Page 120 - Carbonate Sedimentology and Sequence Stratigraphy

CHAPTER 7: SEQUENCE STRATIGRAPHY OF THE T FACTORY 111

HIGHSTAND TRACT
shoaling
skeletal + non-skeletal, muddy
reefs retreat to margins
lagoons increasingly restricted, rims continuous
tidal flats extensive

TRANSGRESSIVE TRACT
deepening
skeletal + non-skeletal
on platform top: mud decreasing, hardgrounds increasing
oolites + reefs spread over top
lagoons normal marine, leaky rims
cliffs, sandy beaches, tidal flats
rare: major mud bodies

LOWSTAND SYSTEM TRACT
narrow (0.2 - 2 km)
deepening or shoaling
skeletal sands, reefs common
lagoons normal marine
cliffs, sandy beaches
rare: non-skeletal grains, mud, tidal flats

Fig. 7.7.— Facies characteristics of carbonate systems tracts based on the Holocene record of the Bahamas and Florida. After
Schlager (2002).

were retreating (Figs 7.6, Hardie and Shinn, 1986). Progra- Significance of shoaling and deepening trends. Much informa-
dation occurred by rapid accumulation of shallow-marine tion in sedimentary geology becomes available in the form
muds and muddy tidal flats. Narrow tidal channels, closed of vertical stratigraphic sections. In such sections, carbon-
off at their seaward end, as well as shore-parallel, inactive ate lowstand tracts appear as exposure surfaces on the plat-
beach ridges attest to this stepwise progradation. form top (or carbonate shelf) and as downstepped shoal-
Shoaling and prograding Holocene systems tracts are water deposits on the slope. Transgressive and highstand
common on other carbonate platforms, too. The tidal flats tracts both are represented by marine deposits on the plat-
and barrier islands of Qatar in the Persian Gulf have built form top. They differ by the change in depositional environ-
a 15 m thick wedge that has prograded 5 – 10 km in the ment observed in vertical section: deepening upward (and
Holocene (Hardie and Shinn, 1986). Reefs and reef aprons increasingly open marine) in transgressive tracts, shoaling
in the Caribbean and the SW Pacific have built to sea level (and increasingly restricted) in highstand tracts.
and prograded both seaward as well as landward (exam- The above examination of Holocene systems tracts shows
ples in James and Macintyre, 1985). This bi-directional once more that in carbonate sections only the lowstand tract
progradation of platform margins is common during the with its exposure surface on marine deposits is a reliable
early phases of highstand deposition when rapid sea-level indicator of sea-level change. The change from transgres-
rise left the lagoons empty. sive to highstand tracts and vice versa is an ambiguous
record. It may be produced by a change of relative sea level