ABSTRACT

Few fossils have been collected from the Bakken Formation in North Dakota until recent years. Originally described as Mississippian, the Bakken has been correlated by lithology and well logs with both Devonian and Mississippian outcropping units in nearby states and provinces. Studies of 95 form-species (assigned to 22 form-genera) of conodonts and more than 500 macrofossils in 50 taxa from 30 cores now allow more accurate Bakken biozonation in North Dakota. The Bakken, up to 45 metres thick, is as old as the Late Famennian Lower Palmatolepis gracilis expansa Biozone and as young as the Kinderhookian Lower Siphonodella crenulata Biozone. A lower, black-shale member, less than 16 m thick, has a brachiopod fauna in the bottom metre, conodonts perhaps as old as the Lower Palmatolepis perlobata postera Biozone, the alga Foerstia 2 m above the base, and abundant conchostracans marking the top. A middle member, lacking age-diagnostic conodonts, has been divided into three lithologic units: a lower, massive, gray, 9 m siltstone unit with a Late Famennian brachiopod fauna; a middle unit (3-11 m thick) of thin, largely unfossiliferous laminae of gray mudstone and siltstone with "blade-like leaves" near the base; and an upper, massive, gray siltstone, less than 3 m thick, with a distinctly Mississippian Spirifer s. s. fauna. The systemic boundary has been placed at the base of the second unit of the middle member. An upper black-shale member, less than 7.5 m thick, contains Siphonodella sulcata at the base, rare macrofossils, and perhaps as many as three Kinderhookian conodont biozones. Deposition of the upper member of the Bakken may have begun as early as the time of the late Upper S. duplicata Biozone and continued through the 5. sandbergi Biozone into the Lower S. crenulata Biozone; although, more likely, deposition of the upper member began in the time of the Lower Siphonodella crenulata Biozone. This biozone is represented in both the upper member of the Bakken and the lower part of the Lodgepole Formation. Above this, suggestion of the S. isosticha—Upper S. crenulata Biozone was found in the Lodgepole; but conodont numbers dwindled rapidly to disappearance by 15 metres above the base of the Lodgepole, before the Kinderhookian-Osagean boundary was reached.