Page 84 - Carbonate Facies in Geologic History

Morphology of Sessile Benthos 71

4. Diagenesis by brines in near-surface evaporitic areas.
a) Processes which preserve void space.
(1) Creation of chalky texture by lack of cementation during lithification. The process
probably involves the transformation, over a long time period, of aragonite to
blocky calcite in brines with a low calcium content. Such brines might occur directly
below or down hydrologic gradient from evaporite deposits.
(2) Enlargement of void space through dissolution and the consequent reorientation
and preservation of porosity by solution and reprecipitation of local C0 3 during
dolomitization. This results in the well known sucrose texture of dolomite (Murray,
1960).
b) Void-filling by gypsum-anhydrite and metasomatic replacement of calcite by sulfate
minerals.
c) Dolomitization in slightly lithified aragonitic sediments due to shallow reflux.
5. Deep subsurface connate waters (poorly understood).
a) Pressure solution after deep burial. Stylolites, grain collapse under load due to solution
(deformed ooids); possible anhydrite solution.
b) Cementation processes, difficult to distinguish from those of phreatic zone diagenesis.
(1) Rim cementation on echinoderm particles, a final filling of intergranular voids
(second generation cements).
(2) Final cementation oflime muds.
(3) Some vein-filling cementation.
c) Perhaps also extensive post-lithification dolomitization. In such fabrics no evidence
occurs that permeability control on dolomitization existed.
d) Neomorphic calcite-microspar development.
e) Replacements by anhydrite and subsequent solution.

Biological Observations

Morphology of Sessile Benthos
In addition to the observations noted during description of basic microfacies,
there are some purely biological considerations which aid in environmental inter-
pretation. One of these is careful attention to morphology of rooted bottom dwell-
ing organisms whose life styles and shapes are adapted closely to current activity
and sediment accumulation. Such organisms may develop extraordinarily robust
forms in rough water. In slight or moderate currents their architecture is adapted
for presenting maximum surface area to the water for feeding and respiratory
efficiency. Other forms have adopted shapes for preventing accumulation of sedi-
ment on the feeding surface.
The following are common growth forms found among such organisms as
corals, stromatoporoids, bryozoans, and sponges. Some suggested ecologic inter-
pretations are given.

1. Massive and irregular in areas of high wave energy: Some stromatoporoids, corals such
as Microsolena.
2. Wavy, erect to encrusting; in areas of high wave energy, but perhaps in more protected
places: Halysites, Agaricia, Millepora.
3. Branched palmate, elongate in downcurrent direction in areas of high wave energy:
Acropora.
4. Tabular or irregularly lamellar, in places encrusting and binding fine sediment; in
deeper quieter water, wide surface area presented for maximum exposure to water and
light: Tabular stromatoporoids, tubular foraminifera, Thbiphytes, Alveolites, sheety Mon-
tastrea.