Page 174 - Carbonate Facies in Geologic History

North American Examples of Waul sort ian Mounds in Mississippian Facies 161

108°

Fig. V-14. Isopach (in meters) and generalized
• facies map of Paine Member, Lodgepole For-
mation of central Montana after Smith (1972).
Iillings
Shows offshelf position of Waul sort ian mounds
in earliest Lodgepole cycle. The Paine member
is characteristically "deeper water" limestone

The layering within the mounds dips from 20 to 35 degrees showing deposi-
tion on somewhat steep slopes. Erosion has truncated the mounds obliquely so
that layering does not outline simple conical bodies. This complexity probably
results as well from different growth centers with different directions of accumula-
tion, inferring lack of persistent currents. In addition to complex layering, several
dike-like fissures, filled from above with horizontally stratified crinoidal pack-
stone, are noted. These are early postdepositional features resulting from dilation
of the mound; They are wider below than above. Irregular, crinoid-filled cavities
several meters across also occur. These were either caused by solution or early
slumping within the mass. The mounds are best interpreted as biogenically accu-
mulated banks formed below wave base in water deeper than 50 m.
Regional relationships give some evidence to show paleogeographic control of
mound deposition. In the Big Snowy Mountains the buildups lie in an intermedi-
ate position between the apex ofthe Devono-Mississippian high, lying to the east,
and the offshelf deeper water facies north and west. In the Bridger Range they lie
off the Wyoming shelf (Fig. V-14). In facies and faunal content these buildups are
identical to those well-known in Europe and in New Mexico. However, the facies
changes are more gradual and subtle and the belts of the facies much wider in the
Montana-Wyoming areas. The mounds formed as local accumulations, on a sea
floor of some moderate depth.