Hell Creek seaway

[bwitzke@gsbth-po.igsb.uiowa.edu]

     Jim Kirkland raised a noteworthy point concerning environmental 
     changes at the end of the Cretaceous (DD 212, 2/21/97):  "There is a 
     distinct possibility that the sea never fully retreated from the 
     western interior at the end of the Cretaceous," noting a sea-level 
     rise in the Triceratops Zone of North Dakota -- "the sea would have 
     been in eastern ND."  His point is well taken, and one we should 
     consider in evaluating "traditional" scenarios of Lance-Hell Creek 
     dinosaur extinction.  In particular, it has become almost gospel that 
     the Western Interior (WI) seaway was gradually retreating during the 
     Maastrichtian, and had completely withdrawn from the interior of the 
     continent well before the K-T boundary.  There are several lines of 
     evidence to suggest that such a scenario may be overly simplistic.
     
     Several stratigraphers had earlier raised the possibility that the WI 
     seaway did not completely withdraw at the end of the Cretaceous, 
     noting that the eastward-thinning Hell Creek-Ludlow nonmarine package 
     is sandwiched between marine units below (Pierre-Fox Hills) and above 
     (Paleocene Cannonball Fm.)[e.g., Feldman, 1972, NDGS Bull 61].  In 
     this regard, the Hell Creek nonmarine wedge resembles other Cretaceous 
     nonmarine wedges (e.g., Two Medicine-Judith River) which are known to 
     grade eastward into marine units.  If the trend of Hell Creek thinning 
     is extrapolated eastward beyond the present-day erosional edge, it 
     seems reasonable to suggest that the WI seaway may have remained in 
     the eastern Dakotas at the time of the K-T boundary.  Because eastward 
     erosion is profound, the extent of any late Maastrichtian seaway 
     remains unknown, but, like earlier Cretaceous seaway configurations, 
     it may have connected northward (Arctic), southward (Gulf), and/or 
     northeastward (Hudson Bay).
     
     Marine strata are known to interfinger with the Hell Creek Fm in North 
     Dakota, primarily the Breien Member (see NDGS, 1995, Rept. Inv. No. 
     98, for a listing of Breien marine fossils compiled by John Hoganson 
     from a locality south of Bismarck).  In addition, an ammonite 
     (Scaphites) and teredo-bored wood are noted from upper Lance strata in 
     eastern Wyoming, leading Clemens (1963, p. 17) to suggest that it is 
     "likely that during deposition of the type Lance formation the sea was 
     not far removed."  Oyster beds also occur in the Lance of southeast 
     Wyoming (Schlaikjer, 1935).
     
     Finally, occurrences of marine-related fish in the Hell Creek of 
     eastern Montana are of special interest (see Bryant, 1989, 
     U.Cal.Geol.Sci., v. 134).  These include sawfish (Ischyrhiza), rays 
     (Myledaphus), hybodont sharks (Lissodus), orectolobid sharks 
     (Squatirhina), and marine teleosts (Belonostomus, pachyrhizodontids).  
     A few taxa of rays and sawfish live in brackish to fresh water in the 
     modern world (in the lower few hundred kms of some tropical river 
     systems), but the overall aspect of the Hell Creek assemblage 
     certainly suggests marine influence -- Bryant "consider[s] these fish 
     indicative of nearby marine conditions."  These marine-related Hell 
     Creek fish occur throughout the Hell Creek sequence, including the 
     uppermost part.  They are known to occur in the Bug Creek and Harbicht 
     Hill channels, which encompass faunas straddling within a few meters 
     of the K-T boundary (latest Cretaceous-earliest Paleocene).  As 
     suggested by Bryant (p. 8), "the presence of sharks and rays in the 
     uppermost Hell Creek formation is indirect evidence that the seaway 
     remained nearby until virtually the terminal Maastrichtian."
     
     Did the Western Interior seaway withdraw from the continent well 
     before the end of the Cretaceous, heralding the inexorable decline and 
     extinction of the last North American dinosaurs in its stead?  Maybe 
     not.  Alternative geographies need to be considered, and a K-T seaway 
     in the continental interior remains a real possibility.
     
     Brian Witzke
     Iowa City