GEOL 340 Sedimentology and Stratigraphy
Lecture 35
Methods for Documenting High-Frequency Sea Level Changes
1. Pinning Point Curves
2. Fischer Plots
3. Milankovitch Cycles and Cyclostratigraphy
Announcements:
1. hand lense, pencils and hammers
2. $18 payment to Geology Department office
3. term papers due
4. return graded exercises
1. Pinning Point Curves
Technique developed for Micoene Carbonate Platforms, Spain
Franseen et al. (1993) and Goldstein and Franseen (1995)
Precise records of sea level change
a. shoreline position
b. marine-nonmarine sediments contact
c. near sea level facies (beaches, tidal flats, reefs)
d. subaerial exposure unconformities overlain by marine sediments
Pinning Point Method
a. document precise elevation of sea level indicators
b. relative elevatiuons provide fixed points on a sea level curve
c. similar to sequence stratigraphy, but uses more geological
data from outcrop
d. more control on corrections (geopetals used to control tectonic
tilting)
Example: Miocene of Spain
a. Nijar cross-section
b. pinning point curve
c. example points
PP1 = subaerial exposure on top of volcanic basement overlain
by marine seds
PP7 = subaerial exposure tracked down paleoslope across shelf
break
PP10 = reef facies encursted by Porites, indicaitng reef
crest environment
2. Fischer Plots (meter-scale cycles)
Technique developed for study of Triassic Alps by Fischer (1964)
Cycle = layer or layers of sediments deposited during consistent environmental changes
Fischer Plot Method (WARNING: all fundamnetal assumptions may
be invalid)
a. plot of cycle thickness versus duration of deposition
b. plot gives estimate of subsidence (assumes constant subsidence
and sed rates)
c. line joining cycle tops interpreted to reflect changing accomodation
space
d. rise in slope = thick cycles = increasing accomodation space
e. decrease in slope = thinner cycles = decreasing accomodation
space
What is actually being plotted (Sadler et al., 1993)
a. actually plots the distribution of thickness over an entire
section
b. vertical axis is deviation from mean cycle thickness
c. horizontal axis is the accumulative cycle thickness
should be labelled cycle number
d. therefore, graphs may finish where they start
e. must have at least 50 cycles to avoid statistical distortions
f. suggest calling them "waves" instead of "cycles"
Practical Problems
a. different sed definitions of the basic "cycles" cause
offsets in the Fischer plot patterns
b. depositional hiatuses cause serious distortions in the Fischer
plots
c. changes from cyclic to noncyclic deposition in the same section
can not be incorporated
d. can not distinguish local and regional effects from global
effects
e. sedimentation rate changes resulting from facies changes cause
significant distortions
f. plots are always asymetric due to flatter thin-bed parts of
waves
Example: Cambrian of Utah (Osleger and Read, 1993)
3. Milankovitch Cycles and Cyclostratigraphy
Milankovitch Concept
a. rhythmicity of glaciation attributed to astronomical forcing
b. variations in earth's orbit affects distribution of solar radiation
(by latitude and season)
c. Serbian mathematician named Milankovitch proved correlation
of orbit/solar radiation
d. Milankovitch band = 104 - 105 years
Pleistocene-Holocene Eustasy
a. glacial advance/retreat causes lowering/rise of global sea
level
b. 100 m sea level drop during glacial advance
c. 50 m sea level rise if remaining ice caps were melted
d. combined effect = 150 m sea level change at rates of 0.01 m/yr
= Milankovitch band
e. however, other effects (i.e. intraplate stresses) maybe in
same band
Components
a. eccentricity (stretch in shape of earth orbit around sun)
major periods = 413 and 100 ka
minor periods = 2035.4, 412.8, 128.2, 99.5, 94.9, 54 ka
b. obliquity (tilt of earth's axis)
major period = 41 ka
minor periods = 53.6 and 39.7 ka
c. precession (wobble of earth's rotation)
major period = 23.7 ka
minor periods = 19 ka (perihelion = earth-sun distance)
Climatic Links
a. low obliquity = more solar energy to equator, less to poles
= steep latitudinal temp
b. changing precession = changing solar insolation
c. each effect has different periodicities, so go in and out of
phase
d. phases control earth ocean-atmosphere circulation cells (Hadley,
Ferrel, Polar)
trade winds develop at contact of cells
more humidity = greater upwelling due to adaibatic cooling effects
more arid = more downwelling
e. prominant controls
low latitudes = precession and eccentricity dominant
mid-latitudes = all orbital parameters affect this zone, changing
length of seasons
high-latitudes = obliquity is dominant
f. Milankovitch modelled these phase relationships
g. cyclostratigraphy compares periodicities of geologic evidence
with Milankovitch phases (i.e. isotopes, bed thickness, grain
size, etc.)