GEOL 340 Sedimentology and Stratigraphy
Lecture 26
1. Comparison of Time Scales
2. Sea Level Curves
Comparison of Time Scales
Haq et al. (1987) versus Berggren et al. (1995)
see handouts
Boundary Age Errors (combined radiometric errors, biostrat, magnetostrat errors)
a. Early Tertiary = + 1.5 m.y.
b. Late Cretaceous = + 6.0 m.y.
c. Early Cretaceous = + 5.0 m.y.
d. Jurassic = + 12 m.y.
e. Triassic = + 24 m.y.
Therefore, it is impossible to correlate 1 m.y. cycles with + 0.5 m.y. precision
Comparison of Sea Level Curves
Specific Problems with the Exxon Curve
a. same biozones given different ages and durations
b. different papers give different versions of the curve, despite same authors (handout)
c. average duration of third-order cycles decreases with time
(handout)
not easily explained by geologic phenomena, so probably methodology
- younger events more easily detected
- loss of resolution with depth on seismics
- more young sed data sets used than old sed data sets
d. Exxon sites chosen for analysis may be dominated by tectonism
(overhead)
Moray Firth, Scotland versus open North Atlantic at Dorset
Examples
a. Cretaceous
b. Jurassic
Why does Exxon Chart have so many more events than other curves?
a. Exxon has ubiquitous 3rd order sea level changes
probably reflects different modes of data synthesis
b. Exxon model allows them to interpret all sea level events as eustatic
c. however, many are probably driven by local plate-margin and intraplate tectonism
d. these high-frequency events are almost impossible to correlate