Page 150 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 7: SEQUENCE STRATIGRAPHY OF THE T FACTORY 141

is at the transition between the T and M type. The margin, exposure produced by a long-term eustatic fall. The max-
an unstratified mass of automicrite and marine cement with imum flooding levels lie in the intervals of maximum sub-
some sponges and corals, is less than 50 m wide and grades mergence near the base and the top of the section. The as-
◦
seaward into a 25-35 slope. The evenly bedded rocks of sumptions in the model on the length of the time interval
the platform interior lack major breaks but clear subdivi- are at variance with recent findings on biostratigraphy and
sions emerged from mapping facies and stacking patterns radiometric dating (Brack et al. 1996; Zühlke et al., 2003, vs.
of the basic bedding units, called cycles by Goldhammer et Preto et al., 2001). However, the basic approach towards a
al. (1990; 1993). Figs 7.45 and 7.46 display the major results. stratigraphy of gradual change remains valid regardless of
The lower part of the 670-m section is shoaling upward as the outcome of the chronostratigraphic debate.
indicated by the increase of supratidal and terrestrial facies. From the Latemar case study emerges a clear strategy for
The upper part shows the reverse trend. No major break or the sequence stratigraphy of gradual change on carbonate
unconformity has been observed on the platform, nor was platforms.
there clear evidence of lowstand system tracts on the slope. ➤ Use bed facies and bed stacking patterns to reveal deep-
As major breaks and geometrically differentiated systems ening and shoaling trends generally considered syn-
tracts were lacking, Goldhammer et al. (1990; 1993) relied onymous to “transgressive/regressive” trends in se-
on proxy indicators instead. They put a sequence bound- quence stratigraphy.
ary in the interval of most frequent exposure – a unit of ➤ Identify turning points in these trends and interpret
stacked tepee horizons and meteoric diagenesis. The under- changes from transgressive to regressive as maximum
lying, shoaling-upward interval was interpreted as a high- flooding levels, changes from regressive to transgres-
stand tract, the deepening interval above as a transgressive sive as sequence boundaries.
tract. The interval of thick subtidal beds with sparse and Similar approaches have been used in many subsequent
thin supratidal layers at the base of the section was inter- studies. Recently, it has become popular to mark shoaling
preted as another transgressive systems tract. In line with and deepening trendsby triangles on the vertical section.
the definitions used here, Goldhammer et al., 1993 defined a This is excellent for quick orientationbut oftenleads to very
lowstand systems tract as a unit whose marine deposits did schematic descriptions. I suggest to at least display quanti-
not extend to the platform top. tative observations along with the triangles (e.g. D’Argenio
An important tool for the sequence stratigraphy of grad- et al. (1999) and specifically identify intervals that lack clear
ual change is the "Fischer plot" (Fig. 7.46A). It plots cu- trends (e.g. Immenhauser et al., 2004).
mulative deviation from mean cycle thickness versus cycle A consequence of defining sequences where significant
number as a qualitative measure of time; in addition, the breaks are not obvious is that sequence boundary and max-
graph plots subsidence for each cycle, assumed to be con- imum flooding surface are no longer surfaces but inter-
stant throughout the section (Fischer, 1964; Read and Gold- vals (Goldhammer et al., 1990; Schlager, 1992; Montanez
hammer, 1988; Sadler et al., 1993). Fig. 7.46B shows the and Osleger, 1993, p. 322). Only stratigraphy with very
Latemar succession of Goldhammer et al. (1993). The se- high resolution or firm evidence for the presence or ab-
quence boundary interval lies at the center, in a succession sence of significant lowstand wedges can determine if the
of very thin cycles. The maximum flooding levels are again gradual trends are punctuated by major gaps. Where large
intervals rather than distinct surfaces and lie in the rising gaps and stratigraphic turning points are lacking, correla-
limbs of the curve. Their positions are difficult to pick be- tion of sequence boundaries and systems tracts becomes
cause they lie near the lower and upper boundary of the much more arbitrary and sequence-stratigraphic analysis
plot. If one were to replace the Fischer plot by a smooth converges with the tried and true sedimentologic practice
curve, the sequence boundary would lie at the inflection of delineating shoaling and deepening, coarsening and fin-
point of the falling limb of the curve, the maximum flood- ing, thickening and thinning trends in a section. In most in-
ing level at the inflection point of the rising limb. stances, these trends simply reflect the gradual change in the
Goldhammer et al. (1993) have taken the study of the balance of the rate of accommodation creation and the rate
Latemar succession a step further by creating a computer of sediment supply. Embry (1993) made a strong case for the
simulation of the observed stratigraphy. For this simula- power of this T-R (“transgressive-regressive”) stratigraphy.
tion they assumed that the bedding rhythm was dictated by However, if transgressive and regressive intervals smoothly
the Earth’s orbital perturbations with individual beds corre- grade into one another, it becomes very difficult to distin-
sponding to the precession cycle with an approximate du- guish between trends caused by relative sea-level fluctua-
ration of 20 ky, and bundles of 4–5 beds corresponding to tions and trends caused by changes in the rate of sediment
the Earth’s short eccentricity cycle. The long-term trend in supply. Thus, the search for major breaks and lowstand sys-
cycle facies and stacking patterns can be adequately repro- tems tracts remains crucial for the reconstruction of sea-level
duced by combining the orbital pulses with a eustatic sea- history.
level wave of about 40 m amplitude and 3 My period (Fig. The above discussion leads to the following conclusions
7.47). With the above assumptions, the measured section on the sequence stratigraphy of gradual change:
would represent a little less than 3 My, the sequence bound- ➤ Sequence stratigraphy of gradual change sometimes
ary in the middle coincides with an interval of maximum is the only way to identify sequences in a responsi-