Abstract The Cretaceous Western Interior Seaway experienced several transgressive/regressive cycles during its existence. The Greenhorn Cyclothem, the sixth such cycle, is significant because of the symmetry of deposition, and because of the expression of cyclical climatically influenced deposits within. This field trip will illustrate evidence of both of these cycles.

Abstract The Greenhorn Cyclothem has become the model for eustatically-generated, third-order, cyclic sedimentation in Cretaceous epicontinental settings. The Rock Canyon Anticline sequence near Pueblo, Colorado, is the standard reference section for this cyclothem. The Greenhorn transgressive hemicyclothem (including the Muddy Sandstone, Mowry Shale, Graneros Shale, and the Greenhorn Formation through the middle Bridge Creek Limestone Member) has been the subject of detailed stratigraphic and paleontologic study for many years. In contrast, the Greenhorn regressive hemicyclothem (including the upper Bridge Creek Limestone Member, Greenhorn Formation, and the Fairport Shale, Blue Hill Shale, and Codell Sandstone Members of the Carlile shale) has received only general stratigraphic study. In this paper, high-resolution lithostratigraphic, geochemical, event-stratigraphic, and paleobiologic data for the Greenhorn regressive hemicyclothem are presented and analyzed in terms of facies and depositional history. The regressive sequence at Pueblo is nearly complete, and in general displays a graded, upward-coarsening suite of facies cut by rare disconformities. Cyclically bedded pelagic carbonates (Bridge Creek Limestone Member) grade upward through chalky and calcareous shales (Fairport Member) to dark non-calcareous shales becoming more silty and sandy upward (Blue Hill Shale Member), and the sequence terminates in lower, middle, and upper shoreface sands of the Codell Sandstone Member. The top of the cyclothem is truncated on middle-upper shoreface sandstones of the Codell by a regional transgressive unconformity at the base of the Niobrara Cyclothem. A second regional disconfor-mity may occur at the Fairport - Blue Hill member contact. Although the Greenhorn regressive facies sequence generally mirrors that of the transgression (the symmetrical cyclothem model), significant differences were recorded in this study which suggest that the Western Interior Basin was tectonically passive during eustatic fall and regional regression; subsidence did not outstrip or equal sedimentation rates and the basin filled; shallow water environments predominated. Thus, the regressive sequence of the Greenhorn Cyclothem is coarser-grained, more bioturbated, less laminated, and contains lower percentages of organic carbon than equivalent transgressive facies. Greater current circulation and higher levels of benthic oxygenation are implied during regression. Volcanic ash (bentonite) beds are extremely common in transgressive facies, and comparatively rare in regressive facies; this suggests a major slowdown in tectonism and associated volcanism during eustatic fall and regional regression. The number and extent of disconformities cutting the regressive sequence is far less than during either the Greenhorn or the Niobrara transgression; eustatic fall and regression were more continuous than the highly punctuated transgressive events.

Abstract The Lincoln Limestone Member of the Greenhorn Limestone is characterized by calcareous shale and calcarenite beds deposited during the transgressive phase of the Greenhorn Cyclothem. At Rock Canyon anticline the Lincoln Member is conformably underlain by the Graneros Shale and overlain by the Hartland Shale Member of the Greenhorn Limestone. A detailed study of the Lincoln divides the member into a lower and an upper interval, based on lithostratigraphic, geochemical and paleobiologic criteria. The lower unit is characterized by laminated, weakly calcareous shales, relatively high organic carbon content (C org ∼ 2.5%), and a macrofaunal assemblage transitional between the Graneros Shale and overlying Greenhorn deposits. The upper Lincoln interval is characterized by frequent alternation of laminated shale and ripple-bedded calcarenite beds with associated high C org values in the shale beds (>2.0%) and lower C org values in the calcarenite beds (<1.0%). Faunal composition through this upper interval records development of the first typical Greenhorn biota in the calcarenite beds. In the upper interval, the interbedding of contrasting lithofacies and biofacies suggests a dynamic paleooceanographic system poised between a relatively quiet water, oxygen-deficient environment during shale deposition, and well-circulated, well-oxygenated benthic conditions during calcarenite deposition. The Lincoln Limestone Member thus represents one complete 4th order marine cycle within the Greenhorn Cyclothem beginning with lithofacies and biofacies characteristic of a stillstand event in the lower Lincoln interval, progressing to deposits characteristic of an overall transgressive pulse in the upper Lincoln interval.

The historic type area of the Dakota Formation lies along the Missouri and Big Sioux River Valleys in northeast Nebraska and northwest Iowa, in the eastern margin area of the Western Interior Province. The type Dakota sequence is characterized to provide a basis of comparison with supposed “Dakota” sections elsewhere in the Western Interior. The general sequence includes (1) a lower sandstone-dominated Nishnabotna Member with coarse-grained and conglomeratic facies, and (2) an upper mudstone-dominated Woodbury Member with sandstone channel bodies and lignites. The depositional sequence is interpreted to record, in ascending order: (1) aggradation of coarse-grained braided and proximal meanderbelt fluvial systems; (2) an episode of lowered base levels, channel incision, and widespread pedogenic alteration; (3) aggradation of channels and flood-basin sediments in meander-belt fluvial systems; and (4) upward transition into deltaic and marginal-marine facies. The Woodbury Member is of Cenomanian age, and is interpreted to have aggraded in response to sea-level rise in the adjacent seaway during transgressive phases of the Greenhorn Cyclothem, with progressive eastward displacement of nonmarine by marine facies. The age of the lower Dakota sequence is known with less certainty, but is probably upper Albian in part. Aggradation of lower Nishnabotna fluvial systems may be related to sea-level rise during transgression of the Kiowa–Skull Creek Cyclothem, although the possibility that the entire Dakota sequence is post-Kiowa cannot be completely discounted. Based on the type sequence, usage of the term Dakota Formation should be restricted to include only eastern-derived detrital sequences and should exclude marginal-marine or marine facies of the Kiowa–Skull Creek Cyclothem. Usage of the term “Dakota” for western-derived sedimentary sequences in the Rocky Mountain area is discouraged.

Abstract The Niobrara Transgressive Hemicyclothem at Pueblo and to the south and west comprises four discrete lithostratigraphic members, separated by four and possibly five regional disconformities. The sequence disappears to the east and north as disconformities expand, anastomose, and become a single regional unconformity, bringing pelagic Fort Hays limestones, representing maximum transgression and eustatic rise, into erosional contact with maximum regressive shoreface sandstones (Codell) of the underlying Greenhorn Cyclothem. Eight molluscan biozones are defined within the complex Niobrara transgressive sequence in the Axial Basin. These biozones progressively disappear eastward within anastomosing disconformities. Near Pueblo only three molluscan biozones are recognized within the pre-Niobrara part of the cyclothem. A sequence of four foraminiferal biofacies are defined at Pueblo and along the Colorado Front Range which reflect rapid development of clear, warm, well circulated waters early in the transgressive history. These biofacies rise stratigraphically to the west, associated with eustatic rise in sealevel, and grade laterally along any time-line from diverse open water assemblages to environmentally stressed, less diverse, assemblages westward. This lateral gradation, and the rise of foraminiferal biofacies westward, is thought to reflect the westward migration of turbid watermasses associated with the active margin of the seaway during eustatic rise of the Niobrara Marine Cycle.

Subsurface information obtained during a recent investigation has been combined with information from previous investigations to establish a stratigraphic framework for the Cretaceous rocks of southwestern Minnesota. This new framework makes possible the reconstruction of Late Cretaceous (Cenomanian to Campanian) paleogeography. In southwestern Minnesota, the Greenhorn cyclothem is partly represented by an eastward transition from mostly marine strata of shelf and nearshore environments to mostly nonmarine strata of fluvial and deltaic depositional environments. Rocks of subsequent depositional cycles are extensively eroded and display less variation in depositional environment. The Upper Cretaceous sequence has been mapped as nine lithostratigraphic units. The combination of paleontologic information from past investigations and the stratigraphic framework yields a depositional history that begins with marine incursion in the late Cenomanian, continues with peak transgression in the early Turonian and regression through the remainder of the Turonian, and ends with erosion, creating an unconformity. A unit of Coniacian age overlies this erosional surface and represents another transgressive phase; although not evident, this unit may contain a lithologically similar regressive phase. An erosional unconformity forms the upper boundary of this unit. Finally, a poorly documented, mostly eroded unit similar to the Pierre Shale of eastern South Dakota occurs as small remnants, indicating a transgression in the Campanian. Distinctive clay-mineral assemblages that correspond to stratigraphic positions are further evidence of the postulated sea-level history.

Abstract The Pueblo-Canon City area provides an outstanding opportunity to study well exposed sections of the marine and marginal marine stratigraphie units which make up the Kiowa-Skull Creek (Late Albian), Greenhorn (latest Albian-Middle Turonian), and Niobrara (Late Turonian -Early Campanian) Cyclothems. Each of these cyclothems represents a third-order eustatic fluctuation that produced regionally synchronous transgression and regression in the Western Interior Seaway. Extensively studied exposures along the Canon City hogback and in the dissected Rock Canyon Anticline west of Pueblo are easily accessible and contain the most typical examples of the stratigraphie units in these cyclothems found in Colorado. Numerous, detailed and interdisciplinary studies of this Cretaceous section have been completed (see papers in this volume and references therein) which collectively allow the dynamics of large-scale Cretaceous eustatic cycles to be interpreted and modeled for epicontinental seas. In view of these factors, the Pueblo-Canon City area has become the standard mid-basin reference section for the Kiowa-Skull Creek, Greenhorn, and Niobrara Marine Cycles. The field trip will examine outcrops of each stratigraphie unit in the Kiowa-Skull Creek, Greenhorn, and Niobrara Cyclothems in the Pueblo-Canon City area. Field examination will focus on high-resolution stratigraphie concepts, facies characteristics, geochemical profiles, and inferred depositional environments. Discussions will emphasize a broad range of possible interpretations and current controversies regarding the depositional history of these cyclothems. The prevailing theme of this excursion will be cyclic sedimentation as expressed in Cretaceous marine and marginal marine facies of the Western Interior Seaway. Detailed discussion of each outcrop section can

Abstract Fine-Grained Deposits and Biofacies of the Cretaceous Western Seaway: Evidence of Cyclic Sedimentary Processes - This volume emphasizes the influence of cyclic sedimentary processes on the distribution of rock types and faunas in the Cretaceous strata in Colorado. As a whole, the volume provides an interdisciplinary view of the causes and consequences of gradual cyclic, periodic, and catastrophic changes in environmental conditions recorded by strata in the Kiowa-Skull Creek, Greenhorn, and Niobrara Cyclothems.