LEARNING FROM THE PAST – USING OLD RECORDS TO GAIN NEW INSIGHTS INTO THE PALEOCENE CANNONBALL FORMATION

The Paleocene Cannonball Formation of North Dakota is important as part of the last sedimentary record of the Western Interior Seaway preserved in North America. A significant record of the Cannonball Formation is the “long core” (220 m) drilled by the U.S. Army Corps of Engineers in 1947 during the early construction of Garrison Dam on the Missouri River, near Riverdale, North Dakota. Drilled from the west end of the dam, "long core" samples were meticulously described and tested by the Corps, with a focus on engineering properties of materials, so no attempt was made to relate the core to the regional geology. For the most part, previous studies treated the Cannonball Formation stratigraphy in a general way, unconcerned about the relative stratigraphic position of fossils (even within the core, e.g., benthic foraminifera by S.K. Fox, Rutgers University, from R.W. Lemke, U.S. Geological Survey; planktic forams by Fox and R.K. Olsson, 1969).

Working from earlier studies on the history of the Cannonball Sea (stratigraphy of ostracods, diatoms, and nannoplankton by Hartman et al., 1999), current reseach reexamines records of the original Corps of Engineers' descriptions, the physical core samples, and an unpublished and undated Fox student (Jackson) senior thesis, which included a hand-drawn chart (to scale) of the core containing brief lithologic descriptions, sample locations, and identification of 27 benthic foram horizons. From a derived version of this chart, precise depths of fossil horizons and of the lithology in which they occurred were determined. These occurrences were correlated with fossil and rock samples and original core descriptions. Reasonably good correspondence of the original core lithologies makes possible the approximate placement of the earlier recognized foram horizons within the Cannonball Formation section of the "long core." Study is underway to correlate more specific core samples and identified organisms with the "long core" log, lithologies, and other markers, and to incorporate the core record into a revised regional chronostratigraphy.