Peter S. Mozley

Oxygen and Carbon Isotopic Composition of Marine Carbonate Concretions: An Overview

ABSTRACT Compilation of stable isotope data (18O and 13C) for marine calcite, siderite, and dolomite concretions from a large number of studies reveals consistent and unanticipated isotopic trends. Although many of the dolomites have 18O values that are compatible with precipitation from unaltered seawater at low temperatures, most of the calcites and siderites are far too depleted in 12O for such an origin. This is surprising, as most of the concretions appear to have formed very early in the diagenetic history of the sediments, often within several meters of the sediment/water interface The most likely cause of the anomalously depleted oxygen values is a decrease in pore-water 18O values during early diagenesis--a result, perhaps, of a significant influx of meteoric water into the marine shelf environment, or water-mineral interaction (e.g., precipitation of 18O-enriched minerals). The carbon isotopic field for dolomite is quite different from that for calcite and siderite. Highly positive 13C values are fairly common only for dolomites. This indicates that, while dolomite concretions often form in sediments where methanogenesis is rapid, precipitation of calcite and siderite concretions is restricted to sediments where either n methanogenesis takes place or where methanogenesis is not extensive enough to generate highly positive pore fluid 13C values. This variation in carbon isotopic composition among the different minerals suggests that the rates of organic-carbon oxidation and sedimentation may control the mineralogy of carbonate concretions. High organic-carbon oxidation rates favor dolomite, and lower oxidation rates favor calcite or siderite.

Relation between depositional environment and the elemental composition of early diagenetic siderite

Early diagenetic siderites from marine and fresh-water depositional environments are characterized by distinctive compositional trends. Siderite from fresh-water environments is often relatively pure (i.e., greater than 90 mol% FeCO{sub 3}) and commonly attains end-member composition. Siderite from marine environments, however, is always extremely impure and has extensive substitution of Mg (up to 41 mol%) and, to a lesser extent, Ca (up to 15 mol%) for Fe in the siderite lattice. In addition, marine siderite generally contains less Mn and has a higher Mg/Ca ratio than fresh-water siderite. This compositional variation appears to result from differences in the chemistry of early marine and meteoric pore waters, inasmuch as early marine pore waters generally have a higher Mg{sup 2+}/Ca{sup 2+} ratio and contain less Mn{sup 2+} and Fe{sup 2+} and more Ca{sup 2+} and Mg{sup 2+} than meteoric waters.

Hydrogeologic Controls on Induced Seismicity in Crystalline Basement Rocks Due to Fluid Injection into Basal Reservoirs

A series of Mb 3.8-5.5 induced seismic events in the midcontinent region, United States, resulted from injection of fluid either into a basal sedimentary reservoir with no underlying confining unit or directly into the underlying crystalline basement complex. The earthquakes probably occurred along faults that were likely critically stressed within the crystalline basement. These faults were located at a considerable distance (up to 10 km) from the injection wells and head increases at the hypocenters were likely relatively small (∼70-150 m). We present a suite of simulations that use a simple hydrogeologic-geomechanical model to assess what hydrogeologic conditions promote or deter induced seismic events within the crystalline basement across the midcontinent. The presence of a confining unit beneath the injection reservoir horizon had the single largest effect in preventing induced seismicity within the underlying crystalline basement. For a crystalline basement having a permeability of 2 × 10(-17)  m(2) and specific storage coefficient of 10(-7) /m, injection at a rate of 5455 m(3) /d into the basal aquifer with no underlying basal seal over 10 years resulted in probable brittle failure to depths of about 0.6 km below the injection reservoir. Including a permeable (kz  = 10(-13)  m(2) ) Precambrian normal fault, located 20 m from the injection well, increased the depth of the failure region below the reservoir to 3 km. For a large permeability contrast between a Precambrian thrust fault (10(-12)  m(2) ) and the surrounding crystalline basement (10(-18)  m(2) ), the failure region can extend laterally 10 km away from the injection well.

Elemental and isotopic composition of siderite in the Kuparuk Formation, Alaska; effect of microbial activity and water sediment interaction on early pore-water chemistry

ABSTRACT The Kuparuk Formation (Neocomian) consists of glauconitic sandstones and mudrocks of marine origin. Authigenic siderite is common throughout the formation, either concentrated in distinct layers or as isolated rhombs. Examination of siderite-rich samples with back-scattered electron imaging reveals complex compositional zonation in the siderite. The samples are principally composed of mixtures of relatively early-formed Fe-rich siderite and relatively late-formed Mg-rich siderite, with mean compositions of (Fe77.5Mg13.8Ca7.9Mn0.7 )CO3 and (Fe55.8Mg32.8Ca10.8Mn0.6 )CO3, respectively. Fe-rich siderites are most abundant in glauconitic sandstone intervals, whereas Mg-rich siderit s are most abundant in non-glauconitic sandstones and mudrocks. The whole-rock isotopic composition of the siderite is highly variable, with 13C values ranging from -20.3 to 7.9 PDB, and 18O values from 21.4 to 31.1 SMOW. This variation in isotopic composition correlates with the relative proportions of the two siderite types in a given sample. The Fe-rich siderites have low 13C and high 18O values, whereas the Mg-rich siderites have high 13C and low 18O values. Both the elemental and carbon isotopic compositions of the siderite result from modification of the original marine pore waters during successive stages of microbial decomposition of organic matter. The oxygen isotopic compositions of the siderite indicate that early pore waters were depleted in 18O, perhaps as a result of water/sediment interaction. Authigenic ankerite associated with the siderite-bearing samples is unzoned and has a mean composition of Ca1.11 Fe0.35Mg0.53(CO3)2. The ankerites have higher 13C values and lower 18O values than the Fe-rich siderite, but have 13C values less than and 18O values greater than (with one exception) that of the Mg-rich siderite. The isotopic data suggest that ankerite precipitation began prior to precipitation of Mg-rich siderite and ended subsequent to or during Mg-rich side ite precipitation.

The internal structure of carbonate concretions in mudrocks: a critical evaluation of the conventional concentric model of concretion growth

Abstract Carbonate concretions have traditionally been viewed as forming concentrically, through the progressive addition of carbonate to the outer edge of the growing concretion. This conventional model of concretion growth is based mainly upon center-to-edge textural and geochemical trends that are consistent with concentric growth. However, a number of recent studies of concretions in mudrocks have documented complex zonation that is not compatible with the conventional concentric model. In some cases complexly zoned concretions contain late-stage cements that precipitated throughout the concretion body. None of the center-to-edge relationships used to support the conventional concentric model exclude more complex modes of origin. As concretions that fit the conventional model may be the exception rather than the rule, high-resolution petrographic analysis of concretions should always be used to characterize internal structure and mode of growth prior to interpretation of geochemical and textural data.

Pore networks in continental and marine mudstones: characteristics and controls on sealing behavior.

Mudstone pore networks are strong modifiers of sedimentary basin fluid dynamics and have a critical role in the distribution of hydrocarbons and containment of injected fluids. Using core samples from continental and marine mudstones, we investigate properties of pore types and networks from a variety of geologic environments, together with estimates of capillary breakthrough pressures by mercury intrusion porosimetry. Analysis and interpretation of quantitative and qualitative three-dimensional (3D) observations, obtained by dual focused ion beam–scanning electron microscopy, suggest seven dominant mudstone pore types distinguished by geometry and connectivity. A dominant planar pore type occurs in all investigated mudstones and generally has high coordination numbers (i.e., number of neighboring connected pores). Connected networks of pores of this type contribute to high mercury capillary pressures due to small pore throats at the junctions of connected pores and likely control most matrix transport in these mudstones. Other pore types are related to authigenic (e.g., replacement or pore-lining precipitation) clay minerals and pyrite nodules; pores in clay packets adjacent to larger, more competent clastic grains; pores in organic phases; and stylolitic and microfracture-related pores. Pores within regions of authigenic clay minerals often form small isolated networks (

Diagenesis, sediment strength, and pore collapse in sediment approaching the Nankai Trough subduction zone

A minor amount of opal cement inhibits consolidation of sediment approaching the Nankai Trough subduction zone at Ocean Drilling Program Sites 1173 and 1177. Secondary and backscattered electron images of sediments from Site 1173 reveal a low-density, silica phase (opal-CT) coating grain contacts. The grain-coating cement is more widespread in the upper Shikoku Basin facies than in the lower Shikoku Basin facies. Numerical models of opal-CT content display increases with depth through the cemented upper Shikoku Basin section. Once temperature increases above ∼55 °C, the rate of opal-CT dissolution outpaces precipitation, the cement can no longer support the overburden, and the open framework of the sediment begins to collapse. Cementation followed by cement failure is consistent with observed anomalies in porosity, seismic velocities, and shear rigidity. Porosity is anomalously high and nearly constant near the base of the upper Shikoku Basin facies, whereas seismic velocity increases with depth in the same interval. Across the boundary between the upper Shikoku Basin facies and the lower Shikoku Basin facies, there are step decreases in porosity from ∼60% to ∼45%, P-wave velocity from ∼1800 m/s to ∼1650 m/s, and S-wave velocity from ∼550 m/s to ∼300 m/s. Similar cementation and porosity collapse may be important in other locations where heating of hemipelagic deposits, with minor amounts of opal, is sufficient to trigger opal diagenesis.

Patterns of cementation along a Cenozoic normal fault: A record of paleoflow orientations

The Sand Hill fault is a steeply dipping normal fault that cuts poorly consolidated sediments of the Albuquerque basin, New Mexico. The fault zone, which varies in width from ∼1 to 6 m, is selectively cemented by calcite. The margins of cemented areas are characterized locally by striking patterns of elongate cementation that have a strong subvertical orientation. We have considered a variety of possible mechanisms for the formation of these elongate cements, including deformation, weathering, rotation of adjacent cemented beds, and precipitation from flowing ground water. All but the latter can be ruled out. In addition, the elongate cements closely resemble elongate concretions that form from flowing ground water in sedimentary rocks. We conclude that the cements precipitated from flowing ground water, and are elongate parallel to the flow direction at the time of precipitation. Thus, these elongate cements provide an important constraint on models of fluid flow along faults in poorly consolidated sediments; i.e., the orientation of outcrop-scale flow.

Internal structure and mode of growth of elongate calcite concretions: Evidence for small-scale, microbially induced, chemical heterogeneity in groundwater

Elongate calcite concretions are thought to form parallel to the direction of groundwater flow at the time of their precipitation, and are potentially very useful in determining the nature of paleogroundwater flow and geological variables that influence flow. This study focuses on the mode of growth of such concretions, which has not been previously investigated in detail. Most of the concretions examined are from the Oligocene to Pliocene-Pleistocene, Santa Fe Group of New Mexico, which is the synrifting basin fill of the Rio Grande rift. Based upon their macroscopic characteristics, elongate concretions can be classified into three main types: uniform, composite, and zoned. The uniform and zoned concretions have similar, relatively smooth exteriors, but differ internally, with the zoned variety displaying internal concentric zonation on broken surfaces. Uniform concretions consist of mixtures of early micrite/microspar and later sparite that have different elemental chemistry (mainly Mg variation) and are present throughout the concretion. Petrographic examination reveals significant porosity (up to 15%) in the interiors of some uniform concretions. Zoned concretions consist mainly of sparite, which commonly varies both texturally and in elemental composition in a concentric manner. The composite variety has a rough “warty” exterior and consists of an amalgamation of pea-sized, spherical poikilotopic crystals of calcite. Uniform and composite concretions appear to have formed from the simultaneous precipitation of calcite throughout the volume of the concretions (i.e., early and later stage cements are present throughout). Zoned concretions appear to have formed from concentric growth in which early calcite precipitated at several interior sites (core zones) and was covered by successive layers of younger calcite. Most of these sites of inferred initial precipitation occur preferentially near the up-gradient ends of the concretions, which suggests that the zoned concretions grew in part through preferential precipitation in the direction of flow. The observed petrographic and geochemical characteristics indicate that the elongation is the result of growth within elongate zones of favorable pore-water chemistry (e.g., zones of elevated carbonate alkalinity). Such zones probably formed down-gradient of fragments of organic matter undergoing microbial decay.

Relationship between Oriented Calcite Concretions and Permeability Correlation Structure in an Alluvial Aquifer, Sierra Ladrones Formation, New Mexico

ABSTRACT Fluvial deposits of the Sierra Ladrones Formation (Pliocene-Pleistocene) contain abundant elongate calcite concretions that are interpreted to have precipitated in the saturated zone parallel to the direction of groundwater flow. The orientation of these concretions is coincident with the orientation of the permeability correlation structure, as determined from architectural element mapping and permeameter measurements. In cases where the paleo-groundwater flow closely corresponds with the direction of fluvial deposition, measurement of concretion orientations in alluvial rocks and sediments may provide a means of rapidly estimating the major axis of the permeability correlation structure over large areas.