Stratigraphic Concepts
Demonstrating equivalency (genetic/time)
between strata (rock units)
• lithostratigraphy - organization of strata on the basis of
their lithologic characteristics
• biostratigraphy - organization of strata on the basis of the
fossils they contain
• magnetostratigraphy - organization of strata on the basis of
their magnetic characteristics
• chemostratigraphy - organization of strata on the basis of
their isotopic characteristics
• seismic stratigraphy - organization of strata on the basis of
their seismic characteristics
• chronostratigraphy - time relationships
• sequence stratigraphy - depositional sequences, packages of
strata bounded by unconformities
San Benito
Gravels
• Age?
• Sedimentation
Rate?
S.R.=∆d/∆t
• Episodic deposit.?
– pause
– erosion
Unconform
ity?
∆d=?
∆t=?
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Absolute vs. Relative Time
• Absolute time - set within the framework of
geologic time
– essential for reconstructing tectonic history,
paleoclimates, etc.
• Relative time - time represented by the outcrop
– essential for computing accumulation rates, etc.
time
time
∆t
depth
∆d
depth
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“Timing is everything”
• Chronology (stratigraphy) is essential for
understanding all earth history & hence
earth system processes,
– tectonic
– climatic
– biologic (evolution).
Lithostratigraphy
the study & organization of strata on the basis of
lithologic characteristics
• lithology - type, color, mineral composition, and
grain size
LITHOSTRATIGRAPHIC UNITS bodies of rocks distinguished on the
basis of observable lithologic
characteristics
– no connotation of age other than
the law of superposition
– separated by contacts
– Stratotype - type section (most
complete)
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formation
Fundamental Units*:
• Group - collection of formations
• Formation- lithologically distinctive unit
that is large enough in scale to be mapable
(single lithology or regular alternation of
lithologies)
• Member - (subdivision of a formation)
characteristics that distinguish it from other
parts of the formation
– e.g. series of phosphatic-rich layers interfingering
with dolomites/cherts
• Beds - subdivision of a member, smallest
unit
*Only applied to land-based sedimentary
sections
Group I
– e.g. Monterey Formation
B
Lithology, members?
C
C
Paleocene-Eocene Strata: Gulf & Atlantic Coast
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Paleocene-Eocene Strata: Gulf & Atlantic Coast
Clayton
Wilson Lake
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Paleocene-Eocene Strata: Gulf & Atlantic Coast
Gl-91
Clayton
Wilson Lake
Purisma
Santa Cruz MS
Monterey
Vaqueros
San Lorenzo
Butano SS
Locatelli
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Contacts
Site 1263A - 13H - S6 - 50.5 cm
boundaries between units
1. plane or irregular surfaces
2. Conformable or unconformable
• Conformity (Conformable) no physical evidence of nondeposition
– Abrupt
– Gradational
• progressive - gradation
• intercalated - gradation is an
inter-bedded interval
Eocene
Paleocene
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Contacts
• Unconformity (unconformable) - break or hiatus in
deposition (erosional, non-deposition)
current scour surface, or sub-aerial weathering surface, slump or
slide surfaces
– Angular - younger sediments atop eroded surface of
tilted or folded rocks
– Disconformity - parallel bedding planes, but erosional
surface (channeled, paleosols, lag-gravel deposit) uplift, sea-level regression
• lithology may change
– Paraconformity - same lithology above and below, non
deposition or dissolution.
• Can only be recognized by other stratigraphic techniques
• Nonconformity - sedimentary / igneous or metamorphic
rock
Angular
? UC
Disconformity
?
?
Nonconformity
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Recognition of Unconformities
•
•
•
•
•
•
•
basal conglomerates
deeply weathered soils horizons
truncated bedding
clasts
burrowing or hardgrounds
channel deposits
truncated fossil ranges (several lineages)
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Lateral Contacts
• pinch-out - progressive thinning of a bed
• intertonguing - lateral splitting of units that
pinch out independently
– shoreline migrating back and forth
• progressive lateral gradation
Stratigraphic completeness:
hiatuses (diastems) - abundant in the rock record – more frequent in high energy environments
• Sedimentation rates high, but episodic (erosion common)
• continental
– non-deposition and erosion - uplift
– episodic deposition - flooding
depth
• shallow marine
– Erosion - regressions (local, global)
– non-deposition, wave erosion
– deposition during storms
• hemi-pelagic
– slumps, turbidites represent episodic
deposition
• pelagic
– sedimentation tends to be lower, but more
continuous
time
gap
gap
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ODP Site 1207,
northwest Pacific
• Drilling Objectives:
– P/E Boundary
– K/T Boundary
– Aptian/Albian
ODP Site 1207, northwest Pacific
Age/Depth
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Facies Relationships in Space and Time
Prograding Delta
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Vertical and Lateral Successions of Strata:
• conformable and unconformable contacts divide
sedimentary rocks into vertical successions.
Walther’s law - to be conformable, vertically
adjacent facies must reflect those facies which
occur side by side
• facies - a body of rock with some consistent
characteristic
• lithofacies - a consistent lithologic characteristic
within a formation (shale facies, evaporite facies)
Walther’s Law
Deepening upward
Shallowing upward
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Cyclic successions
rhythmic sedimentation
(repetitions of strata)
• Temporal scales
annual to m.y.
• All environments
pelagic - limestone/marl
delta - repeated
coarsening upward
cycles
Paleocene Scalia Rosa, Dolomites, Italy
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Cycle Order (Scales)
Cycle Order
Major sedimentary cycle durations as influenced by
eustatic sea level changes
Type
Duration (m.y.)
Cause
1st
200-400
Tectonic: formation and breakup
of supercontinents
2nd
10-100
Mid-ocean ridge spreading
changes - volume
3rd
1-10
Mid-ocean ridge spreading
changes - volume
4th
0.2-0.5
Milankovitch glacioeustatic cycles
5th
0.01-0.2
Milankovitch glacioeustatic cycles
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Cyclic successions
Causal Mechanisms:
• Autocyclic - internal to the
basin
switching delta lobe, storm
beds, floodplain
– beds show limited lateral
continuity
• Allocyclic - mainly external
to the basin
climate, sealevel, tectonic
movements in source area
– beds may show extensive
lateral continuity
Flood Plain Deposits,
Paleocene-Eocene Bighorn Basin, WY
U. Carboniferous, S Wales
Allocyclic Mechanisms
Milankovitch cycles oscillations in earth's orbit
primary periods:
• 19, 23 ky - precession of
the pole (wobble)
• 41ky - obliquity (tilt)
• 100, 410 ky - eccentricity
Perihelion -147x106km
Aphelion - 152x106km
16
Orbital cycles:Effect on Insolation
•
Eccentricity 95, 100, 120, 413 ky (2.3
m.y.)
– Earth - sun distance 0.0 to 0.06
Effect on insolation: ca. 0.7 W/m2
uniform across latitudes
•
Tilt 41 ky (29, 54 ky,1.25 m.y.)
– angle ~ 22.0-24.3°
hotter summers / colder winters in both
hemispheres
Effect on insolation: up to 17 W/m2 at high
latitudes
•
Precession 19, 23 ky
– wobble - gravitational pull of sun on earths’
equatorial bulge
– elliptical precession
hot summers/cold winters in one hemisphere,
and cold summers/hot winters on the other.
Effect on insolation: up to 40 W/m2
Ice-sheets (18 kya, present day)
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Sea-level Change, 150 kya to present
Sea Level
120 meters
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90
10
m
0m
~10 km
Transgressions/Regressions
Transgressions - shoreline moves landward
Regressions - shoreline moves seaward
3 Causes:
1. Sea level - rise and fall
2. Tectonic - uplift /subsidence
3. Sediment Supply
• eustatic changes
– ice-volume (glacioeustatic), basin geometry
Global signal
• relative changes
– Local subsidence/uplift or sediment supply
Regional signal
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Sea level Rise
sand
silt
mud
transgression
coastal onlap
time lines
fining upward sequence
• Transgression
– Sea level rise w/ no change in sediment supply
• Stationary
– Sea level rise w/ balanced by sediment supply
• Regression
– Sea level rise w/ large increase in sediment supply
• all three produce coastal onlap because sea level is rising
Sea level rising
(high sediment supply)
erosional
surface
regression
coastal onlap
• Rising sealevel - coastal onlap,
– regression
Top lap
Regression
no onlap
Sea level stationary
• Standstill of sealevel - no coastal onlap, but top-lap
– regression
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Coast
Section C
Section A
Basin
Section B
Lithofacies are time transgressive
Transgression (Deepnening)
Regression (Shallowing)
Sea level Rise
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Asymmetry of Transgressive and
Regressive Cycles:
• Transgressive - classic fining
upward?
– Rare- fining upward less common
than coarsening upward
non-marine
marine non-marine
regression
transgression
• Rapid rise in SL - erosion/nondeposit during transgressive phase regression
– coastal and shallow marine deposits thin or non-existent
– deposition mainly during regressive
phases
– e.g. delta progradation Carboniferous Cyclothems
marine
(rapid)
transgression
limestone
limestone
shaly
shale
(marine)
coal
shale
(sandy)
channel sand
disconfomity
Delta
Progradation
Rapid
transgression
Carboniferous (299 to 359 Mya)
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Carboniferous
(299 to 359 Mya)
Cyclothems:
Allocyclic vs. Autocyclic
Carboniferous Coals, West Virginia
Correlation of Lithostratigraphic Units
• Lateral Tracing
• most direct method
• only where strata are continuously
exposed
• Lithologic Similarity and Stratigraphic
position
• indirect method
• correlation based on facies sequence
• difficult to apply to cyclic successions
• Event Stratigraphy
• Marker beds
• ash (bentonites)
lithologic
similarity
Key bed
(ash)
(e.g. Bishop Tuff, Long Valley Caldera; 740
ky)
• lava flows
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Correlation of Lithostratigraphic Units
Bishop Tuff
• Long Valley Caldera
• 740 ky
Lithologic Similarity and Stratigraphic position
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