Page 125 - Carbonate Facies in Geologic History

112 The Advent of Framebuilders in the Middle Paleozoic

southwest (windward) side. A zone where the flattish convex trilobite Bumastus is common is
interpreted as a reef flat bordered by a beach on which orthoconic cephalopods accumulated.
A crinoid meadow with scattered large patches of tabulate corals is considered to have filled
in the lagoon leeward of the main southwest facing coral-stromatoporoid ridge. Coarser
crinoid debris occurs on the southwest (windward) side. The presence of echinoderms and
cephalopods indicates water of normal marine salinity and of some depth over the top of the
reef-not an intertidal environment. Relatively fewer bioclastic beds exist down the wind-
ward flank giving a northeast elongation to the mass. Suggested reef-derived breccia is
reported along the southwest quadrant of the mass, an unusual feature in Silurian reefs. Most
of the flanking material consists of thin, rhythmic beds, alternating dolomite mudstone, and
bioclastic beds. Fig. IV -10 is an environmental interpretation by Ingels of a restored Thorn-
ton reef mass.
5. The Michigan Basin" Pinnacle Reifs" :
Superimposed and interlaced reefs of the above types are known to form a Middle
Silurian barrier complex 150-200 m thick around the Michigan basin which was sediment-
starved and accumulated only 30 m of sediment during this time (Fig. IV-11). Sequences of
cored wells passing through this barrier reef encounter the following sequence above basinal
carbonate muds: (1) coarse carbonate sands and gravels with cross-bedding grading up into
massive sands (15-100 m thick), (2) floatstone with large heads of stromatoporoids with the
Tabulate corals Coenites and Favosites in a very coarse sand matrix (15-80 m), (3) coarse
sands with no fossils grading up through green shale, to (4) mottled and laminated backreef
lime muds about 10 m thick. This regressive sequence is Wenlockian in age but on the
basinward side ofthe barrier younger Silurian (Ludlovian) with stromatoporoids and algae is
present.
Within the basin numerous isolated buildups parallel both the north and south flanks of
the barrier. Accumulation on the slopes into the basin managed to keep pace with sedimenta-
tion. Briggs and Gill (1971) and Mesolella et al (1974) described the ideal sequence developed
in these Wenlockian pinnacle reefs (Fig. IV -12). The sequence begins with crinoidal bryozoan
micrite which formed in water of some depth. Piles of crinoidal calcarenite occur in this early
phase. Tabulate corals (the genera Coenites, F avosites, and H alysites) with some stromatopo-
roids developed. They are encrusted by algae. Evidence from cross-bedded detrital sediment
shows a subsequent fall of sea level. Much of the mass may have been subjected to vadose
diagenesis. During this time of lower sea level (Ludlovian) evaporites began to form in the
basin (A-l unit). A sea-level rise then permitted laminar stromatoporoids and algal stromato-
lites to form. Another period of exposure and desiccation is indicated by flat pebble conglom-
erates at the top of this unit. During this time shallow-water evaporites were precipitated
from brine ponds and sabkhas in the basin (A-2 evaporites). The pinnacles were later buried
by accumulation of the evaporites which form the remainder of Salina Formation. Very
similar Middle Devonian stromatoporoid reefs, buried by saline deposits, are also known in
the Zama areas of western Canada where they produce considerable oil and gas. Zechstein
algal mounds in Germany lie peripheral to a basin of Permian age and were also buried by
evaporites. This common association is of consequence. The evaporites were deposited dur-
ing sea-level lowering and basin filling during cyclic reciprocal sedimentation. During these
low sea-level stands the mounds were exposed, leached, and dolomitized. Porosity and
permeability is thereby increased in the organic carbonate buildups intermittently during the
evaporite fill-in of the basin. The same evaporites also act as seals for the reservoirs created
during their formation.

Evidence Determining Trends, Paleocurrents, and Wind Directions
of Silurian Buildups

In the mounds and ecologic reefs of the Silurian shelf of Indiana and Illinois, wind
and wave direction was generally from southwest to west. The evidence for this
offers an illustration of what kind of data may be used for directional interpreta-