STRATIGRAPHIC INVESTIGATIONS OF THE BASAL CRETACEOUS SECTION AT ASH GROVE CEMENT QUARRY, LOUISVILLE, NEBRASKA
R.M. Joeckel, G.A. Ludvigson, R.L. Brenner, B.J. Witzke, R.L. Ravn, E.P. Kvale, and J.B. Swinehart
The Geological Society of America
30th Annual North-Central Section Meeting
Iowa State University, Ames, IA, May 2-3, 1996
1996 Abstracts with Programs, v. 28, no. 6, p. 47
The Ash Grove section (1) provides convincing evidence demonstrating that the lower Dakota Fm in eastern Nebraska (and by inference, Iowa) is of Late Albian age; (2) shows that estuarine sedimentation during the Kiowa/Skull Creek transgression extended eastward to the Iowa-Nebraska border; and (3) preserves "the most detailed tidal record yet available for the Cretaceous of North America" (Zawistoski et al., this session). The section includes: Unit 1, basal conglomerate (4 m) that buries an eroded surface with 3 m of local relief, cut into Pennsylvanian strata. Unit 2, a carbonaceous mudstone (7 m) with weakly-developed pedogenic fabrics. Unit 3, a conglomerate (0 to 3 m), preserves the depositional edge of a complex gravel sheet. Unit 4 drapes units 2 and 3, and consists of 5 m of thinly laminated siltstone and shale (including estuarine tidal rhythmites); it is cut by an erosional surface with 2.5 m of local relief, overlain by Unit 5, a medium- to coarse-grained sandstone (3 m) with abundant mudclasts. In November 1994, quarry workings had fully exposed the 20 m section, and because of concerns about the longevity of the section, it was measured/sampled in December, 1994. By April 1995, palynostratigraphic work showed that Ash Grove mudrocks were Late Albian in age (Witzke et al, this session); the presence of acritarchs and dinoflagellates indicated marine influences. In July 1995, staff members of the Nebraska, Iowa, and Indiana geological surveys collaborated on drilling 3 closely-spaced cores through units 5, 4, and 3. These cores provided study material for the analysis of tidal rhythmites (Zawistoski, et al., this session).