GEOL 340 Sedimentology and Stratigraphy
Lecture 34
1. Sea Level: Methods for Reconstructing
Sea Level: Methods for Reconstructing
Methods for reconstructing sea level changes (Handout from
Kendall and Lerche, 1988):
a. Hypsometry
tracking shore line positions through time
measure aerial extent of marine sediments on continents
b. Sediment Packaging (Sequence Stratigraphy)
geometries of stratal packages in relation to bounding unconformities
c. Paleo-Bathymetry Markers (water depth)
paleoecology
shoaling (shallowing) cycles
strandline markers (beaches, knotches, etc.)
d. Crustal Subsidence
differences from thermo-tectonic curves
peturbations from integrated stacked curves
Current Approaches
a. Theoretical geophysical modelling of structural and tectonic methods to change continent and ocean elevation
b. Basin modelling of subsidence, sea level change, and sediment supply
c. Detailed analysis of stacking patterns - tracking of the vertical and lateral changes in depositional stacking patterns and their associated sedimentologic and geochemical features
d) Shallow-to-deep water correlations
shallow-water: directly affected by SL change, but often severely altered or destroyed in the process
deep-water: basinal sediments are unrivaled for their record of oceanographic response to changes in climate, circulation and chemical, however, the direct linkage is only assumed
Combining records in shallow-water and deep-water allow gauging sea level change along paleoslope to directly constrain paleoceanographic responses generally used as a proxy for sea level change
1. Backstripping
Modifying Factors (show the accommodation space diagram)
a. sedimentation
facies-specific sedimentation rates
changes in lateral sediment supply and dispersion
b. sea level changes
local or regional
eustatic
c. sediment compaction - reduces initial thickness estimates
d. difficult to determine appropriate tectonic driving mechanism
not just simple McKenzie thermal/shear model
Errors Caused by Ignoring Modifying Factors
can create 100% + errors for the timing, rate, and magnitude of
sea level change
The Sea Level-Subsidence Teleology
a. use steady tectonic setting to gauge sea level change
b. use steady sea level change to gauge tectonic setting
Backstripping
a. progressively remove sediment load with time
b. divide section into thickness/age increments
c. add to basement one-by-one
d. correcting each layer for:
compaction and lithification (bulk density)
paleobathymetry
sea level changes
depth to basement
e. loaded isostatic subsidence is calculated for the rock unit
after corrections
f. backstripped depth of basement surface calculated with ONLY
water weight
Example:
a. Western Canadian Shield (Bond and Kominz, 1984)
flexural subsidence model applied
basin deeper than can be explained with model = thermal subsidence
sediments farther inland than can be explained with model = eustasy
does not provide high-resolution controls on timing, rate, and
magnitude
b. Iowa Craton USA (Bond and Kominz, 1991)
stable craton unaffected by tectonic motions
Iowa baseline showed 3 relative sea level rises
results could not be explained with the McKenzie model
interpreted as longterm intraplate stress flexure
however, even "stable" cratonic interiors affected
by tectonic (epeiric) movements