Page 101 - Carbonate Facies in Geologic History

88 Outline of Carbonate Petrography

are of previously well-consolidated lime mud and may be either fecal pellets or
have formed as steinkerns (internal moulds) of ostracod tests. (Note the ostracod
tests in the lower right corner.) However formed, these large clasts were probably
torn out of the sediment and drifted around before redeposition, indicating some
water movement.

Fabric Packing, Diagenesis, and Orientation

The coarse fragments are jumbled together and form a self-supporting pack. The
pack may have been deposited rapidly with fine sand and silty-muddy sediment or
more probably the finer matrix material may have drifted in later. In any event
the finer matrix has altered to microspar. A large calcite area which was originally
void space stands out in the lower right-hand corner. The centripetally enlarging
calcite of this area and the rather frequent occurrence of enfacial angles (Bathurst,
1971) at contacts of calcite crystals argue strongly for a cement infilling. Various
origins are possible for the void: it may be a cavity protected by the framework of
algal plates, it may be a burrow, or the space earlier occupied by a soft-bodied
organism which rotted away. Whatever its origin, sediment fell or was washed to
its bottom, larger pieces following the smaller. This sediment probably came from
roof collapse; its larger pieces are angular and the roof of the cavity is more
jagged than the base. That this partial sediment in-fill occurred early in the
lithification history is evidenced by isolated, loose foraminifera tests as pieces of
the in-fill.
Several other lines of evidence indicate that micritic sediment filled in a pack-
stone which was supported by the larger grains. For one, a settling effect is
noticed. Preferentially one side of the larger, elongate grains contain calcite
mosaic spar-filling which enlarges centripetally. This is void-filling calcite which
was deposited in space caused by the finer intergranular sediment settling down
in protected areas. That this type of early diagenesis occurs on a smaller scale and
throughout the fabric is indicated by the light color of the irregular, almost
clotted character of the general matrix which is a coarse microspar in size. The
grains separated by microspar and constituting the general matrix are fine sand to
silt size (50-100 microns).
This fabric is quite common in packstones with an appreciable amount of
varied sized framework grains and affords a means of orienting the rock sample.
In addition to the above described settling effect under grains, a cavity just left of
the center of the photo contains floored sediment of the "vadose" crystal silt
described by Dunham (1969b). The jumbled large grains show no preferred linea-
tion but diagenetic crystal growth has been controlled by the original fabric, a fact
that shows early cementation. It is not possible to ascertain whether such cemen-
tation was under the influence of meteoric water or was submarine. The surface of
the thick ledge from which the sample comes shows some oxidation (red staining)
and is at least a surface of nondeposition if not subaerial exposure.
To sum up: This sediment represents bioclastic trash which drifted around on
the sea floor for some time in very shallow tropical water of less than 50 feet depth
and of normal marine or just slightly restricted marine salinity. The fabric shows