Page 139 - Carbonate Facies in Geologic History

126 The Advent of Framebuilders in the Middle Paleozoic

narrow shelf and in a narrow shale basin of the Variscan geosyncline. The basin
was bordered on the south by swells within the geosyncline, the Stavelot and
Rocroi massifs. The northern Brabant massif provided reddish terrigenous clas-
tics to the trough. Thickness of the Late Devonian in the basin is 510 m, about
twice the amount on the shelf. Marine regressions and transgressions, coupled
with periodic influx of clastics, resulted in cyclic sedimentation in the area
throughout Late Devonian time. During periods of transgression, carbonate sedi-
mentation prevailed on the shelf. At this time shelf biostromes of stromatoporoids
develop while mounds of carbonate mud began to accumulate downslope in the
subsiding basin. In periods of marine regression or stillstand, clastics crossed the
shelf and flooded into the basin. In some basinal positions continued carbonate
accumulation overcame both subsidence and clastic influx and isolated carbonate
mounds built steadily upward to heights of tens of meters. As this occurred a
faunal sequence adapted to shoaling formed progressively higher in the buildups.
The mounds are surrounded by dark shale and overlie a Middle Devonian
carbonate platform. They built upward through coral limestones to a stromato-
poroid cap whose constituents became massive and irregular as wave base was
approached. The progression through dark goniatite-conodont shales to brachio-
pod-bearing limestone to coral limestone, to tabular stromatoporoids, to massive
irregular forms, is essentially that found in the Canadian buildUps. Lecompte
envisaged this sequence as representing progressive shoaling zones. It is now
accepted that this scheme is too simple and that argillaceous influx and substrate
character played a role. Figure IV-18 is Lecompte's classic presentation of this
faunal succession.
The mounds contain much micrite matrix and stromatactoid structure. The
"organism" Stromatactis was originally described from these beds. It denotes an
area of calcite spar with a flat base and digitate top. Much of the sparry calcite
and internal micritic sediment of stromatactoid structure obviously filled cavities
protected by irregular tabular organism (e.g., bryozoans, Alveolites). For discus-
sion of the origin of these see Chapter V. Major fissures in the mounds occur in
places, presumably caused by large-scale slumping. The reddish color of much
late Frasnian sediment outlines these fissures and the internal fillings of the
stromatactoid structures.
Flank beds in the Belgian mounds consist principally of bioclastic medium-
bedded limestone with brachiopods, solitary corals, and conodonts. Extensive
encrinites are not present in them. The beds intertongue with parts of the massive
mound rock. The relationships indicate mound height of a few tens of meters
above the sea floor.
The Dinant basin mounds offer a good illustration of how effective carbonate
accumulation may be even in conditions of rapid subsidence and the biologically
unfavorable terrigenous mud environment. Even though they began accumulat-
ing below wave base and in muddy water, and were originally not very far above
the sea floor, the upper portions managed to build up into a zone of moderately
active turbulence. Since no clear windward and leeward flank beds and no coarse
foreslope breccias are known, one may assume that the tops of the bioherms
remained under some meters of water. Lack of encrusting forms and a few green
or blue-green stromatolitic algae on the tops support this view.