ABSTRACT

Facies of the Mississippian Sherwood Beds, Mission Canyon Formation along the International Border eastward through the Workman, Sherwood and Elmore oil pools demonstrate a progressive depositional shallowing from subtidal carbonates, through a 6-12 mi (10-20 km) wide intertidal barrier, and a narrow 2.5 mi (4 km) wide upper intertidal/lower supratidal region to a broad 40-60 mi (60-100 km) anhydrite and halite-filled playa. The Sherwood 'interval' represents one distinct sequence of the Mission Canyon regressive mega-sequence of the eastern Williston Basin characterized by basinward progradation of intertidal to supratidal system tracts. Eustatic sea level changes are marked in the Sherwood 'sequence' by Type 2 (Vail and Todd, 1981) sequence boundaries, one significant transgressive system tract, and numerous parasequences.

The broad intertidal barrier in the Sherwood pool area consists of intertidal to supratidal carbonate islands, each with characteristic micritization, exposure surfaces, vadose dissolution and collapse breccias, tepee structures, vadose pisolites, meteoric cements and gypsum crystallization. Across this broad intertidal shelf with limited tidal range occur normal circulation lagoonal sediments of mixed pellet, algal-coated grain, skeletal grain, and vadose pisolitic packstone which form the primary oil reservoir, and more restricted lagoonal argillaceous and carbonaceous dolomitic lime mudstone with abundant ostracods calcispheres, blue-green and dasycladacean algae. Late Sherwood argillaceous dolomitic mudstone and siltstone-filled channels are abundant across the intertidal belt.

The narrow upper intertidal to lower supratidal facies belt lies protected behind the major intertidal barrier in the Elmore and Workman pools area, and is marked by thinly laminated algal and lagoonal dolomitic mudstone, wind and water-borne fine grained oolite sandstone, and finally, interlaminated thin 'chicken-wire' anhydrite. The oolite beds form the most significant oil reservoir within this portion of the sequence. Eastward of the intertidal belt are various anhydrites, in part sabkha in origin. Minor relict carbonate beds also show clear evidence of complete replacement by anhydrite. A broad coastal playa in the eastern 30-50 mi (60-100 km) of the transect within Bottineau and McHenry Counties, North Dakota, lay protected behind the intertidal barrier, possibly below Sherwood sea level. Halite beds, up to 40 ft (12 m) in thickness, are restricted to the distant central position of this region. The Sherwood intertidal barrier-continental playa couplet describes a depositional pattern persistent throughout Mission Canyon time.