Page 151 - Carbonate Facies in Geologic History

138 The Advent of Framebuilders in the Middle Paleozoic

niche dwellers include the stick-like stromatoporoid Stachyoides which seemed to
flourish widely on slope, reeffront, and immediate backreef. Corals are rare in the
reef (particularly in Frasnian parts) and totally absent where the reef is wide. They
do occur commonly in the backreef. Slightly down the seaward slope from the reef
crest several other biological assemblages occur, including brachiopods and other
open marine organisms. The matrix becomes micritic and stromatactoid struc-
tures appear.
In the backreef strata (Pillara limestone) there are both similarities and differ-
ences from the well-known Canadian buildups. These strata are medium, regular-
ly, and horizontally bedded, a striking contrast to the unbedded reef margin.
Sedimentation in this environment is more or less independent of the reef itself.
Little debris was provided from the barrier to the lagoon. Several hundred meters
of such strata exist. The accumulation is typical of a shelf environment in its rapid
alternation of rock types most of whose source material is indigenous to quiet,
clear, tropical water. Several distinctive organic sedimentary types alternate in
upward shoaling cyclic sequences discussed comprehensively by Read (1973). The
sediment types are listed in sequence from base upward.
a) Interbedded with the several special kinds of beds are beds of bioclastic lime packstone
to wackestone. These lie generally at the base of a sedimentary cycle.
b) Coral biostromes with tabular stromatoporoids, are common in the Givetian and
early Frasnian. The coral heads are ball-like to lens-shaped and broken and disarranged in a
micrite matrix; floatstone texture of Embry and Klovan (1971). Hexagonaria, Disphyllum,
Temnophyllum, and Donia are known coral genera. The tabulates Alveolites and 1hamnopora
occur.
c) Biostromes of small (up to 0.5 m) globular stromatoporoids~mainly of the genus
Actinostroma. These beds occur as close as 100 m to the reef. They are commonly micritic.
d) Cyclindrical stromatoporoids of the genus Stachyoides may replace or occur above the
globular stromatoporoids.
e) Biostromes of Amphipora, a thin, twisted stromatoporoid forming "spaghetti rock."
Matrix of such units varies from micrite to sparry calcite. Calcispheres are common. Such
beds may be capped by globular stromatoporoids.
f) Onkoidal beds with irregular algal encrustations on snails or crinoids; matrix is com-
monly lithoclastic calcarenite with finely coated particles. The large clam Eumegalodon is
present here; also thick-shelled gastropods.
g) Fenestral fabric in peloidal grainstone-packstone and wackestone with varying
amounts of micrite matrix; some mud cracks; occurs commonly at top of sedimentary cycles.
h) A so-called oolite subfacies (Playford and Lowry, 1966, Fig. 24) is very probably
synonymous with vadose pisolite of Dunham (1969a) and Thomas (1965) and is not the
typical well-sorted, crossbedded, homogeneous oolite. Interpretations of this particular rock
type, made in the Permo-Triassic beds, as of vadose concretionary origin would indicate more
periodic sea level fluctuation in the Pillara limestone.
The immediate backreef zone is determined where the reefward limit of Am-
phipora and the shelfward limit of Renalcis approach each other or overlap
slightly. Here globular stromatoporoids existed but not the typical irregular
forms. The bedding begins to become thin and regular where the back reef begins.
Shelfward, this whole section grades rapidly to coarse terrigenous conglomer-
ate whose boulders and gravels derive from the nearby Precambrian land mass
along which the reefs formed a barrier. In many places the lagoon was narrow.
Places are known where fanglomerates, washed out from the land with much finer
terrigenous clastics, all but smothered the reef.