Page 312 - Carbonate Facies in Geologic History

The Duperow (Devonian) of the Williston Basin 299

SO.SASK. s.w. NO. OAK. S.E. MONT.

200km
stromatoporoid s

Fig.X-14. Interpreted Duperow facies in ideal cycle from north to south across Williston
basin. Normal marine-burrowed bioclastic lime wackestone (oblique lines) and stromatopo-
roid patch reefs grade to restricted marine laminated lime mudstone with a microfauna (white
area). This probably graded in southeastern Montana to sabkha evaporites which were
leached from the most shelfward areas. From Wilson (1967b)

berta. The sea covering this shelf and shallow basin was more than 1000 km
across in any direction. Sedimentological analysis of its sediments show it to have
been generally very shallow. Boundaries of the Duperow cycles are marked by
sabkha anhydrite and radioactive silty gray-green dolomite mudstone and about
a dozen cycles may be traced from southern Alberta across the Williston basin of
Saskatchewan and southern North Dakota using gamma ray-neutron logs. These
beds are known in outcrops of the Jefferson Formation of the Montana shelf
where the same cycles are identifiable.
Each cycle (Fig. X -13) consists of a lower member with two alternative phases;
either dark brown, burrowed, lithoc1astic-bioc1astic brachiopod-crinoidal wacke-
stone or a stromatoporoid biostrome with a few corals and red algae. In Sas-
katchewan a lower unit of dark shale occurs below these normal marine lime-
stones. The middle part of each cycle is brown lime mudstone lacking megafauna
and bearing a restricted marine or brackish microfauna of ostracods and calci-
spheres interbedded with laminated beds of peloidal or homogeneous lime mud-
stone. The cycle is capped by bedded irregular nodular and laminated anhydrite
and gray-green silty, very fine-grained dolomite displaying intertidal and supratid-
al sedimentary structures. The only grainstone appears in a few thin beds above
the stromatoporoid biostrome.
The Duperow cycles are exceedingly widespread and constituent beds only 3-
5 m thick can be traced for several hundred km across the Williston basin (Fig. X-
14, X-15). Deposition occurred in a vast back-reeflagoon south of the Alberta reef
belt and stretched to a sandy shore in South Dakota and Wyoming. This very
shallow basin was periodically and apparently rapidly flooded with marine water,
permitting certain benthonic organisms to flourish and even patch reefs to grow
at times. Gradual shallowing as sedimentation filled in the basin resulted in
extensive tidal flats and evaporitic sabkhas; extensive dolomitization occurred on