Page 176 - Carbonate Facies in Geologic History

Sparry Calcite and Stromatactoid Structure (Plate XVII) 163

The origin of the cavities through decay of some softbodied organism has been
accepted by many researchers, particularly in Europe where the structure is so
abundant and regular. But the traditional idea that a particular organism formed
the structure is open to question. The form is too peculiar and its top is too
irregular. Its shape precludes an origin as burrows. Neither is it positioned in
most mounds at any certain level, only occurring where micrite matrix abounds.
It holds no consistent relationship to the vertical biological sequence which a few
mounds display. The multiple layers of it and the tendency for their orientation to
be parallel to the outer surface or en echelon and at an angle to it are hard to explain
if the structure is organic.
Other possibilities exist for the cavity such as leaching or solution of lime mud
and subsequent filling with fresh water tufa, or in situ mud collapse and slumping
as the lime mud dewatered (Heckel, 1972a). Some additional facts and arguments
pertain to the problem. Typically, the base of a stromatactoid structure is smooth
and filled in places with calcite mudstone of a different color or texture than that
of the host micrite. This supposed internal sediment is laminated in places and
may even be cross-stratified. Typical stromatactoids are commonly multiple, ori-
ented parallel to each other in layers a few cm apart, and parallel to bedding or at
least to the outer slope of the mound. Significantly, they much resemble, although
on a very large scale, fenestral fabric seen in places in intertidal muds. Lees has
used them and their inclinations to map growth stages of the massive sheet
mounds in Ireland (Fig. V-6). If the structure is organic, the sheet-like organism
was often laid down parallel to the outwardly arched surface of the mound. Sheer
failure, however, of the collapsing host mud might also account for the en echelon
series of oblique crust of spar-like sheet cracks or "zebra rock" seen in many
places in the geologic column.
However, configuration of calcite infills in the mounds may be more complex
and irregular than the parallel and multiple stroma tactoid cavities. Philcox has
pointed out (1963) that cavities in the mud, formed by loops of fenestrate bryo-
zoans, may be variously filled with sparry calcite or internal lime mud sediment.
Progressive gentle collapse of mud from around such partly supported cavities
gives irregular shapes to the holes. The peculiar digitate tops of stromatactoids
could be caused by gravity fall of semilithified sediment off the roof of a hole
created by down-settling of rigid skeletal debris (e.g., bryozoan fronds) or by
slumping of mud which suddenly becomes thixotropic. Later, secondary cracks at
high angles to bedding may also be filled with calcite spar, mimicking the early
sheet crack spar. Similar irregular stromatactoid forms are common in the brec-
ciated algal plate mound rock of the late Paleozoic discussed in Chapter VI.
However created, the cementation of stromatactoids raises a considerable
problem in carbonate geology. All British workers have emphasized the early
formation of most of these cavities. Schwarzsacher (1961) pointed out their pres-
ence in reworked boulders at the foot of the mounds. Lees and Philcox noted the
close association in time and space between collapsed mud matrix (M 1), cavity
formation, internal sediment (M 2), and spar filling. Lees offered a quite reasonable
explanation for the generation of internal sediment as having been held origi-
nally in tissues of sponges or algae and released upon decay. His interpretation
accepts that the spar precipitation was submarine and ended the progressive