Page 84 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 5: RHYTHMS AND EVENTS IN CARBONATE STRATIGRAPHY 75

➤ Changes in precession. The Earth’s axis wobbles like lead to large gaps and “missed beats”, i.e. orbital oscillations
that of a spinning top, i.e. its tilted axis describes a cir- that remain unrecorded because sea-level remained below
cular motion and points in different directions at differ- the platform top (Fig. 5.5). Tests for orbital rhythms, like
ent times. Important precession cycles fall in the range any time-series analysis, are very sensitive to such gaps (e.g.
of 19 - 23 ky. Hinnov, 2000) and sedimentology and standard stratigraphy
The climatic effect of obliquity is the seasonality of cli- need to be used to the fullest to identify such gaps before-
mate. If the axis of rotation were perpendicular to the or- hand.
bit, there would be practically no seasons on Earth. Preces- Problems can be minimized by selecting records from ar-
sion and eccentricity combine to determine the warmth of eas with high subsidence. Fig. 5.6 shows an example from
the seasons. Warmest summers occur if the eccentricity is the Triassic of the Alps where subsidence was on the order
high and the Earth is in perihelion, i.e. the position closest of 100 m/My. The bundling of platform cycles into groups
to the Sun during one orbit. of 4-5 was one of the first observations suggesting possible
The orbital perturbations are quasi-periodic and the most orbital control of these cycles (Schwarzacher 1954). A hierar-
important periods fall in the range of 20 – 400 ky. Or- chy of bedding with bundles and superbundles, i.e. bundles
bital rhythms are highly interesting for stratigraphers be- of bundles, with ratios of 4 or 5, remains one of the best field
cause they are global signals and their periodic nature of- indicators of possible orbital control. To build a strong case
fers a means of measuring time beyond the normal resolu- for orbital control, time-series analysis is essential. Recent
tion of biostratigraphy or radiometric techniques. Thus, “cy- studies indicate that the display of series of data in spec-
clostratigraphy” (House, 1985) has become a widely used trograms, i.e. continuous series of spectra, is a particularly
technique. Orbital cyclicity in the sediment record is fre- powerful tool as it reveals the variation of rhythms with
quently inferred but the claim is less frequently backed by time (Fig. 5.7). Fig. 5.8 shows the spectrogram-technique ap-
good data. plied to the analysis of bedding in a geologic example (Preto
Shoal-water carbonates offer special challenges, mainly and Hinnov, 2003). There is a strong suggestion of orbital
related to the transformation from thickness to time. Strati- rhythms but the result is less than definitive because of the
graphic successions represent rhythms in space while the limited length of the section.
orbital cycles are rhythms in time. Testing a stratigraphic Problems with recognizing orbital rhythms are com-
record for orbital rhythms requires a transformation of the pounded in situations of low subsidence and long-term fall
data from stratigraphic thickness to geologic time. If the of eustic sea level. The Bahama platforms, for instance,
4
time control is 10 y or better, the transformation is rather have not yielded anything even remotely resembling or-
straightforward. In most other instances, one has to assume bital rhythms because the record abounds with hiatuses
that thickness is approximately proportional to time. For and missed beats (e.g. McNeill et al., 1998). The slopes
certain deposits, e.g. fine-grained pelagics, this condition is and basins, on the other hand, have yielded excellent or-
approximately satisfied. Shoal-water carbonates, particu- bital signals both by direct correlation with the pelagic stan-
larly sediments of the T factory, are notoriously problematic dard (Droxler et al., 1988) as well as by time-series analysis
is this respect. The juxtaposition of extremely high produc- (Williams et al., 2002).
tion in the uppermost water column and zero production Orbital cyclostratigraphy has great potential in stratigra-
above sea level (Fig. 2.3) induces a stop-and-go rhythm in phy and is rapidly expanding into the realm of sequence
sedimentation and makes the record very sensitive to rel- stratigraphy. Recently, many sequence stratigraphers re-
ative changes of sea level. Even minor sea-level falls may ported much higher numbers of sequence boundaries than
erosion of foreshore
& shoreface
Fig. 5.3.— Siliciclastic autocycles
generated by migrating mud banks
newly prograded on the coast of Suriname. Each
mud coastline time a mud bank passes, some of
its sediment stays behind and be-
comes attached to the shore; between
interbank
direction of mudbanks, the shore is being gently
prevailing mud bank eroded. In this way, the shore pro-
wind, grades in steps and the record con-
waves and
sists of marine muds punctuated by
inclined 5 m erosional surfaces. A space rhythm
acoustical interbank
layers (mud banks and interbank troughs)
has been turned into a stratigraphic
5 km time rhythm. After Rine and Ginsburg
(1985), modified.
discontinuities vertical
exaggeration 500 x