ESTIMATING MAGNITUDES OF SEA-LEVEL CHANGES ASSOCIATED WITH MIDDLE AND UPPER DEVONIAN SEQUENCES IN THE IOWA AREA

Physical geometries of Middle & Upper Devonian strata in Iowa help to constrain magnitudes of sea level changes associated with “third-“ and “fourth-order” sequences across inner and middle shelves. Since Iowa was tectonically stable, eustasy would have been the primary driver for marine onlap. Inner-shelf sequences (north & west) are capped by peritidal facies, whereas equivalent thinner to condensed sequences on the middle shelf (southeast) are of deeper subtidal origin. The sequential buildup of the inner shelf during the late Givetian-Frasnian constrains vertical relations during onlap of each sequence (T-R cycle). Famennian sequences overstep across the inner shelf. The position occupied during maximum sea-level lowstand for each cycle is drawn where the greatest depth of erosional penetration into underlying strata is observed, or by the position of the lowest exposure surface along the shelf transect. This lowstand position serves as a datum from which the total vertical extent of stratigraphic onlap across the inner shelf and its margin can be measured for each cycle. These measurements give minimum estimates of sea-level change, as the maximum water depth during deposition of each inner-shelf sequence would further increase the range of sea-level change. Because a horizontal datum is used, the values are further underestimated since any depositional slope across the shelf/ramp would also increase the range of sea-level change. Preliminary estimates of sea-level change for various T-R cycles (of Johnson et al., 1989/1992) in Iowa are as follows (* denotes units with evidence of middle-shelf seaway stratification): Ie through IIa-1 (Wapsipinicon-Little Cedar*) not constrained; IIa-2 (Coralville) >15 m; IIb (Lithograph City*) >35 m (with deep basal channel fills); IIc (Shell Rock) >30 m (assumes middle shelf exposure); IId (Lime Creek*) >90-140 m (deep sub-Lime Creek karst; regional overstepping of IIa-IIc edge); IIe-1 (lower Grassy Creek*) >70 m; IIe-2 (upper Grassy Creek*) >30 m (oversteps IIe-1 edge; onlaps 50 m at shelf-margin); IIf (English River) not constrained (onlaps 35 m at shelf margin). Regional crustal subsidence and post-depositional compaction would decrease or modify these estimates by an unknown amount. Potential causes for sea-level changes of these magnitudes need to be evaluated.