Depositional
and Dissolutional Processes and Their
Resulting Thinning Patterns Within the Middle Devonian Prairie Formation,
Williston Basin, North Dakota and Montana
Chris A. Oglesby
Within the Williston basin, thickness variations of the Prairie Formation are
common and are interpreted to originate by two processes: differential
accumulation of salt during deposition and differential removal of salt by
dissolution. Unambiguous evidence for each process is rare because the
Prairie/Winnipegosis interval is seldom cored within the United States' portion
of the basin. Therefore, indirect methods, using well logs, provide the
principal method for identifying
characteristics of the two processes. The
results of this study indicate that the two processes can be distinguished using
correlations within the Prairie Formation.
Several regionally correlative brining-upward and probably shoaling-upward
sequences
occur within the Prairie Formation. Near the basin center, the
lowermost sequence is transitional with the underlying Winnipegosis Formation.
This transition is characterized by thinly laminated basal carbonates that
become increasingly interbedded with anhydrites of the basin-centered Ratner
member. The remainder of the sequence progresses up through halite and
culminates in the halite-dominated Esterhazy potash beds. Two overlying
sequences
also brine upward; however, these
sequences
lack the basal anhydrite
and instead begin with halite and culminate in the Belle Plaine and Mountrail
potash members, respectively. A fourth sequence is indicated by several feet of
halite capping the Mountrail member n some parts of the basin. Subsequent
erosion or dissolution prior to burial may have removed the upper portion of
this sequence.
Cross sections
show that the lower Prairie gradually decreases in thickness
from the basin to its margins. This thickness variation is most simply explained
by decreasing accommodation potential due to decreased basin topography away
from the basin depocenter and by
depositional
onlap of the Prairie toward the
basin margins. A
depositional
onlap pattern is characteristic of the basin's
southern margin. In this case, a progressive increase in areal extent of the
formation occurs from the base to the top. By correlating the four
sequences
within the Prairie, some local thickness variations are shown to originate by
depositional
onlap onto Winnipegosis reefs. Other thickness variations occur in
response to minor vertical movements across basement faults. In particular, the
Brockton-Froid fault zone and the Nesson anticline have affected deposition of
the Prairie and Winnipegosis Formations.
In other areas, such as along the western and eastern basin margins, the basal parts of the formation are more extensive and completely preserved, but upper parts are progressively diminished in areal extent from the top down. This pattern is explained by progressive dissolution of Prairie evaporites from the top downward, suggesting that the aquifers contributing to the dissolution overlie the Prairie Formation. Although no distinct linear pattern emerged, the coincidence between tectonic features such as the Brockton-Froid fault system, the Nesson anticline, and the Superior/Churchill contact with major dissolution areas suggests that dissolution is controlled primarily by tectonic fracturing.
Dissolution was also recognized over the Winnipegosis pinnacle reefs. This dissolution preferentially removed the overlying potash horizons and resulted in only partial dissolution of the Prairie. These observations suggest that dissolution resulted from an upward-directed flow of waters localized by the reef facies.
AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988