Abstract The Southwest Carbon Partnership (SWP), one of seven United States Department of Energy-funded Regional Carbon Sequestration Partnerships, has been tasked with assessing the CO 2 sequestration potential within the southwestern United States. Carbon dioxide is considered a ‘greenhouse’ gas and is emitted, in large volumes, by the burning of fossil fuels and other industrial processes. CO 2 capture from point source emitters and subsequent geological sequestration is being considered as a viable short- to intermediate-range mitigation option to combat the phenomena of global warming. Significant fossil fuel reserves and consumers exist within the seven member states of the SWP and, as such, the Partnership is dedicating a large amount of resources to the challenges posed by large-scale CO 2 sequestration. Three distinct phases of work have been or will be performed by the SWP: a Characterization Phase to identify carbon capture and sequestration potential; a Validation Phase to test small-scale field injection of CO 2 ; and a Deployment Phase to test commercial-scale field injection of CO 2 . Each phase presents challenges and opportunities to the refinement of the best approach to safe and efficient geological storage of CO 2 within the SW region of the United States.

A series of 33 Late Cretaceous (earliest Campanian through Maastrichtian) paleoshoreline maps was developed to document the migrational evolution of the western edge of the North American Western Interior Seaway. The maps represent a geologic span of roughly 18 million years, and portray the estimated positions of the strandline for each standard Western Interior ammonite zone, beginning with the Clioscaphites choteauensis zone and continuing to the end of the Mesozoic. We attempted correlation of all significant mammal-bearing localities known from the Western Interior with the ammonite-based marine zonation. First approximations of correspondence between ammonite zones and North American Land-Mammal “Ages” (NALMAs) include: Lancian ( Sphenodiscus through “Triceratops” zones); “Edmontonian” (a name not yet faunally defined; Didymoceras cheyennense through Baculites clinolobatus zones); Judithian (the smooth, late form of Baculites sp. through Exiteloceras jenneyi zones); and Aquilan ( Scaphites hippocrepis through Baculites asperiformis zones). Correlations emphasize use of provincial biostratigraphic terminology designed specifically for use in the Western Interior. On the basis of temporal constraints suggested herein, known mammalian fossils from the upper Fruitland and/or lower Kirtland Formations of New Mexico probably are of “Edmontonian,” not Judithian age. Although considerable latitudinally based taxonomic diversification of Judithian mammals is now recognizable across the Western Interior, comparative data are inadequate to defend a similar statement for the remaining Late Cretaceous NALMAs. Quantitative evaluation of geographic patterns of shoreline change suggests occurrence of a general, regional regression of the sea during the entire geologic interval represented in the study. We favor explanation by a slow sea-level depression resulting from topographic evolution of the world’s mid-oceanic system of ridges and volcanic plateaus. Local and subregional asynchronous episodes of shoreline transgressions, stillstands, and regressions are superimposed upon the general regressive trend, and probably represent influences of local tectonism, not eustatic changes in sea level. Strandline evolution of the epeiric sea during the last 20 million years of the Cretaceous in the North American Western Interior is inconsistent with: (1) existence of geologically brief (1 to 10 m.y.) global fluctuations in sea level; and (2) the concept that the late Campanian was represented by an unusually high global sea level.

The Kirtland Shale or Fruitland Formation directly underlies the Cretaceous-Tertiary boundary throughout most of the San Juan Basin of northwest New Mexico and southwest Colorado. These formations have been known to be Late Cretaceous in age since the early 1900s. Now, with the greatly renewed interest in rocks adjacent to mass extinction boundaries, it is important to place more precise ages on such rock units as the Fruitland and Kirtland. Deposition of the Fruitland and Kirtland was closely related to deposition of the underlying marine-regressive Pictured Cliffs Sandstone. Because the Pictured Cliffs was deposited as a strandline sandstone in a subsiding seaway, its stratigraphic expression, when related to a time horizon (the Huerfanito Bentonite Bed), is a series of rising-to-the-northeast, time transgressive, stair steps. Thus, time lines (or horizons) drawn parallel to the Huerfanito cut through the marine Lewis Shale, the strandline Pictured Cliffs Sandstone, and the continental Fruitland Formation and Kirtland Shale. Ammonites have been collected and identified from various stratigraphic levels within the Lewis Shale around the northwest, north, and east sides of the San Juan Basin. These fossils can be tied in to the established ammonite zonation of the Western Interior seaway. Because some of these ammonite zones have been radiometrically dated outside the San Juan Basin, it is possible to project these dated faunal zones from the Lewis Shale along time lines into the Fruitland Formation and Kirtland Shale and thereby estimate the age of those rocks. Based on these projections the part of the Fruitland and Kirtland laterally time-equivalent to the Lewis Shale is estimated to range from 73.2 ± 0.7 Ma to 73.9 ± 0.8 Ma. The average age for this interval based on these dates is 73.5 ± 0.5 Ma; the maximum range of the interval at the 95 percent confidence level is 71.8 to 75.5 Ma. This age range puts these rocks in the Campanian Stage of the upper Cretaceous in the San Juan Basin.

Dinosaurs from the Upper Cretaceous Fruitland Formation and Kirtland Shale in the San Juan Basin, New Mexico, pertain to the following taxa: Ornithomimidae, cf. Ornithomimus edmonticus , cf. Struthiomimus altus , Dromaeosauridae, Albertosaurus sp., cf. Tyrannosaurus rex , Alamosaurus sanjuanensis , ?Pachycephalosauridae, Ankylosauria, Ankylosauridae, Nodosauridae, ? Euoplocephalus sp., ? Panoplosaurus sp., Hadrosauridae, Kritosaurus navajovius , Parasaurolophus tubicen , P. cyrtocristatus , Ceratopsidae, cf. Chasmosaurus sp., Pentaceratops sternbergii , P. fenestratus , and Torosaurus cf. T. utahensis. The dinosaur fauna of the Fruitland Formation is temporally equivalent to the dinosaur faunas of the Judith River (Montana) and Oldman (Alberta) Formations and is of late Campanian (Judithian) age. This correlation is based primarily on the absence in the Fruitland Formation of dinosaurs typical of post-Judithian dinosaur faunas elsewhere in western North America. The dinosaur fauna of the Kirtland Shale below the Naashoibito Member is virtually identical to that of the Fruitland Formation. Based on stratigraphic relationships, the Kirtland Shale must be younger than the Fruitland Formation and may be as young as Edmontonian (latest Campanian-early Maastrichtian) below the Naashoibito Member. The Naashoibito Member of the Kirtland Shale contains cf. Tyrannosaurus rex, Alamosaurus sanjuanensis, and Torosaurus cf. T. utahensis, taxa indicative of a Lancian (middle-late Maastrichtian) age. Therefore, Kritosaurus from the Naashoibito Member represents the youngest known occurrence of this genus. The Lancian age of the Naashoibito Member indicates that the unconformity at the base of the Ojo Alamo Sandstone is not of as great a temporal magnitude as most previous workers had believed. Thus, there is a nearly complete record of the Cretaceous-Tertiary transition in the west-central San Juan Basin, indicated by dinosaur-based correlation of the Fruitland and Kirtland Formations. This correlation is consistent with most other evidence, except magnetostratigraphy, that has been used to determine the age of the Fruitland and Kirtland Formations.

Cretaceous mammals have been recovered from the lowermost Kirtland Shale (Quarry 1 of the Fossil Forest study area). Of the 13 therian mammal taxa described, 6 are new ( Alphadon parapraesagus, n. sp., Ectocentrocristus foxi n. gen. and sp., Pediomys fassetti n. sp., Aquiladelphis paraminor, n. sp., Gypsonictops clemensi n. sp., and Cimolestes lucasi n. sp.). Faunal comparison with other known Cretaceous sites show that Fossil Forest mammals are most closely related to faunas described by Sahni (1972) from the Judith River Formation and by Fox (1977, 1979a, 1979b, 1979c, and 1981) from the Oldman Formation (Judith River equivalent). The age of the lowermost part of the Kirtland Shale must be near the Campanian-Maastrichtian boundary based on mammalian evidence. The long normal magnetochrons (30 and 31 of Lindsay and others [1981]) must, therefore, be renumbered. Two alternatives for renumbering are discussed; the fossil mammal evidence favors renumbering the magnetochrons as anomaly 33. This interpretation requires that one or more unconformities be present between the lower part of the Kirtland Shale and overlying formations on the south flank of the San Juan Basin. This results in the loss of approximately 6.4 Ma of rock record that probably includes the Cretaceous-Tertiary boundary event.

Several volcanic ash units that have recently been discovered in the Kirtland Shale (Cretaceous) in the northwestern part of the San Juan Basin, New Mexico, occur some 20 m (sample 62) to 39 m (sample 54) above the uppermost rocks of the Fruitland Formation. The ashes are of extremely great value in attempting to work out the stratigraphy of the rocks in this area, especially as there is some question concerning the interpretation of faunal assemblages and paleomagnetic signatures; contradicting “dates” of earliest Tertiary to latest Cretaceous have been suggested for the part of the Kirtland Shale sampled. A systematic study of the Potassium-Argon geochronology of some of the ashes has been undertaken with the following results: (1) highest ash (sample 54): sanidine concentrates—72.4 ± 3.1 to 74.4 ± 2.6 Ma; biotite concentrates—73.2 ± 2.7 to 76.1 ± 2.8 Ma; (2) lowest ash (sample 62): sanidine concentrate—75.0 ± 2.7 Ma; and (3) upper middle ash (sample 93): sanidine concentrate—69.8 ± 2.5 Ma. Petrographic and scanning-electron microscope studies show the biotite and sanidine to be primary phases, containing negligible amounts of detrital material. The rare earth element and other chemical studies of the ashes show them to be slightly different from each other, and this information may allow rare earth element distribution data to be used for purposes of ash correlation. We interpret the Potassium-Argon ages as suggesting a Late Cretaceous age for the part of the Kirtland Shale sampled, a view consistent with recent paleontological and paleomagnetic studies.

North American Paleocene land mammal ages are the Mantuan, Puercan, Torrejonian, Tiffanian, and Clarkforkian. These ages (and associate stages) are subdivided into 16 zones or subzones, varying in duration from 0.1 to 2.9 m.y., defined by widespread species. Although gross evolutionary changes during the first four of these ages are about equal, their durations are very unequal. As defined by magnetostratigraphy and fossil occurrence, the Mantuan is about 0.2 m.y., the Puercan about 1.1 m.y., the Torrejonian about 3.1 m.y., the Tiffanian about 6.1 m.y., and the Paleocene part of the Clarkforkian about 1.3 m.y. in duration. Puercan encompasses normal magnetozone 29, Torrejonian zones 28 and 27, and the Tiffanian–Clarkforkian boundary falls in zone 25. The type Rio Chico Formation of Patagonia is of mid-Tiffanian to Clarkforkian age. Problems in the identification of magnetozones in the San Juan Basin have arisen because an unconformity is present between the Kirtland Shale and the Ojo Alamo Sandstone, and for some years an extra normal chron was falsely identified. When this hiatus is taken into account, marine and terrestrial fossil correlations agree with magnetozone correlations throughout Upper Cretaceous and Paleocene rocks. The Danian stage in marine rocks in North Dakota is equivalent to Mantuan through early Tiffanian; the Thanetian is exactly equivalent to mid-Tiffanian to early Clarkforkian. The rate of Paleocene sedimentation in the major basins of North America does not depart from linearity much more than the contemporary rate of seafloor spreading. Terrestrial rates of sedimentation vary from a peak of 568 bubnoffs (b) (meters per million years) for the Hoback Formation at the Rocky Mountain front to 99 b in the San Juan Basin and 15 b in the Black Peaks Formation in Texas, compared to 2.7 b at Gubbio, Italy. Sedimentation rates along a transect through the Bighorn, Powder River, and Williston basins follow the equation Y = 200X −0.25 , where Y is the sedimentation rate in bubnoffs and X is the radial distance in kilometers from the Absaroka thrust. Absolute taxonomic and morphologic rates of evolution of the most rapidly evolving mammals during the Bugcreekian–Mantuan interval across the Cretaceous–Paleocene boundary peak at 5 genera per m.y. and 3.85 darwins (a rate of measurement defined in the text), the fastest rates known in the fossil record, and decline exponentially to more normal rates of 1 species per m.y. and 0.5 darwins by the Tiffanian. Range charts of 299 species of ungulates, primates, and multituberculates permit ready identification of zones. Seven new species of multituberculates are described, and shape and metrical properties of latest Cretaceous–Paleocene neoplagiaulacid multituberculates are summarized for ease in identification.

During the past year we have been measuring trace element abundances and searching for anomalously high iridium (Ir) concentrations in continental sedimentary rocks that span the Cretaceous-Tertiary boundary in the Raton and San Juan Basins of northern New Mexico and southern Colorado. Using neutron activation and radiochemical separations, we have identified anomalous concentrations of Ir in samples from two sites in the Raton Basin: in a drill core at York Canyon, about 50 km west of Raton, New Mexico, and in a road cut near the city of Raton. In both cases the anomaly occurs essentially at the base of thin coal beds, across a thickness span of only a few cm and at the same level at which several species of Cretaceous pollen become extinct and the ratio of angiosperm pollen to fern spores drops sharply. The Ir surface density ranges from 8 to 40 × 10 −9 g cm −2 . In the York Canyon core the Ir concentration reaches a value of 5.6 × 10 −9 g/g of rock over a local background of about 10 −11 g/g; the Pt abundance distribution is similar to that for Ir, while Au reaches its maximum concentration about 10 cm below the Ir peak. Se, V, Cr, Mn, Co, and Zn are about two-fold more abundant at the anomaly zone than in adjacent zones, and mass spectrometric 244 Pu analysis showed the 244 Pu/Ir atom ratio ⩽ 1 × 10 7 . In the San Juan Basin we have located a small Ir spike (55 × 10 −12 g/g over a local background of 8 × 10 −12 g/g) that is accompanied by high concentrations of Co and Mn. It is thought to be due to geochemical enrichment processes.

Available evidence suggests that dinosaurs in the San Juan Basin survived the event responsible for the creation of an iridium-enriched zone that has been discovered at or near the Cretaceous/Tertiary boundary at several locations throughout the world. The iridium zone has been found at two separate localities in the Raton Basin coincident with the palynologic Cretaceous/Tertiary boundary. The iridium zone has been searched for in the San Juan Basin at several localities where the palynologic Cretaceous/Tertiary boundary has been bracketed, but it has not been found. It is unlikely that the iridium zone will be found in the San Juan Basin, because stratigraphic studies indicate that the rocks which could have contained the iridium zone were eroded prior to deposition of the Ojo Alamo Sandstone. The significant fact is that dinosaur bone has been found within the Ojo Alamo Sandstone in the San Juan Basin stratigraphically higher than the palynologic Cretaceous/Tertiary boundary; thus, this bone apparently postdates the event that created the iridium zone.