Page 172 - Carbonate Facies in Geologic History

North American Examples of Waul sort ian Mounds in Mississippian Facies 159

UL __ ~~~--- 500m
"

• Wauho,Han lime mud mound.

Fig. V -12. Early Mississippian facies in southern New Mexico, after Armstrong (1962), show-
ing position of known Waulsortian mounds

The Lake Valley may have formed on a shelf of moderate depth occupying an
intermediate position between shelf and starved basin. The basin was bordered on
the east by the Diablo Platform and its northward extension, the Pedernal uplift,
but early Mississippian beds are largely removed from this area. Armstrong's
(1962) map (Fig. V-12) shows that the west side of the suspected early Missis-
sippian starved basin was bordered by a shelf of thick crinoidallimestones (Esca-
brosa Formation) whose facies is almost an exact duplicate of the Great Scar
Limestone bordering the north Pennine block in England. No bioherms occur in
this formation.
Regional facies of early Mississippian strata (Lodgepole Formation) in central
Montana have been studied by Smith (1972). The paleogeography is complicated
by a Devono-Mississippian horst block (Central Montana high) trending east-
west across Montana as a bridge between the Williston Basin and the Cordilleran
miogeosyncline. The apex of this Paleozoic positive area lies east of the present
structural crest of the Central Montana high, the Big Snowy Mountains. This is
indicated by pre-Mississippian erosion of the Devonian.