Page 110 - Carbonate Facies in Geologic History

The Earliest Buildups 97

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11'.Z\~1 Stromatolite r;;:1 Digitote
l' mound. ~ fenestrol fab,ic
I ""-'- I U"dulato~y I ~ <7l lntr.oclasti(
~ stromatolite, gra,nstone
101cm f ~ I Columnor ~ Oolit.
slromotolite.l!.!:J
Fig. IV-I. Facies of Middle Precambrian carbonate platform, Coronation geosyncline, north-
west Canadian Shield, N.W.T., Canada. After Hoffman (1974), with permission of American
Association of Petroleum Geologists
Precambrian turbidites in the Great Slave Lake area (Hoffman, 1974) occurring
on the seaward side of such platforms, are evidence for these steep margins.
The best-known early buildups of North America, however, are small mounds
of lime mud located toward the edges of the wide shelves covered by Cambro-
Ordovician seas. Algae from these masses have been described from the Late
Cambrian of Texas by Ahr (1971) (Fig. IV-2). In Lower Ordovician strata the
possible dasydadacean Calathium, the lithistid sponge Archeoscyphia, a spinose
stromatolitic organism Pulchrilamina, and tiny branching, cylindrical, and en-
crusting algae (?) Nuia, Epiphyton, and Renalcis have been described by Toomey
(1970), Toomey and Ham (1967), and Riding and Toomey (1972) from mounds in
the southwestern U.S.A. The sponges are small basket-shaped, thick-walled forms
(Plate V A). The algae are encrusting in habit and Pulchrilamina forms stromato-
lite structure on the mound tops. Where not dolomitized, the Cambro-Ordovi-
cian strata preserve internal 'structures beautifully and permit detailed micro-
scopic study of these primitive organisms.
The mounds are composed of massive micrite apparently representing lime
mud trapped by the organisms. They are seldom more than 10 m thick and most
are considerably smaller. Their micrite cores may intergrade with bedded bioclas-
tic sediment. On the same buildup, however, bioclastic strata may overlap parts of
the mound showing that in places considerable relief had developed over the
seafloor (Fig. IV-3). Many of the mounds show two stages of formation: (1)An
early massive lime mud accumulation replete with algae which trapped the sedi-
ment. (2) This may be followed by a mound-capping phase of stromatolite. Such
sequences may form composite masses (Fig. IV -2). The mounds may be cut by