Page 298 - Carbonate Facies in Geologic History

Middle Jurassic Cycles of the Paris Basin 285

NW SE
- -------- 350km ---------
C,inoidal biocla.tic. grain.'one Oolite grainltone

'/ Microbioclo.tic, spicule.
:. Tiny peloids
Shale .. Iryozoons, mostly fen.,trate.
11: Crinoid particle.
~ Shelly bioclas". mostly brach iopods
o Ooids
Fig.X-J Idealized Lodgepole Formation cycle from Madison Group across central Montana
from northwest near Great Falls to southeast. After Smith (1972)

Bridger Mountains contains Waulsortian bioherms surrounded by rhythmically
bedded, dark, shaly "deeper water" limestone. The four typical, overlying cycles
have been traced in wells about 350 km from north central Montana, near Great
Falls, across the Central Montana high onto a more shoaling area in southern
Montana (Fig. V-14). Each cycle has a lower member composed of fine-grained
dolomitized unfossiliferous laminated mudstone and ripple cross-bedded calcisil-
tite, a peloid lime packstone, and a grainstone. These lower beds of the cycle are
lighter in color to the south and more dolomitic in places. This grades upward
through very fossiliferous wackestones often with bryozoans and brachiopod
fragments to bioclastic crinoidal grainstone and/or cross-bedded oolite often with
overpacked textures. The oolite-crinoidal grainstone beds cap the cycle. To the
south, and across both the Bridger and Big Snowy Mountain outcrops, oolite is
more common at the tops of the cycle, but to the north and northwest of the
Central Montana high the cycles are capped more commonly by crinoidal bio-
clastic grainstones. Diagenetic studies show very early cementation but no evi-
dence of vadose diagenesis or clearcut subaerial exposure (Jenks, 1972).
Interpretation of the cycles : Figure V-14 is a somewhat interpretative regional
facies map of Lodgepole cyclic units around the western Williston basin and the
Montana shelf. Repeated and apparently somewhat rapid marine invasions of the
shelf area from northerly directions are responsible for the cycles. The lower
member, of fine, rippled, pellet mudstone with few bioclastic fragments, is a
shelfward equivalent of the basinal environment of black, argillaceous, bryozoan
wackestone and siliceous mudstone. The cleanly washed oolite and crinoidal
bioclastic grainstones capping each cycle must represent shoals and banks accret-
ing during regressive phase ofT the shelves.

Middle Jurassic Cycles of the Paris Basin

In outcrops around the southeast Paris basin, Purser (1969, 1972) described very
similar cycles to those of the Mississippian in Montana. These cycles correspond
approximately to each of the three Jurassic stages of the Dogger: Bajocian, Bath-