Page 370 - Carbonate Facies in Geologic History

Belt 5. Organic Reef of Platform Margin 357

on an incline formed seaward as the ramp grows. This may be as steep as 30
degrees. The sediment is somewhat unstable and varies greatly in size and shape.
There may be bedded, fine-grained layers with megaslumps; some foreset bedded
and wedge-shaped strata consist principally of lime sand and of mound- or lens-
shaped masses of trapped and stabilized, fine-grained carbonate sediment.
a) Prevailing rock types: Variable types oflimestone depending on the water
energy upslope; lime muds and sands, boundstone, and sedimentary breccia.
b) Color: Dark to light.
c) Grain type and depositional texture: Lime silts and bioclastic wackestone-
packstone. Lithoclasts of varying shapes and sizes are derived from cemented
strata upslope. Much reworked material with locally derived organic debris, reef
rudstone.
d) Bedding and sedimentary structures: Megaslumps in thinly bedded strata;
large scale foreset bedding (wedges); large exotic blocks interrupting bedding;
slope mounds of fine-grained sediment; syngenetic slumps, pull-aparts, and brec-
cias; clastic injection dikes and fissure in-fills.
e) Terrigenous clastics: Mostly pure carbonate but some shale, silt, fine sand
drifted downslope from above and mixed with carbonate or filling cavities.
f) Biota: Mostly bioclastic debris from upslope but also colonies of in place
encrusting organisms. The facies may be very fossiliferous, its fauna varied and
openmanne.

Belt 5. Organic Reef of Platform Margin
The ecologic character varies dependent on the water energy, steepness of the
slope, organic productivity, amount of frame construction, binding, or trapping,
frequency of subaerial exposure, and consequent cementation. Three types of
profiles with linear shelf-margin organic buildups may be discerned:
Type I: Downslope accumulations of carbonate mud and organic detritus.
Type II: Ramps of knoll reefs with intervening bioclastic sands.
Type III: Frame-constructed reef rims.
These types of shelf margins are discussed in the following section.
a) Prevailing rock type: Massive limestone and dolomite in places consisting
solely of organisms. Also much bioclastic debris.
b) Color: Light.
c) Grain types and depositional texture: Masses and patches of organic bound-
stone. Interstices may be filled with lime mudstone in downslope reefs or banks
and with grainstone and packstone in upslope accumulations. Some mounds
formed by clumps of organisms growing in upslope position have only mud
matrix owing to the protection from winnowing by reef frame. Interstices in
boundstone of higher energy reefs are filled with lime sand and gravel. Inter-
mound areas consist commonly of grainstone and packstone.
d) Bedding and sedimentary structures: Massive organic framework with con-
structed (roofed) cavities. Lamination is caused by organic growth; it swells and
thickens upward. In mounds with considerable lime-mud matrix, stromatactis-
like structure is common. Brecciation and fissuring of massive buildups may
occur and injection dikes are present.