WHAT, IF ANYTHING, IS THE "MUDDY CYCLE" (LATE ALBIAN) IN THE WESTERN INTERIOR?

The recognition of Cretaceous cyclothems during the 1960s and 1970s (especially work by Weimer, Kauffman, Hattin) has proven to be a remarkably helpful innovation for constraining large-scale sedimentary packages of the Western Interior (WI) into genetic units. However, an interval of uppermost Albian strata has been more difficult to classify within this scheme, and these strata have been variously considered in any of three ways: 1) late regressive phase of the Kiowa-Skull Creek Cycle, 2) early transgressive phase of the Greenhorn Cycle, or 3) a separate depositional unit, the so-called “Muddy Cycle” (named after the Muddy Sandstone of CO-WY). The mid Dakota Fm succession in the eastern region of the WI (sequence D₁ of Brenner et al., 2000) correlates palynostratigraphically with the Muddy Sandstone. Marine-influenced sedimentation, mostly estuarine facies, in Dakota sequence D₁ is recognized in at least 3 stratigraphic positions in eastern NB and western IA, as indicated by sedimentary structures (IHS, tidal rhythmites), marine palynomorphs, brackish-water mollusks, trace fossils, and siderite/calcite geochemistry. It is of particular note that marine influence extends as far eastward during D₁ as it does during maximum transgression of the earlier Kiowa Cycle (D₀), suggesting similar magnitudes of sea-level rise and seaway expansion. This conclusion runs counter to some paleogeographic interpretations that show major seaway constriction for the Muddy interval. Holbrook et al. (2002) identified 3 thin but widespread sequences within the Muddy Sandstone (each with marine-influenced facies), and postulated ephemeral seaway connections across WY-CO. Seaway expansion into KS and NB is also suggested for each of the Muddy sequences to account for development of eastern estuarine facies in the Dakota Fm, but pedogenesis and erosional channeling during lowstands have obscured evidence for marine deposition in those areas. The “Muddy Cycle” is interpreted as a succession of at least 3 lower-order marine cycles, each marked by widespread marine transgression in the WI. As the seaway contracted during successive lowstand episodes, eastern-derived sandstone facies (estuarine and marginal-marine facies) prograded into the central region of the WI and bisected the seaway several times during the late Albian.