SEQUENCE STRATIGRAPHIC ARCHITECTURE OF THE MAQUOKETA FORMATION (UPPER ORDOVICIAN) ACROSS IOWA

Recent stratigraphic synthesis of the Maquoketa Fm (Upper Ordovician, Richmondian) across Iowa using outcrop and subsurface core information now identifies four widespread sequences, in ascending order: 1) Elgin Member, 2) Clermont-Fort Atkinson members, 3) Brainard Member, and 4) Neda Member. Each sequence is characterized by recurring facies patterns that include, in ascending order: 1) submarine hardgrounds and condensed phosphatic facies; 2) nonskeletal graptolitic shale facies, in part organic; 3) burrowed shale and nonskeletal mudstone facies; 4) shale and skeletal carbonate facies, part trilobite-rich; and 5) wackestone-packstone carbonate facies with diverse skeletal fauna. These facies patterns define a stack of transgressive-regressive sequences, each with condensed transgressive systems tracts (TSTs). The same patterns are seen laterally within each sequence in a shoreward direction, entirely losing facies 1 through 3 to the northwest. A TST wedge of limited extent is seen in each sequence, but TSTs are seen as thin condensed phosphatic facies across broader areas (includes the widespread basal Maquoketa phosphorite). Each sequence forms a broad shallowing-upward depositional succession labeled in some models as a “regressive systems tract” (HST and FSST). Regression was forced by eustatic sea-level fall across the stable Iowa shelf, an area of relatively low net sediment accumulation. Ironstone and red shale facies are widespread in the Neda Member, but the Neda sequence is now recognized to include additional facies: graptolite shale, burrowed green shale, linguloid shale, and shelly faunas. Graptolite shales are identified in the lower part of each sequence, and these shales interfinger with a unique tract of phosphatic carbonate facies in the Elgin Member of eastern Iowa. A lower anoxic water mass within a stratified epeiric seaway is interpreted to have encroached into Iowa during maximum sea level highstand for each Maquoketa sequence.