Page 321 - Carbonate Facies in Geologic History

308 Shoaling upward Shelf Cycles and Shelf Dolomitization

TypeS Of CYCLES

hgt ... lve with Sifong tid." mod.fot. l.a lent fI",luotioll'll
TYPE I
-..... -. ....... . . . .

Oup.tO W
.. gr .... i •• 0' botin filling wilh almo .. no al,ong tid.l. Mod.fote ... 0 ley.t fluctuation,

TYPE 2
LIME MUD SABKHA

Lof.r

Fig.X-20. Hydrographic features controlling three types of shelf cycles Legend: brick pattern
marks open marine limestone; open circles mark grainstone with indicated organisms;
dashes indicate restricted marine limestone-dolomite

analog for these particles has been described in Holocene sediments. Whatever their precise
origin, these large pisoids are characteristic of this environment of intense diagenesis.
Several factors could be responsible for the particular features listed above, all
within a shallow marine and shoreline area in a strongly seasonal dry and humid
climate with an overall record of more evaporation than precipitation:
1. Intense and sudden temperature changes and alternation of extreme dry-
ness and torrential rains could cause expansion or contraction of the sediment.
2. Frequency, duration, and regularity of the processes could vary from sev-
eral dry periods seasonally alternating with brief heavy rains to equal times of
dryness and rainfall. The latter might result in solution, the former in more
carbonate precipitation.
3. Substrate drainage enhances movements of fluids and karst development
and is controlled by permeability of caIcarenitic strata, elevation above the fresh-
water table, amount of fractures within the substrate and frequency and extent of
sea-level fluctuations.
4. The degree of access of marine saline water to the sediment while it is
undergoing diagenesis by meteoric water may be important; amount of salt spray,
intensity of storms, tidal fluctuations are considerations. Increased ionic activity
of mixtures of salt and fresh water is probably important in speeding up diagene-
sis. Perhaps marine vadose and phreatic alteration is of equal importance to that
by fresh water. Purser and Loreau's (1973) description of aragonite accretion and
cementation in the splash zone of the Trucial coast is important in this context.
Here periodic inundation by the sea and splash zones at the shoreline result in
carbonate crystal growth within the sedimentary fabric.