William A. Cobban

Effect of diagenesis on the Sr, O, and C isotope composition of late Cretaceous mollusks from the Western Interior Seaway of North America

Evaluating the effects of diagenesis on the isotopic compositions of Sr, O, and C in marine carbonates is critical to their use as proxies in reconstructing information on the salinity, temperature and dissolved inorganic carbon of ancient oceans. We have analyzed a series of samples of mollusk shells from the Baculites compressus zone (late Campanian) of the Pierre Shale of South Dakota. Samples included outer shell material and septa of cephalopods collected inside and outside concretions. Preservation was evaluated using light microscopy, scanning electron microscopy (SEM), trace element analysis and X-ray diffraction. All of the material consists of aragonite based on X-ray diffraction. An SEM preservation index (PI) was established based on comparison of the microstructure of the fossil material with that of modern Nautilus. Excellent preservation (PI = 5) was characterized by well-defined nacreous plates with discrete, angular boundaries. In contrast, samples showing fused nacreous plates with indistinct boundaries were rated poor (PI = 1). 87Sr/86Sr ratios vary with preservation and average 0.707648 ± .000021 (n = 10) for excellent preservation (PI ≈ 5), 0.707615 ± .000028 (n = 5) for good preservation (PI ≈ 3), 0.707404 ± .000074 (n=7) for fair preservation (PI ≈ 2), and 0.707261 ± .000053 (n=8) for poor preservation (PI ≈ 1). These data suggest that as the quality of the preservation declines, the mean 87Sr/86Sr ratio decreases and the standard error of the mean increases. Oxygen and carbon isotope analyses of the same specimens also show decreases with preservation, and δ18O, δ13C and 87Sr/86Sr are well correlated, suggesting that these tracers are all altered as the PI decreases. The Sr/Ca ratio increases as preservation decreases, indicating that Sr is added to the shell material during diagenesis. In contrast, Mg/Ca shows no trend with preservation. If the increasing Sr concentration (and decreasing 87Sr/86Sr) of the shell material with decreasing preservation represents the addition of Sr to the shell during diagenesis, we calculate that the added Sr had 87Sr/86Sr ranging from 0.707582 to 0.707032. Potential sources of the added Sr include older marine carbonates and weathering of volcanic ash layers present in the shale. The mechanisms of alteration likely include epitaxial growth of strontianite on the original shell aragonite and isotopic exchange of C and O between alteration fluids and shell carbonate. We conclude that SEM preservation criteria are effective in screening shell material that records original isotopic values and that variations in Sr, O and C isotope composition in well-preserved material can be used to assess paleoenvironmental parameters, such as salinity and temperature. Our results also indicate that assessing preservation is a critical prerequisite to the determination of numerical ages of shell material using strontium isotope stratigraphy.

The Manson Impact Structure: 40Ar/39Ar age and its distal impact ejecta in the pierre shale in southeastern South Dakota

The 40Ar/39Ar ages of a sanidine clast from a melt-matrix breccia of the Manson, Iowa, impact structure (MIS) indicate that the MIS formed 73.8 � 0.3 million years ago (Ma) and is not coincident with the Cretaceous-Tertiary boundary (64.43 � 0.05 Ma). The MIS sanidine is 9 million years older than 40Ar/39Ar age spectra of MIS shock-metamorphosed microcline and melt-matrix breccia interpreted earlier to be 64 to 65 Ma. Grains of shock-metamorphosed quartz, feldspar, and zircon were found in the Crow Creek Member (upper Campanian) at a biostratigraphic level constrained by radiometric ages in the Pierre Shale of South Dakota that are consistent with the 40Ar/39Ar age of 73.8 � 0.3 Ma for MIS reported herein.

40Ar/39Ar age of the Manson impact structure, Iowa, and correlative impact ejecta in the Crow Creek Member of the Pierre Shale (Upper Cretaceous), South Dakota and Nebraska

A set of 34 laser total-fusion 40 Ar/ 39 Ar analyses of sanidine from a melt layer in crater-fill deposits of the Manson impact structure in Iowa has a weighted-mean age of 74.1 ± 0.1 Ma. This age is about 9.0 m.y. older than 40 Ar/ 39 Ar ages of shocked microcline from the Manson impact structure reported previously by others. The 74.1 Ma age of the sanidine, which is a melt product of Precambrian microcline clasts, indicates that the Manson impact structure played no part in the Cretaceous-Tertiary (K-T) mass extinction at 64.5 Ma. Moreover, incremental-heating 40 Ar/ 39 Ar ages of the sanidine show that it is essentially free of excess 40 Ar and has not been influenced by postcrystallization heating or alteration. An age spectrum of the matrix of the melt layer shows effects of 39 Ar recoil, including older ages in the low-temperature increments and younger ages in the high-temperature increments. At 17 places in eastern South Dakota and Nebraska, shocked quartz and feldspar grains are concentrated in the lower part of the Crow Creek Member of the Pierre Shale (Upper Cretaceous). The grains are largest (3.2 mm) in southeastern South Dakota and decrease in size (0.45 mm) to the northwest, consistent with the idea that the Manson impact structure was their source. The ubiquitous presence of shocked grains concentrated in a thin calcarenite at the base of the Crow Creek Member suggests it is an event bed recording an instant of geologic time. Ammonites below and above the Crow Creek Member limit its age to the zone of Didymoceras nebrascense of earliest late Campanian age. Plagioclase from a bentonite bed in this zone in Colorado has a 40 Ar/ 39 Ar age of 74.1 ± 0.1 Ma commensurate with our sanidine age of 74.1 Ma for the Manson impact structure. 40 Ar/ 39 Ar ages of bentonite beds below and above the Crow Creek are consistent with our 74.1 ± 0.1 Ma age for the Manson impact structure and limit its age to the interval ±74.5 0.1 to 73.8 ± 0.1 Ma. Recently, two origins for the Crow Creek have been proposed—eastward transgression of the Late Cretaceous sea and a Manson impact-triggered tsunami. We conclude that most data are in accord with an impact origin for the Crow Creek Member and are at odds with the marine transgression hypothesis.

Scaphites of the “Nodosus Group” from the Upper Cretaceous (Campanian) of the Western Interior of North America

Abstract Scaphitid ammonites (scaphites) are common in the Upper Cretaceous Pierre Shale and Bearpaw Shale of the Western Interior of North America. We redescribe Hoploscaphites nodosus (Owen, 1852) and H. brevis (Meek, 1876) from the Baculites compressus–B. cuneatus zones of the upper Campanian. The types of both of these species were collected in the mid-19th century in what was then called Nebraska Territory, and included parts of present-day South Dakota, North Dakota, and Montana. Based on our present knowledge of the distribution of these species, the type material was probably collected from the B. compressus–B. cuneatus zones in the Pierre Shale at Sage Creek, a tributary of the Cheyenne River, Pennington County, South Dakota. Traditionally, the more robust, more coarsely ornamented scaphites (comprising the “nodosus group”) from the Pierre Shale and Bearpaw Shale were assigned to Jeletzkytes Riccardi, 1983, and the more slender, more finely ornamented scaphites were assigned to Hoploscaphites Nowak, 1911. However, our large collections of these scaphites from the Baculites compressus–B. cuneatus zones reveal a complete intergradation between the two morphological extremes, and for many specimens, the choice of genus is arbitrary. In addition, our studies of other biostratigraphic zones in the Pierre Shale and Bearpaw Shale reveal that cooccurring species of these two “genera” share more in common with each other than they do with congeneric species from other horizons. Furthermore, contrary to earlier assumptions, Jeletkytes is not endemic to the Western Interior Basin of North America and occurs, for example, in the U.S. Atlantic Coastal Plain and Europe. We thus provisionally treat Jeletzkytes as a junior subjective synonym of Hoploscaphites. This expanded definition of Hoploscaphites is consistent with present-day concepts of other scaphitid genera such as Discoscaphites Meek, 1876, and Trachyscaphites Cobban and Scott, 1964. In Hoploscaphites nodosus and H. brevis, the juvenile shell is planispirally coiled with a small umbilicus. The whorl section is initially depressed and becomes more compressed through ontogeny. The angle of the body chamber in juveniles is approximately two-thirds of a whorl. At the approach of maturity, the shell uncoils, forming a relatively long shaft and recurved hook. The ratio of whorl width to whorl height reaches a minimum value at midshaft. The apertural margin at maturity is constricted and terminates in a flared lip. Commonly, the last two or three septa, corresponding to the formation of the hook, are more closely spaced (approximated). These features indicate that the rate of growth decreased and eventually stopped at maturity (“morphogenetic countdown” associated with determinate growth). Both species of scaphites occur as dimorphs, which are referred to as macroconchs (presumably females) and microconchs (presumably males). In samples of specimens of the same species within a single concretion, macroconchs are approximately 20% larger than microconchs. In addition to size, dimorphs are distinguished by differences in shape, including the presence or absence of an umbilical bulge, the size of the umbilical diameter, the outline of the umbilical shoulder relative to that of the venter in side view, and the relative change in whorl height in passing from the mature phragmocone to the shaft of the body chamber. The holotype of Hoploscaphites nodosus, by monotypy, is UC 6381, the original of Scaphites nodosus Owen (1852: 581, pl. 8, fig. 4). Adults exhibit a range of variation in size, degree of compression, and coarseness of ornament. The exposed phragmocone occupies most of the coiled portion of the shell, and is approximately two-thirds of a whorl in angular length. Adults are large (LMAX averages 91.8 mm in macroconchs and 78.0 mm in microconchs) and ellipsoidal in side view, with a strongly recurved hook (apertural angle averages 73° in macroconchs). The ratio of whorl width to whorl height at midshaft averages 0.99 in macroconchs and 1.02 in microconchs. The whorl section is subquadrate/ovoid to reniform, with broadly rounded to flat flanks, and a broadly rounded venter. Ribs are straight to slightly flexuous and cross the venter with a weak adoral projection. There are 5–7.25 ribs/cm on the venter at midshaft in macroconchs and 6–8 ribs/cm on the venter at midshaft in microconchs. Umbilicolateral tubercles usually appear midway on the exposed phragmocone, but may already be present near the point of exposure. At midshaft, they occur at one-third to one-half whorl height and are relatively widely and evenly spaced, and usually extend to the aperture. Ventrolateral tubercles are generally present at the point of exposure. They are unevenly spaced on the exposed phragmocone, becoming more evenly spaced on the shaft. They attain their maximum size at midshaft, sometimes forming large, subspinose clavi that project out to the side. They become smaller and more closely spaced on the hook, and usually extend to the aperture. The suture is characterized by a broad, asymmetrically bifid first lateral saddle and narrow, symmetrically to asymmetrically bifid first lateral lobe. The holotype of Hoploscaphites brevis, by original designation and monotypy, is USNM 367, the original of Scaphites nodosus var. brevis Meek (1876: 428, pl. 25, fig. 1). Adults exhibit a wide range of variation in size, degree of compression, and coarseness of ornament. Adults are small to large (LMAX ranges from 29.5 to 101.5 mm in macroconchs and from 26.7 to 81.2 mm in microconchs), and rounded to ellipsoidal in lateral view. The body chamber consists of a relatively short shaft and a weakly recurved hook (apertural angle averages 59° in macroconchs). The ratio of whorl width to whorl height averages 0.73 at midshaft in both macroconchs and microconchs. The whorl section is subquadrate/ovoid, with fairly flat flanks, and a broadly rounded venter. Ribs are fine and flexuous and cross the venter with a weak adoral projection. There are 6–14 ribs/cm on the venter at midshaft in macroconchs and 8–18 ribs/cm on the venter at midshaft in microconchs. In most small specimens, umbilicolateral tubercles are absent on the phragmocone and, instead, the primary ribs are strong and adorally concave in this area. In large specimens, umbilicolateral tubercles usually appear midway or near the adoral end of the exposed phragmocone. They are small and relatively evenly spaced. In almost all specimens, umbilicolateral tubercles are present on the body chamber, and occur at one-fourth to one-third whorl height. They are more or less uniformly spaced with, occasionally, some approximation near the point of recurvature. In specimens that are rounded in lateral view, the umbilicolateral tubercles are arranged in a broad arc, which is one of the hallmarks of this species. Ventrolateral tubercles are stronger than umbilicolateral tubercles, and are usually present on the phragmocone starting anywhere from the point of exposure to the adoral end of the phragmocone. In general, they are unevenly spaced on the phragmocone, commonly occurring in pairs or clusters, becoming more evenly spaced on the shaft. As in H. nodosus, they attain their maximum size at midshaft. The ventrolateral tubercles usually die out near the point of recurvature, but if not, they become smaller, more bullate, and more closely spaced toward the aperture. The suture is the same as that in H. nodosus. Hoploscaphites landesi Riccardi, 1983, is a junior subjective synonym of H. brevis. The holotype is a small, compressed, finely ornamented microconch of H. brevis. It grades into larger, more robust specimens, with coarser ornament. We thereby expand the definition of H. brevis to include a wide range of variation in adult size, and argue that such variation reflects variation in the age (and size) at which individuals reach maturity. Establishment of separate species for different size specimens, given that all other aspects of their morphology (shell shape, whorl cross section, pattern of ribs, distribution of umbilicolateral and ventrolateral tubercles, and suture) are the same, seems unwarrented. Hoploscaphites nodosus and H. brevis are widespread in the Baculites compressus–B. cuneatus zones in the Western Interior of North America, which represent a time interval of approximately 580 ky (Cobban et al., 2006). They are also present in parts of the underlying Didymoceras cheyennense Zone and the overlying B. reesidei Zone, but their exact distribution in these zones is not yet known. Hoploscaphites nodosus and H. brevis occur in the Bearpaw Shale in Alberta and Saskatchewan, the Bearpaw Shale and Pierre Shale in Montana, and the Pierre Shale in North Dakota, South Dakota, Wyoming, Colorado, and Kansas. They occur in nearshore deposits such as the unnamed shale member of the Pierre Shale in Grand County, Colorado, in offshore deposits such as the DeGrey Member of the Pierre Shale in Buffalo County, South Dakota, and in cold methane seeps in the Pierre Shale in Custer County, South Dakota. They are absent in the U.S. Gulf and Atlantic Coastal plains and northern Europe, although similar forms are present in both these areas in strata above and below the biostratigraphic interval containing the ranges of H. nodosus and H. brevis. Thus, although H. nodosus and H. brevis are endemic to the Western Interior of North America, they are part of a more broadly distributed clade that is also present in the U.S. Gulf and Atlantic Coastal plains and northern Europe. As adults, these scaphites probably lived just above the sea bottom. They preferred oxygenated water, as indicated by the fact that they are generally associated with a diverse molluscan community. Habitat depths are estimated at less than 100 m, based on studies of the mechanical properties of the septa and siphuncle (Tsujita and Westermann, 1998). The high angle of orientation of the aperture a

Ammonites and inoceramid bivalves from close to the middle-upper Albian boundary around Fort Worth, Texas

The Goodland/Comanche Peak Limestone, Kiamichi Formation and basal Duck Creek Limestones around Fort Worth Texas yield a limited number of cosmopolitan ammonite and inoceramid bivalve taxa that allow precise correlation with the sequence that has been used as a standard in northwest Europe. The upper part of the Goodland/Comanche Peak Limestones yields species of Dipoloceras that show the base of the Upper Albian substage, provisionally defined as the first appearance of D. cristatum (Brongniart, 1822), to lie within this unit. Brancoceras aff. cricki Spath, 1934, Mortoniceras (Deiradoceras) beloventer new species, and Actinoceramus cf. concentricus (Parkinson, 1819) parabolicus Crampton, 1996a, co-occurs with D. cristatum in the Comanche Peak Limestone. The Kiamichi Formation yields rare Mortoniceras (Mortoniceras) pricei (Spath, 1922), M. (Deiradoceras) prerostratum Spath, 1921, M. (D.) bipunctatum Spath, 1933, and Actinoceramus sulcatus (Parkinson, 1819) morphotypes that allow correlation with the European Hysteroceras orbignyi and Hysteroceras varicosum subzones of the Mortoniceras inflatum zone. The basal Duck Creek Limestone yields Mortoniceras (Deiradoceras) sp. and Hysteroceras cf. varicosum (J. de C. Sowerby, 1824), and can also be correlated with the varicosum subzone.

Colorado Shale of Central and Northwestern Montana and Equivalent Rocks of Black Hills

The Colorado shale of central and northwestern Montana and the equivalent rocks on the north flank of the Black Hills are dominantly dark gray marine shales. It has been customary to treat the Colorado shale as a single formation, with one or two members. More detailed studies show that the Colorado shale is divisible into many lithologic units that can be correlated with the standard sequence in the Black Hills. The formations in the Black Hills that are equivalent to the Colorado shale are the Fall River sandstone, Skull Creek shale, Newcastle sandstone, Mowry shale, Belle Fourche shale, Greenhorn formation, Carlile shale, and Niobrara formation. Each formation and the equivalent part of the Colorado shale are briefly described, compared, and contrasted, and the importa t fossils are listed.

Ammonites from the Saratoga Chalk (Upper Cretaceous), Arkansas

The Saratoga Chalk of Arkansas yields a rich ammonite fauna of 17 species, referred to the Nostoceras ( N. ) hyatti zone. Recognition that the northwest European N. ( N. ) pozaryskii Blaszkiewicz, 1980, is a synonym of N. ( N. ) hyatti Stephenson, 1941, and N. ( N. ) helicinum (Shumard, 1861) dates the zone as latest Campanian on the basis of co-occurrence with Belemnitella langei Jeletzky, 1948, in Poland. Most previous estimates of the Campanian–Maastrichtian boundary in the Gulf Coast have been drawn at too low a level, at least in terms of ammonite faunas. Elements of the N. ( N. ) hyatti zone fauna occur in the United States Western Interior, and show the base of the Maastrichtian there to lie above the Baculites jenseni zone.

Ammonites from the Upper Part of the Pierre Shale and Fox Hills Formation of Colorado

Abstract The upper part of the Pierre Shale and Fox Hills Formation were deposited in the Late Cretaceous (Maastrichtian) Western Interior Seaway. They crop out in a belt that roughly parallels the Front Range of the Rocky Mountains from Douglas to Weld County, Colorado. These rocks consist of sandy shales and sandstones and are overlain by the nonmarine Laramie Formation. A sparse assemblage of ammonites is present consisting of Coahuilites sheltoni Böse, 1928, Sphenodiscus pleurisepta (Conrad, 1857), Trachybaculites sp. cf. T. columna (Morton, 1834), Hoploscaphites birkelundae Landman and Waage, 1993, Hoploscaphites sp. cf. H. birkelundae, Jeletzkytes dorfi Landman and Waage, 1993, and Jeletzkytes sp. cf. J. dorfi. Hoploscaphites birkelundae and Jeletzkytes dorfi define the H. birkelundae Zone in the Western Interior, which represents the lower part of the upper Maastrichtian. These rocks are thus equivalent in age to the Fox Hills Formation in Niobrara County, Wyoming, and older than the type Fox Hills Formation in north-central South Dakota. An analysis of the ratio of 87Sr/86Sr in a belemnite from this zone in Morgan County, Colorado, yields a value of 0.707790 ± 0.000008 (2-sigma SE), nearly identical to that of a bivalve from the same zone in Niobrara County, Wyoming (McArthur et al., 1994). The western shoreline of the seaway during the time of H. birkelundae extended as far west as northwestern Colorado and southwestern Wyoming.

Frontier Formation, Wyoming and Adjacent Areas

The Frontier formation in its type area in southwestern Wyoming consists of about 2,000 feet of sandstone and siltstone interbedded with softer units of shale and mudstone containing minor beds of sandstone, coal, bentonite, and porcellanite. At Cumberland Gap, Wyoming, 15 miles south of Frontier, the lower half of the Frontier formation consists of non-marine sandstone, siltstone, mudstone, and water-laid volcanic rocks with some coal, carbonaceous shale, and limestone. The upper half of the formation consists of marine sandstone, siltstone, and shale, and, near the top, a 260-foot non-marine unit of sandstone, mudstone, and some coal. The formation is underlain by the marine Aspen shale of late Early Cretaceous (Albian) age and is overlain by 6,000 feet of marine Hillia d shale of middle Late Cretaceous (Coniacian-Santonian) age, which in turn is overlain by 4,000 feet of largely non-marine Adaville formation. The non-marine lower half of the Frontier formation can not be precisely dated, but it is probably Cenomanian. The marine beds in the upper half of the formation are dated as of late Greenhorn (Turonian), early Carlile (Turonian), and early Niobrara (Coniacian) ages. The non-marine unit near the top of the formation is believed to be of late Carlile age (Turonian). Northward from the type area the Frontier formation does not appear to change greatly as far as Fontanelle Basin, but the upper half of the overlying Hilliard shale changes to sandstones and shales of Adaville aspect. Still farther north in Snider Basin a relatively thin equivalent of the typical Frontier formation can be identified, but it is overlain by a great thickness of sandstone and shale that is not greatly different from the Frontier. Surface measurements suggest as much as 13,000 feet, but there may be some duplication of beds not readily seen on the surface. The uppermost part has not yielded fossils, but the main part of these beds contains Niobrara fossils, and the whole sequence above the Aspen has been called Frontier by some authors. Though the name Frontier was first applied in southwestern Wyoming, its use has spread over most of Wyoming and into northern Utah, northwestern Colorado, southeastern Idaho, and southern Montana. The formation passes eastward into dominantly marine rocks with fewer and thinner beds of resistant sandstone. Inasmuch as the sandstone beds that locally form the top of the Frontier formation pass eastward into shale, the upper limit of the formation changes in age. Ordinarily one of three large units of sandstone forms the top of the formation. The youngest sandstone unit, of early Niobrara age, is tongue-shaped, extending from the southwest and northwest corners of Wyoming into the central part of the state. Sandstone beds of about middle Carlile age form the top of the Frontier formatio for some distance eastward in Wyoming, northeastern Utah, and northwestern Colorado, beyond the eastern limits of the sandstone beds of early Niobrara age. In a much smaller area in north-central Wyoming and south-central Montana sandstone beds of pre-Carlile age form the top of the Frontier formation. Comparisons of the Frontier formation at many widely separated localities with the standard Great Plains Cretaceous sequence reveal many apparently local breaks in the Frontier sections. In central Wyoming, rocks equivalent to the lower part of the Carlile shale of the Black Hills seem to be either missing or represented in the Frontier formation by abnormally thin lithologic units. Probably as more sections are studied in detail many time-gaps will be discovered. Much remains to be done before the details of Frontier paleogeography can be worked out.

Maastrichtian ammonites from the Hornerstown Formation in New Jersey

The base of the Paleocene Hornerstown Formation at the Inversand pit and certain other localities in New Jersey yields a diverse phosphatised fauna of Maastrichtian age, including the ammonites Pachydsicus ( Neodesmoceras) mokotibensis Collignon, 1952, Sphenodiscus lobatus (Tuomey, 1854), Baculites spp., and Eubaculites carinatus (Morton, 1834). S. lobatus is known from the older Red Bank Sand and Tinton Sand in New Jersey; the other species are known only from the basal Hornerstown. Occurrences at the Inversand pit are regarded as either reworked or remainie, although details of Cretaceous/Paleocene boundary events have been destroyed by pervasive burrowing that pipes the Hornerstown down into the underlying Navesink Formation.