David A. Osleger

Relation of eustasy to stacking patterns of meter-scale carbonate cycles, Late Cambrian, U.S.A.

ABSTRACT An interbasinal study of Late Cambrian cyclic carbonate successions in the Appalachian and Cordilleran passive margins suggests that superimposed orders of eustasy controlled the development of large-scale depositional sequences and the component peritidal to subtidal meter-scale cycles that comprise them. The focus of this paper is on the small-scale cyclicity, its probable control by Milankovitch-forced sea-level oscillations, and how stacking patterns of meter-scale cycles can be used to define internal components of larger-scale sequences and estimate variations in relative sea level. Fining-upward peritidal cycles showing evidence of episodic emergence grade seaward into coarsening-upward subtidal cycles which lack evidence of emergence and form a continuum across the Cambrian carbonate platforms. Eustasy appears to exert the dominant control on the simultaneous development of peritidal and subtidal cycles on Late Cambrian carbonate platforms. Evidence for Milankovitch forcing of glacio-eustatic sea-level oscillations is provided by a 4:1 bundling of fifth-order meter-scale cycles ( 96 ky) within fourth-order cycles spanning tens of meters ( 440 ky) within the Big Horse Member of the Orr Formation in the House Range of Utah. The 4:1 bundling may manifest the short e centricity to long eccentricity ratio of the Milankovitch astronomical rhythms. Systematic changes in the stacking patterns of meter-scale cycles can be used in conjunction with Fischer plots to define long-term sea-level cycles. On Fischer plots of peritidal cyclic successions, long-term relative sea-level rises are characterized by thick, subtidal-dominated cycles with thin laminite caps. Long-term relative sea-level falls are defined by stacks of thin, laminite-dominated cycles that show brecciated cycle caps and quartz sands toward the relative sea level lowstand. On Fischer plots of dominantly subtidal cyclic successions, long-term sea-level rise is characterized by storm-dominated, open marine carbonate cycles or thick, deep ramp, shale-based cycles. Falling segments of the Fischer plot are characterized by thin, shallow subtidal cycles compos d of restricted lithofacies. Cycle stacking patterns (parasequence sets) provide the crucial link between the meter-scale cycles (parasequences) and the larger scale sequences and their component systems tracts. One- and two-dimensional models of peritidal and subtidal cycle development indicate that, whereas peritidal cycle thickness is primarily controlled by accommodation space, deeper subtidal cycle thickness is primarily controlled by sedimentation rate. Lithofacies within peritidal cycles reflect the sedimentologic response to fluctuations in sea level; lithofacies within subtidal cycles may be related to fluctuations in the zones of fairweather and storm-wave reworking that oscillated in harmony with sea-level fluctuations and may have acted as a subtidal limit to upward aggradation. The 2-D modelling illustrates how stacked peritidal to deep subtidal carbonate cycles with thicknesses, compositions and stacking patterns similar to the Late Cambrian of North America can be generated by ilankovitch-driven composite eustasy.

Dynamic carboniferous climate change, Arrow Canyon, Nevada.

The Phanerozoic9s longest-lived and most widespread glaciation, the late Paleozoic ice age, is undergoing a resurgence in interest. Long-held models of the timing, duration, and magnitude of glaciation are being reevaluated due to emerging evidence from former high latitudes, evidence that the late Paleozoic ice age was punctuated by long-lived glacial minima or possibly ice-free times. The history of the late Paleozoic ice age is archived within the biostratigraphically well-constrained, carbonate-dominated succession of Arrow Canyon, Nevada, United States. In this paleo-tropical succession, the distribution of lithofacies, flooding surfaces, and subaerial exposure horizons and their stacking into meter-scale cycles record a detailed climate history. The onset of this phase of glaciation during the middle Mississippian was followed by a dynamic evolution of glacioeustasy through the late Mississippian to late Pennsylvanian. Moderate- to high-amplitude glacioeustasy was likely interrupted by an earliest Pennsylvanian short-lived glacial minimum, but otherwise appears to have persisted through the middle Pennsylvanian. Upper Pennsylvanian strata record low- to moderate-amplitude relative sea-level changes, suggesting a long-lived interval of diminished ice volume. This proposed glacial minimum is coincident with a notable minimum in glaciogenic sedimentation near the former southern pole, aridification across paleo-tropical Pangea, and significant floral and faunal turnover, suggesting a link between tropical environmental change and high-latitude glaciation. These conclusions, however, are at odds with those traditionally inferred from Euramerican cyclothems, i.e., persistent high-amplitude glacioeustasy driven by a single, large ice sheet. Rather, the Arrow Canyon archive of varying depositional facies and cycle stacking patterns records major changes in the magnitude of short-term glacioeustasy. This finding contributes to recent and growing near- and far-field evidence for a more dynamic glaciation history than previously inferred from the classic Euramerican cyclothems.

Evolution of Cupido and Coahuila carbonate platforms, Early Cretaceous, northeastern Mexico

The Cupido and Coahuila platforms of northeastern Mexico are part of the extensive carbonate platform system that rimmed the ancestral Gulf of Mexico during Barremian to Albian time. Exposures of Cupido and Coahuila lithofacies in several mountain ranges spanning an ~80 000 km 2 area reveal information about platform morphology and composition, paleoenvironmental relations, and the chronology of platform evolution. New biostratigraphic data, integrated with carbon and strontium isotope stratigraphy, significantly improve chronostratigraphic relations across the region. These data substantially change previous age assignments of several formations and force a revision of the longstanding stratigraphy in the region. The revised stratigraphy and enhanced time control, combined with regional facies associations, allow the construction of cross sections, isopach maps, and timeslice paleogeographic maps that collectively document platform morphology and evolution. The orientation of the Cupido (BarremianAptian) shelf margin was controlled by the emergent Coahuila basement block to the northwest. The south-facing margin is a highenergy grainstone shoal, whereas the margin facing the ancestral Gulf of Mexico to the east is a discontinuous rudist-coral reef. A broad shelf lagoon developed in the lee of the Cupido margin, where as much as 660 m of cyclic peritidal deposits accumulated. During middle to late Aptian time, a major phase of flooding forced a retrograde backstep of the Cupido platform, shifting the locus of shallow-marine sedimentation northwestward toward the Coahuila block. This diachronous flooding event records both the demise of the Cupido shelf and the consequent initiation of the Coahuila ramp. The backstepped Coahuila ramp (AptianAlbian) consisted of a shallow shoal margin separating an interior evaporitic lagoon from a low-energy, muddy deep ramp. More than 500 m of cyclic carbonates and evaporites accumulated in the evaporitic lagoon during early to middle Albian time. Restriction of the platform interior dissipated by middle to late Albian time with the deposition of peloidal, miliolid-rich packstones and grainstones of the Aurora Formation. The Coahuila platform was drowned during latest Albian to early Cenomanian time, and the deep-water laminites of the Cuesta del Cura Formation were deposited. This study fills in a substantial gap in the Cretaceous paleogeography of the eastern Gulf of Mexico coast, improving regional correlations with adjacent hydrocarbon-rich platforms. The enhanced temporal relations and chronology of events recorded in the Cupido and Coahuila platforms significantly improve global correlations with coeval, economically important platforms worldwide, perhaps con

THE LEGACY OF MERCURY CYCLING FROM MINING SOURCES IN AN AQUATIC ECOSYSTEM: FROM ORE TO ORGANISM

Clear Lake is the site of an abandoned mercury (Hg) mine (active intermittently from 1873 to 1957), now a U.S. Environmental Protection Agency Superfund Site. Mining activities, including bulldozing waste rock and tailings into the lake, resulted in approximately 100 Mg of Hg entering the lakes ecosystem. This series of papers represents the culmination of approximately 15 years of Hg-related studies on this ecosystem, following Hg from the ore body to the highest trophic levels. A series of physical, chemical, biological, and limnological studies elucidate how ongoing Hg loading to the lake is influenced by acid mine drainage and how wind-driven currents and baroclinic circulation patterns redistribute Hg throughout the lake. Methylmercury (MeHg) production in this system is controlled by both sulfate-reducing bacteria as well as newly identified iron-reducing bacteria. Sediment cores (dated with dichlorodiphenyldichlorethane [DDD], 210pb, and 14C) to approximately 250 cm depth (representing up to approximately 3000 years before present) elucidate a record of total Hg (TotHg) loading to the lake from natural sources and mining and demonstrate how MeHg remains stable at depth within the sediment column for decades to millenia. Core data also identify other stresses that have influenced the Clear Lake Basin especially over the past 150 years. Although Clear Lake is one of the most Hg-contaminated lakes in the world, biota do not exhibit MeHg concentrations as high as would be predicted based on the gross level of Hg loading. We compare Clear Lakes TotHg and MeHg concentrations with other sites worldwide and suggest several hypotheses to explain why this discrepancy exists. Based on our data, together with state and federal water and sediment quality criteria, we predict potential resulting environmental and human health effects and provide data that can assist remediation efforts.

ANTHROPOGENIC STRESSORS AND CHANGES IN THE CLEAR LAKE ECOSYSTEM AS RECORDED IN SEDIMENT CORES

Sediment cores were collected to investigate multiple stresses on Clear Lake, California, USA, through the period of European occupation to the present day. Earlier workers suggested the hypothesis that the use of mechanized earthmoving equipment, starting in the 1920s and 1930s, was responsible for erosion, mercury (Hg) contamination, and habitat loss stresses. Cores (approximately 2.5 m in depth) were collected in 1996 and 2000 from each of the three arms of the lake. Carbon-14 dating suggests that these cores represent as much as 3000 years of the lakes history, beginning long before European settlement. Total mercury (TotHg) and methylmercury (MeHg), dry matter, water, carbon, nitrogen, phosphorus, sulfur, and the stable isotopes 13C and 15N were measured at 5-cm intervals. Nearly all parameters show major changes at depths of 58-135 cm, beginning at ca. 1927 (dated with 210Pb). Accepting this date for concomitant major changes in seven cores yields an estimated 8.6 mm/yr average sedimentation rate after 1927. Pre-1927 sedimentation rates were approximately 1 mm/yr. Total mercury and MeHg, dry matter, phosphorus, and 15N increase significantly, whereas nitrogen, sulfur, carbon, and water content decrease significantly above the 1927 horizon. Both TotHg and MeHg show extremely large increases (roughly 10-fold) above the 1927 horizon. A peak in inorganic deposition rate and minimum values for percentage of water is present at depths corresponding to ca. 1970. Interestingly, the first 75 years of European settlement in the Clear Lake basin (including the most productive years of the Sulphur Bank Mercury Mine) appeared to have had undetectable effects on lake cores. Changes since 1927 were dramatic. The large increase in Hg beginning about 1927 corresponds to the use of heavy equipment to exploit the ore deposit at the mine using open-pit methods. Increases in sediment deposition from increased earthmoving in the basin and sulfate loading from the mine are the most likely explanations for the dramatic changes seen in the post-1927 sections of the cores.

CLEAR LAKE SEDIMENTS: ANTHROPOGENIC CHANGES IN PHYSICAL SEDIMENTOLOGY AND MAGNETIC RESPONSE

We analyzed the sedimentological characteristics and magnetic properties of cores from the three basins of Clear Lake, California, USA, to assess the depositional response to a series of land use changes that occurred in the watershed over the 20th century. Results indicate that distinct and abrupt shifts in particle size, magnetic concentration/mineralogy, and redox conditions occur concurrently with a variety of ecological and chemical changes in lake bed sediments. This coincidence of events occurred around 1927, a datum determined by an abrupt increase in total mercury (Hg) in Clear Lake cores and the known initiation of open-pit Hg mining at the Sulphur Bank Mercury Mine, confirmed by 210Pb dating. Ages below the 1927 horizon were determined by accelerator mass spectrometry on 14C of coarse organic debris. Calculated sedimentation rates below the 1927 datum are approximately 1 mm/yr, whereas rates from 1927 to 2000 are up to an order of magnitude higher, with averages of approximately 3.5-19 mm/yr. In both the Oaks and Upper Arms, the post-1927 co-occurrence of abrupt shifts in magnetic signatures with color differences indicative of changing redox conditions is interpreted to reflect a more oxygenated diagenetic regime and rapid burial of sediment below the depth of sulfate diffusion. Post-1927 in the Oaks Arm, grain size exhibits a gradual coarsening-upward pattern that we attribute to the input of mechanically deposited waste rock related to open-pit mining activities at the mine. In contrast, grain size in the Upper Arm exhibits a gradational fining-upward after 1927 that we interpret as human-induced erosion of fine-grained soils and chemically weathered rocks of the Franciscan Assemblage by heavy earthmoving equipment associated with a road- and home-building boom, exacerbated by stream channel mining and wetlands destruction. The flux of fine-grained sediment into the Upper Arm increased the nutrient load to the lake, and that in turn catalyzed profuse cyanobacterial blooms through the 20th century. The resulting organic biomass, in combination with the increased inorganic sediment supply, contributed to the abrupt increase in sedimentation rate after 1927.

Meteoric diagenesis and fluid-rock interaction in the Middle Permian Capitan backreef: Yates Formation, Slaughter Canyon, New Mexico

This study examines the geochemical record of diagenesis in the Middle Permian Yates shelf, exposed in Slaughter Canyon, New Mexico. This diagenetic history significantly modified lithologies, depositional fabrics, and pore systems. Early diagenesis was dominated during sea level highstands by marine cementation and reflux dolomitization, and during sea level lowstands by meteoric cementation and stabilization—the focus of this study. This early diagenesis variably overprinted primary marine isotopic signatures, potentially leading to erroneous chemostratigraphic correlations or paleoclimate reconstructions. Four correlative sections through one m-scale cycle were analyzed for their and values. They show significant (2–4‰) and variability in coeval, texturally well-preserved calcites. The and values of marine cements, brachiopods, bulk carbonate, micritic matrix, and the first generation of meteoric spar (from high to low values) delineate an “inverted J curve,” indicating the variable alteration of components by diagenetic fluids. Numerical models indicate that the observed stable isotope trend is most consistent with diagenetic alteration in a partially closed system by meteoric fluids mixed with a progressively diminishing contribution of recycled marine waters. In the Yates shelf, marine cements provide a more robust primary isotopic record than micritic matrix; however, neither preserves primary seawater isotopic values. Furthermore, common criteria used to diagenetically screen samples proved inadequate (e.g., textural preservation, staining, luminescence, depletion near sequence boundaries). Instead, diagenetic resetting is resolved by analyzing multiple, closely spaced, independently correlated sections, and by delineating trends between primary and later diagenetic components in populations of isotopic data.

Modified Fischer Plots as Graphical Tools for Evaluating Thickness Patterns in Stratigraphic Successions

Fischer plots are a simple, objective graphical tool for illustrating thickness trends in cyclic successions and are particularly amenable for use in stratigraphy courses. Their utility in stratigraphic analysis is making its way into textbooks, and the large body of literature discussing the strengths and limitations of Fischer plots provides fertile ground for discussions about cyclic successions and concepts such as time-versus-thickness in undergraduate and graduate classes. In an effort to enhance the utility of Fischer plots and to reduce some of their inherent limitations, we suggest two modifications to the fundamental technique. First, we suggest that the cumulative departure from mean cycle thickness should be plotted against the individual measured cycle thicknesses rather than the unit-mean cycle thickness. This time-independent modification has the advantage of being directly comparable to measured thicknesses of the stratigraphic columns from which they were derived and also may, in some cases, enhance the sharpness of the waveform. The second suggested correction to the traditional Fischer plot is to decompact individual lithofacies prior to plotting. A correction for compaction is especially useful in successions that consist of heterogeneous facies that exhibit clear differences in compaction through time. In certain cases, decompacted Fischer plots reveal higher magnitude changes in thickness patterns that may not otherwise be recognizable in traditional or modified Fischer plots. In the spirit of “the only way to understand it is to do it,” we provide a simple LISP subroutine for automatically drawing each type of Fischer plot under any Autocad environment.

Depositional Sequences on Upper Cambrian Carbonate Platforms: Variable Sedimentologic Responses to Allogenic Forcing

Six third-order depositional sequences are documented for Late Cambrian time by interbasinal correlation of cyclic carbonates from tectonic settings in the Appalachian and Cordilleran passive margins, the Texas cratonic embayment, and the southern Oklahoma aulacogen. Paleobathymetric interpretation, integrated with graphic correlation, is used to establish the relative synchroneity of Upper Cambrian depositional sequences and is crosschecked with two quantitative techniques that provide an approximation of the accommodation history independent of fluctuations in carbonate sediment production. The first technique, Fischer plots, graphically illustrates systematic changes in the stacking patterns of meter-scale cycles that presumably reflect third-order changes in accommodation potential. The second technique, subsidence analysis, determines the accommodation remaining after the isostatic and thermo-tectonic components of total decompacted subsidence have been removed. Integrating the three methods enhances the interbasinal correlation of individual third-order depositional sequences and permits the construction of a robust relative sea-level curve for the Upper Cambrian of North America.