Page 152 - Carbonate Facies in Geologic History

Morocco Devonian 139

All in all, the backreef Pillara limestone is more normal marine and of more
open circulation than in the Alberta banks whose interior facies is a rather mo-
notonous repetition of the special rock types "e", "r', and "g", listed above. The
narrowness of the shelf is evidenced by the proximity of clastic source. A change-
able environment with fluctuating sea levels is indicated by the cycles. What type of
reef protected this lagoon? It was relatively narrow and somewhat incomplete. In
many places it probably "broke water" inasmuch as it provided an immense
amount of detritus, some of it very large. Its profile is steep and sharp and,
therefore, in many respects it more resembled a modern barrier reef than other
Paleozoic and Mesozoic marginal buildups (profile Type III).
A few tens of km farther southeastward along the shelf, the belt of buildups
broadens and shelf platforms or ramps grew over and around highs in the Pre-
cambrian basement. Perfectly circular faros with quaquaversal dips can be seen
on maps and air photos. Various stages of development around, over, and out-
ward from positive elements can be observed. The reef development is upward or
outward depending on the ratio of production compared with subsidence. Chan-
nels between platforms filled with interreef sediments; depositional dips off the
platform are not so steep into the protected lagoons as on the outer, southwest
shelf edge, nor are flank beds so extensive. Some north-south orientation of the
platforms is seen as if wind or currents might have shaped their trends. The
buildups might have grown on bars of detrital sediment (e.g., crinoidal sand).
Satellite algal bioherms up to a few meters high occur on the west and southwest
(windward?) sides of the platforms. Brachiopods are extremely abundant down-
slope of these shelf platforms and faros and are represented by at least 10 common
genera.
Playford described and pictured an almost perfectly circular faro (shallow
atoll) with protected lagoon, stromatoporoid rim and steeply dipping flank beds.
Fig. IV-26 shows a cross section across one ofthese shallow platforms.

Morocco Devonian

Platform-like bioherms and circular faros very similar to those of Australia, are
known in western Morocco and the northern part of the province of Spanish
Sahara. These range in age from Eifelian to Frasnian and are also spectacularly
exposed. They lie east of a Devonian north-south positive axis trending along the
Moroccan coast, are up to 100 m high and 1.5 km in maximum diameter (Elloy,
1972). Dips are quaquaversal and from 30 to 40 degrees. Some of the bioherms are
micrite mounds capped by stromatoporoids. The micrite cores with abundant
stromatactoid structure, have 10-15% each of bryozoan and crinoid remains and
traces of corals, stromatoporoids, brachiopods, and trilobites. Tabular bodies
contain more stromatoporoids and corals. An upward faunal progression is seen
from disphyllid-Thamnopora to tabulate corals and Hexagonaria or from the dis-
phyllid-Thamnopora assemblage through lamellar corals and stromatoporoids to
massive forms of both stromatoporoids and corals. Such bioherms are known in
Late Devonian strata for hundreds of kilometers to the west in southern Morocco
and Algeria.