ABSTRACT

The Williston Basin is one of the largest structural basins in North America. The United States part—the southern Williston Basin—underlies about 110,000 square miles in North Dakota, South Dakota, and eastern Montana and is included within a larger intracratonic sedimentary basin, which was intermittently downwarped during the Paleozoic and Mesozoic.

Although the Williston Basin was shaped by Laramide orogeny during the latest Cretaceous and early Tertiary, many of the present structural features were initiated during the Precambrian and reactivated by several subsequent epeirogenies and orogenies, of which the latest was the Laramide. The intermittent differential movements of several major structural features, such as the ancestral Cedar Creek and Nesson Anticlines and the ancestral Central Montana Uplift, and of many smaller structural features greatly influenced not only sedimentation but also preservation of Cambrian to Mississippian rocks.

The intracratonic Williston Basin had its inception in the Middle Ordovician and its general down-warping was interrupted twice during the middle Paleozoic. The following sequence of Precambrian to Mississippian events is interpreted from isopach maps of selected rock units, structure contour maps, and related stratigraphic studies.

(1) The Precambrian landscape, on which the initial Paleozoic sediments were to be deposited, developed as a low to moderate, irregular surface blanketed by a thick regolith and studded with isolated monadnocks. A low plateau probably occupied the east end of the ancestral Central Montana Uplift and low ridges marked the trends of the ancestral Cedar Creek and Nesson Anticlines.

(2) The Deadwood Formation and related rocks of Middle Cambrian to Early Ordovician age were deposited in a sea that transgressed eastward from the Cordilleran miogeosyncline across a broad shelf that included the entire Williston Basin area. These rocks form a wedge that is as much as 1,300 feet thick on the west and becomes progressively thinner and younger eastward.

(3) In Early to Middle Ordovician time epeirogenic uplift and erosion produced a farily even surface on the Deadwood Formation. A small area in which a 1,000-foot-thick sequence of the Deadwood was preserved just south of the present Nesson Anticline marked the center of the incipient intracratonic Williston Basin.

(4) The Winnipeg Formation of Middle Ordovician age, which is 350 feet thick at its depositional center just south of the present Nesson Anticline, was laid down in a shallow epicontinental sea that extended far south and east of the Williston Basin. The Winnipeg affords the first evidence for an intracratonic Williston Basin and for retarded subsidence of the ancestral Cedar Creek and Nesson Anticlines.

(5) Sedimentation was continuous from Middle Ordovician until Late Silurian time. The Red River and Stony Mountain Formations of Late Ordovician age, which together are as much as 900 feet thick, and the Interlake Formation (formerly Group) of Baillie (1951) of latest Ordovician to Silurian age, which is more than 1,200 feet thick, were laid down in a deeply subsiding intracratonic basin surrounded by a wide stable shelf. Depositional centers of the Red River and Stony Mountain Formations migrated about 65 miles southeast of those of the Winnipeg and Interlake Formations. Retarded subsidence along a small northward-trending ancestral Nesson Anticline continued, but the ancestral Cedar Creek Anticline apparently was inactive.

(6) Epeirogenic uplift ended early Paleozoic sedimentation^ and a low landmass occupied most of the basin area from latest Silurian to Middle Devonian time. The Interlake Formation was widely and deeply eroded, and although Upper Silurian strata may once have been present, they apparently are not preserved in North Dakota. On the margins of the former basin: the Interlake Formation and underlying Stony Mountain Formation were truncated and the Red River Formation was deeply eroded and locally truncated. Shallow southward- and southwestward-trending valleys were cut, and sinkhole topography was widely developed. The Beartooth Butte Formation of Early Devonian age was laid down as channel and sinkhole fillings in southern Montana and Wyoming. Lower Devonian deposits may have extended into the basin area, but they are not differentiated from similar basal Middle Devonian deposits there.

(7) In late Middle Devonian time a sea transgressed southward from northern Alberta into North Dakota and eastern Montana within a restricted intracratonic basin. As much as 875 feet of carbonate and evaporite rocks was deposited in northern North Dakota, but the depositional center lay much farther north. The northwest-trending ancestral Cedar Creek Anticline largely controlled the southwestern margin of the basin, and erosion continued on the west. The ancestral Nesson Anticline was expressed at this time as a large area of thinning that paralleled the ancestral Cedar Creek Anticline and the present Sanish Anticline.

(8) Deposition was continuous from Middle into Late Devonian time in the central Williston Basin while erosion and valley cutting progressed on the west and south. An early Late Devonian sea spread southward and westward from the area of the restricted Middle Devonian basin and joined a sea that simultaneously had transgressed eastward from the Cordilleran seaway. Basal channel-fill deposits, similar to those of Middle and Early Devonian age, were laid down in Montana and northern Wyoming. By middle Late Devonian time the Williston Basin had lost much of its identity as a depositional center and was but part of a huge shelf across which a shallow, pulsating sea transgressed southward. Upper Devonian rocks, as much as 1,140 feet thick along the Canadian border, were deposited as a nearly uniform southward-thinning wedge. This wedge was interrupted only by areas of anomalous thinning caused by retarded subsidence of the parallel ancestral Cedar Creek and Nesson Anticlines and of an ancestral anticline in the vicinity of Poplar Dome. As the intracratonic basin filled and deposition drew to a close, the sea gradually became shallower and more restricted.

(9) Between latest Devonian and earliest Mississippian time several local uplifts occurred sporadically around the margins of the Williston Basin area while deposition continued in a restricted central Williston Basin. The ancestral Central Montana Uplift was elevated, and compressive forces caused high-angle reverse faulting on the steep west flank of the ancestral Cedar Creek Anticline. Accompanying these movements, short episodes of moderately intense erosion stripped as much as 750 feet of Devonian and older Paleozoic rocks from parts of these ancestral features. Elsewhere erosion generally did not cut below Upper Devonian rocks.

Thin dark carbonaceous clastic strata were deposited penecontemporaneously in several shallow restricted-marine basins interspersed between areas of uplift. In the central Wiliiston Basin area, the Bakken Formation of Late Devonian(?) and Early Mississippian age attained a maximum thickness of about 140 feet at its depositional center on the east flank of the present Nesson Antcline. During episodes of little or no uplift, Bakken deposition spread southward and westward and for a time was continuous with deposition of the Exshaw Shale in Alberta and the Sapping-ton Member of the Three Forks Formation in western Montana. Seas, in which the dark shale unit of Devonian and Mississippian age was deposited in Wyoming and southern Montana and in which the Engle-wood Formation was deposited in South Dakota, eastern Wyoming, and southeastern North Dakota, transgressed northward in narrow northeast-trending basins. These marginal seas probably coalesced with each other and with the Bakken sea in earliest Mississippian time. Renewed uplift destroyed the thin connecting deposits and considerably eroded the margins of these units. The concluding uplift apparently took place along the southern part of the ancestral Cedar Creek Anticline just north of the present Black Hills.

(10) As orogenic activity diminished, a Mississippian sea spread eastward from the Cordilleran geosyncline into a moderately subsiding intracratonic basin. The axis of subsidence of the trough that permitted the eastward advance of the sea lay direcdy along the trace of the then eroded ancestral Central Montana Uplift, but the connection between the geosyncline and the basin probably was much wider. The widespread Madison Group of Mississippian age, which was laid down in this sea, attained a maximum thickness of 2,300 feet at its depositional center south of the present Nesson Anticline and exceeded 1,600 feet in thickness in the connecting trough. This thick accumulation of sediments largely masked all evidence of many of the preceding local uplifts. However, retarded subsidence over the trace of the eroded ancestral Cedar Creek Anticline caused slight depositional thinning of the lower part of the Madison Group. Later, renewed intermittent activity of this same ancestral anticline may have periodically restricted the eastward flow of water and caused halite to be precipitated in the upper part of the Madison Group in the intracratonic basin.