Post on 28-Oct-2018
1
Stratigraphic ConceptsDemonstrating 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 BenitoGravels
• Age?• Sedimentation
Rate?S.R.=∆d/∆t
• Episodic deposit.?– pause– erosion ∆d=?
∆t=?
Unconformity?
2
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
depth
time
depth
∆t
∆d
3
“Timing is everything”
• Chronology (stratigraphy) is essential forunderstanding all earth history & henceearth system processes,– tectonic– climatic– biologic (evolution).
Lithostratigraphy
LITHOSTRATIGRAPHIC UNITS -bodies of rocks distinguished on thebasis of observable lithologiccharacteristics– no connotation of age other than
the law of superposition– separated by contacts– Stratotype - type section (most
complete)
the study & organization of strata on the basis oflithologic characteristics
• lithology - type, color, mineral composition, andgrain size
4
Fundamental Units*:
• Group - collection of formations• Formation- lithologically distinctive unit
that is large enough in scale to be mapable(single lithology or regular alternation oflithologies)– e.g. Monterey Formation
• Member - (subdivision of a formation)characteristics that distinguish it from otherparts 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
Gro
up I
C
B
C
Lith
olog
y, m
embe
rs?
form
atio
n
Paleocene-Eocene Strata: Gulf & Atlantic Coast
6
Paleocene-Eocene Strata: Gulf & Atlantic CoastClayton Wilson LakeGl-91
Butano SS
San Lorenzo
Monterey
Vaqueros
Santa Cruz MS
Purisma
Locatelli
7
Contactsboundaries between units1. plane or irregular surfaces2. Conformable or unconformable
Site
126
3A -
13H
- S6
- 50
.5 c
m
Paleocene
Eocene
• Conformity (Conformable) -no physical evidence of non-deposition– Abrupt– Gradational
• progressive - gradation• intercalated - gradation is an
inter-bedded interval
8
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 metamorphicrock
Angular UC
Disconformity
Nonconformity
?
?
?
9
Recognition of Unconformities
• basal conglomerates• deeply weathered soils horizons• truncated bedding• clasts• burrowing or hardgrounds• channel deposits• truncated fossil ranges (several lineages)
10
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
gap
gaptimedepth• 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 morecontinuous
11
ODP Site 1207,northwest Pacific
• Drilling Objectives:– P/E Boundary– K/T Boundary– Aptian/Albian
ODP Site 1207, northwest Pacific
Age/Depth
13
Vertical and Lateral Successions of Strata:
• conformable and unconformable contacts dividesedimentary rocks into vertical successions.Walther’s law - to be conformable, vertically
adjacent facies must reflect those facies whichoccur side by side
• facies - a body of rock with some consistentcharacteristic
• lithofacies - a consistent lithologic characteristicwithin a formation (shale facies, evaporite facies)
Walther’s Law
Deepening upward Shallowing upward
14
Cyclic successionsrhythmic sedimentation
(repetitions of strata)• Temporal scales
annual to m.y.• All environments
pelagic - limestone/marldelta - repeated
coarsening upwardcycles
Paleocene Scalia Rosa, Dolomites, Italy
15
Cycle Order (Scales)
Cycle OrderMajor sedimentary cycle durations as influenced by
eustatic sea level changes
Milankovitch glacioeustatic cycles0.01-0.25th
Milankovitch glacioeustatic cycles0.2-0.54th
Mid-ocean ridge spreadingchanges - volume
1-103rd
Mid-ocean ridge spreadingchanges - volume
10-1002nd
Tectonic: formation and breakupof supercontinents
200-4001st
CauseDuration (m.y.)Type
16
Cyclic successions
Causal Mechanisms:• Autocyclic - internal to the
basinswitching delta lobe, storm
beds, floodplain– beds show limited lateral
continuity• Allocyclic - mainly external
to the basinclimate, 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 -147x106kmAphelion - 152x106km
17
Orbital cycles:Effect on Insolation• Eccentricity 95, 100, 120, 413 ky (2.3
m.y.)– Earth - sun distance 0.0 to 0.06Effect 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
hemispheresEffect on insolation: up to 17 W/m2 at high
latitudes• Precession 19, 23 ky
– wobble - gravitational pull of sun on earths’equatorial bulge
– elliptical precessionhot 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)
19
100 m
90 m
~10 km
Transgressions/RegressionsTransgressions - shoreline moves landwardRegressions - shoreline moves seaward3 Causes:
1. Sea level - rise and fall2. Tectonic - uplift /subsidence3. Sediment Supply
• eustatic changes– ice-volume (glacioeustatic), basin geometryGlobal signal
• relative changes– Local subsidence/uplift or sediment supplyRegional signal
20
Sea level Rise
• 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
transgression
fining upward sequence
sandmudsilt
time lines
coastal onlap
• Standstill of sealevel - no coastal onlap, but top-lap– regression
coastal onlap
Sea level rising (high sediment supply)
erosional surface
regression
Top lap Regression
Sea level stationary
no onlap
• Rising sealevel - coastal onlap,– regression
21
Lithofacies are time transgressive
Section A Section BSection C
Coast Basin
Sea level Rise
Transgression (Deepnening)Regression (Shallowing)
22
Asymmetry of Transgressive andRegressive Cycles:
• Transgressive - classic finingupward?– Rare- fining upward less common
than coarsening upward• Rapid rise in SL - erosion/non-
deposit during transgressive phase– coastal and shallow marine deposits -
thin or non-existent– deposition mainly during regressive
phases– e.g. delta progradation -
Carboniferous Cyclothems
non-marine marinenon-marine marine
transgression
regression(rapid)
transgression
regression
Delta Progradation
Rapid transgression
coal
shale(marine)
limestoneshaly
limestone
shale (sandy)channel sand
disconfomity
Carboniferous (299 to 359 Mya)
23
Carboniferous(299 to 359 Mya)
Carboniferous Coals, West Virginia
Cyclothems:Allocyclic vs. Autocyclic
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)
(e.g. Bishop Tuff, Long Valley Caldera; 740ky)
• lava flows
lithologicsimilarity
Key bed(ash)