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Primary StructuresGeologic Structure A definable shape or fabric
in a rock
Primary Structure: A structure formed during or shortly after
deposition (sedimentary) or formation (igneous) of rocksSecondary
Structure: A structure formed after its host rock is formedTectonic
Structure: A structure formed as a result of strain due to tectonic
deformation
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Primary Sedimentary StructuresBedding: The primary surface in a
sedimentary rock, separating beds with different composition,
texture, color, cement (make sure you recognize beds based on these
criteria!)Different beds represent different source, sedimentary
processes, and environments of depositionEmphasized in outcrop by
parting and differential weathering and erosionA plane of
separation, along which the rock has a tendency to split or
fracture parallel to bedding (dont confuse with fracture!)Commonly
due to the weak bonds between different beds, or preferred
orientation of claysCommonly, there is a bedding-parallel fracture
which forms due to unloading or rocksClosely-spaced parting is
called fissility (e.g., in shale).
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Bedding between inter-bedded sandstone and conglomerate
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Reasons Why Clays or Clasts are Preferrably-Oriented?Sedimentary
settling of elongate or planar flakes in the gravity field
(syn-depositional)
Rotation and reorientation of flakes in a flowing fluid
(syn-depositional). Flakes reorient so that the traction is
minimized This may lead to imbrication (grains overlap like roof
singles) which may be used for paleo-current analysis (finding past
flow direction and regime) e.g., Pebble Imbrication where shingled
flat pebbles indicate current direction Reorientation (rotation)
due to post-depositional compaction (squeezing of unlithified
sediment due to the weight of the overlying rocks).
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Bedding is Important in Structural AnalysisBedding is used as a
paleo-horizontal, or nearly horizontal reference frame (recall the
principle of original horizontality)
Bedding as a primary structure (S0, or original surface) is the
first object that becomes deformed. The subsequent deformation
surfaces created (S1, S2, S3) are compared relative to the S0
Structures, textures, fossils, etc, in beds provide clues as to
the:Depositional environmentStratigraphic facing (younging
direction) to identify right-side-up or overturned bedsCurrent
directionBeds help us to better map stratigraphic contacts, and
identify large structures such as folds, faults, and
unconformities.
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Bedding between sandstone & conglomerate
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Graded BedsGraded beds: Progressive fining of clast grain size,
from the base to the top of a bed; form as a consequence of
deposition by turbidity currents (e.g., in turbidite)
Can indicate which way is up provided the bed is not inversely
gradedProvide information for stratigraphic facing and possibly
current direction, e.g., if cross-beds are present
Must know what kind of depositional environment deposited the
bed example: debris flows - deposit inverse graded beds, storm
deposits (tempestites) & turbidites are typically graded
beds
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Graded Bedding
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Cross Beds provide information for facing and possibly current
directionCross beds: Are surfaces within a thicker, master bed that
are oblique to the bedding in the master bedDefined by subtle
parting or concentration of grainsForm when grains move from the
windward or upstream side of a dune ripple, toward the leeward or
downstream sideTopset: thin, usually concave upward, laminations
parallel to the upper master bedding.Foreset: inclined, curved,
laminations or beds deposited parallel to the slip face. These
merge with the topset and bottomset beds. Foresets define the cross
beds. Current direction is perpendicular to the strike of the
foresetBottomset: thin laminations parallel to the bottom master
bedding
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Cross Bedding
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Cross BedsErosion truncates the topset and upper part of the
foreset, juxtaposing younger bottomsets on the older foreset; this
forms higher foreset angles at the upper bedding compared to the
tangential angles below (used for facing).
The foreset beds are inclined at an angle to the main planes of
stratification.- Truncated at top- Tangential at bottom .- Dip
direction indicates transport direction
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Ripple Marks
Ridges and valleys on the surface of a bed, formed due to
current flow. Cross stratification with wave amplitude < 6
(1) Oscillation or Symmetric Ripple MarksOscillation wave
produced ripples (current moving in two opposite directions)Crests
are pointed and troughs are curvedSymmetrical concave up small
scale (amplitude < 6") cross stratification.Good facing
indicator(2) Current or Asymmetric Ripple MarksAsymmetric cross
stratification produced by current moving in one direction; i.e.,
uniformly flowing currentGood current direction indicator
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Ripple Marks
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Mud CracksPolygon shape in map view. Result from desiccation
into an array of polygons separated by mud cracks.
Thin (typically sand filled) fractures that taper down in cross
section because each polygon curls upwards along its margin.
Good facing indicator (individual cracks taper downward.
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Mud Cracks
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Other CastsErosion or scraping, filling, subsequent erosion
produce positive relief casts. Good indicators of current
direction
Groove casts - Elongate nearly straight ridges
Bounce, Brush, Skip marks All are discontinuous type of groove
cast
Flute Cast Asymmetric troughs formed by fluid vortices or eddies
(mini-tornadoes) that dig into unconsolidated sediment Stronger
vortex at the upstream end cuts deeper and narrower than the
downstream part which is shallower and wider. Thus, flute casts
taper down-stream!
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Sole Marks - Load CastsBulbous protrusions of denser sand into
less dense mud layers
Forms due to density instability when sediment is still soft
(i.e., still unlithified) The sinking is triggered by the
disturbance during earthquake, storm, or slump
At greater depths, partially consolidated mud breaks into pieces
and sink into underlying sand, forming disrupted bedding
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ContactsContact: Boundary between two geologic units of any
kind.
Depositional contact: a sedimentary unit is deposited on top of
another.
Fault contact: two units are juxtaposed by a fault.
Intrusive contact: an igneous cuts across another rock body.
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UnconformitiesConformable contact: The boundary between adjacent
beds or units does not represent substantial gap in timeA
succession of beds of nearly the same age that represent nearly
continuous deposition
DiastemErosion surfaces within a conformable succession of
strata
Unconformable contact (unconformity):Represents an interruption
in sedimentation, such that there is a substantial gap in time
(called hiatus), few years to billions of years, across the
contactContact represents erosion or non-deposition of strata
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Unconformity
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Four Types of UnconformityAngular unconformity - Beds below and
above the unconformity have different attitudes. Beds below are
truncated by the unconformity.
Buttress (onlap) unconformity New beds lie on areas with
significant pre-depositional topography. The younger layers are
truncated by the rugged unconformity (difference with angular
unconformity). Beds above and below the unconformity may or may not
parallel the unconformity. There is an angular discordance between
the beds above and below the unconformity
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Types of UnconformityDisconformities Beds above and below the
unconformity are parallel, but there is a hiatus, created by
non-deposition or erosion. A disconformity is hard to recognize in
the field Fossils, paleosols, or scour features help!
Nonconformities Strata deposited on older, crystalline (igneous
or metamorphic) basement rocks
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Identifying UnconformitiesBasal conglomerates, rest on
unconformable surface and contain fragments (clasts) of underlying
rocks
Topographic relief
Paleosols - Ancient soils, weathered zone just below the
unconformityRecognized by color change, and soil structures
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Soft SedimentPenecontemporaneous StructuresSediments may be
deposited with a gentle initial dip. In this case, gravity may pull
them down during storm or earthquake. The downslope movement is
helped by fluid pressureIf sediments that move down the slope are
soft, they may produce a slurry of clasts suspended in a matrix
called debris flow. When the debris flow comes to rest, it forms a
poorly-sorted conglomerateIf the sediments are compacted
sufficiently before they are dislodged by gravity, they maintain
their cohesion, and produce what is called slumping
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Penecontemporaneous structuresThe folds and faults formed during
slumping are called penecontemporaneousPenecontemporaneous means
that they formed almost (hence pene) at the same time as the
original deposition of the layers
Penecontemporaneous folds and faults are characteristically
chaotic
They are intra-formational, i.e., bounded above and below by
relatively undeformed strata
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Growth FaultsSynsedimentary faulting - fault displacement
continues as sediment is deposited on top of the fault blocks
Thickness of sedimentary units varies across the fault
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Volcanic StructuresFlow LayeringLayers of volcanic flows defined
by color, texture and weathering.
Flow structures Pahoehoe; Ropy lava - Good flow direction
indicator
Pillow StructuresFlat bottomed, curved top basalt encased in
thin obsidian coverGood facing indicator
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Volcanic Structures, contdVesiclesVoids formed by gas bubbles
typically more numerous at the top of the flowGood facing
indicator
Columnar JointingFractures formed in basaltic lava due cooling
and shrinkagePolygonal columnsProduct of slow cooling, top of flow
does not have as well defined columnar joints as base of flow. Good
facing indicator
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Intrusive - Plutonic StructuresFlow FoliationAligned minerals in
intrusive igneous rocks occurs while rocks are still melted or
partially melted and flowing.
Indicates flow direction