Page 111 - Carbonate Facies in Geologic History

98 The Advent of Framebuilders in the Middle Paleozoic

Fig. IV-2. Idealized Late Cambrian algal mound sec-
tion in lower Point Peak Member, Wilberns Forma-
tion, southwest Llano uplift, Texas. Upward progres-
sion indicated schematically from nonlaminated
mounds of algal bamestone to algal sheets and cap-
ping stromatolites. From Ahr (1971, Fig. 7), with per-
m DETRITAL INTERBED mission of Society of Economic Paleontologists and
Mineralogists

tidal channels and fissures caused by slumping. These are filled with coarse bio-
clasts of echinoderms, brachiopods, and trilobites, and lithoclasts (pebble con-
glomerates of reworked desiccated tidal flat sediment).
It is probably paleogeographically significant that even such small mounds
are known mainly around the edges of the North American craton, in the lime-
stone facies of the Arbuckles and Wichita Mountains of Oklahoma, as exotic
blocks in the Marathon Formation, and in the EI Paso limestone of West Texas.
Such mounds occur as well in the upper Pogonip of the Cordilleran geosyncline in
Utah and Nevada where Church (1974) described the same sponge-algal assem-
blage. Ross and Cornwall (1961) noted carbonate masses almost 100 m thick in
southern Nevada in the Miekeljohn Peak area. These consist of laminated micrite.
The rare megafauna includes only a few brachiopod layers, sponges, and cephalo-
pods. These large mounds also lie toward the cratonic margin. Lower Ordovician
mounds are rarer and smaller in the more interior platform dolomitic facies, such
as in the Ellenburger Formation of Texas, the type Ozark uplift strata, and the
Oneota and Manitou Dolomites of Wisconsin and Colorado. Presumably the
continental interior contained too restricted an environment and too shallow
water for the biological assemblage to flourish.
The above algal-sponge buildups prevailed at shelf margins and, in the form of
small mounds, over shallow shelves for a very long period in earth history. More
than one billion years passed from the time of the Great Slave Lake deposition