Geology Reference
In-Depth Information
Fig. 21.18
Supratidal facies: (
a
-
b
) Rosettes and laths of
calcite pseudomorphs after anhydrite within a fenestral lime
mudstone from the Cave Hill Member of the Mississippian
Kinkaid Formation, southern Illinois. Note that in a (thin sec-
tion photograph) birdseyes are almost totally fi lled with vadose
silt. Note also that in (
b
) (photomicrograph of the enlarged
portion of the
upper left
of (
a
) under normal light) the rosettes
are fi lled with coarsely crystalline calcite cement.
(
c
) Field photograph of a thickly laminated fenestral dolomud-
stone containing lenticular gypsum casts (the silver end of the
pen in the upper left is 3 cm long); Middle Triassic middle unit
of the Elika Formation in the Ghoznavi section, eastern Alborz
Mountains, northern Iran
and rapid infi lling of the subtidal carbonate factory.
Subsidence and subsequent sea level rise result in the
resumption of carbonate deposition (after a lag period)
and formation of a new asymmetric shallowing-upward
cycle. An alternative autocyclic mechanism, the tidal
fl at island model, has been proposed for peritidal
shallowing-upward cycles that are laterally discon-
tinuous (Pratt and James
1986
; Pratt
2010
) . In this
model, deposition would take place on small low relief
islands and intertidal banks separated by subtidal
source areas. Progradation and lateral growth of the
islands may generate shallowing-upward peritidal
cycles of limited areal extent with random stratigraphic
distribution and variable thicknesses (Pratt et al.
1992
;
Pratt
2010
) .
Lateral migration of tidal channels is another
autocyclic process forming discontinuous small-scale
cycles. Migration of intertidal channel leads to point
bar deposits which are laterally discontinuous, erosive-
based, fi ning-upward successions of gravel- and sand-
size sediments capped by a laminated tidal fl at facies
(Fig.
21.16b, c
). Tidal channel migration in platform
margin shoals generates an erosive-based fi ning-
upward succession (Fig.
21.8b
), capped by offshore,
tidal fl at, lagoonal or coarsening-upward beach facies
(Inden and Moore
1983
) .
