David E. Grandstaff

Chemistry and mineralogy of Precambrian paleosols at Elliot Lake, Ontario, Canada

Two profiles from the paleosol underlying the Huronian Supergroup at Elliot Lake, Ontario were studied to determine conditions present at the time of their formation. (1) One paleosol profile (> 10.5 m thick), developed on greenstone, is exposed within the Denison Mine. It is tentatively classified as an intrazonal gley soil formed in an area of poor drainage and reducing groundwater conditions. It is characterized by extensive loss of Fe, Mn, Cu, Ni, and Mo, as well as Na, Ca, and Mg. An increase in K is attributed to diagenetic K metasomatism. Paleosol structures indicate clay translocation forming an illuvial Bg horizon and suggests a fluctuating water table to depths of 6m. (2) The second paleosol profile (> 5.5 m thick), developed on granite, is exposed near the Pronto Mine. This profile exhibits iron oxidation and enrichment. This indicates that free oxygen was present in the mid-Precambrian atmosphere, although a precise determination of pO2 is not possible from evidence from the paleosols. This paleosol exhibits many characteristics of modern spodosols. The soils probably formed in a temperate humid climate during tectonically stable conditions. The difference in oxidation and behavior of iron in these soils is inferred to result from differences in paleotopography and drainage. The preponderance of gley paleosols found underlying the Huronian Supergroup might result either from lower mid-Precambrian pO2 or from preferential preservation. Evidence suggests that by 2.3 Ga/ago, soil forming processes were capable of producing soils similar to Recent types.

Some kinetics of bronzite orthopyroxene dissolution

Abstract The kinetics of bronzite orthopyroxene dissolution were investigated in HClKCl solutions having a total concentration of 0.1 M, over the pH range 1–4.5, at temperatures between 42 and 1°C. Dissolution of the pyroxene was incongruent and followed a parabolic rate law. The activation energy of the reaction is 10.5 ± 0.6 kcal mole −1 . The rate dependence on hydrogen ion activity is one-half order. The rate of dissolution is unaffected by substitution of sodium for potassium, or sulfate or nitrate for chloride anion, or by addition of citrate or acetate ions. However, traces of fluoride increase the dissolution rate. The rates observed are one to two orders of magnitude slower than those previously reported by Luce et al. (1972) for dissolution of enstatite.

Changes in surface area and morphology and the mechanism of forsterite dissolution

Abstract Scanning electron microscope (SEM) photographs of olivine grains shown that dissolution of olivine may occur more rapidly on some surfaces than on others and that initial dissolution of freshly crushed grains occurs primarily along lattice imperfections such as dislocations or cleavage planes. The SEM photographs generally do not show the presence of thick or continuous residual or precipitated layers which might render the dissolution reaction diffusion controlled. The specific surface area of olivine grains increased greatly during initial dissolution due to formation of etch features. However, despite the increased surface area the rate of dissolution decreased during the experiments. These observations suggest that some assumptions underlying derivation of the diffusion controlled models are invalid for olivine dissolution and suggest that dissolution is controlled by rates of surface reactions.

Origin of uraniferous conglomerates at Elliot Lake, Canada and Witwatersrand, South Africa: Implications for oxygen in the Precambrian atmosphere

Although uraninite is thermodynamically unstable at oxygen pressures greater than approximately 10−21 atmosphere, calculations based on the dissolution kinetics suggest that uraninite from the Witwatersrand Sequence, South Africa may have survived as a detrital mineral at oxygen pressures as high as 0.01 PAL (Present Atmospheric Level). Therefore, the deposition of the mid-Precambrian uraniferous conglomerates at Elliot Lake, Canada and Witwatersrand, South Africa does not require an essentially anoxic atmosphere as previously proposed, but may have occurred under an atmosphere containing small amounts of oxygen, consistent with photodissociation of water vapor and limited aerobic photosynthesis.

Chemistry and mineralogy of Precambrian paleosols at the base of the Dominion and Pongola groups (Transvaal, South Africa)

Abstract We have studied alteration zones found between sedimentary rocks of the (lowest Proterozoic) Dominion and (Archean) Pongola Groups and underlying granitic rocks (Transvaal, South Africa). The alteration zones have a transitional lower boundary, primary igneous minerals are gradually destroyed upward toward the contact with overlying sedimentary rocks. Ca, Mg, Na, P, and Mn are progressively lost upwards in the zone. The chemical and mineralogical variations, and available stratigraphic evidence are all consistent with the interpretation that these zones are paleosols formed by Precambrian weathering 2.8-3.0 Ga ago. After formation the paleosols underwent metasomatism and metamorphism. Sericite now present in the paleosols was probably formed by low temperature potassium metasomatism of original clays. Andalusite in the pre-Pongola paleosol was probably formed by metamorphism of original clays in impermeable zones which prevented groundwater flow and potassium metasomatism. The Dominion paleosol was investigated at three localities. The effects of weathering were similar at the three sites except for the behavior of iron. One site lost about 20% of iron whereas the other sites lost 75% and 80% of initial iron content. The differences in iron loss may be due to variation in soil ventilation and diffusion of atmospheric gases as a function of grain size and topography. Variations in iron loss may be used to place some constraints on the composition of the Precambrian atmosphere. Given constraints imposed by Dominion paleosols, overlying uraniferous conglomerates, requirements for chemical weathering and the greenhouse effect, and possible presence of non-diffusing photochemical oxidants, the partial pressures of oxygen and carbon dioxide gases probably are in the ranges: PO1 - 0.02% to 0.5% PAL (Present Atmospheric Level) and PCO2 - 5 to 30 PAL.

REE Signatures in Vertebrate Fossils from Sewell, NJ: Implications for Location of the K-T Boundary

Abstract Rare Earth Element (REE) signatures have been used to test whether mosasaur bones in a basal Hornerstown Formation bonebed (the Main Fossiliferous Layer, or MFL) in New Jersey were reworked from the underlying Maastrichtian beds or deposited synchronously with the bones of other taxa in the Hornerstown Formation. The interpreted age of the bonebed (Maastrichtian vs. Danian) and the position of the K-T boundary in New Jersey are affected by the possible reworking. Statistical techniques, such as ANOVA and Discriminant Analysis, show that signatures of REE in MFL bones are different from those of bones in either the underlying Navesink Formation or the upper part of the Hornerstown Formation, suggesting a unique depositional setting for this bonebed. REE signatures of the MFL mosasaur bones conform with signatures in bones from other taxa within the MFL, suggesting that their deposition was contemporaneous with that of the other taxa. Thus, the MFL bonebed appears to be Cretaceous in age and the K-T boundary must be in or above the MFL, within the Hornerstown Formation.

Effect of paleosol formation on rare earth element signatures in fossil bone

The rare earth element (REE) content of fossil bones was analyzed and compared with the degree of ancient pedogenic development and depositional environments from several locations in the Orellan Scenic Member of the Oligocene Brule Formation in Badlands National Park, South Dakota. Paleosols ranged from weakly developed Entisols to more strongly developed Inceptisols, all typical of fluvial environments and possible paleocatena variation. Paleosols were alkaline and well drained. Sediments with sparse soil features from an oxbow lake system suggest that conditions were too waterlogged and sedimentation rates too rapid for significant pedogenesis. The variance of REE signatures in fossil bones from the paleosol sites was significantly greater than that of fossils from minimally altered sediments of the former oxbow lake. Positive Ce anomalies were associated with low U concentrations and indicate paleoredox conditions. Greater degrees of pedogenesis, regardless of the horizon in which the bone was found, systematically correlated with increased heavy REE enrichment in fossil bones. The fossil-bone REE signatures from the different paleosols and depositional environments were significantly different and distinguishable.

Profiles of elemental concentrations in Precambrian paleosols on basaltic and granitic parent materials

Abstract Profiles of elemental concentrations are interpreted for four Precambrian paleosols, two developed on basalt and two on granodiorite. All four paleosols appear to be the erosional remnants of originally thick soil-saprolite regoliths. The granitic paleosols are in South Africa where they underlie the 2.9-3.0 Ga Pongola Supergroup and the 2.8-2.9 Ga Dominion Reef Conglomerate. One of the basaltic paleosols also occurs in South Africa where it caps the Ventersdorp Basalt and underlies the 2.3 Ga Black Reef Quartzite. The other basaltic paleosol underlies sandstone of the 2.7 Ga Timiskaming Group in the Abitibi belt of the Canadian Shield. All four paleosols exhibit pronounced upward loss of sodium but no consistent loss of the heavier alkali elements, rubidium and cesium. Iron decreases upward above iron-rich basaltic parent rocks but there is no consistent loss of iron above iron-poor granitic parents. The rare-earth elements are less fractionated than during intensive modern weathering. Uranium locally has been fractionated from thorium, possibly due to oxidative dissolution during Precambrian weathering.

Rare earth element geochemistry and taphonomy of the Early Cretaceous Crystal Geyser Dinosaur Quarry, east-central Utah

Abstract The Crystal Geyser Dinosaur Quarry contains a large monospecific accumulation of bones from a basal therizinosaur, Falcarius utahensis. The quarry is located approximately 16 km south of Green River, Utah, at the base of the early Cretaceous (Barremian) Yellow Cat Member of the Cedar Mountain Formation. Fossil bones in the quarry occur in three units that have distinct taphonomic, lithologic, and geochemical characteristics. Rare earth element compositions of fossils suggest that bones from each unit were drawn from different reservoirs or sources having distinctly different compositions, and fossils were not reworked between units. Compositions of bones differ greatly within Units 1 and 2, even within the same 1-m2 quarry grid. These chemical differences and taphonomic characteristics, such as current orientation, hydraulic sorting, and occasional extensive abrasion, suggest that bones from these two units are allochthonous and were fossilized at other localities, possibly over an area of several kilometers, and were then eroded, transported, and concentrated in a spring-influenced fluvial environment. Bones in Unit 3 have very similar rare earth element signatures, suggesting that they were probably fossilized in situ at a separate time from bones in Units 1 and 2. At least two mass mortality events were responsible for the monospecific assemblage of bones at the quarry. Because bones may have been concentrated from a wide area, causes of mass mortality must have been regionally extensive, possibly owing to seasonal drought, sudden changes in weather, or disease.

SEDIMENTOLOGY, STRATIGRAPHY, AND DEPOSITIONAL ENVIRONMENT OF THE CRYSTAL GEYSER DINOSAUR QUARRY, EAST-CENTRAL UTAH

Abstract The Crystal Geyser Dinosaur Quarry, near Green River, Utah, is located at the base of the Lower Cretaceous (Barremian) Yellow Cat Member of the Cedar Mountain Formation. The quarry preserves a nearly monospecific accumulation of a new basal therizinosauroid, Falcarius utahensis. We used field descriptions and petrographic analysis to determine the depositional environment and development of the quarry strata. Results of these analyses suggest that the quarry represents multiple episodes of bone accumulation buried by spring and overbank flood deposits. Evidence for these previously undescribed spring deposits includes calcite macroscopic structures within the quarry strata—such as pisolites and travertine fragments—and calcite micromorphologies—including radial-fibrous, feather, and scandulitic dendrite morphologies and tufa clasts. At least two episodes of bone incorporation are preserved in the quarry based on their stratigraphic position and lithologic associations. The unique depositional setting in and around the Crystal Geyser Dinosaur Quarry appears to have been favorable for the preservation of vertebrate fossils and provides insight into early Cretaceous environments in North America.