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NARROW
GeoRef Subject
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all geography including DSDP/ODP Sites and Legs
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United States
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New York
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Cattaraugus County New York (1)
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fossils (1)
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geologic age
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Paleozoic
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Silurian
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Lockport Formation (2)
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Middle Silurian (2)
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Niagaran (2)
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Upper Silurian
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Ludlow (1)
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minerals
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carbonates
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calcite (1)
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minerals (1)
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silicates
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framework silicates
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silica minerals
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quartz (1)
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Primary terms
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crystal growth (1)
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diagenesis (2)
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geochemistry (1)
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minerals (1)
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Paleozoic
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Silurian
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Lockport Formation (2)
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Middle Silurian (2)
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Niagaran (2)
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Upper Silurian
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Ludlow (1)
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sedimentary petrology (2)
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sedimentary rocks
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carbonate rocks
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dolostone (2)
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limestone
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micrite (1)
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chemically precipitated rocks
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evaporites (2)
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United States
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New York
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Cattaraugus County New York (1)
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sedimentary rocks
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oolite (1)
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sedimentary rocks
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carbonate rocks
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dolostone (2)
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limestone
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micrite (1)
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chemically precipitated rocks
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evaporites (2)
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sediments
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oolite (1)
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Front Matter
Intorduction
Introduction Techniques and Experimental Studies
Abstract Electron Spin Resonance (ESR) spectroscopy can determine the absolute amounts of Mn(II) in the Ca and Mg sites in dolomite and in associated calcite. The ESR spectra of Mn(II) in dolomite can be qualitatively divided into three types that have little overlap. Type 1 spectra have sharp peaks, and the partitioning of Mn into cation sites can be determined. Such spectra are common in stoichiometric and nonstoichiometric dolomites from both lithified and unconsolidated deposits (14 of 20 modern dolomites; 13 of 27 deep-marine dolomites). There is no apparent relation between Mn partitioning ratios and the absolute amount of Mn, the presence of a free radical center peak, or the total amount of dolomite in the sample. Modern dolomite, deep-marine dolomite, and nonstoichiometric Phanerozoic dolomites have average Mn partitioning ratios of 2, 5, and 6, respectively, suggesting that the ratios are not age dependent. Stoichiometric dolomites have an average partitioning ratio of approximately 30; thus, ratios and stoichiometry may be related. Type 2 spectra were observed in six of 20 modern dolomites and in 10 of 27 deep-marine dolomites. These spectra have broad peaks, and the Mg and Ca sites cannot be individually resolved. Because they are not found in older lithifield Phanerozoic dolomites, type 2 spectra may be related to lattice disorder. Type 3 spectra, observed in four deep-marine dolomites, do not have interpretable Mn peaks. A center peak assignable to radiation damage and/or free radicals may be present, independent of the Mn spectra. Age and thermal history data can be obtained from this peak.
The Elucidation of Dolomitization Events Using Nuclear-Track Mapping
Abstract The concentrations and distribution of uranium and boron have been measured in dolomites and limestones from a core taken on the island of San Salvador in the Bahamas. The analyses reveal a wide range of concentrations both within and between the two predominant types of dolomite. The crystalline dolomites show unexpectedly high concentrations of U in skeletal components (2 to 7 ppm), but low values in void-filling cements (0.5 to 1 ppm). In contrast, the fabric-destructive microsucrosic dolomites are uniformly low in U (0.5 to 1 ppm) with occasional red algal fragments exhibiting concentrations as high as 1.5 ppm. Data presented here suggest that the U concentrations of the dolomites are inherited from original sedimentary and diagenetically altered components. It is suggested that the rocks that have higher concentrations of U, and in which the original fabrics are largely preserved, were dolomitized directly from the aragonite and high-Mg calcite (HMC) precursors. The U concentration is retained during dolomitization because in carbonite-rich fluids the uranyl ion (UO 2 2+ ) is complexed principally with the carbonate ion (CO 3 2- ). As the activity of CO 3 2- is usually limiting in producing solutions supersaturated with respect to dolomite, CO 3 2 produced from the dissolution of metastable precursors is reincorporated into dolomite. In contrast, dolomites with lower U concentration formed from a low-Mg calcite (LMC) precursor which previously lost U during stabilization by meteoric waters. Concentrations of B in the dolomites were similar (1 to 3 ppm) to values determined for modern LMC organisms (this study) and therefore suggest dolomitization from predominantly marine fluids. Comparisons with ranges reported in the literature show B concentrations in this investigation to be much lower. This is attributable to the ability of the nuclear-track technique to recognize contamination within the sample and consequently to allow it to be eliminated from the analysis.
Experimental Investigation of Sulfate Inhibition of Dolomite and its Mineral Analogues 1
Abstract Time series experiments relating to the dolomitization of calcite at 215° to 225°C in saline solutions of near-seawater salinity were conducted to ascertain the influence of sulfate and carbonate in solution on the rate of calcite dolomitization. A concentration of about 0.004M sulfate in solution prevented the dolomitization of calcite. At concentrations of less than 0.004M, dolomitization proceeded at a slower rate than in experiments where no sulfate was present. The final concentration of sulfate was controlled by the precipitation of anhydrite. The presence of sulfate in solution did not prevent the direct precipitation of dolomite in experiments in which the solid reactants were carbonate minerals other than calcite (BaCO 3 and 2PbCO 3 .PbOH). Also, the presence of sulfate in the calcite dolomitization experiments slowed the rate of calcite dissolution from 3 days in sulfate-free solutions to 6 or 7 days in sulfate-bearing solutions. These observations indicate that sulfate in solution may inhibit dolomitization primarily by retarding the rate of calcite dissolution, rather than by inhibiting the direct precipitation of dolomite from solution. The rate of calcite dolomitization was greater in solutions with higher carbonate/bicarbonate concentrations. This provides some confirmation for hypotheses regarding the importance of carbonate in solution as a kinetic factor that expedites dolomitization.
Introduction Organogenic Dolomites
Abstract The Drakes Bay Formation is an upper Miocene sequence of siliceous mudstones containing many small dolomite nodules. The nodules probably formed without a precursor biogenic calcite supplying Ca or HCO 3 for dolomitization. Dolomite formation preferentially took place in sediment layers slightly richer in organic C than the surrounding sediments. More extensive sulfate reduction in these layers raised the porewater HCO, concentration and caused carbonate precipitation. The initial carbonate may have been dolomite or calcite, which was later converted to dolomite. Carbon and oxygen stable isotope ratios vary systematically and clearly illustrate changes in the isotopic composition of dissolved CO 2 that occurred with depth. The isotopic analyses show that dolomite formation did not begin until the pore waters were free of dissolved sulfate. The Ca contents of the dolomites decrease, and both the Mg and Fe contents increase, with depth of formation. Manganese contents correlate with Fe contents. Sodium contents of the dolomites are relatively high, probably reflecting their poor ordering and nonstoichi-ometry. Strontium contents of the dolomites are typical of those from hemipelagic sediments with moderate sedimentation rates.
Sediment Composition and Precipitation of Dolomite and Pyrite in the Neogene Monterey and Sisquoc Formations, Santa Maria Basin Area, California
Abstract A 1.2-km-thick section of the Miocene Monterey and overlying Pliocene Sisquoc formations in the Santa Maria basin area of California contains highly variable amounts of biogenic silica, detrital clay and silt, organic matter, carbonate, pyrite, and francolite. Organic-matter diagenesis resulted in the early precipitation of dolomite, pyrite, and francolite, and the concentration of trace metals. Dolostone horizons occur 1 to 10 m apart and consist of 50 to 95 weight percent pore-filling dolomite. The dolomite is low in Fe and Mn and contains an average of 0.8 to 5.3 mole percent excess Ca. Dolomite composition is related to texture in some samples and suggests several different episodes of dolomitization. There is a positive correlation between organic matter, pyrite, and the trace metals V, Cr, Ni, Cu, and Zn. Pyrite formation probably occurred below the sediment/seawater interface (noneuxinic basin) in the microbial-sulfate reduction zone, and was limited by Fe in sediment having a high organic matter-to-clay ratio and by reduced sulfur in sediment having a low organic matter-to-clay ratio.
Introduction Dolomites in MVT Deposits
Origins of Dolomite in the Offshore Facies of the Bonneterre Formation (Cambrian), Southeast Missouri
Abstract The Bonneterre Formation (Cambrian), southeast Missouri, is characterized by dolomitized algal bioherms and associated shelf carbonates that were deposited around the Precambrian St. Francois Mountains, which were islands during Late Cambrian time. West of the dolomitized shelf carbonates, the offshore facies of the Bonneterre consists of a deeper water limestone and shale sequence composed of oolitic and skeletal wackestones and packstones interbedded with silty lime mudstones and green illitic shales. Individual limestone and shale beds range in thickness from less than 1 cm to several meters. At the base of the offshore facies, immediately overlying the Lamotte Sandstone, is a regionally extensive basal dolomite that averages about 6 m thick. The basal dolomite contains coarse crystalline, nonplanar dolomite. This dolomite is relatively low in iron and nearly stoichiometric with regard to CaCO 3 . Stable carbon and oxygen isotope values for the basal dolomite are similar to values obtained for epigenetic dolomite associated with nearby sulfide ore bodies. The interbedded limestones and shales of the offshore facies contain abundant ferroan dolomite occurring as individual crystals and patches of crystals replacing limestones and floating in shale beds. This dolomite is commonly concentrated near solution seams, in argillaceous seams in limestones, in shale beds, and also may selectively replace allochems. The ferroan dolomite is commonly enriched in CaCO 3 and zoned with respect to iron. Stable carbon and oxygen isotope values are low, indicating elevated temperature and an organic source of carbon. The basal dolomite was formed by warm basinal brines circulating through the Lamotte Sandstone aquifer. This water may have been genetically related to the fluids that produced nearby Mississippi Valley-type ore deposits. The presence of impermeable overlying shale beds had restricted these fluids (and the resultant dolomite) to the lower few meters of the offshore facies. The ferroan dolomite was formed after burial by Mg +2- and Fe +2 -rich water evolved during the illitization of smectite in the interbedded shale.
Abstract A major concern with Viburnum Trend lead-zinc deposits is the nature of the mineralizing fluids. The chemical compositions of secondary recrystallized and sparry dolomites in the Bonneterre and Davis formations were determined to help define the nature of the coexisting mineralizing fluids in the Viburnum Trend and surrounding areas. Chemical compositions of the recrystallized host basal dolomite in the lowermost part of the Bonneterre Formation show a general south to north decrease of iron and manganese and an associated increase of strontium. This is consistent with a southern source for the mineralizing fluids. A similar trend of decreasing iron and manganese and increasing strontium upsection in the Viburnum Trend and the back reef indicates that the fluids moved from the underlying Lamotte Sandstone into the Bonneterre Formation. Low-iron and manganese concentrations in the backreef sparry dolomite, as well as relatively constant strontium values in the entire area, suggest the mixing of basinal brines with meteoric waters. A proposed sequence of dolomitization and ore-forming events can be summarized as: (1) dolomitization of the backreef unit by depositional marine or diagenetic waters, (2) updip movement of mineralizing brines from a southern source, possibly the Ouachita-Arkoma Basin, causing epigenetic dolomitization and ore deposition in the Viburnum Trend, and (3) changing of mineralizing conditions due to the mixing in the back reef of principal basinal brines with dilute meteoric waters from an eastern source.
Abstract Two sequences of pervasive dolomitization are preserved in the Mississippian Burlington-Keokuk Formation of Iowa, Illinois, and Missouri. Cathodoluminescent petrography reveals (1) an early, post-depositional, dolomite-forming episode (dolomite I), and (2) a later dolomite (dolomite II), which replaced the first generation. These texturally and temporally distinct dolomites are correlative over 100,000 km 2 of outcrop and subsurface (see Cander and others, this volume) and have distinguishing isotopic and trace-element characteristics. Calculation of the simultaneous isotopic variations that occur during water-rock interaction demonstrates important differences in the relative rates at which the O, C., Sr, and Nd isotopic compositions of diagenetic carbonates are altered. These quantitative models are used to place constraints on the water-rock interaction history of the Burlington-Keokuk dolomites. Dolomite I samples have a range of δ 18 O (–2.2 to 2.5‰ PDB), δ 13 C (-0.9 to 4.0‰ PDB) and є Nd (342) values (–6.0 to –4.7), and initial 87 Sr/ 86 Sr ratios (0.70757 to 0.70808) that encompass estimated marine dolomite isotopic compositions. These samples also have 107 to 123 ppm Sr, slightly lower than that of modem marine dolomites. Dolomite I formed from predominantly seawater-derived constituents with a small but significant non-marine component. A mixed-marine meteoric-fluid model can quantitatively account for the variations in dolomite I isotope and trace-element compositions, but the origin of the non-marine component is not well constrained. Compared to dolomite I, dolomite II samples have radiogenic initial 87 Sr/ 86 Sr ratios (0.70885 to 0.70942), lower δ 18 O values (–6.6 to –0.2‰ PDB), depleted Sr concentrations (50 to 63 ppm), similar δ 13 C values (-1.0 to 4.1‰ PDB) and similar є Nd (342) values (–6.5 to –5). The isotopic composition and concentration of Sr in dolomite II preclude a source within the Burlington-Keokuk Formation for the Sr in dolomite II. Dolomite II apparently formed as a result of the recrystallization of the less stoichiometric dolomite I by extraformational subsurface fluids that migrated to shallow burial depths. The results suggest that the recrystallization process effectively exchanged nearly all of the Sr from dolomite I. Oxygen isotopes equilibrate between dolomite and fluid at relatively low extents of water-rock interaction, and as a result, the δ 18 O values of dolomite II may reflect only the last stages of recrystallization. The results of model calculations also suggest that the 87 Sr/ 86 Sr ratios of dolomite II preserve an earlier and larger record of water-rock interaction, whereas their C and Nd isotopic signatures are inherited from dolomite I precursors. Late-stage, vug-filling carbonates appear to have formed from extraformational fluids that experienced minimal interaction with Burlington-Keokuk host rocks. The petrology and geochemistry of Burlington-Keokuk dolomites document multiple episodes of pervasive water-rock interaction that can be correlated on a regionally extensive scale.
Fluid Flow Direction During Dolomite Formation as Deduced from Trace-Element Trends
Abstract A qualitative mathematical model applying a variant of the Heterogeneous Distribution Law indicates that elements with distribution coefficients smaller than one (e.g., Sr) should increase downflow if (1) dolomite is formed exclusively or predominantly as a cement, and if (2) dolomite replaces calcium carbonate and the dolomitizing fluid has a molar Sr/Ca ratio that is equal to or lower than that of the calcium carbonate. If the dolomitizing fluid has a molar Sr/Ca ratio greater than that of the calcium carbonate precursor, Sr should decrease downflow. Trace elements with distribution coefficients greater than one (e.g., Mn) should decrease in the downflow direction for dolomite cementation. In the case of dolomitization of calcium carbonate, Mn trends could be sympathetic or antipathetic to those of Sr, depending on the composition of the fluid. Furthermore, trace-element trends may be pronounced, weak, or absent, depending on several interacting factors, such as fluid composition, flow rate, flow direction, water/rock ratio, degree of recrystallization, redox potential, and amount of clay or organic impurities. Trace-element trends, such as those predicted by the model, occur in massive dolostones that range from the Cambrian to Eocene. Such trends may be more common than previously recognized, and they may be useful indicators of the direction of fluid flow during dolomitization.
Abstract Cathodoluminescence petrography defines three regionally extensive dolomite generations in the Mississippian Burlington-Keokuk Formation over an area greater than 100,000 km 2 in Illinois and Missouri. These dolomites represent widespread replacement of older dolomite generations by younger generations, each with distinct petrographic, geochemical, and distributional characteristics. Dolomite I is the oldest and most abundant generation and is characterized by fine-scale cathodoluminescent zoning that is correlative within a measured section but is not correlative between sections. Dolomite II is the second generation and is characterized by unzoned red to brown cathodoluminescence. Dolomite II replaced dolomite I and is primarily restricted to the lower part of Burlington-Keokuk strata. Dolomite III is the youngest generation and occurs as nonluminescent, syntaxial overgrowths on, and partial replacements of, the older two generations. Dolomite I is pre-Pennsylvanian, on the basis of geometric relation with pre-Pennsylvanian calcite cements and with pre-Pennsyl-vanian cherts. The age of dolomite II is uncertain, but its occurrence within chert nodules suggests that it also is pre-Pennsylvanian. On the basis of geometric relations with pre-Pennsylvanian and post-Mississippian cements, dolomite III probably formed after the Mississippian but before late Pennsylvanian/early Permian time. Dolomite I is proposed to have formed in a seawater-freshwater mixing environment associated with a regional meteoric groundwater system that developed beneath late Mississippian/early Pennsylvanian unconformities. This model is supported by the early timing, restriction of correlatable zonation to single measured sections, and by isotopic characteristics. Dolomite II is proposed to have formed as a replacement of dolomite I by warm, relatively Fe- and Mn-rich subsurface fluids. These fluids interacted with pre-Burlington rocks and invaded the Burlington-Keokuk sediments from below in "plumes," which had an irregular regional distribution. Dolomite III probably represents continued influx of progressively more Fe- and Mn-rich, warm subsurface fluids, which caused replacement and syntaxial overgrowth of the older two generations. These models imply that the Mg for dolomite I was derived from sea water and from intraformational skeletal Mg-calcites, whereas the Mg for dolomites II and III was derived mainly from precursor dolomites I and II.
Sedimentology and Geochemistry of a Regional Dolostone: Correlation of Trace Elements with Dolomite Fabrics
Abstract The Interlake Formation (Silurian) in North Dakota is a 366-m-thick dolostone unit representing dolomitization of subtidally deposited lime grainstones and minor lime mudstones. Dolomitization was early and resulted from: (1) hypersaline brines in supratidal settings; and (2) mixing of marine fluids with hypersaline brines and/or meteoric water. Burial dolomites are quantitatively minor. Three types of dolostone fabrics are described: grain-supported fabrics (GSF), pervasive-dolostone fabrics (PDF), and particle-relict fabrics (PRF). Grain-supported fabric (GSF) dolostones result from replacement of lime grainstones or packstones, whereas PDF do-lostones lack evidence of precursor depositional textures and fabrics; PRF dolostones have intermediate fabrics consisting of ghosts and relicts of particles in a mosaic of dolomite crystals. Grain-supported fabric (GSF), PDF, and PRF dolostones were analyzed for Sr, Mn, Fe, Na, and B. In general, GSF have elevated Mn and Na, whereas Fe, Sr and B are depressed. In contrast, PDF have elevated Fe, Sr and B, whereas Mn and Na are depressed. A more complicated picture emerges if PRF dolostones are considered. These dolostones have elevated B and Sr, whereas Mn, Fe, and Na are depressed. In contrast to fabric, dolomite texture does not show systematic trends in element distribution. These variations cannot be easily explained, because factors controlling the incorporation of these trace elements into various dolomite fabrics are poorly understood. Nevertheless, significant differences between PDF and GSF dolostones indicate that concentrations of some trace elements (Mn and Na) in dolostones may be influenced by the concentration of those elements in precursor minerals.
Introduction Dolomite Diagenesis
Dolomitization of Siluro-Devonian Limestones in a Deep Core (5,350 M), Southeastern New Mexico
Abstract One hundred thirty-two feet (40 m) of continuous, conventional core are divisible into an upper part (71 ft; 21.5 m) of limestone, dolomitic limestone, and minor intervals of dolomite sharply separated from a lower (61 ft; 18.5 m) completely dolomitized sequence. Limestone lithotypes, such as stromatoporoid rudstone and peloidal skeletal packstone to grainstone, indicate a generally shallow-water depositional setting of near-normal salinity, whereas laminated to massive, fenestral, peloidal mudstone and packstone, suggest a tidal flat environment. Intraclast breccia consisting of rip-up clasts of this latter lithology, commonly occurring above that in situ sequence, and of skeletal grainstone also associated with laminated fenestral units are interpreted as supratidal deposits analogous to Holocene deposits in Shark Bay, Australia. Obliterative replacement dolomite in the upper part occurs as concentrated seams to thicker bands focused along stylolites, disseminated rhombs, and completely replaced intervals of various unfossiliferous dolomite types, such as brecciated (intraclastic?), laminated, fenestral, burrowed, and stylolitic, which also compose the completely dolomitized lower 61 -ft (18.5 m) sequence. Near-stoichiometric, non- to very slightly luminescent dolomite crystal size ranges from mud to millimeter-size saddle void-filling cement, but the main mode is coarse decimicron- to fine centimicron-size. Average δ 13 C for calcite = -1.13‰, for dolomite = +0.51 ‰; average δ 18 O for calcite = -7.43‰, for dolomite = -6.69‰ (PDB). Average trace-element content (in ppm) for dolomites is Fe = 313, Na = 985, Sr = 31, and Mn = 73. Average homogenization temperatures (pressure uncorrected) of fluid inclusions for selected groups of dolomite crystals suggest a general direct relation to crystal size and range from 130°C for fine centimicron-size crystals to 193°C for saddle dolomite. A sequence containing dolomitized tidal flat features and intimate associations of calcite and dolomite intraclasts suggests early dolomitization. Some deep burial dolomitization is indicated by dolomite growth along stylolites; more pervasive late dolomitization is suggested by broader bands of dolomite, whose geometry suggests stylolite control. Coarse crystallinjty, xenotopic fabric, relatively depleted δ 18 O values for all dolomite types, trace-element content, and limited fluid inclusion data strongly suggest the influence of hot and deep subsurface solutions, but it is unclear whether the dolomite resulted from mesogenetic replacement, early dolomitization followed by neomorphism in the burial environment, or some combination of those two "end-member" models.