Page 176 - Petrology of Sedimentary Rocks

far from shore. Another very good criterion is the Iithology of associated beds. If the
micri te occurs i nterbedded with high-energy calcareni tes and calcirudites, cemented
with sparry calcite and showing evidences of current action, then the micrite is very
likely of shallow water origin; the alternation might represent a fluctuation between
lagoon and barrier-bar or beach environments. If, on the contrary, monotonous
thicknesses of micrite occur with no interbedded -sparites, a deeper water origin is
implied. Burrowed or churned up dismicrites seem to be characteristic of lagoonal or
shallow-water sediments, as are micrites that have been torn up and reworked to form
intraclasts. If the climate was semiarid with development of evaporitic tendencies, the
lagoonal micrites might be associated with beds of more saline indicators such as
replacement dolomite, or, if restricted circulation, primary dolomite, anhydrite,
gypsum, celesti te, etc. If the climate was humid, influxes of clay may be associated
with lagoonal micrites because of river drainage into bays and lagoons.

In many beds of biomicrite and fossiliferous micrite, the fossils are badly broken;
how does this happen in supposedly calm-water environments? Sometimes these beds
would represent a normal sequence from shoreline well-rounded biosparites into more
poorly sorted biosparites made of broken but non-rounded shells, to biomicrites and
finally micrites in deeper or calmer waters. But often the shells are broken not by
wave or current action but by the activities of predator organisms, which crunch up the
shells in search of food and in some cases pass the debris through their digestive tracts.
Many beds of micrite show badly broken and randomly oriented shells distributed
patchily throughout the rock, often arranged along vague “flow-lines.” These probably
represent a lime mud riddled with burrowers and scavengers, which break the shells and
churn them up with micrite.

Beds of intramicrite and oomicrite might be formed by a similar mechanism. A
bed of lime mud might be laid down, followed by deposition of a well-sorted layer of
intraclasts or oolites. Before the rocks have a chance to get cemented, the whole
sediment might be churned up by burrowers so that the allochems would get mixed with
micrite. In these rocks the allochems often show a patchy, swirled distribution.

The significance of color in limestones is not too well known. Ordinarily, in well-
oxygenated (shal I ow and wave-agi toted) waters, bacteria thrive and eat up decayed
organic matter, resulting in light-colored rocks. In waters of poor oxygenation, either
restricted, stagnating lagoon or deeper offshore marine waters, reducing conditions
dominate and organic matter accumulates faster than bacteria can remove it, and dark,
pyri tic limestones result. The Buda limestone in central Texas grades from white
micrite in the nearshore facies to dark gray micrite in deeper wates farther offshore
(Hixon). Many deep geosynclinal limestones are dark colored, but so are many shallow
lagoonal limestones.

Notes to Classification Table:

* designates rare rock types.

I. Names and symbols in the body of the table refer to limestones. If the rock
contains over IO percent replacement dolomite, prefix the term “dolomitized” to
the rock name, and use DLr or DLa for the symbol (e.g. Dolomitized Intrasparite,
Ii:DLa). If the rock contains over IO percent dolomite of uncertain origin, prefix
the term “dolomitic” to the rockname and use dLr or dLa for the symbol (e.g.
Dolomitic biomicrite llb:dLa). If the rock is a primary dolomite, prefix the term
“primary dolomite” to the rock name, and use Dr or Da for the symbol (e.g.

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