Page 175 - Carbonate Facies in Geologic History

162 The Lower Carboniferous Waulsortian Facies

Composition of Typical Waulsortian Facies

The Waulsortian facies thus occupied a shelf marginal position, occurring as
massive sheets of limestone, or was deposited in basinal areas where it developed
as large individual micrite mounds. Figure V-IO diagrams these two forms. Dips
toward the basin on mound flanks may range from 30 degrees to 50 degrees,
which is considered to be somewhat beyond the angle of repose of normal lime
mud and suggests stabilization of the mud by some organic film or frond-like
organisms.
Petrographically Waulsortian facies are remarkably similar both in North
America and Europe. The most detailed study has been done in Britain by Ba-
thurst, Lees, Philcox, and Schwarzsacher, and in the United States by Pray and
Meyers, in studies of the New Mexico mounds. Only four basic rock types make
up the facies although they may occur in very unequal portions.

Lime Mudstone or Wackestone (Plate XXI)

Between 50 and 80% of a mound is clotted, vaguely peloidal lime mudstone. Pray
(1958) estimates that about two-thirds of the core rock ofthe Sacramento mounds
is micrite and subsequent experience confirms this. This micrite may occur in
several generations: as originally deposited lime mud and as later successive
infillings of cavities. Most of the remainder of the rock mass is equally divided
between a meshwork of fenestrate bryozoan fronds, both intact and fragmentary,
and masses of stromatactoid sparry calcite, whose origin is discussed below.
Much of the micrite is finely bioclastic. Stringers of shelly debris and pockets of
whole shells include crinoids, dendroid and lacy bryozoans, orthoconic nauti-
loids, small horn corals (less common than in shelf limestone), brachiopods, tri-
lobites, and ostracods. Many shells in these pockets have both valves intact. Algae
of all sorts are notably absent.

Sparry Calcite and Stromatactoid Structure (Plate XVIII B)

The origin of the sparry calcite component of the Waulsortian is much debated. It
commonly has stromatactoid form, i.e., extended sparry patches several cm long
with a flat base and irregular, digitate top and centripetally arranged coarse
crystals. The name Stromatactis was first proposed in 1881 by Dupont for what
was considered a calcite-filled fossil form whose growth encrusted and paralleled
the sides of Devonian mounds in Belgium. Lees (1964, p. 509) lists eight different
names applied to it, or very similar sparry patches, by various authors. In the past
it has been interpreted as a deposit from meteoric water (hence the British name
"reef tufa"). It has also been considered to result simply from aggrading recrystal-
lization of lime mud. There seems now, however, general agreement that the
feature results from calcite filling of some type of preexisting hole. The centripetal
crystal arrangement and the prevalence of lime mud lying like internal sediment
at the bottom of the area argue for this.