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Cedar Creek is the major anticlinal structure demarcating the south-west flank of the Williston basin. This pronounced fold developed through a geologic history of recurrent tectonic movements along a northwest-southeast striking fault zone. The four major periods of tectonism documentable in the Cedar Creek area from early Paleozoic through mid-Tertiary affected the local and regional distribution, erosion, and/or preservation, and, though moderately, the depositional facies of sedimentary strata since Ordovician time.
Uplift and fault movement accompanied north and east tilting of the main Cedar Creek block. Several hundreds of feet of Silurian strata were eroded prior to Middle Devonian time, and a karst plain developed on the Silurian surface. Middle and Upper Devonian sediments progressively onlapped and infilled the uplifted, northwest plunging element.
During latest Upper Devonian--possibly earliest Mississippian--pronounced fault movement occurred along the major fault zone. The Cedar Creek block was uplifted and tilted north and east; maximum displacement along the zone appears to have occurred in the vicinity of the Pine-Gas City-Glendive oil fields. Extensive erosion resulted in the near peneplanation of the structure and significant truncation of Upper Devonian strata. Continued paleostructural influence during Mississippian time is indicated by the distribution and facies of early and middle Mississippian rocks.
Late Mississippian (Chester) through Triassic:
In late Mississippian (Chester) and early Pennsylvanian time, the central and northern portion of the Cedar Creek area underwent gentle downwarping, and periods of subsidence occurred with relative down-to-the-east fault movement along parts of the ancestral master and subsidiary faults. Similar fault movement(s) and subsidence which continued to occur during the Permian and Triassic periods significantly influenced the deposition and preservation of these evaporite rich, red-bed sequences. Relative tectonic stability was attained by the Middle Jurassic and essentially maintained until post-Paleocene time.
The Cedar Creek block underwent its greatest magnitude of uplift during the post-Paleocene; younger Tertiary beds are not present in the area. This uplifting was accompanied by major flexuring, the extensive linear belt of asymmetric drape-folding generally aligned with the ancestral fault zones, and deep fault adjustment. Northwest plunge along the crestal portion was significantly increased. The entire area was subsequently uplifted during epeirogenic phases of the mid-Tertiary in the northern Rocky Mountain region and about 1,500 ft (460 m) of Paleocene and Upper Cretaceous strata eroded along the axis of the present structure.
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