Maya Elrick

Chuar Group of the Grand Canyon: record of breakup of Rodinia, associated change in the global carbon cycle, and ecosystem expansion by 740 Ma

The Chuar Group (approximately 1600 m thick) preserves a record of extensional tectonism, ocean-chemistry fluctuations, and biological diversification during the late Neoproterozoic Era. An ash layer from the top of the section has a U-Pb zircon age of 742 +/- 6 Ma. The Chuar Group was deposited at low latitudes during extension on the north-trending Butte fault system and is inferred to record rifting during the breakup of Rodinia. Shallow-marine deposition is documented by tide- and wave-generated sedimentary structures, facies associations, and fossils. C isotopes in organic carbon show large stratigraphic variations, apparently recording incipient stages of the marked C isotopic fluctuations that characterize later Neoproterozoic time. Upper Chuar rocks preserve a rich biota that includes not only cyanobacteria and algae, but also heterotrophic protists that document increased food web complexity in Neoproterozoic ecosystems. The Chuar Group thus provides a well-dated, high-resolution record of early events in the sequence of linked tectonic, biogeochemical, environmental, and biological changes that collectively ushered in the Phanerozoic Eon.

Cyclic Ramp-to-Basin Carbonate Deposits, Lower Mississippian, Wyoming and Montana: A Combined Field and Computer Modeling Study

ABSTRACT The Lower Mississippian Lodgepole/lower Madison Formations (20-225 m thick) developed along a broad (> 700 km), storm-dominated pericratonic ramp. Three types of fifth-order upward-shallowing cycles are recognized across the ramp-to-basin transition. Peritidal cycles consist of very shallow subtidal facies overlain by algal-laminated tidal flat deposits, which are rarely capped by paleosol/breccia layers. Shallow subtidal cycles consist of stacked ooid grainstone shoal deposits, or deeper subtidal facies overlain by ooid-skeletal grainstone caps. Deep subtidal cycles located along the outer ramp consist of basal sub-storm wave base limestone-argillite, overlain by storm-deposited limestone, which are capped by hummocky stratified to massive skeletal-ooid grainstone. Deep subtidal c cles pass downslope into ramp-slope facies composed to rhythmically interbedded limestone and argillite, with local deep-water mud mounds; no upward-shallowing cycles occur within the ramp-slope facies. Average cycle durations calculated along the outer ramp are between 30-110 ky. The fifth-order cycles are stacked to form three third- to fourth-order depositional sequences, which are recognized by regional transgressive-regressive facies trends and cycles stacking patterns. Ramp-margin wedges (RMW) developed during long-term sea-level fall and lowstand and consist of cyclic crinoidal bank and oolitic shoal facies, which pass downdip into deep subtidal cycles. Transgressive systems tracts (TST), which onlapped the ramp during long-term sea-level rise, include thick shallow and deep subtidal cycles; peritidal cycles are restricted to the inner ramp. Highstand systems tracts (HST) developed during long-term sea-level highstand and fall, and along the ramp are composed of early HST shallow subtidal cycles overlain by late HST peritidal cycles; shallow through deep subtidal cycles characterize the HST along the ramp-slope. Two-dimensional computer modeling of the cyclic sequences suggests that for the assumed water depths of facies, fifth-order sea-level oscillations of at least 20-25 m were required to generate deep subtidal cycles along the ramp-slope. Synthetic sequences run with fifth-order sea-level amplitudes End_Page 1194----------------------- and TST deposition, a feature not observed in the actual sequences. Other factors, in addition to fifth-order sea-level oscillations, likely played a role in generating synchronous peritidal and deep subtidal cycles during HST deposition. These factors may include short-term climatic changes, which influenced the depths to storm-wave reworking. The moderate-amplitude sea-level oscillations suggested by the cyclic sequences may reflect the initial effects of Carboniferous glaciation that was occurring in Gondwana.

Cyclostratigraphy of Middle Devonian Carbonates of the Eastern Great Basin

ABSTRACT Middle Devonian carbonates (250-430 m thick) of the eastern Great Basin were deposited along a low energy, westward-thickening, distally steepened ramp. Four third-order sequences can be correlated across the ramp-to-basin transition and are composed of meter-scale, upward-shallowing carbonate cycles (or parasequences). Peritidal cycles (shallow subtidal facies capped by tidal-flat laminites) constitute 90% of all measured cycles and are present across the entire ramp. The peritidal cycles are regressive- and transgressive-prone (upward-deepening followed by upward-shallowing facies trends). Approximately 80% of the peritidal cycle caps show evidence of prolonged subaerial exposure including sediment-filled dissolution cavities, horizontal to vertical desiccation cracks, rubble and ka st breccias, and pedogenic alteration; locally these features are present down to 2 m below the cycle caps. Subtidal cycles (capped by shallow subtidal facies) are present along the middle-outer ramp and ramp margin and indicate incomplete shallowing. Submerged subtidal cycles (64% of all subtidal cycles) are composed of deeper subtidal facies overlain by shallow subtidal facies. Exposed subtidal cycles are composed of deeper subtidal facies overlain by shallow subtidal facies that are capped by features indicative of prolonged subaerial exposure (dissolution cavities and brecciation). Average peritidal and subtidal cycle durations are between approximately 50 and 130 k.y. (fourth- to fifth-order). The combined evidence of abundant exposure-capped peritidal and subtidal cycles, transgressive-prone cycles, and subtidal cycles correlative with updip peritidal cycles indicates that the cycles formed in response to fourth-to fifth-order, glacio-eustatic sea-level oscillations. Sea-level oscillations of relatively low magnitude (< 10 m) are suggested by the abundance of peritidal cycles, the lack of widely varying, water-depth-dependent facies within individual cycles, and the presence of noncyclic stratigraphic intervals within intrashelf-basin, slope, and basin facies. Noncyclic intervals represent missed subtidal beats when the seafloor lay too deep to record the effects of the short-term sea-level oscillations. Exposure surfaces at the tops of peritidal and subtidal cycles represent one, or more likely several, missed sea-level oscillations when the platform lay above fluctuating sea level, but the amplitude of fourth- to fifth-order sea-level oscillation(s) were not high enough to flood the ramp. The large number of missed beats (exposure-capped cycles), specifically in Sequences 2 and 4, results in Fischer plots that show poorly developed rising and falling limbs (subdued wave-like patterns); consequently the Fischer plots are of limited use as a correlation tool for these particular depositional sequences. The abundance of missed beats also explains why Milankovitch-type cycle ratios ( 5:1 or 4:1) are not observed and why such ratios would not be expected along many peritidal-cycle-dominated carbonate platforms.

High-resolution δ13C stratigraphy of the Chuar Group (ca. 770–742 Ma), Grand Canyon: Implications for mid-Neoproterozoic climate change

A high-resolution C-isotope record based on δ 13 C org from organic-rich shales and δ 13 C carb from dolomites in the ca. 770–742 Ma Chuar Group provides important new data for evaluating the signifi cance of large-magnitude C-isotope anomalies in Neoproterozoic climate change. Three successive, large-magnitude isotopic excursions (8–15‰) are interpreted to represent primary seawater values based on a series of diagenetic tests, and they are not associated with evidence of signifi cant long-term (10 6 –10 7 m.y.) sea-level change nor glaciomarine deposits. Intrabasinal correlation of δ 13 C org values suggests that most Chuar shales record primary values and is consistent with previously reported H/C ratios of >0.49 indicating that Chuar shales experienced minimal thermal alteration. Although some Chuar dolomites reveal early diagenetic alteration, their δ 13 C dol values typically fall near those of coeval “least-altered” dolomites or organic-rich shales (relative to dolomite values). The Chuar carbon record is interpreted to refl ect predominantly primary organic carbon δ 13 C values and contains suf

Sequence stratigraphy and platform evolution of Lower-Middle Devonian carbonates, eastern Great Basin

Lower–Middle Devonian carbonates (270–400 m thick) of the eastern Great Basin were deposited along a low-energy, westward-thickening carbonate platform. Six regional facies representing peritidal, shallow subtidal, stromatoporoid biostrome, deep subtidal, slope, and basin environments are recognized. Four third-order (≈1.5–2.5 m.y. durations), transgressive-regressive sequences are identified across the platform-to-basin transition based on deepening and shallowing patterns in regional facies, intensity and stratigraphic distribution of subaerial exposure features, and stacking patterns of fourth- to fifth-order, upward-shallowing peritidal and subtidal cycles. Transgressive systems tracts along the basin/slope are characterized by upward-deepening successions of proximal through distal turbidites overlain by fine-grained, hemipelagic deposits. Shallow-platform transgressive systems tracts are composed of stacks of thicker-than-average peritidal cycles overlain by subtidal cycles or noncyclic deep subtidal facies. Maximum flooding zones along the shallow platform are composed of stacked peritidal cycles dominated by subtidal facies, noncyclic deep subtidal facies, or distinct deeper subtidal units within successions of restricted shallow subtidal or peritidal facies. Highstand systems tracts along the basin/slope are composed of hemipelagic deposits overlain by distal through proximal turbidites. Highstand systems tracts along the shallow platform are characterized by upward-shallowing succession of cyclic peritidal through shallow subtidal facies. Sequence boundary zones (2–16 m thick) along the shallow platform are composed of exposure-capped peritidal and subtidal cycles that exhibit upsection increases in the proportion of tidal-flat subfacies and increases in the intensity of cycle-capping subaerial exposure features. Sequence boundary zones along the basin/slope (6–20 m thick) are composed of upward-shallowing successions of proximal turbidites or by platform-margin peloid shoal deposits; the absence of exposure features and meter-scale cycles within basin/slope sequence boundary zones indicates that the combined rates of third- through fifth-order sea-level fall rates were less than tectonic subsidence rates. Sequence stratigraphic correlations between contrasting facies belts of the basin/slope (section NA) and the edge of the shallow platform (section TM) were independently verified with high-resolution conodont and brachiopod biostratigraphy. Correlation of sequences 1–4 with transgressive-regressive sequences of similar age in the western, midwestern, and eastern United States, western Canada, and Europe indicates they are eustatic in origin. Systems-tract scale correlations across the study area indicate that the platform evolved from a homoclinal ramp to a distally steepened ramp, then into a flat-topped platform (sequences 1–2). An incipiently drowned, intraplatform basin developed during sequence 3 as the result of third-order sea-level rise and differential sediment accumulation rates between the platform margin and intraplatform basin. During deposition of highstand systems tract 3, progradation infilled the intraplatform basin, resulting in a flat-topped platform. A distally steepened ramp developed during transgressive systems tract/maximum flooding zone 4 and evolved into a flat-topped platform during highstand systems tract 4 deposition. The four sequences stack in an aggradational to slightly progradational pattern (“keep-up” style sedimentation) and are bound by sequence boundary zones rather than unconformities, suggesting that greenhouse climate modes and second-order accommodation gains related to the lower portion of the second-order Kaskaskia sequence controlled sequence-scale stacking patterns.

Magnetic record of Milankovitch rhythms in lithologically noncyclic marine carbonates

Rock magnetic variations record cyclicity within lithologically homogeneous basinal lime mudstones of the Lower Cretaceous San Angel Limestone, northeastern Mexico. Variations in ferromagnetic mineral concentrations, as measured by anhysteretic remanent magnetization (ARM), occur at frequencies consistent with Milankovitch orbital rhythms. Magnetic mineral compositions, grain-size distributions, and grain shapes from digested samples are congruent with far-traveled atmospheric dust. Prevailing winds and the proximity of the Cretaceous basin to an African eolian source support the encoding of orbitally modulated changes in wind intensity or source-area aridity. ARM measurements offer great potential to calibrate the pace of depositional processes in carbonates and to investigate high-frequency orbitally driven climate change in basinal strata throughout geologic time.

Millennial-scale climate origins for stratification in Cambrian and Devonian deep-water rhythmites, western USA

Abstract Basinal facies of the Middle Cambrian Marjum Formation (western Utah) and Middle Devonian Denay Limestone (central Nevada) are characterized by thin, rhythmically interbedded limestone and marl layers (deep-water rhythmites). Limestone layers (average thickness range of 4.3–5.5 cm) are composed of laminated to massive pelleted lime mudstone; marl layers (average thickness range of 0.7–1.7 cm) are characterized by laminated, argillaceous, dolomitic mudstone. The absence of current- or wave-reworked features, paucity of bioturbation and skeletal fossils, stratigraphic relationships with adjacent facies, and the dark color indicate that limestone and marl layers in both rhythmite successions were deposited in quiet, dysaerobic waters below storm-wave base. Both limestone and marl layers are composed of submillimeter-thick graded laminae which represent discrete depositional events from dilute density currents generated by high-frequency storms or distal turbidity currents. Fluctuations in primary pelagic productivity cannot account for variations in carbonate influx because calcareous microfossils did not evolve until the Mesozoic. Instead, the rhythmic interbedding is interpreted to reflect climatically controlled variations in fluvial and/or eolian influx, or changes in marine currents (storm or distal turbidity currents) which transported shallow platform-derived carbonate material into the deep-water region. Spectral analysis of carbonate time series were compared to lithologic rank series and, particularly for the Middle Cambrian rhythmites, were found to be statistically indistinguishable. The durations of significant spectral peaks were estimated from calculating the average sedimentation rates from the two biostratigraphically controlled stratigraphic sections; these rates range between 4.3 ± 4.1 cm/k.y. and 21.2 ± 29.2 cm/k.y. for the Cambrian, and 3.6 ± 1.8 cm/k.y. to 5.1 ± 7.4 cm/k.y. for the Middle Devonian. Application of these sedimentation rates to the Marjum couplets suggests that individual couplets represent between ∼190–2100 years of time. The Denay couplets represent ∼800–1900 years. The good agreement between empirical Holocene hemipelagic sedimentation rates and the sedimentologic evidence of relatively high depositional rates for the Paleozoic rhythmites supports this millennial-scale interpretation. These results, combined with that from previous work in Paleozoic evaporite and carbonate successions, suggest that millennial-scale paleoclimatic variations affected marine and marginal-marine sedimentation as far back in time as the Cambrian. Millennial-scale climatic change appears to be a permanent feature affecting the oceans and atmosphere, and is apparently largely unaffected by major changes in Phanerozoic paleogeography, tectonics and atmospheric composition.

Orbital-scale climate change and glacioeustasy during the early Late Ordovician (pre-Hirnantian) determined from δ18O values in marine apatite

This study focuses on the ∼10 m.y. before the latest Ordovician (Hirnantian) glaciation; we test whether orbital-scale climatic fluctuations controlled the growth and melting of continental glaciers, resulting in glacioeustatic sea-level changes and the development of widespread marine sedimentary cycles. δ18O values of conodont apatite from 14 Late Ordovician (Katian) cycles range from ∼17‰ to 21‰. Isotopic values decrease and are lowest in the deepest water facies and increase and are highest in shallow-water facies, supporting the hypothesis that glacioeustasy was the dominant control on water-depth changes. Measured intracycle δ18O changes of 0.7‰–2.5‰ were controlled by changes in ice volume (<60 m sea-level changes), sea-surface temperatures (<5 °C), and potentially local increases in seawater evaporation during drier and/or windier glacial stages. These interpreted orbital-scale climate changes and resultant large glacial ice-volume changes support recent interpretations of a dynamic and prolonged Ordovician greenhouse to icehouse transition.

Global-ocean redox variation during the middle-late Permian through Early Triassic based on uranium isotope and Th/U trends of marine carbonates

Uranium isotopes (238U/235U) in carbonates, a proxy for global-ocean redox conditions owing to their redox sensitivity and long residence time in seawater, exhibit substantial variability in the Daxiakou section of south China from the upper-middle Permian through the mid-lower Triassic (∼9 m.y.). Middle and late Permian ocean redox conditions were similar to that of the modern ocean and were characterized by improving oxygenation in the ∼2 m.y. prior to the latest Permian mass extinction (LPME), countering earlier interpretations of sustained or gradually expanding anoxia during this interval. The LPME coincided with an abrupt negative shift of >0.5‰ in δ238U that signifies a rapid expansion of oceanic anoxia. Intensely anoxic conditions persisted for at least ∼700 k.y. (Griesbachian), lessening somewhat during the Dienerian. Th/U concentration ratios vary inversely with δ238U during the Early Triassic, with higher ratios reflecting reduced U concentrations in global seawater as a consequence of large-scale removal to anoxic facies. Modeling suggests that 70%–100% of marine U was removed to anoxic sinks during the Early Triassic, resulting in seawater U concentrations of <5% that of the modern ocean. Rapid intensification of anoxia concurrent with the LPME implies that ocean redox changes played an important role in the largest mass extinction event in Earth history.

Short-term paleoclimatic fluctuations expressed in lower Mississippian ramp-slope deposits, southwestern Montana

Lower Mississippian ramp-slope deposits (Paine Member) of southwestern Montana are composed of thin, rhythmically interbedded limestone and argillaceous limestone (argillite). Millimeter-thick graded layers typical of limestone beds represent distal storm deposits, whereas argillite layers containing abundant whole, delicate fossils represent quiet-water deposition during times of little or no storm activity. Spectral analyses of the fluctuating insoluble-residue content (quartz, muscovite-illite, organic matter) indicate a dominant periodicity of 0.6-2.85 ka in the ramp-slope deposits; no spectral peaks corresponding to typical Milankovitch-type periods ({approximately}20-100 ka) were observed. Similar {approximately}2.5 ka paleoclimatic periodicities are recorded in Quaternary continental and alpine glaciers, Quaternary deep-sea sediments, C variations in Holocene tree rings, and Permian deep-water evaporite varves. These short-term paleoclimatic fluctuations may represent one of several harmonics of the precessional (19-23 ka) or obliquity (41 ka) orbital cycles or may be related to variations in solar activity.