The Stratigraphy of Selected Mission Canyon Wireline Log Markers, U.S. Portion of the Williston Basin, North Dakota
HENDRICKS, MICHAEL L., Hendricks and Associates, Inc., Denver, CO
The Mission Canyon Formation along the northeast flank of the U.S. Williston basin has been informally subdivided into intervals
(members) based on wireline log markers. Wireline log responses of the markers are produced by both lithologic changes and radioactive elements present within these thin stratigraphic intervals.
The wireline markers were originally described as transgressive events. Detailed stratigraphic analyses of the Sherwood and State A markers indicate they were deposited during progradation and sea level stillstand. The sediments which comprise these log markers are the middle and upper portions of thin depositional cycles. The base of these cycles are deepening events (transgressions) but are not normally included within the log marker.
The entire upper Mission Canyon sequence is characterized by "stacked" regressive cycles. Within each upper Mission Canyon member, progradational units are capped by deepening or backstepping events. These deepening events are the bases of succeeding progradational cycles. From base to top of a member, the thickness of progradational cycles decreases.
The Sherwood and State A markers differ from normal Mission Canyon cycles in two ways. First, these markers (cycles) are abnormally thin. Second, the middle and tops of the markers (commonly correlated on wireline logs) were deposited during sea level stillstand.
A typical facies tract from east to west within the Sherwood marker contains anhydrites and anhydritic dolomites deposited in sabkha environments; patterned dolomudstones along shoreline trends ("the Sherwood argillaceous marker"); and limestones in shoaling environments along the Mission Canyon shelf ("Sherwood gamma marker"). During stillstand, brines produced in sabkha environments (east of the Sherwood shoreline) were enriched in magnesium and potassium. These brines migrated basinward first, dolomitizing mudstones. These brines were magnesium depleted by the time they reached shoals along the shelf. Potassium, however, remained in the system and is present within the marker along the shelf, as shown by a slight increase in API units on Spectrologs.
AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)