John F. Hubert

Chemistry, Microstructure, Petrology, and Diagenetic Model of Jurassic Dinosaur Bones, Dinosaur National Monument, Utah

ABSTRACT The fossil-bone mineral in dinosaurs from the Morrison Formation is well-crystallized, stoichiometric francolite. The closely packed, subparallel francolite crystals have equidimensional cross sections mostly 10-40 nm wide. In contrast, crystallites in unaltered mammal bone tissue are poorly crystallized, nonstoichiometric carbonate hydroxyapatite, a few nanometers thick, and plate-, rod- or needle-shaped. The diagenetic francolite crystals are elongated in the c axis direction, as are bone-tissue crystallites. Along permeable cracks, francolite crystals grew to 250 nm wide. Compared to unaltered crystallites in mammal bones, the francolite has higher concentrations of Ba, Ce, Cr, F, La, Mn, Ni, Pb, Rb, Sr, Th, U, V, and Y, and perhaps Zr. Na and Mg are lower in concentration. Diagene ic francolite grew from groundwater enriched in ions from silicic ashes in the Brushy Basin Member and from dissolution of bone-tissue crystallites. After burial, francolite grew on crystallite seeds, filling the space formerly occupied by collagen, perhaps half the tissue volume, and dissolved crystallites. The micron-scale structures such as Haversian canals and laminae are preserved because the diagenetic francolite retains the orientations of the seeds. The laminae, 1-10 µm thick, commonly preserve an orthogonal plywood structure. The diagenetic growth of francolite during this type of bone fossilization implies that oxygen isotopic ratios measured in dinosaur bones, even those that appear fresh and unaltered, may dominantly reflect groundwater temperature rather than dinosaur body temperature. Trace-element concentrations in the francolite may reflect groundwater composition rather than dinosaur diet. Pore-filling cements precipitated in the sands and bones at shallow burial depths, as evidenced by the 35-52% minus-cement porosities of the sandstones and numerous uncompacted oversize pores filled by cements. Gypsum and calcite were precipitated from shallow alkaline groundwater. As the groundwater became increasingly dominated by ions from silicic ash, argentite, chalcopyrite, native gold, native silver, tiemannite, and uraninite replaced areas of bone a few microns to tens of microns in size along cracks. A thin chlorite crust commonly mantled francolite crystals along bone cracks and lined openings in bones and sandstones. Pervasive precipitation of a chalcedony crust followed, with pseudomorphs of chalcedony after gypsum. In larger cancellous openings, the remaining space was fi led by the various forms of silica. The bones are brown due to goethite-filled cracks that cut pore-filling minerals.

Stability of atmospheric CO2 levels across the Triassic/Jurassic boundary

The Triassic/Jurassic boundary, 208 million years ago, is associated with widespread extinctions in both the marine and terrestrial biota. The cause of these extinctions has been widely attributed to the eruption of flood basalts of the Central Atlantic Magmatic Province. This volcanic event is thought to have released significant amounts of CO2 into the atmosphere, which could have led to catastrophic greenhouse warming, but the evidence for CO2-induced extinction remains equivocal. Here we present the carbon isotope compositions of pedogenic calcite from palaeosol formations, spanning a 20-Myr period across the Triassic/Jurassic boundary. Using a standard diffusion model, we interpret these isotopic data to represent a rise in atmospheric CO2 concentrations of about 250 p.p.m. across the boundary, as compared with previous estimates of a 2,000–4,000 p.p.m. increase. The relative stability of atmospheric CO2 across this boundary suggests that environmental degradation and extinctions during the Early Jurassic were not caused by volcanic outgassing of CO2. Other volcanic effects—such as the release of atmospheric aerosols or tectonically driven sea-level change—may have been responsible for this event.

Debris-flow deposits in alluvial fans on the west flank of the White Mountains, Owens Valley, California, U.S.A.

Abstract Sections measured in incised channels on 10 alluvial fans show that debris-flow beds, mostly 30–200 cm thick, are more important than stream-flow deposits in construction of fans along the west flank of the White Mountains. The debris-flow beds have a matrix-supported fabric with a sandy mud matrix that comprises about 40% of individual beds. Complete grain-size analyses of 19 debris-flow beds show that they average 50% gravel, 25% sand, 11% silt, and 4% clay. Inverse grading at the base of most beds is interpreted as due to a layer of high-shear stress beneath an overlying semi-rigid, high-strength plug that supported cobbles and boulders. Sticks and logs embedded in the plugs are oriented parallel with flow directions, reflecting the laminar-viscous motion of the plugs. In the shear layer, the average inclination of discoidal clasts is subhorizontal, varying from 2 to 7° in the up flow direction. In the plugs, discoidal clasts have a subhorizontal fabric with average dips of 5–13°. Compared to the shear layer, plugs have more scatter in clast orientation with numerous clasts dipping at 60–90°. Levees along the margins of debris-flow lobes contain concentrations of the larger clasts in the flows. In the leeves, discoidal clasts have an average up flow dip of 21–31° with substantial variability in orientation similar to the plugs. Maximum clast size in individual debris-flow beds is fairly constant down fan until the flows spread and thinned on the sandflat at the fan toe. Clast lithologies show that debris flows originate on steep slopes underlain by granitic and metavolcanic rocks at high elevations in the canyons. In contrast, stream flows obtained most of their clasts from talus slopes on metasedimentary rocks near the apices of the fans. Debris flows are generated during intense rainstorms in the spring and summer when landslides in the water-saturated regolith move down slope, shear, dilate, and by adding water are transformed into debris flows which then move with surging laminar motion along canyon flows to the fans. The recurrence interval for debris flows is about 320 years as evidenced by 14C dates on plant material buried beneath debris-flow beds.

Paleosol caliche in the New Haven Arkose, Newark Group, Connecticut

Abstract The New Haven Arkose is a 1500–2500 m sequence of fluvial red beds of Late Triassic and probably Early Jurassic age that accumulated in a tropical rift valley. Streams flowing from highlands along the valley deposited braided channel sand interbedded with overbank mud. At most outcrops in Connecticut, some of the beds of red mudstone and sandstone contain paleosol caliche with densely packed root casts and rhizoconcretions. These calcified mudstones and sandstones commonly show the initial stages of calcification where subspherical to vertically elongate calcite nodules with crystic plasmic (microspar) fabric replace up to 50% of the host rock. In more mature paleosols, coalescing calcite nodules replace 50–95% of the rock, accompanied by paleosol mosaics of pedotubules, peds, and laminated crystallaria. The best developed caliche profiles are capped by thin (0–10 cm) layers of nearly pure limestone with complex fabrics of nodules, microspar, veins, caliche clasts, laminae, and calcite cement. The caliche profiles accumulated when rates of sedimentation were low. Much of the time the tropical paleoclimate was semiarid, perhaps with 100–500 mm of seasonal rain and a long dry season.

Eolian dune field of Late Triassic age, Fundy Basin, Nova Scotia

Recently discovered eolian sandstones in the Wolfville redbeds are definitive evidence of aridity in Late Triassic time in the Fundy Basin. The paleowinds consistently blew to the southwest, reflecting the location of the rift valley in the subtropical belt of northeast trades. These data help establish a Late Triassic climatic pattern of increasing aridity from south to north for the basins of the Newark Supergroup.

The Triassic-Jurassic Hartford Rift Basin, Connecticut and Massachusetts: Evolution, Sandstone Diagenesis, and Hydrocarbon History (1)

During initial subsidence in the Late Triassic, rivers flowed from both sides of the subtropical Hartford basin to deposit the red beds of the lower part of the 2-km-thick New Haven Arkose. Subsequently, the fault-bounded eastern highlands became the major source of detritus for the fluvial red beds of the upper New Haven Arkose. Extensional crustal thinning increased dramatically in the Early Jurassic, producing volcanism and a closed, asymmetrical half-graben in which accumulated Milankovitch-type cycles of oligomictic gray/black mudstones and playa red beds. These strata form most of the 150-m-thick Shuttle Meadow, 170-m-thick East Berlin, and lower half of the 2-km-thick Portland formations. Rivers and sheetfloods from the western hinged margin spread sediment over th valley floor, up to small alluvial fans along the eastern highlands. As extension eased, the fault-bounded eastern highlands shed detritus for the fluvial and alluvial-fan red beds of the upper Portland Formation. Basinal sandstones are oligoclase-rich arkoses and lithic arkoses whose compositions reflect erosion of the eastern and western highlands. Ubiquitous albite overgrowths were precipitated in the sands, followed by ferroan calcite/calcite in fluvial sands and mostly ferroan dolomite/dolomite in playa and lacustrine sands. Na{+} for albite cement came from weathering of oligoclase-rich gneisses and schists around the basin. Topography-driven flow circulated groundwater through the sands, precipitating albite and locally albitizing feldspar grains. In the Early to Middle Jurassic high heat flow generated hydrocarbons in the lacustrine, organic-rich mudstones. These source rock mudstones are locally mature for hydrocarbons and are interbedded with or adjacent to potential reservoirs of lacustrine and playa sandstones. In the Middle Jurassic, hydrocarbons migrated into secondary porosity in some of these sandstones.

Isotopic Imprint of Climate and Hydrogeochemistry on Terrestrial Strata of the Triassic-Jurassic Hartford and Fundy Rift Basins

ABSTRACT Late Triassic to Early Jurassic terrestrial sequences in the Hartford and Fundy rift basins have distinctive carbon and oxygen isotopic compositions of calcite and dolomite. The isotopic data mostly reflect paleoclimatic fluctuations and hydrogeochemistry of the lacustrine, playa, and fluvial environments. Dolomites from laminae in three sequences of playa red mudstones and lacustrine gray to black mudstones in the Hartford basin have variable isotopic compositions (13C = -5.8 to + 1.8 PDB; 18O = -7.2 to +0.7 PDB). Within any single symmetrical cycle of playa red mudstone--lacustrine gray, black, gray mudstone--playa red mudstone, there is a systematic change to relatively enriched 13C compositions in dolomite in the grayish black and black mudstones in the center of the cycle. These carbon isotopic data suggest that the lacustrine sequences formed as the lakes changed from well mixed with anoxic bottom waters to stratified with anoxic bottom waters where 13C-depleted carbon was concentrated in organic matter that was then buried. Calcites from lacustrine, micritic, and biomicritic limestones of the Scots Bay Formation of the Fundy basin have stable isotopic compositions (13C = -4.6 to -2.2 PDB; 18O = -6.1 to -3.0 PDB) that become more enriched in 18O and 13C upward in shallowing depositional sequences. These isotopic data reflect initial calcite precipitation when a high inflow of fresh water produced high lake levels, followed by progressively lower inflow, resulting in lower lake stands and higher salinity due to continuing evaporative loss of surface wat r. The lake waters were well oxygenated at all times. In the Hartford basin, caliche calcites in fluvial mudstones and sandstones have isotopic compositions (13C = -7.3 to -3.8 PDB; 18O = -8.0 to - 5.6 PDB) that reflect paleosol processes during climatic conditions that varied from warm and dry in Late Triassic time to relatively cooler and probably wetter in the Early Jurassic. Isotopic compositions of caliche calcites in redbeds in the Fundy basin indicate a parallel climate change from Late Triassic to Early Jurassic time, but also that the climate was relatively hotter and probably drier over the entir interval, as compared to the Hartford basin.

Biological markers in Lower Jurassic synrift lacustrine black shales, Hartford basin, Connecticut, U.S.A.☆

Abstract The East Berlin Formation (Lower Jurassic, Hartford basin, Connecticut, U.S.A.) is distinctive for its six cyclic units of lacustrine black shale and gray mudstone, separated by playa and fluvial redbeds. The black shales are each about a meter thick and were deposited in subtropical, thermally strastified, oligomictic lakes, the youngest of which (lakes 3 through 6) were large enough to flood most of the basin and attained depths of several tens of meters. The saturate fractions of solvent extracts of organic-rich black shales from each of the six lakes, collected at fresh roadcuts near East Berlin, are dominated by extended homologous series of n-alkanes, alkylcyclohexanes, and branched chain alkanes. A striking feature of the black shales is the presence of a series of extended tricyclic terpanes from C20 to at least C41. Hopanes are either not detectable or present only in subordinate quantities relative to the tricyclic terpanes. The samples are depleted in hopanes in part because of the elevated maturity level (mid to late oil window). Tricyclic terpane concentrations may also have been enhanced by fractionation effects related to oil expulsion out of the black shales. In addition, the original organic matter may have been exceptionally rich in tricyclic terpane precursors, i.e. fossil lipids of prokaryotes present in anoxic, moderately saline, alkaline lakes.

Eolian Sandstones in Upper Triassic-Lower Jurassic Red Beds of the Fundy Basin, Nova Scotia

ABSTRACT Eolian sandstones occur interbedded with alluvial-fan and braided-river red beds in the Upper Triassic Wolfville Formation and at the base of the Lower Jurassic McCoy Brook Formation in Nova Scotia. Extensive semiaridity and aridity in the subtropical rift valley favored deposition of eolian sandstones, caliche paleosols, alluvial-fan conglomerates, braided-river alluvium, and playa mudstones. In postdrift configuration, paleowinds blew to the southwest, west, and northwest down the valley, reflecting the direction of trade winds at 25°N paleolatitude. A three-step hierarchy of surfaces truncates cross-beds. First-order bedding planes of low relief extend for at least hundreds of meters, truncating all eolian structures. These surfaces were sculptured by desert flash floods and then deflated to ventifact-bearing bimodal regs. Second-order surfaces bound cross-bed sets and are mostly concave-up and dip downwind, recording shifts in migration direction of individual dunes or dunes superimposed on larger dune structures. Third-order surfaces within cross-bed sets formed on the lee sides of dunes due to fluctuations in wind direction and strength. Barchans initially developed on the alluvial-fan and braided-river alluvium, later coalescing into barchanoid transverse ridges, as evidenced by the internal geometry of the eolian sandstones. /P>

Red-bed diagenesis in the East Berlin Formation, Newark Group, Connecticut Valley

ABSTRACT The East Berlin Formation is a 200-m fluvial and lacustrine sequence of red and gray sandstone and mudstone that accumulated in a tropical rift valley during Early Jurassic time. The hematite pigment that colors the red beds is authigenic, produced by four post-depositional processes. (1) Brown and yellow-brown limonite that stained the surfaces of the detrital particles of sand and mud converted to hematite by aging. (2) In the sandstone, pervasive intrastratal solution of Fe-silicate grains, especially pyroxene, amphibole, epidote, chlorite, and biotite provided iron for precipitation of hematite, or a red ferric oxide precursor that then aged to hematite. These two processes were volumetrically the most important in generating hematite pigment. (3) In all the red beds, abundant mag etite grains were oxidized to hematite; ilmenite grains were oxidized to hematite-rutile. (4) In the sandstones, replacement of Fe-silicate grains by dolomite cement yielded additional iron for hematite cement. Conversion of limonite stains on clay particles to hematite by aging was the major source of hematite in flood-plain grayish red mudstone, a darker red (lower value and less yellow hue) than the interbedded stream channel sandstone of pale red and pale yellowish brown colors. Intrastratal solution of Fe-silicate grains in the fluvial and lacustrine sandstones produced an average of about 3 percent by volume of hematite cement. Pyroxene, amphibole, epidote, chlorite, and biotite grains were consistently protected from post-depositional solution in the impermeable dolomite concretions and grayish red mudstone of the flood-plain deposits and also in lacustrine gray mudstone. The limonite surface stains on detrital particles in the lakes were removed in the reducing, organic-rich bottom w ter so that the impermeable lacustrine gray mudstone and black shale are not reddened by aging of limonite to hematite nor by intrastratal solution.