Stacy C. Atchley

High-precision U-Pb zircon geochronology of the Late Triassic Chinle Formation, Petrified Forest National Park (Arizona, USA): Temporal constraints on the early evolution of dinosaurs

The Triassic successions of the Colorado Plateau preserve an important record of vertebrate evolution and climate change, but correlations to a global Triassic framework are hampered by a lack of geochronological control. Tuffaceous sandstones and siltstones were collected from the Upper Triassic Chinle Formation exposed in the Petrified Forest National Park, Arizona, USA, within a refined stratigraphic context of 31 detailed measured sections. U-Pb analyses by the isotope dilution–thermal ionization mass spectrometry (ID-TIMS) method constrain maximum depositional ages for nine tuffaceous beds and provide new insights into the depositional history of the Chinle fluvial system. The base of the Blue Mesa Member of the Chinle Formation is placed at ca. 225 Ma, and the top of the Petrified Forest Member is placed at 208 Ma or younger, bracketing an ∼280-m-thick section that spans nearly the entire Norian Stage of the Late Triassic. Estimated sediment accumulation rates throughout the section reflect extensive hiatuses and/or sediment removal by channel erosion. The new geochronology for the Chinle Formation underscores the potential pitfalls of correlation of fluvial units based solely on lithostratigraphic criteria. A mid-Norian age (ca. 219–213 Ma) for the distinctive Sonsela conglomeratic sandstone bed constrains the Adamanian-Revueltian land vertebrate faunachron boundary. Our new data permit a significant time overlap between the lower Chinle sequence and the dinosauromorph-rich Ischigualasto Formation of northwestern Argentina. Near-contemporaneity of the trans-American deposits and their faunal similarities imply that early dinosaur evolution occurred rapidly across the Americas.

Eustatic Control on Alluvial Sequence Stratigraphy: A Possible Example from the Cretaceous-Tertiary Transition of the Tornillo Basin, Big Bend National Park, West Texas, U.S.A.

Paleosol-bearing alluvial strata of latest Cretaceous and earliest Tertiary age are continuously exposed along Dawson Creek, in Big Bend National Park, west Texas, U.S.A., and exhibit a three-tier hierarchy of depositional cyclicity. Meter-scale, fluvial aggradational cycles (FACs) occur as fining-upward successions that are gradation- ally overlain by paleosols or are sharply overlain by the coarser- grained base of the succeeding FAC without an intervening paleosol. FACs stack into decameter-scale, fluvial aggradational cycle sets (FAC sets) that also fine upward, and from base to top contain either a grad- ual upsection increase in soil maturity and soil drainage or a somewhat symmetrical pattern of increasing and decreasing paleosol maturity. Longer-period trends of FAC thickness, lithologic proportions, paleosol maturity, and paleosol drainage indicate that two complete, and two partial, hectometer-scale fluvial sequences occur within the study in- terval. From base to top, each sequence is characterized by an asym- metric increase and decrease in FAC thickness, a decrease in the pro- portion of sand-prone fluvial facies, an increase in paleosol maturity, and better paleosol drainage. Whereas FACs and FAC sets are interpreted to record cyclic epi- sodes of channel avulsion and stability, and longer-term avulsive chan- nel drift within the alluvial valley, respectively, fluvial sequences may coincide with third-order sea-level changes within the North American Western Interior Seaway. As such, the Cretaceous-Tertiary (K-T) transition within the Tornillo Basin may provide an example of me- gascale stratigraphic cyclicity that is controlled by eustatic sea level within a fully fluvial succession. Thickening and thinning successions of FACs record a third-order period of accelerating (transgressive- equivalent) and decelerating (highstand-equivalent) base-level rise, and subsequent base-level fall (falling stage- to lowstand-equivalent). Se- quence boundaries are placed at the sharp inflection between thinning and thickening FACs. Sand-prone facies and immature, more poorly- drained paleosols are associated with the transgressive-equivalent por- tion of each sequence, and mudrock-dominated overbank facies and their associated mature, well-drained paleosols are associated with the highstand- and falling stage-equivalent.

Paleosol barometer indicates extreme fluctuations in atmospheric CO2 across the Cretaceous-Tertiary boundary

We present an atmospheric p CO 2 ( p is partial pressure) curve showing extreme fluctuations for the interval between ca. 77 and 63 Ma in southern Alberta, Canada, using a paleosol barometer. Paleosol carbonate nodules (micrite) were collected from 40 Bk horizons among 6 stratigraphic sections for stable carbon isotope analysis. Based on results from the study area, declining atmospheric p CO 2 from 1200 ppmV (V is volume) in the Campanian to 780 ppmV in the Maastrichtian correlates with Late Cretaceous climate cooling and falling sea level as documented in global records. The remarkable rise in atmospheric p CO 2 near 65.5 Ma (1440 ppmV) correlates with volcanic activity associated with the Deccan Traps, rising sea level, and warmer global climates. The decline in atmospheric p CO 2 (760 ppmV) at the Cretaceous-Tertiary boundary and subsequent sharp rise into the Danian (1000 ppmV) occurred during static terrestrial temperatures and sea level. This work provides compelling evidence that atmospheric p CO 2 curves modeled for the Phanerozoic do not offer the resolution needed to understand environmental conditions during catastrophic events in Earth9s history.

Pedogenic carbonate isotopes as evidence for extreme climatic events preceding the Triassic-Jurassic boundary: Implications for the biotic crisis?

The Triassic-Jurassic boundary is associated with widespread marine and terrestrial extinctions, but there is disagreement regarding the existence and extent of climatic changes that may have driven the biotic crisis. Here, we apply quantitative isotopic climate proxies in order to construct two age-equivalent, relatively continuous temperature and p CO 2 records that span the eight million years preceding the Triassic-Jurassic boundary and that supersede previous terrestrial records in temporal resolution. The δ 18 O data suggest that mean annual temperatures (MAT) increased by 7–9 °C from the late Norian to the Rhaetian in association with the peak increases in p CO 2 levels. The δ 13 C data suggest relatively low late Norian p CO 2 levels ( 1500 ppmV), and at least two periods of extreme p CO 2 levels (~3000 ppmV) preceding the Triassic-Jurassic boundary. These estimates are consistent with a recent Late Triassic climate model that suggests the effects of increased p CO 2 levels on Pangea would cause severe climatic consequences, including a global MAT increase of 6 °C (>10 °C in some regions). While it is possible that periods of increased aridity could have resulted in erroneously high estimates of both temperature and p CO 2 levels, it is likely that climate was still fluctuating during the end of the Triassic. Although our data precede the Triassic-Jurassic boundary, many studies conclude that the mass extinction took place over a more prolonged period beginning in the Late Triassic. Thus, climate may have been a significant driving mechanism of the Late Triassic extinctions.

Collapse of the Late Triassic megamonsoon in western equatorial Pangea, present-day American Southwest

During the Late Triassic, western equatorial Pangea, in the present-day American Southwest, was unusually humid, as indicated by sedimentological evidence and inferred from general circulation models. These studies show that once equatorial Pangea was assembled, cross-equatorial summer air flow penetrated into western equatorial Pangea, bringing abundant megamonsoon rainfall and warm temperatures. However, many of these investigations indicate that gradual aridification began sometime during the Late Triassic to Jurassic. We show from field properties, geochemical transfer functions, and isotopic analysis of paleosols in the Chinle Formation of Petrified Forest National Park, Arizona, that the western equatorial Pangea megamonsoon collapsed by 214.7 Ma during the “middle Norian climate shift.” Paleosols include Entisols, Inceptisols, Vertisols, Aridisols, and Alfisols, with transfer functions for mean annual precipitation (chemical index of alteration minus potassium [CIA–K]) and mean annual temperature (NAK)(salinization) developed from analogous modern data sets. The most notable shift is the appearance of carbonate-enriched paleo-Inceptisols and paleo-Vertisols after 214.7 Ma and paleo-Aridisols after 210 Ma. Prior to the middle Norian climate shift, the region is classified as humid (humidity province) based on rainfall estimates. Initiation of the middle Norian climate shift was characterized by an increase in paleosol carbonate content and rapidly declining rainfall as the region shifted to subhumid, and eventually to semiarid and arid after 210 Ma. Paleosol-derived temperatures are indicative of warm temperate to subtropical ecozones, whereas general circulation models show higher temperatures and tropical conditions, perhaps because boundary conditions were set to atmospheric p CO 2 4–5× present. Elevations of >1.5–2 km adjacent to the Cordilleran magmatic arc complex located ~400 km west of the study area may account for nontropical temperatures. After 210 Ma, however, temperatures began to increase, possibly as a result of escalating atmospheric p CO 2 from shale oxidation during marine regression and from coal and paleosol organic matter oxidation during prolonged aridification. Paleomagnetic studies have suggested northward continental drift of western equatorial Pangea to outside of the Intertropical Convergence Zone as the cause of monsoonal collapse during the Late Triassic to Early Jurassic. The formation of a rain shadow as a result of the evolving Cordilleran magmatic arc as the cause of aridification is supported by recent magnetostratigraphic work substantiating that the region remained in the tropics through the Triassic. According to our age model, the middle Norian climate shift is dated to near the same time as the Manicouagan impact crater, but there is no evidence of such an event in the study area, either geochemically or sedimentologically at our scale of observation. However, a regionally defined faunal turnover may have been a response to rapidly changing climates in the region.

Depositional and diagenetic controls on reservoir attributes within a fluvial outcrop analog: Upper Triassic Sonsela member of the Chinle Formation, Petrified Forest National Park, Arizona

The Upper Triassic Sonsela member of the Chinle Formation in the Petrified Forest National Park was evaluated using sedimentologic, stratigraphic, paleopedologic, and petrographic criteria along a continuous 0.5-km (0.3-mi) outcrop. The study interval consists of interbedded sandstones and mudstones and is composed of a two-tiered hierarchy of cyclic alluvial deposits with bounding paleosols. The succession is composed of 15 fluvial aggradational cycles (FACs) that comprise two FAC sets (FAC-Sets). The FAC-Sets are composed of architectural elements suggestive of a mixed-load fluvial system that is alternately dominated by bed-load deposits and suspended-load deposits. A thinning and fining stacking pattern within FAC-Sets is accompanied by an upward increase in pedogenic modification, suggesting that cycles systematically stack in response to a longer period decrease in the rate of accommodation gain. Sandstones are classified as litharenites, feldspathic litharenites, and lithic subarkoses, and occur within recycled orogen, dissected arc, and transitional arc provenance fields. Sandstone compositional maturity increases upward through the FAC-Sets. Point counts of intergranular volume (as a proxy for primary porosity) within channel facies and subsequent transform to syndepositional permeability provide a two-dimensional depiction of the lateral variability in reservoir quality. Paleosols are weakly to moderately developed and have little stratigraphic variation. These characteristics suggest that climatic fluctuations are not responsible for evolving fluvial depositional style or associated reservoir quality. Trends in sandstone compositional maturity suggest that fluvial stacking patterns and depositional style are related to pulses of tectonism. Sandstones are volcanogenic rich and have undergone an almost complete diagenetic loss of porosity caused by the precipitation of authigenic clays. Paragenetic reconstruction suggests that porosity loss occurred contemporaneous with the silicification of fossil logs in channel deposits. Log compaction at the time of silicification averaged 9.1%, suggesting that log silicification and porosity loss occurred soon after deposition.

Reservoir characterization and facies prediction within the Late Cretaceous Doe Creek Member, Valhalla field, west-central Alberta, Canada

Oil resources at Valhalla field of west-central Alberta, Canada, are stratigraphically trapped within the Upper Cretaceous Doe Creek Member of the Kaskapau Formation. The reservoir is subdivided into four thin (1–10 m [3–33 ft]), cyclic alternations of offshore mudrock and shoreface sandstone that are designated the I 1, I, I + 1, and I + 2 units. The thickest and most widespread I sandstone is the primary reservoir. Optimum reservoir quality corresponds to coarser grain shoreface sandstone; however, reservoir quality may be diminished by postdepositional calcite cement commonly observed near the top of shoreface sandstones. Open-hole well logs are used to predict depositional facies and calcite cement occurrence in wells that lack core control. Decreasing shale volume (Vsh) and increasing deep resistivity values correspond to progressively shallower water deposits. Zones of calcite-cemented shoreface sandstone greater than 0.5 m (1.6 ft) thick are interpreted when the neutron porosity exceeds the density porosity by more than 7%. Facies distributions predicted for the I sandstone closely match trends of the sandstone gross pore volume and daily total fluid production, and suggest that open-hole well logs may be used to anticipate reservoir quality and continuity. Regional and local observations support previous interpretations that attribute the Doe Creek to forebulge erosion and southwestward sediment transport toward a foredeep where shoreface sandstones accumulated within a coastal embayment to the Western Interior seaway. Regionally, the Doe Creek interval thins northeast of Valhalla and is truncated beneath the K1 unconformity, and shoreface sandstone bodies are encased within offshore mudrocks and detached from their contemporaneous shoreline. Locally at Valhalla, the Doe Creek reservoir progrades toward the southwest and is extensively and commonly uniformly burrowed by a relatively diverse assemblage of trace makers.

Reserves growth in a mature oil field: The Devonian Leduc Formation at Innisfail field, south-central Alberta, Canada

Oil has been continuously produced from the Devonian Leduc Formation at Innisfail field since its discovery in 1957. To date, cumulative production at Innisfail is 98.9% of recoverable oil in place (84.3 MMBO) and indicates that the Leduc is near the end of its productive life. A sequence-stratigraphic interpretation produced from core, well logs, and three-dimensional (3-D) reflection seismic data, however, suggests that production may be extended through the development of two previously undetected scenarios of bypassed oil entrapment: (1) attic oil accumulations associated with small buildups atop the isolated Innisfail platform and (2) backstep-edge accumulations located in positions structurally low to currently producing wells. In both cases, oil accumulations are related to a Leduc depositional history that was characterized by rates of long-term carbonate sediment accumulation exceeded by the rate of sea level rise. Positive bathymetric relief created at the time of deposition and, subsequently, present-day stratigraphic traps were produced during the final phase of stratal retrogradation that immediately preceded the drowning of the isolated Innisfail platform. High-quality 3-D seismic data are essential in the exploration for both attic and backstep-edge oil accumulations at Innisfail. Integration of 3-D seismic data, well-log, and core data indicates that although depositional relief on both types of features may be low, they are nonetheless seismically resolvable. The small platform-top buildups are up to 0.16 km2 (40 ac) in diameter and have up to 10 m (33 ft) of independent closure above spill point. Backstep-edge oil accumulations occur within the structurally highest parts of the Leduc high-frequency sequence G downlap limit. Depositional relief along the downlap limit and associated trap closure ranges from approximately 5 to 15 m (16 to 49 ft). During 2003 and 2004, two platform-top buildups were directionally drilled, and one existing well located near a backstep-edge position and suspended since 1988 was reactivated. All three wells are successful and, to date, have cumulative production of 71 thousand bbl of oil. A total of 17 platform-top buildups and backstep-edge development drilling prospects exist across Innisfail and have expected-case recoverable reserves of approximately 960 thousand bbl of oil. These reserve additions suggest the potential for 1.14% field growth at Innisfail and are representative of the potential value that may yet exist within all mature Leduc fields in western Canada.

Ecosystem response to soil biogeochemical behavior during the Late Cretaceous and early Paleocene within the western interior of North America

Paleosol properties are routinely characterized by whole-rock geochemistry, compromising the interpretation of important biogeochemical information in deep time. As a consequence, we employ a new pedotransfer function approach to the characterization of paleosols and apply this methodology to Late Cretaceous (Campanian and Maastrichtian) and early Paleocene (Danian) landscapes from the Dawson Creek study area of the western interior to: (1) reconstruct collodially based physical and chemical soil properties, and (2) assess climate and soil biogeochemical controls on evolving terrestrial ecosystems. Nine paleoseries (i.e., pedotypes) characterize the range of soil properties within the fluvial stratigraphic succession, which includes Entisols, Inceptisols, and Vertisols. All soils had optimal water-holding potential as inferred from low bulk densities, whereas poorly drained and colonizing landscapes likely suffered from poor aeration during seasonal water logging. Even with high water-holding capacity, Maastrichtian soils experienced seasonal moisture stress because of lower rainfall than Campanian and Danian soils. Fertility levels were sufficient for the growth of most plants judging from high cation exchange capacity and base saturation, negligible aluminum toxicities because of nonacid pH, and limited salinity and sodicity from relatively low exchangeable sodium and soluble salts in solution. Unlike warm-temperate and forested paleosols with neutral pH from the Campanian and Danian, subtropical and alkaline paleosols from the Maastrichtian apparently supported a woodland plant formation adapted to low availability of iron and manganese, which were fixed with calcium in carbonate, and low availability of phosphorous because it formed insoluble compounds with iron and manganese. Carbon, nitrogen, phosphorous, and sulfur cycling through microbially mediated mineralization of soil organic matter was limited from low litter inputs in both early and midsuccessional ecosystems, particularly in woodland soils. Results do not reveal demonstrable changes in soil characteristics through the K-T transition.

A Predictive Model for Reservoir Distribution in the Permian (Leonardian) Clear Fork and Glorieta Formations, Robertson Field Area, West Texas

Through the collaborative efforts of a multidisciplinary team, various subsurface data types have been integrated to produce a conceptual geologic model for reservoir prediction within the upper Clear Fork and Glorieta formations of the Robertson field area, west Texas. Detailed description of 1434 m of core, 109 thin sections, and 241 line-kilometers of 2-D (two-dimensional) seismic indicates the stratigraphic interval accumulated as a progradational succession of platform-interior subtidal and intertidal facies. Reservoir intervals preferentially occur within subtidal facies having intercrystalline porosity associated with replacement by coarsely crystalline dolomite. Intertidal facies were replaced by a finely crystalline phase of fabric-preserving dolomite and are dominated by an ineffective fenestral pore system. Facies and reservoir distribution is largely controlled by antecedent topography. Structural highs generated during the terminal phase of Ouachita-Marathon compression were the preferred site of intertidal facies deposition. Adjacent structural lows have a higher proportion of reservoir-prone subtidal facies. Analysis of compensated-neutron log porosity indicates that subtidal-prone intervals may be broadly characterized as having less than 11% porosity, whereas intertidal-prone intervals generally exceed 11% porosity. From this observation, a simple computer algorithm allows facies interpretation within wells lacking core. Intertidal-subtidal facies ratio maps generated from this algorithm closely match the distribution of pre-Wolfcampian structural elements and present-day structure. Intertidal facies are more common across the crest of structural highs. Future drilling in the North Jenkins area should pursue structural-flank positions where subtidal facies are volumetrically more abundant. Care should be taken, however, to avoid completions below the composite oil-water contact. Subtidal facies are compartmentalized within individual sequences of a progradational sequence set. Progradational stacking and compactional drape over deeper structures produces discrete flow units with potentially independent oil-water contacts.