VOLUME 42 NUMBER 2

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A tribute to W.A. "Bill" Cobban

Neil H. Landman¹ and Neal L. Larson²

¹ Division of Paleontology (Invertebrates), American Museum of Natural History, 79^th St. and Central Park West, New York, NY 10024, U.S.A.
² Black Hills Institute of Geological Research, P.O. Box 643, 117 Main St., Hill City, SD 57745, U.S.A.

In August, 2006, a two-day symposium was held at the Colorado School of Mines in Golden, Colorado, on "The Paleontology, Geology, and Stratigraphy of the Late Cretaceous Western Interior Seaway: A tribute to the life of W.A. Bill Cobban." This symposium was attended by nearly 100 participants and featured 25 oral and poster presentations. Several of the papers that grew out of this symposium are published in the present issue of Rocky Mountain Geology (see following articles by N. H. Landman and W. A. Cobban; E. A. Merewether, W. A. Cobban, and J. D. Obradovich; and J. W. Grier, J. C. Grier, N. L. Larson, and J. G. Peterson.

Redescription of the Late Cretaceous (late Santonian) ammonite Desmoscaphites bassleri Reeside, 1927, from the Western Interior of North America

Neil H. Landman^1,* and William A. Cobban²

¹ Division of Paleontology (Invertebrates), American Museum of Natural History, 79th St. and Central Park West, New York, NY 10024, U.S.A.
² Research Associate, Division of Paleontology (Invertebrates), American Museum of Natural History, 79th St. and Central Park West, New York, NY 10024, U.S.A.; Home address: 70 Estes St., Lakewood, CO 80226, U.S.A.

 

^* Correspondence should be addressed to: landman@amnh.org

 

The Late Cretaceous scaphitid ammonite Desmoscaphites bassleri Reeside, 1927, is redescribed based on newly collected material from Montana. This species is an important index fossil and demarcates the uppermost Santonian in the U.S. Western Interior. It is widely distributed and extends from Alberta to New Mexico, primarily along the western margin of the Interior Seaway. The adult shell is moderately stout and closely coiled, with small ventrolateral tubercles on the body chamber. Microconchs are smaller than macroconchs but otherwise similar in morphology. There is an abrupt change in ornamentation on the late juvenile whorls from coarse, widely spaced ribs to threadlike, more closely spaced ribs, with a pronounced sinuosity. The ribs later become more widely spaced on the shaft of the body chamber and more closely spaced again near the aperture. The early whorls are characterized by constrictions, which represent flexures in the shell wall. The constrictions first appear at a shell diameter of approximately 3 mm and occur regularly thereafter at intervals of 6590 up to a shell diameter of approximately 15 mm. The suture is characterized by a trifid first lateral lobe. This lobe does not pass through an asymmetrically bifid stage during ontogenetic development.

Key Words: ammonite • Desmoscaphites • Montana • ontogeny • Santonian • U.S. Western Interior

Regional disconformities in Turonian and Coniacian (Upper Cretaceous) strata in Colorado, Wyoming, and adjoining statesbiochronological evidence

E. Allen Merewether^*, William A. Cobban and John D. Obradovich

U.S. Geological Survey, MS 939, Box 25046, Denver Federal Center, Denver, CO, 80225, U.S.A.

 

^* Correspondence should be addressed to: merewether@usgs.gov

 

Siliciclastic and calcareous sedimentary rocks of early Late Cretaceous age in the Western Interior of the United States have been assigned to, in ascending order, the Graneros Shale, Greenhorn Formation, Carlile Shale, Niobrara Formation, and their lateral equivalents (including members of the Frontier Formation and overlying formations). This sequence of formations was deposited intermittently within and near an epicontinental seaway during the Cenomanian, Turonian, and Coniacian stages of the Cretaceous. It encloses three conspicuous and widespread disconformities that reflect regional marine regressions and transgressions as well as moderate tectonism. The disconformities and associated lacunae occupy three large areas within Wyoming, Colorado, and adjoining states. In parts of that region, as in northwestern Wyoming, a lacuna can represent more than one period of erosion and more than a single disconformity. Evidence for these disconformities was obtained from about 175 collections of molluscan fossils and from sedimentological studies of outcrops and borehole logs, supplemented by previously published data.

The oldest of the three disconformities, within the Frontier Formation and partial age-equivalents (including the Carlile Shale), separates Cenomanian or lower Turonian beds from middle Turonian beds in central and western Wyoming, northwestern Colorado, and adjoining areas of Montana and Utah. In parts of that region, the maximum duration of the associated lacuna is about 3 m.y. Erosion of the region in the late early Turonian was associated with a marine regression and transgression as well as mild local tectonism. The area where strata underlying the unconformity are oldest is partly overlain by the youngest of the succeeding transgressive beds. These youngest overlying beds presumably were deposited in an uplifted area where the eroded surface had a slightly higher elevation.

A younger disconformity, within the Frontier Formation and lateral equivalents, separates upper Cenomanian or lower or middle Turonian strata from middle or upper Turonian strata in central and eastern Wyoming, southwestern South Dakota, western Nebraska, and central and eastern Colorado. Locally in that region, the duration of the lacuna is as much as 5 m.y. The oldest beds underlying this contact are of late Cenomanian age and are distributed in north-central and southeastern Wyoming and in north-central Colorado, where the erosional surface was affected probably by slight uplifts and by fluvial drainage systems. In that region, the oldest beds are partly overlain by the youngest (late Turonian) of the transgressive strata. The areal distribution of the younger overlying beds in central Wyoming could indicate a westward migration of marine prodelta environments during the late Turonian.

 

At the youngest of the three disconformities, strata of middle or late Turonian ages in the Carlile Shale and lateral equivalents are overlain by upper Turonian or lower or middle Coniacian beds of the basal Niobrara Formation in Wyoming, Colorado, Nebraska, and parts of adjoining states. The maximum duration of the associated lacuna is more than 4 m.y. in northwesternmost Wyoming and northeasternmost Nebraska. Beds underlying this disconformity are oldest (early middle Turonian) in northwestern Wyoming, northeasternmost Nebraska, and possibly elsewhere in Nebraska, which apparently were areas of comparatively higher elevation and greater truncation. The underlying beds are youngest in a northeast-trending area that extends at least from eastern Utah to northeastern Wyoming. This area presumably was uplifted less than adjoining areas possibly in the late Turonian. Strata overlying this disconformity are oldest in northeastern New Mexico and much of Colorado and are youngest in northeastern Utah, northwestern and east-central Wyoming, north-central Kansas, and northeastern Nebraska, which indicates a marine transgression that progressed mainly northward.

 

Most of the ages used for the following calculations are estimates; consequently the resulting quantitative interpretations are speculative. The duration of the lacuna between the uppermost Carlile and the basal Niobrara increased northwestward from about 0.8 m.y. in south-central Colorado to about 4.3 m.y. in northwesternmost Wyoming. It also increased northeastward from 0.8 m.y. in Colorado to about 5.1 m.y. in northeastern Nebraska. Ages of basal beds of the Niobrara decrease northwestward from about 89.3 Ma in southeastern Colorado and northeastern New Mexico to about 88.7 Ma in northwesternmost Wyoming. Apparently, the Niobrara sea transgressed northwestward about 500 mi (805 km) from southeastern Colorado to northwesternmost Wyoming in about 0.6 m.y. Ages of the basal Niobrara also decrease toward the northeast, from 89.3 Ma in southeastern Colorado to 87.6 Ma in northeasternmost Nebraska. The Niobrara sea in that region, where chronologic data are notably sparse, possibly transgressed more than 480 mi (772 km) in about 1.7 m.y.

Key Words: biostratigraphy • Coniacian • Cretaceous • disconformities • marine regressions and transgressions • mid-Cretaceous tectonism • Turonian • Western Interior U.S.A. 

Synonymy of the ammonite genus Ponteixites Warren with Rhaeboceras Meek

James W. Grier^1,*, Joyce C. Grier², Neal L. Larson³ and Jack G. Petersen⁴

¹ Department of Biological Sciences, North Dakota State University, Fargo, ND 58102-3400, U.S.A.
² 17648 57th Ave N, Hawley, MN 56549, U.S.A.
³ Black Hills Institute of Geological Research, Inc., 117 Main Street, Hill City, SD 57745, U.S.A.
⁴ 1256 Hammond, Waterloo, IA 50702, U.S.A.

 

^* Correspondence should be addressed to: james.grier@ndsu.edu

 

The scaphitid ammonite genus Ponteixites Warren, 1934, has remained poorly known and poorly represented in collections up to the present. All previously described specimens of Ponteixites, including the original type specimens, are small and appear to be juveniles. A larger, clearly adult specimen was recently discovered in the Pierre Shale of eastern Montana. If not for the larger size, it would be identified as P. robustus Warren, 1934. A comparison of the new, larger specimen with hundreds of typical, smaller specimens of P. robustus provides evidence for replacing the name Ponteixites with Rhaeboceras Meek, 1876. An alternative interpretation of the large size of the new specimen would be gigantism. However, it is within the size range of other known Rhaeboceras species and has normal scaphite adult characteristics, thus disqualifying it as a candidate for gigantism. The species P. robustus should be renamed R. robustus. Another species, R. coloradoense Cobban, 1987, strongly resembles R. robustus and might be the latter species. The species formerly described as P. gracilis Warren, 1934, is believed either to represent the juvenile stage of another Rhaeboceras species or to be within the range of morphologic variation for R. robustus.

Key Words: ammonite • Cedar Creek • Cobban • gigantism • Montana • Ponteixites • Rhaeboceras • scaphite • suture approximation • synonymy • uncoiling • Pierre Shale 

Geochemistry of the Bonito Lake stock, Lincoln County, New Mexico Petrogenesis and hydrothermal alteration

James Constantopoulos

Department of Physical Sciences, Eastern New Mexico University, Portales, NM 88130, U.S.A.

 

email: jim.constantopoulos@enmu.edu

 

The Bonito Lake stock is one of three stocks that intrude the Sierra Blanca Volcanics in the Nogal-Bonito mining district of New Mexico. The 26.6-Ma stock consists primarily of porphyritic syenite with subordinate quartz syenite and monzonite. SiO₂ concentrations range from 54 to 66 weight percent. Na₂O exceeds K₂O and combined alkalis average 9.4 weight percent. The rocks are alkaline and metaluminous and plot in the syenite and syenodiorite fields on an R₁R₂ multicationic plot. The chondrite-normalized extended trace element diagram has a negative slope with Ba values 167 to 286 times chondritic values and Yb values eight to 14 times chondritic values. The chondrite-normalized REE plot has a negative slope with mild LREE enrichment. (La/Yb)_N values range from 15 to 22 and the mean Eu/Eu* is 0.91. Clinopyroxene, zircon, and apatite fractionation, along with feldspar accumulation were the most important factors in controlling elemental abundances. Quasi Pearce Element Ratio (PER) analysis confirms the role of clinopyroxene fractionation, as well as indicates that an increase in K and a loss of Ca Na occurred during alteration. The quasi PER analysis also shows that the rocks are slightly metasomatized. Whole-rock ¹⁸O values for the Bonito Lake stock range from 2.0 to 8.4 per mil. Oxygen isotope values for the more permeable surrounding volcanic rocks range from 1.0 to 7.8 per mil. The lower values of the volcanic rocks are reflective of meteoric-hydrothermal water interaction. These data help us better understand not only this stock, but also the Sierra Blanca igneous complex and the Lincoln County Porphyry Belt. This study also contributes to our understanding of the Nogal-Bonito mining district and the larger Rocky Mountain alkalic province.

Key Words: alkaline rocks • Bonito Lake stock • geochemistry • hydrothermal alteration • New Mexico • REE • syenite

New ⁴⁰Ar/³⁹Ar age determinations and paleomagnetic results bearing on the tectonic and magmatic history of the northern Madison Range and Madison Valley region, southwestern Montana, U.S.A.

Karl S. Kellogg^1,* and Stephen S. Harlan²

¹ U.S. Geological Survey, Mail Stop 980, Box 25048, DFC, Lakewood, CO 80225, U.S.A.
² Department of Environmental Science and Policy, George Mason University, MS 5F2, 4400 University Drive, Fairfax, VA 22030-4444, U.S.A.

 

^* Correspondence should be addressed to: kkellogg@usgs.gov

 

Detailed ⁴⁰Ar/³⁹Ar dating and paleomagnetic analysis of dacite porphyry sills and dikes that intrude Cretaceous sedimentary rocks in the northern Madison Range in southwestern Montana show that Laramide shortening was essentially complete by 69 Ma. A negative paleomagnetic fold test indicates that Laramide folding occurred before cooling of the dacite sills and dikes at 69 Ma. Laramide deformation began synchronous with deposition of the Livingston Formation rocks at 79 Ma. These results are consistent with previous observations in the region that show the onset of Laramide deformation in the northern Rocky Mountains becoming progressively younger toward the east. ⁴⁰Ar/³⁹Ar dating of additional igneous rocks in the northern Madison Valley and around Norris, Montana better define post-Laramide tectonomagmatic events in the region, including EoceneOligocene volcanism and Basin and Range crustal extension. Dates from three rhyolitic intrusions near Red Mountain are between 48.71 0.18 Ma and 49.42 0.18 Ma, similar to the dates from basal silicic flows of the Virginia City volcanic field (part of the southwest Montana volcanic province), suggesting that the Red Mountain intrusions may have been the sources for some of the early extrusive rocks. Magmatism in the Virginia City volcanic field became generally more mafic with time, and a 30-Ma basalt flow near Norris is considered a late, outlying member of the volcanic field. A tuff along the east side of the Madison Valley half graben yielded a early middle Miocene date (16.2 0.19 Ma), suggesting that accelerated crustal extension and associated rapid basin sedimentation probably began in the early Miocene, slightly earlier than previous estimates.

 

Key Words: ⁴⁰Ar/³⁹Ar dating Laramide Madison Range Madison Valley Montana Norris paleomagnetism Red Mountain Virginia City volcanic field