Geology Reference
In-Depth Information
signifi cant. That is, it is conceivable that
sea-level dropped below the platform margin
and failed to produce a supratidal record, but the
facies cannot be formed without subareal
exposure. Therefore, the observed number of
occurrences of Facies 1 represents the minimum
number of intervals of platform exposure. Given
that 162 occurrences of Facies 1 were recognized
in the Cimon del Latemar section, and the dura-
tion of accumulation for that succession has been
interpreted to range from ~0.6 Myr (Zühke
et al
.,
2003) to 3.1 Myr (Preto
et al
., 2001) the most
elementary calculation would indicate that the
Latemar platform experienced subareal exposure
at an average recurrence interval ranging from
3700 years to some 19,100 years, values that are
midway between the short and long precessional
frequencies calculated by Preto
et al
. (2001).
Exclusion from this calculation of the eight occur-
rences not included in the Preto
et al
. (2001)
analysis lengthens the recurrence estimate by 5%.
Establishing meaningful thickness-duration rela-
tionships for this Facies 1 is extremely diffi cult
(Drummond, 2002) because of its importance in the
interpretation of platform exposure and submer-
gence, and because vadose diagenesis and caliche
formation are not constrained to follow the law of
superposition due to the fact that diagenetic altera-
tion and soil formation occurs in a penetrative way
as an alteration of existing sediments. Thus, the
spatio-temporal occurrence of this facies is much
more signifi cant than the thickness of individual
occurrences. Accounting for potential computa-
tional biases resulting from exclusion of very thin
but palaeoenvironmentally signifi cant facies is a
critical topic for future work. This could be espe-
cially critical in peritidal ramps exhibiting more
lithological complexity than the bilithological
cycles found at Cimon del Latemer.
As fi eld geologists, stratigraphers and
sedimentologists are well-trained in the tech-
niques of observation and interpretation of
outcrops. One of the most important aspects of
fi eld geology is the ability to observe and under-
stand the geological 'signal' that is often over-
printed by various forms of 'noise' that can
impede high-quality interpretations (Fichter,
1987). Specifi cally, the effects of weathering,
erosion, and cover by scree or vegetation can
adversely affect the quality of observation and
interpretation in the fi eld. These problems are
compounded when the dataset is reduced from
an integrated sedimentological and stratigraphic
analysis to a set of pixellated grey-scale data
drawn from across an image of an outcrop.
The grey-scale technique assumes that there is
a direct relationship between lithological com-
position and grey-scale value. As discussed by
Schwarzacher (2005), the shade of grey of any
pixel in a photograph is the result of a complex
set of variables that include lithology, weath-
ering and cover. Additionally, there are less
obvious factors that infl uence grey-scale intens-
ity that include sharpness of contrast due to
ambient light conditions, angle of the sun and
depth of shadows, as well as the effect of any
ground or surface waters on the outcrop. Taken
together, these factors make interpretation of a
stratigraphic section by grey-scale analysis an
extremely complex and potentially unreliable
process.
In a study of the Upper Triassic Dachstein
Limestone of Italy, Cozzi
et al
. (2005) conducted a
spectral analysis of 830 grey-scale measurements
from a fi eld photograph. The three segments of
the Picco di Carnizza outcrop included in the
study cover approximately 270 m of section and
are interpreted to be composed of 112 Lofer-type
cycles with an average thickness of 2.41 m. Given
the scale of the image and the resolution of the
scan, each pixel is calculated to represent 0.135 m
of section and thus, on average, a cycle is com-
posed of 18 pixels or approximately 0.225 mm
on the photograph (Fig. 4). Thus, this technique
creates over a 10,000 fold reduction in strati-
graphic resolution, prior to the spectral analysis
of the data. Variations in the grey-scale values of
this record are interpreted by Cozzi
et al
. (2005)
to be the product of differential weathering
of the cycles, wherein lighter shades are taken
to be resistant ledges of subtidal limestone and
the darker shades are recessive weathered and
vegetated portions of the outcrop.
GREY-SCALE ANALYSIS OF OUTCROP
PHOTOGRAPHS
Recently stratigraphic analyses have been con-
ducted using digital outcrop photographs as a data
source. In this technique, a fi eld photograph of an
outcrop is sampled along a continuous transect to
create a series of grey-scale intensity value meas-
urements. Since the data are generated from a
pixilated image, the result is a uniformly sampled
continuous series of data. Such a data series could,
in the ideal, present a very useful foundation for
quantitative analysis.
