Geology Reference
In-Depth Information
specifi c attention to assessing the validity and
reliability of analytical results derived from them.
Facies rank
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DISCRETE FACIES RANK TECHNIQUE
One of the central objectives of stratigraphy is the
analysis of lithologically complex stratigraphic
successions. The selection of an appropriate and
representative suite of lithofacies, the parsing
of the section according to occurrences of these
lithofacies, and the ranking of the lithofacies by
depth or some other palaeoenvironmentally sig-
nifi cant characteristic requires a comprehensive
synthesis of all the observable sedimentological
and stratigraphic characteristics possessed by the
succession. Of the many and varied depositional
environments recognized in sedimentary rocks,
shallow-water and peritidal carbonate settings
present perhaps the greatest degree of textural
and compositional variability and complexity.
From the Proterozoic to the modern, sediments
deposited in temperate and tropical carbonate
environments display an astonishing variety of
sedimentary features. In some cases, individual
facies can be tied directly to specifi c depth ranges
or palaeoenvironments. For instance, the frame-
stone textures of reef environments are readily
recognizable, as are the desiccation features of
supratidal laminites. Diffi culty arises, however,
in distinguishing and ranking the depth or posi-
tional relationships between the large number of
apparently substitutable subtidal facies that form
across the range of depths affected by wave action.
Such lagoonal, shallow-shelf or ramp sediments
make up signifi cant percentages of ancient car-
bonate rocks and therefore have been the subject
of extensive study and debate.
Ranking of sedimentary facies by depth is a
common and well-established practice in strati-
graphic analysis (Fischer, 1964; Olsen, 1986;
Bond
et al
., 1991). Qualitative and quantitative
analysis of patterns of depth-dependent facies
transitions have likewise been conducted in a
wide range of carbonate settings and the analysis
and interpretation of carbonate facies cyclicity
has been the subject of debate for several decades
(Schwarzacher, 1975, 1993). More recently, how-
ever, workers have begun to approach the quan-
titative analysis of depth-ranked successions in a
new way. By converting a standard stratigraphic
column into a lithofacies series, where the depth-
ranked value of the succession is established at
a constant thickness interval
Deeper
Fig. 1.
Graphical example of the discrete facies rank
technique where a stratigraphic column is parsed into fi ve
distinct facies types, ranked by depth or some other palaeo-
environmentally signifi cant parameter, and then sampled
at a constant spacing
t
. This technique allows for the
creation of a continuous proxy sea-level curve derived
from standard stratigraphc data.
series of stratigraphic data is readily evaluated
for spectral characteristics (Fig. 1). This tech-
nique has, for example, been used by Preto
et al.
(2001) to argue for the presence of Milankovitch
forcing of eustatic sea-level recorded by shal-
low carbonate sediments of the middle Triassic
Latemar platform. In that example, the classic
bi-lithological cycles of the Latemar platform were
further subdivided into four depth-ranked sub-
lithologies. The 160-m Cimon del Latemar section
was parsed into 472 occurrences of four sublitho-
facies with an average thickness of 34 cm. This
measured stratigraphic column was then sampled
at 0.5 cm intervals to create a ranked lithofacies
series of 32,000 equally spaced secondary obser-
vations. The resultant series was then taken as a
proxy sea-level curve for the duration of the Cimon
del Latemar deposition. After tuning the results
of a spectral analysis of the lithofacies series to a
precessional duration of 21.7 kyr, the Latemar car-
bonates are then interpreted to have formed under
conditions of periodic low-amplitude (2-3 m)
sea-level oscillations driven at Milankovitch
orbital frequencies.
While this approach has yielded compelling
results in the case of the Latemar succession, there
remain several important questions to be consid-
ered regarding the validity and reliability of the
t
, the resultant
