Page 148 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 7: SEQUENCE STRATIGRAPHY OF THE T FACTORY 139

floor and developed gently landward dipping clino- and Ward (1995) subdivided the highstand tract into
forms (Fig. 7.41). (2) Seaward of the margin, the open- the aggrading systems tract, and the high stillstand sys-
shelf sediment frequently onlapped and partly buried tems tract. The lowstand tract was split into an offlap-
tongues of reef talus, indicating a temporary halt of ping systems tract and a low stillstand systems tract.
progradation. This classification is useful as a subdivision of the stan-
➤ The dominance of sigmoidal margin profiles and lack of dard categories. It should be noted that the aggrading
steep lowstand cliffs indicate that mechanical rework- and high stillstand tracts both satisfy the definition of
ing was very important and the debris-to-framework the highstand tract of Figs 7.3 and 7.4. Similarly, the
ratio in the material was relatively high. offlapping tract and the low stillstand tract both meet
➤ The fine depth zonation on the clinoforms (Figs 7.42, the criteria of the lowstand tract. The offlapping tract
7.43, Pomar, 1991) allows one to at least crudely esti- as proposed requires one to distinguish between a sys-
mate the amount of lowstand erosion from the depth tems tract deposited while sea level was falling (the of-
level of truncation on the clinoforms. The estimated flapping tract) and a prograding tract whose top part
volumes are quite considerable, again suggesting a sys- was merely shaved off during the subsequent lowstand.
tem with major mechanical erosion and efficient sea- The latter simply represents an eroded highstand tract.
ward transport of debris. Even the detailed depth zonation by corals often re-
➤ The high resolution of the observations triggered a dis- mains ambiguous in this respect (e.g. Fig. 7.41).
cussion of the classification of systems tracts. Pomar

0 m
Fig. 7.42.— Depth zonation on
massive
fore-reef clinoforms using coral
morphology. See Fig. 2.7 for re-
cent analogue. Based on Pomar
branching
10 (1991).

dish/branching

20
dish

30

Fig. 7.43.— Field evidence for
relative sea-level fall at boundary
estimated sea-level fall
between two sigmoid units. Zone
of branching corals, indicating 10
0 m
– 20 m water depth, is overlain at
SB
massive/encrusting the same depth by massive and
dish/branching encrusting corals indicating 0 –
SB 10 m depth in the younger sig-
10 moid. After Pomar (1991), mod-
ified.
dish

10 m