Page 149 - Carbonate Sedimentology and Sequence Stratigraphy

140 WOLFGANG SCHLAGER

THE SEQUENCE STRATIGRAPHY OF GRADUAL sive trends and vice versa. These cycles tend to be symmet-
CHANGE rical and the boundaries gradational.
The best known examples of symmetrical cycles with
The title of this section sounds like an explicit contradic- gradational boundaries are the sequences of the 1st and
tion in terms. The definition of stratigraphic sequences as 2nd order (p. 96–98). The 1st order sequences have
conformable successions bounded by unconformities clearly been recognized from the beginning as nearly symmetri-
implies that breaks in the record are necessary to define se- cal, transgressive-regressive successions with gradational
quences. Major breaks, on the other hand, are incompatible boundaries, linked to a smooth eustatic sea-level cycle (Vail
with a stratigraphy of gradual change. et al. 1977; Haq et al., 1987). More recently, Duval et al. (1998)
The standard sequence model (chapter 6) has abrupt depicted the 2nd order sequences, too, as nearly symmetri-
breaks with lapout at the sequence boundary and, less pro- cal units. But the phenomenon of symmetrical sequences
nounced, at the transgressive surface and the maximum with gradational boundaries does not end there. There are
flooding surface. Consequently, a stratigraphic succession also 3rd and 4th order sequences that show almost symmet-
consisting of standard sequenes shows highly asymmetric rical alternations of transgressive-regressive (or deepening-
cycles punctuated by pronounced breaks at the sequence shoaling) trends (examples in Embry, 1993). Carbonate sed-
boundary. Numerous case studies have documented these imentologists described examples of this phenomenon from
punctuations and the asymmetric anatomy of sequences flat-topped platforms (Read, 1989; Goldhammer et al. 1990;
conform the standard model (e.g. Van Wagoner et al., 1990 Montanez and Osleger, 1993). A classic example is the Tri-
and Anderson and Fillon, 2004 for siliciclastics; Eberli et assic Latemar platform in the Southern Alps (Goldhammer
al. 2002, and Pomar et al. 1996, for carbonates). How- et al. 1990; 1993). We will use the Latemar to illustrate both
ever, sequence-stratigraphic case studies also showed that the phenomenon and the sequence-stratigraphic approach
the asymmetric, strongly punctuated standard model has towards it.
its limitations and that there is a fair number of succes- The Latemar Mountain (Fig. 7.44) was a rimmed platform
sions with gradual transitions from transgressive to regres- with a flat top virtually at sea level and a composition that

Fig. 7.44.— Triassic Latemar platform. Bedding is virtually horizontal, smooth and devoid of significant angular unconformities. Gold-
hammer et al. (1990) describe nearly 700 m of measured section in these deposits that form the basis for a sequence stratigraphy of
gradual change.