Geology 528; Autumn, 2002
Introduction to sedimentary basin analysis, crustal
profiles, and basin types
What is a
sedimentary basin?
thick accumulation (>2-3 km) of sediment
physical setting allowing for sed accumulation
e.g. Mississippi Delta up to 18 km of sediment
accumulated
significant element of vertical tectonics which cause formation of sed basins,
uplift of sed source areas, and reorganization of sediment dispersal
systems.
Vertical tectonics
caused primarily by
- plate tectonic
setting and proximity of basin to plate margin
- type of
nearest plate boundary(s)
- nature of
basement rock
- nature of
sedimentary rock
three broad
categories of sed basins
-
convergent
-
divergent
- strike-slip
basins
Sedimentary Basin
Analysis:Study of history of sedimentary basin and
processes that influence nature of basin fill
Requires working or
expert knowledge on wide variety of geologic
subdisciplines
sedimentology
(basis of interpretation of depositional systems
depositional
systems analysis
paleocurrent
analysis
provenance
analysis
floral/ faunal
analysis
geochronology
crustal scale
tectonic processes, geophysical methods
thermochronology (Ar/Ar, apatite F-T, etc.)
special
techniques
- organic geochemical analysis
- paleosol analysis
- tree ring analysis
involves both surface and subsurface
data
involves large changes in scale and may involve long
temporal histories
II. Basin models:Development recently of actualistic basin
models (basins models derived from the study of modern basins and applied to
interpretation of ancient basins)
Basin model should act
as:
1) a norm, for
purposes of comparison
2) a framework
and guide for future observation
3) a
predictor
4) an integrated
basis for interpretation of the class of basins it
represents
Basin classification is
complicated and has produced a large number of basin
types
Testing and refining
models based on observations is the primary way that advancement of science
occurs.
Francis
Bacon: Truth emerges more
readily from error than from confusion.
S.J.
Gould: Classifications are
theories about the basis of natural order, not dull catalogues compiled only to
avoid chaos.
III.Vertical crustal controls on sed basins
(Subsidence mechanisms)
Crustal thinning: extensional
stretching, erosion during uplift, magmatic withdrawal
Mantle-lithospheric thickening:
cooling of mantle following cessation of stretching or heating due to
asthenospheric melts
Sedimentary or Volcanic
loading
Tectonic
loading
Subcrustal loading (underthrusting of
dense lithosphere)
Asthenospheric flow, for example due
to descent of subducted lithosphere; emplacement of high density melts into
lower density crust
difference in thickness and density
between oceanic and continental crust (isostacy)
thermal history of continental &
oceanic crust
combinations of
above
More specific details of sedimentary basin fill
patterns governed by
geometric
shape and size of basin and evolution of floor and flanks of
basin
nature of stratigraphic
fill
structures that develop within basin
during its evolution (e.g. growth faults, salt diapirs
Basins should be classified according to their tectonic
setting at the time of deposition of given stratigraphic interval; basins may
change their teconic seting rapidly and often.
A complete basin
analysis must incorporate all phases of development of a basin and must consider
both proximal and distal tectonic influences.
Sedimentary successions
(basin fill) may accumulate due to subsidence of a shallow substrate (sinking
substratum) OR from filling of a space below base level (usually sea level;
filling hole)Most basins are
hybrid.
Preservation potential of
a basin is an important factor in basin analysis.e.g. trench-slope basins have low preservation
potential, whereas intracratonic basins are likely to be preserved.Also: difference between preservability of a
tectonostratigraphic assemblage vs. the basin itself. (e.g. Bay of Bengal vs.
Bengal Fan turbs).
IV.BRIEFSURVEYOFBASINS
Divergent
settings:
Most intraplate (passive
margin) continental margins originated as divergent plate margins formed during
breakup of supercontinent Pangea.Likewise, many Paleozoic plate margins (e.g.
Cordillera, Appalachians) originated from breakup of Rodinian
supercontinent.
Supercontinent cycle
350-400 Ma.Two different hypotheses to
explain breakup of supercontinent
1) random
motions of continents around earth
2)
Supercontinent acts as a thermal blanket, inducing thermal upwelling of mantle
to initiate rifting and eventual breakup.
3) Subduction
principally of old, cold crust during times of supercontinent formation.Slab roll-back (i.e. density driven slab
pull) = important phenomena that may induce supercontinent
breakup.
Rift
phase (East African Rift, Rio
Grande Rift) vs. Drift phase (Red Sea, Gulf of
California)
Rift phase is tectonically active, with normal faulting,
crustal thinning, volcanism, high heat flow and locally high rates of subsidence
and sediment accumulation.
Drift phase
(post-rift) = dominated by lithospheric cooling, thermal subsidence, and
development of broad flexural basins dominated by sediment loading (e.g.
continental embankments).
Active vs. Passive
Rifting:
Active: Kinsman (1975): early, domal uplift
preceeded crustal stretching; subaerial erosion of thermal dome thinned upper
crust enough to result in major subsidence once margin rafted away from heat
source.
Test: check amount of crustal erosion at base of
rift sequence.Should be >15 km (amount
cont. crust has been thinned by rifting).Drainage patterns should be centrifugal; should
result in sediment starvation (unless significant volcanism).Basaltic volcanism should predominate in early
rift stages.
Passive: thermal domes that precede rifting are in
response to crustal thinning, rather than the cause of crustal thinning.Doming is caused by mantle
upwelling.
Test: Basal unconformity of major proportions not
necessary; initial drainage patterns should be centripetal due to sagging of
rift zone.Volcanism should coincide with
more advanced stages of rifting.
Passive thinning is
thought to be responsible for most rift systems, although the mechanisms
responsible for extension are still unclear (e.g. supercontinent
breakup)
Geometry of
rifts
simple shear
(no mechanism for bringing mid-crustal rocks to shallow
levels)
low angle
detachment faults: either as through-going (cut entire lithosphere) or
intracrustal
Asymmetry of rift
systems by presence of major detachments will produce upper plate and lower
plate margins
Upper plate: originate in
hanging walls of detachments; thick continental crust with narrow continental
shelves and thin sedimentary cover; structurally simple with only weakly rotated
normal faults
Lower plate margins:
originate in footwalls of detachments; thin continental crust with broad
shelves, thick sedimentary cover, and exhumed middle to lower crustal rocks and
remnants of upper plate in strongly tilted blocks.
Rift may change
polarity across transfer faults
If more than one
detachment system is involved, marginal plateaus and continental slivers may
result (Ingersoll and Busby, Fig. 1.9).Continental slivers may also result because
continental crust is weaker under tensile stress than oceanic
crust.
Rifts may be localized
along zones of pre-existing weakness, as Triassic rift systems in eastern
U.S.
Characteristics of
Rift basins
very early
stages of rift development
high heat
flow
extensional
interstratified lavas and redbed sediments; evaporites common due to screening
of rivers by rift shoulders
e.g. Red Sea,
Rio Grande Rift
Red Sea is only true
proto-oceanic basin on earth, so our understanding of these basins is strongly
weighted towards examination of that basin.
Post-rift
basins (as opposed to syn-rift)
are formed mainly from subsidence resulting from thermal
relaxation
Miogeoclinal
Prisms
require
one-sided open ocean setting
transition of
sedimentary facies
- nonmarine to shallow marine deltaic
- shelf and paralic sediments of continental terrace
- marine turbidites of slope and rise
may be characterized
by salt diaparism and growth faulting
Continental
Embankments
advancement of
shelf break to point over oceanic crust (Mississippi embankment is 1000 km wide
from OK coastal plain to edge of Sigsbee escarpment)
series of
lensoidal sedimentary packages
immense
sediment accumulation (16-18 km) which loads continental and oceanic
margins
unstable:
results in growth folding, gravitational failure, salt diaparism, etc.Sigsbee salt nappe is one of largest single
structural features of the North American
continent
usually result
from drainage of large continents toward mouths of failed rifts and away from
normal rifted continental margins (e.g. Mississippi delta, Nile
delta)
Rapid subsidence
(10-100m/Ma) results from
sedimentary
load which induces lithospheric flexure
listric normal
faulting (growth faulting)
salt
withdrawal during diapirism
compaction of
unconsolidated sediments.
Progradation of
rise-slope-shelf triad results in predictable sedimentary sequence: deep sea
fan, slope mud with diapirs, shelf and shoreline, continental (primarily
fluvial).
Active Ocean
basins
largely a
function of cooling and contraction of oceanic
lithosphere
development of
thicker strata on older, colder (and more subsided) oceanic
crust
sediment
largely pelagic and hemipelagic; influenced by position of CCD and presence of
local bathymetric features like seamounts.
Dormant ocean
basins
underlain by
oceanic crust
plate margin
rearrangement results in cessation of actively spreading centers or subducting
margins
dominant mode
of subsidence is sediment loading
classic
filling hole basin
weakest part
of basin would be along residual continental margins related to extinct rifting
or subduction; subsequent orogeny might create intermontane basins corresponding
to ancient dormant ocean basins
Intracratonic
basins
overlie fossil
rift, underpinned by transitional crust?
probably high
heat flow early on
doming, normal
faulting followed by thermal subsidence
sedimentary
fill usually shallow marine, mature sediment; occasional basin
starvation
e.g. Michigan
basin, Illinois basin
Changes in stress regime appears to change the viscosity of
crust beneath the basement, resulting in accelerated subsidence during times of
orogeny in nearby mobile belts.
Marginal Aulacogens
failed
rift arm, underpinned by oceanic or transitional crust
early high heat
flow
rifting, normal faulting followed by
thermal subsidence
sedimentary fill usually thick
accumulations of shallow marine sediment
subsequent closure of associated oceanic basin "inverts" aulacogen, changes
dispersal patterns and provenance trends
e.g. Oklahoma Aulocogen; Benue
Trough
Arc-trench
systems
Can be extensional, neutral, or
compressional
Extensional: trench rollback
is faster than trenchward migration of overriding plate; characterized by low
relief, thin sediments, and deep trenches.
Compressional: overriding plate
advances trenchwrd faster than trench rollback; characterized by high relief,
abundant sediments, and shallow trenches.
Neutral: trench rollback is about
equal to trenchward advance of overriding plate.
Kanamori (1986) concluded
that a relationship existed between convergence rates, plate age, and
seismicity.Backarc opening generally
associated with subduction zones of low seismicity. Busby and Ingersoll, Fig.
1.20
Jarrard, 1986 suggested
that major factors that affect tendency for arc-trench system to be extensional,
compressional or neutral include:
- convergence
rate
- slab
age
- slab
dip
Jarrard (1986) plotted
slab descent angle (trajectory) against their dip angle (architecture) to
demonstrate that:
1) most slabs do
not descend at angles parallel to their dip angle
2) most slabs
descend at angles steeper than their descent angle (slab
roll-back)
3) a few
slabs descend at angles shallower than their descent
angle
4) a few slabs
descend at angles greater than 90 degrees
5) no
tendency for old slabs to descend steeply and young slabs to descend more
shallowly
6) probably
greater slab age does not result in swinging of slab descent towards
vertical.
Conclude, strain regime
is probably a complex function of slab age, slab dip, and convergence
rate, but more specifics await further research.
Trenches, trench slope
basins, and Forearc basins
Oceanic trenches and
slope basins:
formed by
flexure of downgoing slab
dominated by
accretion tectonics
sediment fill
dominated by gravity flow deposits, rafted oceanic
sediments
local
variations in bathymetry of subduction complex may result in slope
basins
subduction complex highly deformed by imbricate thrusting and gravitational
spreading
contacts between sediment fill and accreted material may be either depositional
or tectonic
very low
geothermal gradient
Forearc
basins
occur within
the arc-trench gap
not as highly
deformed as oceanic trench and slope basins
very low
geothermal gradient
variety of
types
Factors affecting
forearc geometry include
sediment
thickness on subducting plate
rate of
sediment supply to trench
rate of
sediment supply to forearc
rate and
orientation of subduction
time since
initiation of subduction
Arc-trench gaps tend to
widen with time due to accretion of sediments; thus general tendency for
forearcs is to enlarge with time (e.g. Great Valley forearc basin,
California)
Intra-arc
basins:
basins within an
arc system
usually filled
with volcanic material resulting from:
- magmatic explosions due to exsolution of volatiles
- hydroclastic fragmentation (magma-water interactions)
- autoclastic lava fragmentation
- weathering products from emergent arc
form in
several possible settings
- low regions between volcanoes and their flanks
- when axis of volcanism shifts to new position on an oceanic arc
platform
- fault-bounded basins within arc itself (relief created by tectonic structure,
not constructional volcanic features
poorly
understood basin type due to thermal and metamorphic overprinting and
susceptibility of volcanic seds to diagenesis
Back-Arc
Basins:
Oceanic basins
behind intraoceanic magmatic arcs
Continental
basins behind continental-margin arcs that lack foreland foldthrust
belts.
Most are
extensional, forming by rifting and sea-floor spreading
May result in
remnant arc behind BAB (if subduction does not jump)
Probably have
relatively low preservation potential due to eventual closing of BAB by
subduction
Many
ophiolites of geologic record may have originated from BABs floored by oceanic
crust.
Evolutionary
model for BABs includes periods of major extension, minor extension, and major
compression
Remnant Ocean
basins:
form during
intense deformation associated with attempted subduction of nonsubductable,
buoyant continental crust during terminal ocean closure.
irregularity
of continental margins tends to create great variability of timing, structural
deformation, and preservability occurs along strike
most sed from
orogenic highlands pours longitudinally through deltaic complexes into remnant
ocean basins as turbidites that are subsequently deformed and incorporated into
orogenic belts as collisional sutures lengthen
Foreland
Basins:
formed by
flexure of continental crust by tectonic and sedimentary
loads
may be formed
either in retro-arc or peripheral positions
strongly
asymmetric transverse profile
orogenic
flanks of basin undergo deformation during life of basin
cratonal
flanks of basin merge with platform sequences
composite
foreland basins may reflect net effects of successive orogenic episodes (e.g.
Appalachian basin)
usually
dominated by nonmarine or shallow marine strata
compositional
differences in sandstones of foreland basins (recycled) relative to BABs
(volcanic), dominantly due to presence or absence of foldthrust belt (i.e.
compressional or extensional back-arc)
distinction
between ancient retroarc and ancient peripheral foreland basins based on:
- polarity of magmatic arc
- presence of oceanic subduction complex assoc with early phase of peripheral
dev.
- greater water depths in peripheral stage
- protracted development of retroarc (e.g. Appalachians) vs. discrete
development of peripheral foreland (terminal ocean
closure)
- possible volcanic input to retroarc forelands, minimal to
peripherals
Broken Foreland
Basins
basement
involved uplifts that partition foreland basin
probably
related to shallow angle of subducting slab
generally 3
types:
- Green River type (= ponded): large equidimensional to elliptical, bounded on 3
or more sides by uplift, commonly containing lakes
- Denver type (= perimeter): elongate, open, asymmetric synclinal downwarps with
uplift on one side.
- Echo Park type (= axial): narrow, highly elongate, fault-bounded, through
drainage and strike-slip origin.
Transpressional and
transtensional basins in strike-slip fault system
en echelon or
curved strike-slip fault at constraining or releasing
bends
occur in wide
variety of settings
characterized
by rapid, complex facies changes