited on a basement of older crystallinerocks. The crystalline rock may be either plutonicor metamorphic.


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Buttress Unconformity

occurs where beds of the younger sequencewhere deposited in a region of significantpredepositional topography. Note that a buttressunconformity differs from an angularunconformity in that the younger layer is

truncated at the unconformity surface.


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Other contact indicators Channeling

Basal conglomerate

A coarse gravelly sandstone or conglomerate forming the lowestmember of a series of related strata which lie unconformably on

older rocks; records the encroachment of the seabeach on dryland.

Time discordance / age disordance

Paleosol

A layer of fossilized soil, usually buried beneath layers of rock ormore recent soil horizons.


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CHANNELING


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Basal Conglomerate


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Time disordance / age disordance


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Paleosol


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Penecontemporaneous Structures

Debris flows / Debris fall

are fast moving, liquefied landslides of mixed and

unconsolidated water and debris that look like flowing

concrete.

Slumping

a form of mass wasting movement that occurs when acoherent mass of loosely consolidated materials or rock

layers moves a short distance down a slope.


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Debris flows / Debris fall


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Slumping


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Igneous structures

Terminologies

Igneous rock

- rocks formed by solidification of cooled magma (moltenrock), with or without crystallization.

Types of Igneous Rock

Intrusive - form by the crystallization of magma at adepth within the Earth

Extrusive - igneous rocks which form by thecrystallization of magma at the surface of theEarth


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Sheet Intrusion

- is where a mass of molten magma takes advantage of a pre-existinglinear feature in a host rock, such as a long rupture or fault, and forces its

way into these spaces

Batholith

- is a large emplacement of igneous intrusive rock that forms fromcooled magma deep in the Earth's crust.

Pluton

- a body of intrusive igneous rock that is crystallized from magmaslowly cooling below the surface of the Earth.

Dike - or dyke

- a type of sheet intrusion referring to any geologic body that cuts

discordantly across.


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Sill

- a tabular sheet intrusion that has intruded betweenolder layers of sedimentary rock, beds of volcanic lava or tuff, or evenalong the direction of foliation in metamorphic rock.

Stock

- a discordant igneous intrusion having a surface exposure of less

than 40 sq mi (100 km2), differing from batholiths only in being smaller.

Hypabyssal

- Solidifying chiefly as a minor intrusion, especially as a dike or sill,

before reaching the earth's surface. Used of rocks


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Igneous structures extrusive features

Lava

- molten rock expelled by a volcano during an eruption.

Pahoehoe Lava & Aa Flow

Pahoehoe lava

more fluid (low viscosity) and has a distinctive ropy to smooth surface. Some lava flows

begin as hot pahoehoe, then change to a'a' flows as they cool or slow down.

Aa Flow

are covered by an extremely rough jumble of broken lava blocks. Beneath thisforbidding cover is a dense core of basalt. 'A'a flows form from relatively cool, sticky,pasty (high viscosity) lava. The surface and base of an 'A'a flow solidifies to form acrusty sheet that breaks into rough blocks as it flows away from its volcanic vent.


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Pahoehoe Lava & Aa Flows


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Pillow basalts

- are bulbous bodies with quenched, glassy rinds.

Ignimbrites

- is the deposit of a pyroclastic density current,or pyroclastic flow.

Flow foliation


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Pillow basalts


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Ignimbrites


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impact structures

Craters

- a large, bowl-shaped cavity in the ground or on the surfaceof a planet or the moon caused by an explosion, the impact of ameteorite, etc.

Shatter cones

- a fluted conical structure produced in rock by intensemechanical shock, such as that associated with meteoriticimpact.

stishovite (quartz)

- a dense tetragonal mineral SiO2 consisting of silicondioxide that is a polymorph of quartz and that is formed undergreat pressure


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Pseudotachylite

A black rock that resembles tachylite; carriesfragmental enclosures and shows evidence of havingbeen at high temperature.

Fault rock that has a basaltic glass appearance

Shock metamorphism

Also known as impact metamorphism describes the effects of shock-wave related deformation

and heating during impact events.


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Craters


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Shatter cones


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Pseudotachylite


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Shock metamorphism


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PRIMARY & NON-TECTONICSTRUCTURES

Group 1 (PetE-3203)

Mandocdoc, Jennifer

Bagon, Juan Paulo

Casas, Leigh Andrei

Alcantara, Jose Mari

Gonzales, Sherwin

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