Fort Hays Limestone Research Articles

Chemostratigraphy of unconventional shale reservoirs: A case study of the Niobrara Formation within the Denver-Julesburg basin

The Upper Cretaceous carbonate-rich Niobrara Formation is a prominent petroliferous unconventional reservoir that is generally characterized by fine-scale heterogeneities that complicate local and regional correlation of target zones, well placement and completion strategies. Within rhythmically interbedded reservoirs like the Niobrara Formation, the occurrence of thin marl and chalk beds within chalk- or marl-dominated intervals respectively, further impedes identification of marl or chalk benches. This study uses a combination of petrophysical and geochemical data from 6 oil and/or gas wells within the Niobrara interval of the DJ basin to identify and correlate eight (8) chemozones (C1, C2, C3, C4, M1, M2, M3 and M4) and four (4) chemostratigraphic packages (Sharon Springs, Smoky Hill, Fort Hays and Codell Sandstone) that are essential for constructing a robust and dynamic stratigraphic framework.The results showed synchronous variation in petrophysical trends, elemental, mineralogical and organic composition at contacts, ubiquitous cyclicity within, and across the Niobrara Formation in the northern DJ basin. The stratigraphic units are geochemically separated based on TOC, total clay, total carbonate, and quartz + feldspars, and Ca, Sr, Si, Al, Ti, Rb, Zr, Nb, K, and Cr, while the internal stratigraphic units within the Niobrara Formation are defined by integrating gamma-ray (GR), resistivity, carbonate proxy, marl proxy, carbonate/marl proxy and terrigenous input. The chalk and limestone units (C1–C4) have higher Ca, Sr, total carbonate concentrations and relative brittleness index relative to the marl units (M1-M4). This multiproxy approach reveals variable calcareous and terrigenous/detrital input, and paleoredox conditions as the main controlling factors behind the changing lithologies/cyclicity within the Niobrara Formation. Hence, an adequate characterization and correlation of inherent heterogeneities within marls and chalks of the Niobrara Formation may require incorporation of a multiproxy approach at various scales. By integrating color scale, geochemistry, TOC and wireline well log curves responses, higher resolution identification of chalk reservoirs and marl source beds can be achieved. Spatial distribution of chalks and marls consistently show a trend of total carbonate and Ca increasing southwards towards the Tethys Sea with highest average concentrations recorded in Fort Hays Limestone while quartz + feldspars, and total clay concentrations increasing northwards, suggesting a southern and northern provenance, respectively.

A Stratigraphic Analysis of the Codell Sandstone in Ellis County, Kansas

This is a study of a Codell Sandstone outcrop in the Northern part of Ellis County, Kansas. The Codell Sandstone outcrop is part of a much larger section of the Codell Sandstone that is a part of the southern Denver Basin. It extends over the states of Colorado, Wyoming, Kansas, Nebraska, South Dakota, and Minnesota. This section of the Codell overlies the Blue Hill Shale and is overlain by the Fort Hays Limestone. The Codell was formed during the Late Cretaceous period. In Colorado and Wyoming, it is mainly a gas and oil reservoir, while in Kansas it is a domestic aquifer for many agricultural areas. The outcrop was formed during the time of the Western Interior Seaway. One study has been done over the exact outcrop of what I viewed, called “The type section of the Codell Sandstone” written by James Hagadorn , Mark Longman, Richard Bottjer , Virginia Gent, Christopher Holm Denoma , and Jonathon Sumrall. The discussion portion is a surface look at each of the layers instead of focusing on one particular layer .

Stratigraphic distribution of the Codell Sandstone in the Denver Basin using wireline logs and core

Core data from five key wells spanning the Denver Basin were tied to wireline log data and used to interpret the distribution of the Middle Turonian Codell Sandstone Member of the Carlile Formation across the Denver Basin. The character of the Codell’s upper contact is sharp with a localized top-down truncation across the basin, which is consistent with an associated unconformity surface. In contrast, the Codell’s lower contact varies from being gradational in most of the southern Denver Basin to being unconformable in the northern basin. Log correlations reveal that the Codell is absent within an elongate northeast-trending swath up to 125 miles wide in northeastern Colorado. This elongate gap is herein referred to as the ‘No Codell Zone’ abbreviated as NoCoZo. Hypotheses to explain the absence of the Codell Sandstone in the NoCoZo include a lateral facies change from sandstone to shale, non-deposition of Codell-equivalent sediments across this area, post-depositional erosion, or a combination of these processes. Correlation of wireline logs across the northern and southern limits of the NoCoZo, combined with outcrop and core observations, suggest top-down erosion of the Codell increasing into the NoCoZo. However, the overlying Fort Hays Limestone is laterally continuous and has a relatively consistent thickness across the NoCoZo, suggesting two tenable hypotheses: 1) The NoCoZo represents an area of post-Codell erosion due to short-lived growth of a broad, low relief uplift that was no longer active during Fort Hays deposition; or 2) A stepped sea level fall and forced regression resulting in non-deposition of the Codell over this broad swath. North of the NoCoZo, the Codell thickens northward to more than 40 ft into adjacent parts of Wyoming and Nebraska. In this northern area, the Codell has two main lithofacies in three laterally correlative zones, in ascending order: a lower bioturbated siltstone to very fine-grained sandstone ranging from 2 to 20 feet thick, a middle 2 to 10-foot thick laminated to bedded siltstone to fine-grained sandstone, and an upper 5 to 20-foot thick bioturbated siltstone to very fine-grained sandstone. Southeast of the NoCoZo the Codell thickens to as much as 80 feet in an east-trending belt from Pueblo, Colorado, into west central Kansas. The southern Codell can be divided into two coarsening upward parasequences, from a basal muddy coarse siltstones to very fine-grained sandstones. The siltstones and sandstones in the southern Codell are mostly bioturbated with locally developed bedded facies at the top.

The type section of the Codell Sandstone

We formally assign, describe and interpret a principal reference section for the middle Turonian Codell Sandstone Member of the Carlile Shale near Codell, Kansas. This section, at the informally named Pumpjack Road, provides the thickest surface expression (9 m, ~30 ft) of the unit in Ellis County. The outcrop exposes features that typify the Codell throughout the southern Denver Basin and vicinity. At this reference section, the Codell conformably overlies the Blue Hill Shale Member of the Carlile Shale and is unconformably overlain by the Fort Hays Limestone Member of the Niobrara Formation or locally by a thin (<0.9 m, <3 ft) discontinuous mudstone known as the Antonino facies. The top contact of the Codell is slightly undulatory with possible compaction features or narrow (<30.5 m, <100 ft), low-relief (0.3-0.6 m, 1-2 ft) scours, all of which hint that the Codell is a depositional remnant, even at the type section. At Pumpjack Road, the Codell coarsens upward from a recessive-weathering argillaceous medium-grained siltstone with interbedded mudstone at its base to a more indurated cliff-forming muddy, highly bioturbated, very fine-grained sandstone at its top. The unit contains three informal gradational packages: a lower Codell of medium to coarse siltstone and mudstone, a middle Codell of muddy coarse siltstone, and an upper muddy Codell dominated by well-sorted very fine-grained sandstone. The largest grain fractions, all <120 mm in size, are mostly quartz (40-80%), potassium feldspar (7-12%), and albite (1-2%), with some chert (<15%), zircon, and other constituents such as abraded phosphatic skeletal debris. Rare fossil fish teeth and bones also occur. Detrital and authigenic clays make up 9 to 42% of the Codell at the reference section. Detrital illite and mixed layer illite/smectite are common, along with omnipresent kaolinite as grain coatings or cement. As is typical for the Codell, the sandstone at the type section has been pervasively bioturbated. Most primary structures and bedding are obscured, particularly toward the top of the unit where burrows are larger, deeper and more diverse than at its base. This bioturbation has created a textural inversion in which the larger silt and sand grains are very well sorted but are mixed with mud. Detrital zircons from the upper Codell are unusual in that they are mostly prismatic to acicular, euhedral, colorless, unpitted, and unabraded, and have a near-unimodal age peak centered at ~94 Ma. These characteristics suggest they were reworked mainly from Cenomanian bentonites; their ultimate source was likely from the Cordilleran orogenic belt to the west and northwest.

Sequence stratigraphy and regional context of the Mancos-Niobrara in the northern San Juan Basin

In the northern San Juan Basin, the Niobrara Formation is represented by the upper half of the Mancos Shale (the Smoky Hill Member and Cortez Member). This section is generally equivalent to the Niobrara Formation along the Colorado Front Range. Although the Fort Hays Limestone is absent west of Pagosa Springs, the C Chalk and B Chalk are well-expressed as two resistant bench-forming calcareous units in the northern San Juan Basin. These two calcareous units have also been established as prospective hydrocarbon targets by operators in the area. Calcareous facies equivalent to the A Chalk were not deposited in the northern San Juan Basin due to siliciclastic dilution during the regressive latter half of the Niobrara marine cycle. The overall third-order Niobrara marine cycle includes these members of the Mancos Shale: the Juana Lopez, Montezuma Valley, Smoky Hill, and Cortez members. The Smoky Hill Member sits just above the basal Niobrara unconformity in most of the study area, and the entire section also has greater thickness and siliciclastic content than its equivalent farther east along the Front Range. Several extensive outcrop locations (in and around Pagosa Springs, Piedra, and Durango, CO) along with three new cores along the CO-NM border form the foundation for sequence stratigraphic interpretation of the Niobrara marine cycle in this study. All these locations and cores were tied back to the Mancos reference section at Mesa Verde National Park established by Leckie et al. (1997) which provides detailed description and biostratigraphy for the entire Mancos Shale. Establishing and applying a sequence stratigraphic framework to any section creates consistent reference standards for communication, research, and further correlation. Comparisons of chemostratigraphic data from equivalent strata between the northern San Juan Basin and Denver-Julesburg (DJ) Basin reveal significant differences in the timing and style of source-rock deposition (and associated low-oxygen conditions). The sequence stratigraphic framework also emphasizes tremendous lateral facies changes in the basal Niobrara section (i.e., Fort Hays Limestone to Tocito Sandstone). Once refined and applied, this stratigraphic framework can be used for predicting the distribution of reservoir properties, in addition to enhancing understanding of the Niobrara marine cycle and the Western Interior Seaway.

Petrophysical rock typing in unconventional shale plays: The Niobrara Formation case study

Production from organic-rich shale petroleum systems is extremely challenging due to the complex rock and flow characteristics. An accurate characterization of shale reservoir rock properties would positively impact hydrocarbon exploration and production planning. We integrate large-scale geologic components with small-scale petrophysical rock properties to categorize distinct rock types in low-porosity and low-permeability shales. We then use this workflow to distinguish three rock types in the reservoir interval of the Niobrara Shale in the Denver Basin of the United States: the Upper Chalks (A, B, and C Chalk), the Marls (A, B, and C Marl), and the Lower Chalks (D Chalk and Fort Hays Limestone). In our study area, we find that the Upper Chalk has better reservoir-rock quality, moderate source-rock potential, stiffer rocks, and a higher fraction of compliant micro- and nanopores. On the other hand, the Marls have moderate reservoir-rock quality and a higher source-rock potential. The Upper Chalks and the Marls should have major economic potential. The Lower Chalk has higher porosity and a higher fraction of micro- and nanopores; however, it exhibits poor source-rock potential. The measured core data indicate large mineralogy, organic richness, and porosity heterogeneities throughout the Niobrara interval at all scales.

Chemostratigraphic and depositional characterization of the Niobrara Formation: Techniques for assessing lateral heterogeneity, CEMEX Quarry, Lyons, Colorado, Denver-Julesburg Basin

The Late Cretaceous (89–82 Ma) Niobrara Formation is the primary target for unconventional horizontal well activity in the Denver Basin. With an abundance of horizontal activity, the question of well-length scale lateral heterogeneity becomes paramount addressing steering practices, completion, and ultimately well performance. We have incorporated detailed core description, high-resolution chemostratigraphy, and petrophysical logs from four near-surface cores taken at the CEMEX Lyons Quarry to assess lateral heterogeneity. Cores were originally intended to determine the geochemistry and areal extent of the Fort Hays Limestone Member of the Niobrara Formation, CEMEX’s primary target for cement production. However, due to the cores’ proximity to one another (91.4–609.6 m [300–2000 ft]) and the presence of not only the Fort Hays Limestone but also the petroliferous chalks and marls of the Lower Smoky Hill Member (including the D Chalk, C Marl, and a portion of the C Chalk), the data set can also be used as a proxy to model changes along a pseudohorizontal wellbore. Observations include lateral bentonite discontinuity, trace element and organic carbon variations, and distinct changes in sedimentary structures. Well-length sedimentological and stratigraphic changes may have unique implications for unconventional oil and gas development.

Paleostructural control of hydrocarbon production from the Cretaceous Niobrara in a part of the Denver-Julesburg Basin of Colorado and Wyoming

Horizontal drilling and fracture stimulation of the Niobrara Formation chalks within the last decade have resulted in a widespread resource play in the Denver-Julesburg (DJ) Basin where over 50,000 vertical wells had already penetrated the Niobrara. The first fracture-stimulated horizontal Niobrara well in the DJ Basin was drilled in 2005. By the end of 2015, over 2000 horizontal wells had been drilled targeting the Niobrara and these new wells have made over 120 MMBO and nearly 500 BCF. Thickness changes correlate with the varying success of hydrocarbon production from the Niobrara in a part of the DJ Basin of Colorado. In the study area, the Niobrara comprises four chalks with interbedded marlstones; from top to bottom the A, B, and C chalks, and the Fort Hays Limestone. The Niobrara B chalk is the primary target for horizontal drilling; both the A and C chalk are secondary targets. The Niobrara Formation is self-sourcing and the hydrocarbons in the study area are not thought to have migrated. Within the study area, productivity in the Niobrara may be directly related to thermal maturity. Regionally thinner Niobrara trends are more likely to contain more productive wells than where thicker Niobrara is present. Thin intervals also coincide with higher resistivity values in the Niobrara B chalk and higher bottom hole temperatures. Temperatures were likely elevated in these locations in the past which led to increased organic maturity. Reservoir porosity and permeability may be enhanced along thin trends where shallow water currents winnowed sediments. Mapping the interplay of thickness, resistivity, and temperature of the Niobrara Formation can greatly improve the success rate of drilling in this play.

Micropaleontological evidence for redox changes in the OAE3 interval of the US Western Interior: Global vs. local processes

The Coniacian-Santonian interval has been proposed as the youngest of the Cretaceous ocean anoxic events (OAE3), but this designation has long been debated. OAE3 is associated with a long-lasting (∼3 myr) succession of black shales from the central and South Atlantic, Caribbean region, and the North American Western Interior; in the Western Interior it is characterized by an abrupt increase in total organic carbon (TOC) and corresponding trace metal indicators for anoxia. However, the modern concept of OAEs is predicated on detection of global carbon cycle perturbations as recorded by substantial carbon isotope excursions (CIE), and the protracted Coniacian-Santonian black shale interval does not have a large CIE. A more conservative definition of OAE3 might limit the event to the modest positive carbon isotope excursion restricted to the upper Coniacian Scaphites depressus Ammonite Zone. Trace metal proxies suggest that oxygen levels abruptly declined prior to the onset of this CIE in the Western Interior Sea (WIS), but it is unknown whether regional anoxic conditions were confined to sediments/pore waters, or how anoxia may have affected the biota. In an effort to characterize the oxygenation history of the WIS and to better understand the nature of the hypothesized OAE3, we present micropaleontological evidence of declining oxygen in bottom waters prior to the event using benthic foraminifera, which are sensitive to dissolved oxygen. Changes in benthic foraminiferal abundances suggest a decline in oxygen at least 1-myr prior to the CIE (including a nadir immediately below the start of the excursion), improving bottom water oxygen during the CIE, and re-establishment of persistent anoxia following the isotope excursion. Anoxia endured for nearly 3 myr in the central seaway, showing some signs of recovery toward the top of the Niobrara Formation. Our findings suggest that declining oxygen concentrations in the seaway eventually reached a tipping point, after which dissolved oxygen quickly dropped to zero.The late Coniacian CIE is an exception to the trend of declining oxygen in the WIS, and part of a larger pattern in the oxygenation history of the Niobrara Formation which suggests that it does not adhere to standard black shale models. Transgressive periods, including the Fort Hays Limestone and the lower limestone unit of the Smoky Hill Shale (which corresponds to the CIE) are relatively oxic, while periods of highstand (i.e., most of the Smoky Hill) correspond to deteriorating oxygen conditions. This contrasts with the standard black shale model for sea level and oxygen, where transgressions typically correlate with maximum TOC enrichment, interpreted to result from both sediment condensation and oxygen deficiency. The association of global carbon burial/anoxia (as indicated by carbon isotopes) with a regional increase in oxygen and decrease in organic matter preservation is reminiscent of the Cenomanian-Turonian Greenhorn Limestone, which contains OAE2. In both cases, the facies are not typical black shales, but instead have appreciable carbonate content. Western Interior redox trends support the rejection of the original concept of a protracted Coniacian-Santonian OAE3 because it is not a distinct “event.” Increasing local oxygen during the late Coniacian CIE also argues against a narrower OAE designation for this event, because the excursion can't be tied to anoxia here or anywhere else it has been described. Nevertheless, the Late Coniacian Event (as we prefer to call this CIE) still represents an important perturbation of the global carbon cycle. This is emblematic of the shift away from widespread, discrete anoxic events during the ongoing paleoceanographic reorganization of the Late Cretaceous, even as large carbon cycle perturbations continued.

Interpretation of wrench faulting and faultrelated pressure compartmentalization, Wattenberg Field, Denver Basin Colorado

A structural interpretation of a 128 km2 3D seismic data set from Wattenberg Field, Colorado indicates a northeast-trending step-over fault system in the Niobrara Formation. These step-over faults are the product of a fault block rotation that is caused by two large-scale dextral wrench faults: the Longmont and the Lafayette wrench fault zones. An investigation into different fault orientations between the Niobrara and the overlying Pierre Shale indicates that a rotation of the maximum horizontal stress has occurred during the Laramide Orogeny. The step-over faults and stress rotation result in reservoir compartmentalization in the Niobrara. Seismic inversion shows the different stress field compartmentalization. Low impedance occurs in a lower effective stress environment caused by high pore pressure. These compartments may constitute ‘sweet spots’ in exploration and production in the Niobrara Formation. The Niobrara Formation is a major hydrocarbon reservoir deposited during the Late Cretaceous (Figure 1a). This formation consists of the Smoky Hill Member and Fort Hays Limestone. The Smoky Hill member consists of A, B, and C chalks, and A, B, and C marls. Bratton (unpublished work, 2015) indicates that the overpressure regime extends into the overlying Sharon Springs Member of the Lower Pierre Formation. Figure 1B shows the geologic events of the Late Cretaceous Petroleum System (Higley and Cox, 2007). The Laramide Orogeny occurred during the Late Cretaceous and Early Tertiary (between 70-50 ma.). This event is thought to be responsible for the structural features in the study area (Higley and Cox, 2007). The products of this movement include several wrench fault zones located in the area mapped by Weimer (1996).

Interregional correlation of disconformities in Upper Cretaceous strata, Western Interior Seaway: Biostratigraphic and sequence-stratigraphic evidence for eustatic change

We are pleased to have this opportunity to address the main points raised by [Krause and Braunberger (2015)][1]. In 1964, Scott and Cobban, in their study of the Peck Creek section, noted that the top of bed 29 was “very smooth,” a description not applied by them to any other bed in the Fort

A new skeletal remain of the durophagous shark, Ptychodus mortoni, from the Upper Cretaceous of North America: an indication of gigantic body size

Ptychodus mortoni Mantell is a Late Cretaceous shark that possessed pavement-like tooth plates that were used to feed on hard-shelled macroinvertebrates (durophagy). Here, we describe a new specimen of P. mortoni from the Fort Hays Limestone Member of the Niobrara Chalk in Kansas, USA, that consists of associated teeth, placoid scales, and a portion of the right upper jaw. Although the specimen is fragmentary, this fossil supports the previously proposed contention that P. mortoni was a gigantic animal that likely reached at least 10 m in total body length with an estimated jaw length of nearly 1 m.

Earliest record of the genus Tylosaurus (Squamata; Mosasauridae) from the Fort Hays Limestone (Lower Coniacian) of western Kansas

Abstract Fossil vertebrates are extremely rare in the Fort Hays Limestone member of the Niobrara Chalk in comparison to the rich variety of well-preserved fish, turtles, mosasaurs, pteranodons, and toothed birds collected from the overlying Smoky Hill Chalk, and to a lesser extent, the underlying Carlile Shale. Although mosasaurs are well documented from the Smoky Hill Chalk (Upper Coniacian – Lower Campanian), and occur rarely in the Carlile Shale (Middle Turonian), they had not been previously reported from the intervening Fort Hays Limestone (Lower Coniacian). Here we report the remains of a mosasaur (FHSM VP-2297) preserving 14 articulated vertebrae, the left coracoid, ribs, and fragments of sternal cartilage collected by M.V. Walker in 1967 from Ellis County, Kansas. Serrated bite marks on several of the ribs are attributable to the anacoracid shark, Squalicorax falcatus, and suggest postmortem scavenging of the remains. The specimen is identified as Tylosaurus sp. on the basis of the characteristic ...

Ptychodus mammillaris (Elasmobranchii) and Enchodus cf. E. shumardi (Teleostei) from the Fort Hays Limestone Member of the Niobrara Chalk (Upper Cretaceous) in Ellis County, Kansas

Vertebrate fossils are rare components of the Fort Hays Limestone Member (uppermost Turonian - Lower Coniacian) of the Upper Cretaceous Niobrara Chalk. Herein, two fish taxa are described that are new to the unit: Ptychodus mammillaris Agassiz and Enchodus cf. E. shumardi Leidy. These additions increase the total number of documented vertebrate species to 11 for the Fort Hays Limestone and provide additional insights into the paleoecology (e.g. community structure) of the Western Interior Seaway during the deposition of the Fort Hays Limestone.

Stratigraphic Record of the Late Cretaceous Lamniform Shark, Cretoxyrhina mantelli (Agassiz), in Kansas

The Late Cretaceous lamniform shark, Cretoxyrhina mantelli (Agassiz), occurs in the Graneros Shale, Lincoln Limestone, Jetmore Chalk, Pfeifer Shale, Fairport Chalk, Fort Hays Limestone, and Smoky Hill Chalk in Kansas. The crown heights of anterior teeth of geologically older C. mantelli generally are smaller than those of geologically younger ones, but taxonomic division at the subspecific level cannot be made. Cretoxyrhina mantelli is thought to prefer offshore environments.

Selachians from the Fort Hays Limestone Member of the Niobrara Chalk (Upper Cretaceous), Ellis County, Kansas

Vertebrates from the Fort Hays Limestone Member (uppermost Turonian?-Middle Coniacian) of the Upper Cretaceous Niobrara Chalk in Kansas are poorly known. Six selachian taxa from the member in Ellis County, Kansas, are described herein: Ptychodus mortoni, cf. Scapanorhynchus sp., Cretolamna appendiculata, Cretoxyrhina mantelli, Paranomotodon sp., and Squalicorax falcatus. These selachians probably occupied the highest positions in the food chain during the maximum invasion of the Niobrara sea. Because the gradationally overlying Smoky Hill Chalk contains a rich vertebrate fauna, including P. mortoni, C. appendiculata, C. mantelli, and S. falcatus, other fossil vertebrates may eventually be discovered in the Fort Hays Limestone.

Calcareous Nannofossil Biostratigraphy and Paleoecology of the Fort Hays Limestone Member (Upper Cretaceous), Niobrara Formation, from Jewell County, Kansas, Wolcott, Colorado, and Wagon Mound, New Mexico

Calcareous Nannofossil Biostratigraphy and Paleoecology of the Fort Hays Limestone Member (Upper Cretaceous), Niobrara Formation, from Jewell County, Kansas, Wolcott, Colorado, and Wagon Mound, New Mexico

Dakota Sandstone and Associated Rocks Adjacent to San Juan Sag Near Gunnison, Colorado: ABSTRACT

The stratigraphy and depositional systems of the Dakota Sandstone and associated rocks were studied in outcrop at the northeastern margin of the San Juan volcanic field in southern Gunnison and northern Saguache Counties, Colorado. This study fills in a major gap in regional Mesozoic stratigraphy and provides a last view of these rocks before they are concealed to the south by the volcanic cover of the San Juan sag, a frontier hydrocarbon basin. Locally, the Burro Canyon Formation is interpreted as a dominantly meandering fluvial system formed under oxidizing conditions similar to the underlying Morrison Formation. The Burro Canyon Formation pinches out along a roughly east-west line just south of US Highway 50 and is missing for about 15 mi south to the edge of the continuous volcanics. The Dakota Sandstone consists of a lower, low-sinuosity fluvial system abruptly overlain by an upper, mostly nearshore marine sequence which grades upward into the offshore Mancos Shale. While the total Dakota thickness is relatively constant, the fluvial system thickens markedly to the south where marine rocks are quite thin. To the north, marine rocks thicken irregularly and fluvial rocks are thin. In this area, the Mancos Shale can be readily subdivided intomore » Graneros, Greenhorn, Carlile, Juana Lopez, and upper Niobrara formations, although the Fort Hays limestone is locally missing. The upper Mancos Shale and Mesaverde Group are also missing, presumably due to late Eocene prevolcanic erosion.« less

Use of Rhythmic Bedding Patterns for Locating Structural Features, Niobrara Formation, United States Western Interior

Milankovitch-type bedding cycles are well developed in the Upper Cretaceous Fort Hays Limestone Member, Niobrara Formation. These time-equivalent cycles can be correlated across much of Colorado, Kansas, and northeastern New Mexico by combining subsurface and outcrop data. Documentation of thickness variations within the regionally persistent Fort Hays bedding sequence furnishes a basis for fine-scale analysis of Late Cretaceous crustal movements within the eastern ramp region of the Western Interior foreland basin. Regional thickness changes in groups of shale-limestone couplets were correlated and mapped in outcrop and in the subsurface to locate structural elements that influenced Fort Hays deposition. In the Denver-Julesburg basin of Colorado and western Kansas, up to 6.1 m (20 ft) of thinning of the section occurs dominantly along northeastwardly trending belts formed during Late Cretaceous reactivation of the Transcontinental arch. Mapping of these small-scale thickness changes in the Fort Hays demonstrates that Cretaceous reactivation of the Transcontinental arch was not restricted to the northern part of the Denver-Julesburg basin. Additional structures may occur as far south as the Colorado-New Mexico border. A northwestwardly thinning trend is also apparent and may have resulted from increased compaction and diagenesis, reduction of sediment input during transgression, or from uplift in the vicinity of the Colorado Front Range.

Fort Hays State University Natural Areas

Two natural areas are supervised by the Department of Biological Sciences at Fort Hays State University and are known as the and the Cedar Bluff In 1900 the Federal government granted old Fort Hays to Kansas. The 35 acre (14.2 ha) relict natural area was included in the 4160 acres (1683.5 ha) that became Fort Hays State University. The area is located in the SE 1/4 Sec. 1, T. 14S, R. 19W in Ellis County. The relict area has not been grazed since 1902. Ruts made by wagons coming to and from the Fort remain and can be seen cutting across the relict area. The topography of the relict area is characterized by steep to gentle slopes bordering a central drainage area. The site contains strata of the Niobrara Chalk, particularly the Fort Hays Limestone Member. A cap of Loveland Loess overlies the Fort Hays Limestone on upland areas. Parent materials consist of loess, limestone, alluvium, and shale. Four major communities are found in this area. The upland shortgrass community is dominated by buffalograss (Buchloe dactyloides), blue grama (Bouteloua gracilis) and western wheatgrass (Agropyron smithii), and occurs on deep loessial soils of the Mento series. The hillsides are dominated by big bluestem (Andropogon gerardii), little bluestem (Andropogon scoparius), and side-oats grama (Bouteloua curtipendula). The shallow calcareous Brownell soils underlying the hillside community support diverse forb populations. A homogeneous big bluestem community parallels the major drainage and is developed on alluvial and colluvial Armo soils. A large ravine traverses the relict area and is dominated by big bluestem, kochia (Kochia scoparia), and common sunflower (Helianthus annuus). The weediness of the lowland community is possibly the result of seepage from the farm pond located on the upper end of the drainage. The lowland has alluvial Roxbury soils.