Page 100 - Carbonate Facies in Geologic History

Water Energy 87

Physico-Chemistry of Water Mass during Deposition
(as Derived from Ecological Interpretation of Biota)

The biota indicate warm tropical water of normal marine or perhaps slightly
higher salinity for the following reasons: Echinoid spines and echinoderms indi-
cate generally normal marine salinity; Foraminifera are abundant and varied;
Triticites (fusulinid), Paleotextularia, Globovalvulina are large foraminifera and are
presumably characteristic of warm open marine water; Thberatina, Calcitornella,
and Archeodiscus are all encrusting forms and appear to have grown on plant life
or soft-bodied creatures-as well as on the abundant platy algae; platy codiacean
algae today flourish in warm, tropical, sun-lit water generally less than 15 m deep;
common encrusting blue-green algae, in corsortium with foraminifera, also indi-
cate shallower photic zone water; ostracods-many scattered throughout, colo-
nies of bryozoans in the form of hollow spheres embedded in micrite and intro-
duced in the flanking beds from a quieter environment; worn, broken, gastropod
fragments, encrusted inside, obviously introduced from outside the immediate
environment.
Note certain characteristic bioclasts that are lacking: brachiopods, common
bryozoans, crinoids, red algae, corals, common mollusks. The lack of, or rarity of,
these forms indicates that the bioclastic material does not derive from organisms
living in fully open marine conditions, but probably from water on a shelf, water
somewhat warmer than open marine, nutrient depleted, and even slightly more
saline than in the open sea.

Water Energy

Note the texture: a packstone with a wide range of bioclasts of all sizes, and
lithoclasts of several shapes and sizes and obviously of different origins. The
unsorted grains constitute about 50% of sediment volume. The matrix consists of
clotted silt and fine sand-size carbonate with many tiny patches of coarser sparry
calcite. Larger fragments such as algal plates and echinoid spines are rounded,
coated, and micritized indicating considerable movement of the grains in water
before burial. Note the recrystallization and the cement infilling the interior of
algal plates. No cortical structure is preserved.
Smaller bioclasts are less damaged, protected from mechanical abrasion by
surface tension effects. Lithoclasts comprise additional coarse fragments. The left
center and center field of Plate I contains two rounded lumps about 3 mm across
in actual size. Their matrix is micrite as indicated by the denser, darker color than
the general matrix of the sediment. The lithoclasts contain bioclastic and pelletoid-
al material. These lumps may have been formed by crabs or other arthropods
which "ball-up" excavated material. The larger lump contains a bryozoan colony
in the shape of a hollow sphere. The lump may have been shaped around the
bryozoan colony. The central lump was still soft when deposited. It is pressed flat
against the algal plate above it. This is not a stylolitic contact formed through
later pressure solution. About twenty dark micritic peloids 300-400 microns in
diameter are scattered about within the lumps and in the general matrix. These