Page 141 - Carbonate Facies in Geologic History

128 The Advent of Framebuilders in the Middle Paleozoic

with considerable relief above the bottom, existed toward the shelf margin (Type
2b on Fig. IV-19). Total thickness of Dorp facies limestone here is close to
1000 m. Individual limestone banks with less sea floor relief and whose "reefy"
facies grades to interreef strata, exist in the northern, most interior parts of the
shelf (Type 3c). All of these banks are flat and wide, may be hundreds of meters
thick and up to 100 km. square. A shallow shelf area like that of the northern
Dinant basin, existed West of the Rhine and contains biostromes of corals and
stromatoporoids long termed meadow (rasen) reefs, ruben (carrot) reefs, knobby
(knollen) reefs and blocky reefs by the Germans, depending on growth forms of
organisms composing them (Fig. IV-19).
Farther south in the Eifel Variscan trough, reefy buildups also exist which
differ from the steep-sided bioherms of the Dinant shale basin. These are larger
and flatter mounds on volcanic rises capped by atoll-like stromatoporoid build-
ups (Type 1 b).
Despite the geosynclinal tectonic setting, the familiar facies pattern of Middle-
Late Devonian buildups occurs in the Eifel and can be favorably compared with
that of both Belgium and the Alberta banks (Krebs and Mountjoy, 1972). The
facies sequence characteristic of the Dorp reef complex in the Eifel region is as
follows:
Basin: dark shales, black, thin-bedded limestone, some turbidites.
Lower Slope: fossiliferous calcarenite and local breccias, brachiopods,
bryozoan, crinoid faunas.
Upper Slope: reef flank, coarse breccia with drusy lined cavities, dendroid and
tabular stromatoporoids.
Reef: massive, irregular-tabular stromatoporoids with some rugose and tabu-
lar corals. Algae not important. Some Solenopora.
Backreef: mainly reef detritus. Stachyoides, Amphipora, tabular corals, echi-
noderm peloid micrite facies.
The buildups and banks are framed by a relatively narrow reef band with in
situ stromatoporoids, interstitial fragments, and coarse drusy linings. Unlike the
Alberta reefs, echinoderms and corals are present in the bank margins and back-
reef areas. This indicates considerably less restriction on the external shelf. Some
of the Eifel banks do not contain an interior lagoonal facies but only a thick cap
of stromatoporoids and corals. Parts of the shelf area possess vertically alternat-
ing beds with the usual backreef Amphipora, fenestral laminites, globular stroma-
toporoids, and micritic limestones.

Alberta Basin Banks
The Middle and Late Devonian typical reefoid facies exists along edges of huge,
flat banks, and wide shelves surrounding the Devonian Alberta basin and along
narrow banks and linear trends extending into it. Oil discoveries, beginning in
1950, in the vast plains and foothills area of western Canada resulted in extensive
study of the subsurface Devonian to the east, in front of the magnificent Devonian
outcrops lying westward in the Canadian Rockies. Since carbonate banks are