Page 85 - Carbonate Sedimentology and Sequence Stratigraphy

76 WOLFGANG SCHLAGER

previously observed. For instance, Van Wagoner et al. (1990, quency bands of orbital cycles and stratigraphic sequences
p. 52) estimate that type-1 unconformities are spaced at in- broadly overlap and the two approaches complement each
tervals of 100,000 - 150,000 years and that global curves such other. In the Neogene and Quaternary, where a continuous
as the one by Haq et al. (1987) refer to sets of sequences. orbital time scale exist, many standard sequences have been
The carbonate record confirms this notion in many instances correlated to and dated by the orbital clock (e.g. Lourens and
(e.g. Fischer, 1964; Read and Goldhammer, 1988; Goldham- Hilgen, 1997).
mer et al., 1990). These observations imply that the fre-

A) march 20
B) Fig. 5.4.— Principles of orbital pertur-
dec 21 23.5°
bations. A) The Earth in its elliptical orbit
around the Sun. Tilt of the Earth’s rota-
june 21
tional axis causes the seasons. The an-
gle of tilt varies with cycles of 40 and 54
ky. B) The direction of tilt of the Earth’s
axis slowly changes, completing a circu-
lar loop every 26 ky. The climatically ob-
servable result of this cycle of axial pre-
cession is a bundle of “precession cy-
september 22
cles” of19to23ky. AfterImbrie and Im-
brie (1979), modified.

periodic platform carbonate deposition in tune to Milankovitch rhytms
Fig. 5.5.— Scenarios for deposition of
carbonate cycles dictated by the Earth’s
time orbital perturbations (Milankovitch cy-
100,000 y 0 y cles). Top panel shows the record
platform top of flooding and exposure of a platform
top by cycles that consist of a ba-
relative sea level sic rhythm of 20,000-y (orbital preces-
sion) plus a 100,000-y eccentricity cy-
cle. The 100,000-y cycle has low ampli-
tude and consequently all sea-level fluc-
tuations in the 20,000-y rhythm are be-
exposure of deposition on ing recorded by deposition on the plat-
platform platform form top (shaded peaks of cycles). Bot-
tom panel shows the same superposition
of cycles but with high amplitude oscilla-
aperiodic platform carbonate depsosition with Milankovitch sea level oscillations
tion in the 100,000-y cycle. This leads
to a very erratic record whereby only few
time
20,000-y cycles are recorded on the plat-
200,000 y 100,000 y 0 y
form top and most of the time is repre-
sented by exposure of the platform top.
After Hardie and Shinn (1986).
platform top
relative sea level

deposition on
platform