John D. Humphrey

Coastal mixing zone dolomite, forward modeling, and massive dolomitization of platform-margin carbonates

ABSTRACT Dolomite of mixed-water origin occurs in three late Pleistocene raised reef terraces in southeastern Barbados, West Indies. Dolomite concentrations range from trace quantities to locally complete, with a general increase in dolomite percentage with increasing terrace age. Most of the dolomite is replacive, both mimetic and fabric selective, with limpid pore-lining dolomite cements common in the oldest terrace. Stratigraphic relationships suggest that mixing zone dolomitization occurred locally during three temporally separate sea level events. The rapidity of Quaternary glacio-eustasy constrains dolomitization to have occurred within about 5,000 yr in each terrace. Rapid, recurring dolomitization resulted from development of similar hydrologic and hydrochemical conditions during each ea level event. Barbados data are used as conceptual input for a computer-based forward model which simulates the development of thick, platform-margin sections of mixing zone dolomite through the interaction of sea level fluctuations, sedimentation, rapid recurring dolomitization, and subsidence. Sensitivity tests of the model indicate that the most important parameters for producing thick sections of platform-margin dolomite are: (1) dolomitization rate, (2) sea level still stand duration, and (3) rate of sea level change. Dolomite thus formed may provide nuclei for later burial dolomitization or may provide an intrabasinal magnesium source for stepwise stabilization of platform interior dolomites.

New Geochemical Support for Mixing-Zone Dolomitization at Golden Grove, Barbados

Dolomite occurs in the Golden Grove reef terrace of late Pleistocene age in southeastern Barbados, where fore-reef lithologies have been partially to completely dolomitized. Interpretation of the possible modes of formation of this dolomite has been a subject of disagreement for over a decade. Among the points of contention are the timing of deposition, dolomitization and dolomite cement precipitation, and the relationship of replacement dolomite with dolomite cement. Data from this study indicate that dolomitization occurred during a single relative sea-level fall following reef development at marine oxygen isotope stage 7.3, at approximately 216 ka. Replacement dolomitization affected both micritic matrix and skeletal grains. During or after replacement dolomitization, dolomite and calcite cements filled primary and secondary pores. The cement paragenesis comprises early dolomite cement, complexly banded cements of alternating calcite and dolomite, late dolomite cement of near-stoichiometric composition, and a final phase of blocky calcite cement. Replacement dolomitization and cement precipitation represent a continuum of processes resulting from progressive modification of diagenetic fluids. Stable carbon and oxygen isotope and strontium elemental compositions of replacive dolomite, dolomite cements, and calcite cements indicate that dolomitization occurred in a mixing zone between marine and meteoric groundwaters occupying the reef terrace shortly after deposition. Dolomite oxygen isotope values range from +0.18 to +3.85‰ PDB, and become progressively more depleted throughout the paragenesis, indicating a greater influence of meteoric water through time. Remarkably depleted carbon isotope compositions, for both replacement dolomite and dolomite cement (−9.53 to −23.04‰ PDB), originated from oxidation of thermogenic methane advected from the underlying accretionary complex. Methane was oxidized in marine waters that subsequently mixed with meteoric waters, as evidenced by a progressive enrichment in carbon isotope composition throughout the paragenesis. Dolomite strontium contents are substantially elevated above those expected for marine dolomitization, ranging from 640 to 1650 ppm, and increasing throughout the paragenesis. Meteoric water provided the excess strontium, as aragonite-to-calcite transformation in the exposed reef terrace increased the fluid Sr/Ca ratio. The continuum of geochemical compositions between the replacement dolomite and dolomite cement argues for their genetic association. These new data provide additional support to the mixing-zone interpretation for Golden Grove, and that the model of mixing-zone dolomitization remains viable.

Lithofacies, stable isotopic composition, and stratigraphic evolution of microbial and associated carbonates, Green River Formation (Eocene), Piceance Basin, Colorado

Lacustrine carbonates of the Eocene Green River Formation crop out on the western margin of the Piceance Basin and the eastern margin of the Uinta Basin, in western Colorado. This area allows tracing of vertical and horizontal facies variation over hundreds of meters. Limestone beds consist of littoral to sublittoral lithofacies: bioclastic and oolitic grainstones, oolitic wackestone, intraclastic rudstone, stromatolites, and thrombolites. Facies form upward-deepening cycles that start with sharp-based grainstones and packstones followed by stromatolites or thrombolites and capped by fine-grained stromatolites and/or oil shale deposits. The vertical succession of carbonate deposits correlates with evolutionary lake stages. The succession starts with grainstone deposits rich in ostracods and gastropods that correspond to an initial freshwater lake. Thrombolites capped by laminated stromatolites or coarse-agglutinated stromatolites correlate with a higher-salinity transitional lake. Deepening-upward cycles, as much as 5 m (16 ft) thick, of thrombolites, agglutinated stromatolites, and fine-grained stromatolites occur in the highly fluctuating lake. The upper section is dominated by laminated stromatolites that correspond to a rising lake. Stable isotope 18O and 13C values covary and range from 8 to +0.8 and 3 to +5, respectively. The 18O values indicate carbonate-precipitating water evolved from fresh to saline and became less saline in the upper Green River. Negative excursions of 13C values correspond to lake level rises, and positive excursions of 13C values occur during lake level falls. Syndepositional to burial diagenesis modified carbonate porosity. Early dissolution is followed by burial compaction and fracturing. Compaction and late calcite cements occluded primary and secondary porosity.

FORMATION OF DOLOMITE IN RECENT ISLAND-ARC SEDIMENTS DUE TO GAS-SEAWATER- SEDIMENT INTERACTION

Dolomite occurs as a primary precipitate that cements Re- cent volcaniclastic sands in the shallow-marine environment of Luise Harbor, Lihir Island, Papua New Guinea. Field observations and geo- chemical data suggest that dolomite precipitates through complex min- eral reactions in response to discharge of gas ( . 90% CO2) and heated seawater from hydrothermal vents in the seafloor. Gas discharge caus- es the formation of shallow circulation cells, and cold seawater is drawn into the sediment, where it is heated to approximately 100 8C. Reaction between hydrothermal CO 2 (g), primary sediment grains, and heated seawater causes the precipitation of primary dolomite. Once temperatures are above 70 8C the Mg/Ca ratio in seawater increases because CaSO 4 precipitates, while at the same time the dissolution of iron oxide minerals increases the pH; both reactions favor dolomite precipitation. Ferrous iron reacts with H 2S to form co-precipitated Fe- sulfide minerals. The dolomite is nearly stoichiometric and occurs as zoned inclusion-rich and inclusion-poor spherulitic cements, many of which are present as coalesced composite spherules. Oxygen isotope values for dolomite of 28.8 to 210.2‰ VPDB constrain the temper- ature of precipitation to be between about 77 to 1108C. Strontium iso- tope ratios for dolomite (average 0.70886) are close to those of modern seawater; their slightly less radiogenic compositions implicate minor addition of magmatic strontium. Hydrothermal dolomite may develop in shallow, near-surface sediments in active tectonic settings.

Lower-latitude mammals as year-round residents in Eocene Arctic forests

The Arctic is undergoing rapid warming, but the impact on the biosphere, in particular on large terrestrial mammals, is not clear. Among the best deep time laboratories to assess biotic impacts of Arctic climate change, early Eocene (ca. 53 Ma ago) fossil assemblages on Ellesmere Island, Nunavut (~79°N), preserve evidence of forests inhabited by alligators, tortoises, and a diverse mammalian fauna most similar to coeval lower-latitude faunas in western North America. By analyzing carbon and oxygen isotope ratios of mammalian tooth enamel, we show that large herbivores were year-round inhabitants in the Arctic, a probable prerequisite to dispersal across northern high-latitude land bridges. If present-day warming continues, year-round occupation of the Arctic by lower-latitude plants and animals is predicted.

Geochemistry and Crystal Morphology of Aragonite Cements of Mixing-zone Origin, Barbados, West Indies

ABSTRACT Isopachous aragonite ray cement (ARC) lines large rugs (> 5 cm) through a 4 m interval recovered in core samples from a borehole drilled along the southeast coast of Barbados. The vuggy/cement interval is 12-16 m below core top, and most of the interval is below the modern water table (13 m). The top of the interval corresponds to the top of the 194 ka coral reef terrace, which is unconformably overlain by the 122 ka terrace. ARC was precipitated after a period of meteoric diagenesis during which vuggy porosity was produced. ARC shows variable ultrastructures, including (1) loosely packed, randomly oriented rays ( The 100% ARC has uncharacteristically low 13C values relative to predicted and reported values for marine aragonite cement. Likewise, the 18O values are lower than both reported and most predicted O18O values for marine aragonite. Stable-isotope compositions of this cement fall along a mixing line between aragonite equilibrium compositions calculated for meteoric and marine end-member fluids. Strontium concentrations are greater than in most known marine aragonite; this suggests precipitation from fluids with an elevated Sr/Ca ratio. Such fluids are found in modern Barbados mixed waters whose elevated Sr/Ca ratios result from updip meteoric dis olution of aragonite. Magnesium concentrations are slightly higher and more variable than in reported Pleistocene marine aragonite. Sodium concentrations are lower than expected marine values and covary with changes in 13C and 18O values. Isotope and minor-element geochemistry of these cements suggests precipitation from mixed meteoric-marine waters. Fluid mixing models for modern Barbados waters suggest precipitation from mixed fluids ranging from 50% to 75% seawater. Precipitation of aragonite from mixed waters may be facilitated by degassing of CO2-charged mixed waters within large vugs and by Mg/Ca ratios sufficiently elevated to inhibit precipitation of calcite.

Stable isotopic evidence for relative and eustatic sea-level changes in Eocene to Oligocene carbonates, Baldwin County, Alabama

Eocene to Oligocene marine carbonate shelf deposits in a continuous core in Baldwin County, Alabama, were evaluated within a sequence stratigraphic framework to determine characteristic stable isotopic responses to inferred fluctuations in sea level. Due to a greater sampling density per unit of time, stable isotopes in this core reveal more defined trends than have previously been reported from DSDP/ODP (Deep Sea Drilling Project/Ocean Drilling Project) records and from more land-ward sections that have undergone intense sub-aerial alteration or erosion. The δ 18 O signature records eustatic sea level. δ 18 O becomes heavier as eustatic sea level falls and lighter as eustatic sea level rises. δ 18 O records a secular shift to heavy from the early Eocene to Oligocene of 3.0‰. The δ 13 C signature records relative sea-level change (paleobathymetry). δ 13 C generally becomes heavier in shallower water deposits and lighter in deeper water deposits. However, in planktonic oozes, δ 13 C may shift to heavy. δ 13 C also records a secular shift to heavy from the early Eocene to the Oligocene of 3.5‰ Although minor diagenetic perturbations in the stable isotopes are present, the depositional signal dominates the isotopic signal. Spar cements generally result in lighter δ 18 O and δ 13 C. If the dolomite is >20% dolomitization results in heavier δ 18 O and δ 13 C.

Sedimentology and Sequence Stratigraphy of Upper Pleistocene Carbonates of Southeastern Barbados, West Indies (1)

Upper Pleistocene reef-associated carbonates of southeastern Barbados have been studied in outcrop and core. Reef terraces, formed during glacio-eustatic sea level highstands and subsequently uplifted, are characterized by thick and areally extensive sequences of allochthonous and autochthonous fore-reef calcarenites. Depositional textures are primarily packstones, and grainstones, wackestones, and coral floatstones are volumetrically less significant. Sediments are coarse- to fine-grained reef-derived allochems and micrite, and autochthonous benthic foraminifera and coralline red algae. Rates of sediment accumulation of fore-reef calcarenites range from about 1 to 4 m/1000 yr. Although of relatively small scale, the carbonate terraces of southeastern Barbados provide excellent analogs for sequence stratigraphic concepts in carbonate settings. The terraces are primarily highstand systems tract deposits separated by type 1 unconformities. These highstand deposits are characterized by reef development and the progradation of fore-reef calcarenites. Extensive fore-reef deposits resulted from mechanical erosion of the reef framework on this high-energy, windward coastline. Type 1 unconformities are characterized by thin caliche layers developed during lowstand subaerial exposure. Thin basal transgressive systems tract deposits are characterized by incorporation of extraformational clasts derived from the underlying sequence during sea level rise. Slope-front ero ion, vertical shift in the position of freshwater lens, and shift in the position of coastal onlap are all consequences of the interplay between eustasy and tectonics. These effects and the development of facies geometries on Barbados are primarily controlled by the glacio-eustatic component, inasmuch as rates of eustatic changes of sea level are at least two orders of magnitude greater than the maximum average rates of tectonic uplift.

Depositional environments and sequence stratigraphy of carbonate mudrocks using conventional geologic observations, multiscale electrofacies visualization, and geochemical analysis: The case of the Tuwaiq Mountain and Hanifa Formations in a basinal setting, Saudi Arabia

ABSTRACT Depositional interpretation and sequence stratigraphic analysis of carbonate mudrocks requires numerical analysis and data integration to achieve quantitative, predictive stratigraphic and geochemical models. A depositional and sequence stratigraphic analysis is built for a basinal interval of the Tuwaiq Mountain and Hanifa Formations, Saudi Arabia. Conventional geologic interpretation, automated electrofacies analysis, and geochemical interpretation are integrated using quantitative means. Cluster analysis of well logs using self-organizing maps and hierarchical clustering allowed for a multiscale electrofacies analysis. This is useful for identifying major lithological surfaces, which commonly correspond to sequence stratigraphic surfaces. Geochemical data were used for depositional environment interpretation, such as sediment provenance, redox, and paleoproductivity conditions. Factor analysis is used to group element data. Redox and paleoproductivity indices were calculated using electrofacies clustering of different elemental groups. Electrofacies analysis shows good correlation with the lithofacies; that is, lithofacies can be identified from logs. Five major lithofacies have been identified in the studied interval: (1) wispy laminated skeletal wackestones to mudstones; (2) differentially cemented skeletal wackestones, packstones, and grainstones; (3) laminated peloidal mudstones to wackestones; (4) bioturbated packstones and wackestones; and (5) palmate anhydrite. The majority of the interval is interpreted to be deposited by gravity processes. Four major sequences have been identified in the Tuwaiq Mountain and Hanifa Formations made of transgressive and highstand systems tracts. The uppermost bioturbated packstones of the Hanifa Formation are interpreted to be a lowstand systems tract with subsequent restriction leading to the deposition of gypsum. In the studied interval, total organic carbon (TOC) content correlates well with suboxic to anoxic intervals that have high paleoproductivity. Complete anoxia is not a prerequisite for organic matter preservation. These high TOC intervals are mainly in transgressive systems tracts.

End-Mississippian paleotopography and interpreted drainage pattern: Eastern Colorado

Eastern Colorado is the site of Pennsylvanian production from Morrowan, fluvial-deltaic and estuarine valley-fill sandstones. At least seven sequences of transgression-regression have been recognized. During regressions, erosional down-cutting carved valley systems on the flood-plain surface. During transgressions, the valleys were filled. The underlying pre-Pennsylvanian unconformity influenced the location of the Morrowan valley systems. Using 1263 well logs, this paleotopographic surface has been defined by detailed correlation of the Mississippian System and by contouring the isopach values of each formation at its erosional subcrop locale. Cross-sections have been prepared to show: (1) that regional formation thicknesses are reasonably constant, therefore, isopach irregularities at the subcrop are mainly due to erosional effects, and (2) that a paleotopographic valley-ridge system was incised on the eastern Colorado shelf at end-Mississippian time. A trellis type drainage pattern, probably controlled by parallel fracturing and jointing of the subaerially exposed carbonate shelf, was established that drained southeastward toward the proto-Anadarko Basin. A major erosional valley, trending mainly north-south, may have existed in far western Kansas that served as a catch-all for the eastern Colorado, karst valley-ridge system.