DEPOSITIONAL ENVIRONMENTS AND SEQUENCE STRATIGRAPHY OF UPPER ORDOVICIAN EPICONTINENTAL DEEP WATER DEPOSITS, EASTERN IOWA AND SOUTHERN MINNESOTA

by
W.D. Raatz and G.A. Ludvigson

The Geological Society of America
Special Paper 306, 1996, p. 143-159
Paleozoic Sequence Stratigraphy: Views from the North American Craton
B.J. Witzke, G.A. Ludvigson, and J.E. Day (eds.)

ABSTRACT


This study examines depositional environments, stratigraphy, paleontology, and petrology of the Upper Ordovician upper Dubuque Formation and Elgin Member (basal Maquoketa Formation) in eastern Iowa and southern Minnesota within a sequence stratigraphic framework. Dubuque-Elgin deposition occurred within a subcycle of the Maquoketa depositional cycle and records a single transgressive-regressive event; the Dubuque Formation and lowermost Elgin Member deposits are transgressive, lower Elgin deposits are highstand, and middle and upper Elgin deposits are regressive. The transgressive Dubuque Formation grades from an open marine benthic environment in its lower portions, to slightly oxygen stressed environments in its upper portions, with an associated loss of calcareous algae and tempestites, and increase in trilobite grain frequency and mud matrix. The Dubuque-Elgin contact is marked at all but the northernmost localities by a regional condensed section consisting of transgressive phosphatic hardgrounds and overlying highstand dark brown pelagic shale. The nature of overlying regressive Elgin Member deposits is dependent upon geographic location, with thick carbonates (~25 m [~82 ft]) present in the north and north-central areas, mixed carbonates and shales in the south-central area (~15 m [~48 ft]), and thin shales in the southern area (~8m [~25.6 ft]). Depositionally, this wedge represents shelf (thick carbonates), shelf-slope boundary (mixed carbonates and shales), and slope-basin (shales) environments.

The epicontinental Maquoketa seaway is interpreted to have had significant maximum depths (>200 m [640 ft]), and to have contained a density-stratified water mass. Upwelling and associated phosphate deposition resulted from a gyre circulation pattern driven by Taconic fresh-water runoff and surface winds, with net surface currents in the study area flowing basinward, replaced by deep upflowing waters. Depths are estimated from regional facies associations, depth-dependent nautiloid septal implosion measurements, and whole-rock carbonate d13C isotopic trends indicating increasing burial of organic carbon.