B. J. Witzke
The Geological Society of America
33rd Annual North-Central Section Meeting
Clarion Hotel and Convention Center, Champaign, IL, April 22-23, 1999
1999 Abstracts with Programs, v. 31, no. 5, p. A-81
Lower Galena Group carbonate strata (Decorah, Dunleith formations) show a general northwestward increase in argillaceous content shoreward towards siliciclastic source areas on the Transcontinental Arch. An overall upward decrease in clay content in the Galena Group further reflects progressive drowning of source areas during deposition of these strata. However, regionally persistent argillaceous to shaley intervals within the Dunleith carbonate succession suggest complex aspects to this onlap. Regional expansion of carbonate facies occur at several stratigraphic levels which likely correspond to times of transgressive deepening (including the Guttenberg, Beecher, Rivoli, Sinsinawa members). Offshore patterns within Galena Group strata show a general decrease in argillaceous content and a marked increase in grain-rich facies into middle shelf areas of Missouri and Illinois (Kimmswick Fm). Upper Galena Group strata (Dubuque, Cape formations) show significant thinning in an offshore direction indicating increasing sediment starvation offshore. Consistent with this pattern, it is proposed that sub-Cape Galena units also thin offshore, and the magnitude of the supposed sub-Cape/sub-Maquoketa unconformity is thereby brought into question. Although previous studies have suggested that upper Dunleith and Wise Lake equivalents are erosionally truncated southward, progressive southward thinning of stratigraphic packages within the Galena/Kimmswick raises the possibility that Wise Lake equivalents are not absent in the upper Kimmswick succession. This proposal can be tested biostratigraphically. Overall sediment accumulation rates during Galena Group deposition are extremely low, with the thickest and most complete succession present in shoreward mud- and clay-rich facies. Offshore sediment starvation is supported for the Decorah interval, and numerous hardground surfaces in overlying Galena strata document recurring episodes of sediment starvation. Relative sediment condensation is suggested for offshore grain-rich facies, analogous to later Paleozoic inner-shelf/middle-shelf cratonic facies patterns.