Page 173 - Petrology of Sedimentary Rocks

to bits by wave action to form rounded, polished coquinas. Such supermature pelecypod
biosparites formed a semicircular beach coquina around the north flank of Pilot Knob, a
Cretaceous submarine volcano near Austin (R. White). Often, the well-sorted (mature)
ones are made up of one fossil type, e.g., pelecypod fragments, crinoid columnals,
forams, or fusulinids; the poorly sorted (submature) ones are made up of diverse
mixtures of fossils of inherently different size, e.g. forams plus crinoids, or brachiopods
plus bryozoans plus fusulinids. Thus sorting is believed, as in terrigenous rocks, to be
largely a function of source (i.e. available types of fossils), with environment (i.e.
strength of waves and currents, or water depths) playing a modifying, but not
controlling role. Sorting is best in shallower waters or on the beach itself, and gets
worse offshore. Thus in moving from a beach surf zone into deeper waters one might
theoretically expect the sequence to go from supermature biosparite (well rounded and
sorted) to mature (sorted but only slightly rounded) to submature (unsorted, fragments
not rounded but possibly broken) to biomicrite (immature, with unsorted, broken or
whole fossils) and finally micrite. This sequence would undoubtedly be modified by
types of fossils available and wave energy of the coast. See I962 AAPG Mem I’/ I and
1964 JG (Folk & Robles) for details.

In some areas, shoal regions and littoral zones are characterized by algal, coral,
or other types of reefs. Most reef-forming organisms live in the shallow waters where
there is still sunlight available for photosynthesis of the algae, which make up the bulk
of living tissue on most reefs (Odum). Reef rocks (biolithites) are exceedingly complex,
because of (I) abundant life and many different types of fossils, making rock
petrographically varied, especially the algae (there is probably as much complexity and
variety of external form and internal structure in algae as there is in all other fossils
put together), (2) the fantastic growth forms assumed by many of the reef organisms,
with weird branching, bushy or encrusting habits, (3) the reef rock is riddled with
cavities which may be empty (and later filled with spar in fibrous or mosaic forms), or
cavities may be partly filled with terrigenous sand, intraclasts, lime mud, pellets,
organisms that lived in the cavity, and inswept broken fossil debris. Add to this the
fact that reefs are prone to patchy recrystallization (because of the high aragonite
content) and one can see that a thin section will be of the utmost complexity. Reefs,
although they usually thrive in high-energy zones, act as baffles to the current, and the
water movement down in the crevices and pockets between branching organisms may be
relatively calm even in a surf zone, just as the interior of a forest may be calm in a
windy day. Hence reefs are associated with much micrite and fine debris which filters
down between the branching growths and cannot be dislodged. In fact, stromatolitic
reefs, probably formed by blue-green algae, are largely micrite, presumably trapped by
the slimy algal mats and then firmly bound together by the filaments.

Rocks of type II and type III (either pure micrite or rocks with a micrite matrix)
indicate formation in areas of ineffective winnowing and calm currents. These can
form in four important ways: (I) in protected lagoons, in which the water is very
shallow, perhaps not more than a few feet deep; (2) in broad, shallow platforms on the
lee side of barriers (e.g. west side of Andros Island, Bahamas), where the great width of
the platform prevents any permanent removal of lime mud and it is merely shifted
around; (3) in moderately deep waters in geosynclines (probably down to a few hundreds
of feet); (4) as open-ocean Chalks; (5) as Ginsberg has shown, lime mud may accumulate
locally around organic baffles (marine grasses, coral or algal growths) even in fairly
high-energy environments. To tell these micrite-rich environments apart is often a
difficult problem as they may represent very shallow or moderately deep areas. Several
criteria are useful. Among the best criteria is the fossil content. For example, clams
or oysters in micrite would indicate a shallow, perhaps lagoonal area, whereas an
exclusively pelagic fauna of small forams would probably be indicative of deeper water

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