Jay M. Gregg

Classification of Dolomite Rock Textures

ABSTRACT Dolomite rock textures can be classified according to crystal size distribution and crystal boundary shape. The classification scheme presented here is largely descriptive but carries genetic implications because size distribution is controlled by both nucleation and growth kinetics, and crystal boundary shape is controlled by growth kinetics. Size distributions are classified as unimodal or polymodal. Crystal boundary shapes are classified as planar or nonplanar. If the evidence permits, a complete classification includes a description of recognizable allochems, matrix, and void filling. Allochems and preexisting cements may be unreplaced, partially replaced, replaced mimically, or replaced nonmimically. Allochems may be dissolved, leaving molds. Matrix can be unreplace, partially replaced, or replaced by a unimodal or polymodal size dolomite. Unimodal size distributions generally indicated a single nucleation event on a unimodal substrate. Polymodal sizes can be formed by multiple nucleation events on a unimodal or polymodal substrate or differential nucleation on an originally polymodal substrate. Planar crystal boundaries develop when crystals undergo faceted growth, and nonplanar boundaries develop when crystals undergo nonfaceted growth. Nonplanar boundaries are characteristic of growth at elevated temperature (> 50°C) and/or high supersaturation. Both planar and nonplanar dolomite can form as a cement, replacement of CaCO3, or neomorphism of a precursor dolomite.

Dolomitization and Dolomite Neomorphism in the Back Reef Facies of the Bonneterre and Davis Formations (Cambrian), Southeastern Missouri

The back reef facies of the Bonneterre and Davis Formations consists of dolomitized cryptalgalaminates and partially dolomitized peloid mudstones. These subfacies were deposited on tidal flats and in shallow lagoons that existed in the St. Francois Mountains of southeast Missouri during late Cambrian time. The back reef facies is bounded on the seaward side by dolomitized algal bioherms and grainstone banks which host the southeast Missouri Mississippi Valley-type (MVT) lead-zinc-copper ore deposits. Cryptalgalaminates are replaced by both fine crystalline, planar dolomite and coarse crystalline, nonplanar dolomite. Peloid mudstones are partially replaced, but mainly by coarse crystalline, nonplanar dolomite. Planar dolomite, replacing cryptalgalaminates, shows gradational transitions into coarse crystalline, nonplanar dolomite. The transitions are accompanied by progressive increase in average crystal size and an increased skewness toward the coarse size fraction. Planar dolomite is enriched in 18 O relative to stratigraphically equivalent marine limestones. Coarse crystalline, nonplanar dolomite, which replaces both cryptalgalaminates and peloid mudstones, has delta 13 C and delta 18 O values transitional between those of planar dolomite and late dolomite cements associated with MVT mineralization. Fluid inclusion analysis of nonplanar dolomite and associated dolomite cements indicates precipitation at elevated temperatures (60 to 114 degrees C) by saline, basinal fluids. These data indicate that the back reef facies was subjected to a complex diagenetic history involving both early and late diagenetic dolomitization and dolomite neomorphism. Fine crystalline, planar dolomite was formed during early diagenetic dolomitization of the cryptalgalaminate subfacies by a fluid approaching seawater composition. Coarse crystalline, nonplanar dolomite was formed as a result of neomorphism of pro-existing planar dolomite and dolomitization of peloid mudstones, after burial, accompanying MVT mineralization in southeast Missouri.

Regional epigenetic dolomitization in the Bonneterre Dolomite (Cambrian), southeastern Missouri

A dolomite bed averaging 6 m in thickness forms the base of a limestone and shale facies of the Bonneterre Dolomite (Cambrian), southeastern Missouri. The dolomite immediately overlies the Lamotte Sandstone and covers an area of more than 16 600 km 2 . It is proposed that this basal dolomite formed as a result of interaction with warm, basin-derived water circulating through the underlying Lamotte Sandstone, and that the dolomitization event coincided with the emplacement of Mississippi Valley–type sulfide ores in the nearby Viburnum Trend. This hypothesis is supported by (1) petrographic evidence that suggests an origin at temperatures >50 °C, (2) cathodoluminescent microstratigraphy of dolomite cements which is correlative with that in gangue dolomite cements in the nearby orebodies, and (3) stable oxygen and carbon isotope values that are consistent with a basinal origin of the dolomitizing water. The basal dolomite bed may represent one of the most extensive epigenetic dolomites yet documented. This implies that a basinal water can alter rocks over a wide area at a considerable distance from its source.

Fluid-inclusion studies of regionally extensive epigenetic dolomites, Bonneterre Dolomite (Cambrian), southeast Missouri: Evidence of multiple fluids during dolomitization and lead-zinc mineralization

Sources of basinal fluids that precipitated the dolomite-hosted Mississippi Valley-type orebodies of southeast Missouri historically have been a subject of debate. This study presents microthermometric data for fluid inclusions in the regionally extensive epigenetic dolomite at the base of the ore-hosting Bonneterre Dolomite and in gangue dolomite of the Viburnum Trend Pb-Zn district. Samples of epigenetic dolomite cover an area of more than 25,000 km 2 west of the St. Francois Mountains and permit determination of regional variations of temperature and composition of the mineralizing fluids and possible fluid interactions. Homogenization temperature-ice-melting relationships (temperature-salinity) among these inclusions document at least two end-member fluid components: a warmer, less saline fluid (120 to 187 °C; 5 wt% equiv. NaCl) and a cooler, more saline fluid (60 to 80 °C; >30 wt% equiv. NaCl). Intermediate temperatures and fluid compositions indicate that the end-member fluids likely mixed as they flowed through the region. Mixing was not confined to a stationary front but occurred throughout the study area. Comparison of homogenization temperatures to distance from possible basinal fluid sources indicates no discernible temperature gradient over the >25,000 km 2 study area. The data are interpreted to indicate multiple basinal-fluid interactions coeval with dolomitization and associated Pb-Zn ore formation.

Minor- and trace-element distributions in the Bonneterre Dolomite (Cambrian), southeast Missouri: Evidence for possible multiple-basin fluid sources and pathways during lead-zinc mineralization

The sources and flow paths of basinal fluids that precipitated the dolomite-hosted Mississippi Valley-type orebodies of southeast Missouri have historically been a subject of debate. This study presents geochemical data for epigenetic dolomites of the Bonneterre Dolomite directly above the Lamotte Sandstone aquifer and from the Viburnum Trend orebodies. Samples of epigenetic replacement dolomite were collected at the Bonneterre Dolomite-Lamotte Sandstone contact from 35 drill cores covering an area of more than 25,000 km 2 west of the St. Francois Mountains. There are distinct aerial trends in Fe, Mn, and Sr contents of the dolomite from values of 3.47 wt%, 0.38 wt%, and 27 ppm in the south to 0.82 wt%, 0.12 wt%, and 54 ppm in the north, respectively. In and near the Viburnum Trend lead-zinc subdistrict, the distributions of Fe, Mn, and Sr are reversed, from values of 0.50 wt%, 0.10 wt%, and 105 ppm in the south to 3.15 wt%, 0.55 wt%, and 40 ppm in the north, respectively. Fe and Mn contents of gangue dolomite cement in the Viburnum Trend orebodies show a similar south-to-north enrichment, ranging from 1.41 mole% FeCO 3 and 0.11 mole% MnCO 3 in the south to 2.34 mole% and 0.23 mole% in the north, respectively. On the basis of fractionation of minor and trace elements between fluids and dolomites, the data are interpreted to indicate a regional south-to-north flow of water from the Arkoma Basin through the Lamotte Sandstone. A second fluid-flow system, with a northern source (possibly the Illinois Basin), precipitated the late phases of gangue dolomite cement in the Viburnum Trend and may have been active during the earlier precipitation of the dolomite at the Lamotte-Bonneterre contact. The inferred fluid-flow paths suggest a more complex, multiple-basin fluid involvement than previously suggested for the Mississippi Valley-type Pb-Zn mineralization of southeast Missouri.

Cold water coral mounds revealed

The discovery of mounds and reefs hosting cold-water coral ecosystems along the northeastern Atlantic continental margins has propelled a vigorous effort over the past decade to study the distribution of the mounds, surface sediments, the ecosystems they host, and their environments [Hovland et al., 1994; Freiwald and Roberts, 2005].This effort has involved swath bathymetry, remotely operated vehicle deployments, shallow coring, and seismic surveys. Global coverage is difficult to gauge, but studies indicate that cold-water corals may cover as large an area as the better known warm-water corals that form shallow reefs (284,300 square kilometers) [Freiwald et al., 2005]. Cold-water corals occur in a variety of forms and settings, from small isolated colonies or patch reefs to giant mound structures such as those found west of Ireland.

Dolomitization of Holocene Mg-calcite supratidal deposits, Ambergris Cay, Belize

Dolomitized crusts and associated sediments 845–2925 yr old on Ambergris Cay, Belize, Central America, compose a significant portion of the Holocene sediment section on many supratidal flats. The dolomitic sections are as thick as 0.7 m and contain an average of 70% calcic dolomite. Dolomite occurs as a replacement of high-Mg calcite micrite matrix and allochems, and as passively precipitated cements. Isotopic analyses suggest that dolomitization appears to be promoted primarily by reactions of the magnesian calcite supratidal sediments with essentially near-normal marine waters. The effects of seasonal influxes of meteoric and hybrid fluids, however, are also clearly evident in the crusts as indicated by isotopes, dolomite-crystal etching, the selective leaching of aragonite, and the local loss of Mg 2+ from the host micrite and included skeletal fragments. The amounts, rapid rates of dolomitization, and initial Mg-calcite mineralogy of the sediments make the Ambergris Cay supratidal flats exceptional in the Holocene, and this area may be a model for the genesis of some ancient peritidal dolomites.

Dolomitization and Dolomite Neomorphism: Trenton and Black River Limestones (Middle Ordovician) Northern Indiana, U.S.A.

ABSTRACT The Trenton and Black River Limestones are dolomitized extensively along the axis of the Kankakee Arch in Indiana, with the proportion of dolomite decreasing to the south and southeast of the arch. Planar and nonplanar dolomite replacement textures and rhombic (type 1) and saddle (type 2) void-filling dolomite cements are present. Three stages of dolomitization, involving different fluids, are inferred on the basis of petrographic and geochemical characteristics of the dolomites. Nonferroan planar dolomite has relatively high 18O values (-1.8 to -6.1o/oo PDB) and has 87Sr/86Sr ratios (0.70833 to 0.70856) that overlap those of Middle Ordovician seawater. Petrography, geochemistry, and the geometry of the dolomitized body suggest that the planar dolomite was formed in Middle and Late Ordovician seawater during the deposition of the overlying Maquoketa Shale. Ferroan planar and nonplanar dolomite occurs in the upper few meters of the Trenton Limestone, confined to areas underlain by planar dolomite. This dolomite contains patches of nonferroan dolomite with cathodoluminescence (CL) characteristics similar to underlying planar dolomite. Ferroan dolomite has relatively low 18O values (-5.1 to -7.3o/oo PDB) and has slightly radiogenic 87Sr/86Sr ratios (0.70915 to 0.70969) similar to those obtained for the overlying Maquoketa Shale. These data indicate that ferroan dolomite formed by neomorphism of nonferroan planar dolomite as fluids were expelled from the overlying Maquoketa Shale during burial. The absence of ferroan dolomite at the Trenton-Maquoketa contact, in areas where the earlier-formed nonferroan planar dolomite also is absent, indicates that the fluid expelled from the overlying shale did not contain enough Mg2+ to dolomitize limestone. Type 1 dolomite cement has isotopic compositions similar to those of the ferroan dolomite, suggesting that it also formed from shale-derived burial fluids. CL growth zoning patterns in these cements suggest that diagenetic fluids moved stratigraphically downward and toward the southeast along the axis of the Kankakee Arch. Type 2 saddle dolomite cements precipitated late; their low 18O values (-6.0 to -7.0o/oo PDB) are similar to those of the type 1 dolomite cement. However, fluid-inclusion data indicate that the saddle dolomite was precipitated from more saline, basinal fluids and at higher temperatures (94° to 143°C) than the type 1 cements (80° to 104°C). A trend of decreasing fluid-inclusion homogenization temperatures and salinities from the Michigan Basin to the axis of Kankakee Arch suggests that these fluids emerged from the Michigan Basin after precipitation of type 1 cement.

Basin-Wide Diagenetic Patterns: Integrated Petrologic, Geochemical, and Hydrologic Considerations

This volume contains papers, many of which were presented at the SEPM Research Conference entitled Basin-Wide Diagenetic Patterns: Integrated Petrologic, Geochemical, and Hydrologic Considerations which was convened May 21 to 25, 1994 at Lake Ozark, Missouri, U.S.A. Some of the issues addressed at this conference and in this volume include: factors governing the temporal evolution of hydrodynamic systems, the origin and evolution, and spatial distribution of paleoflow conduits and their diagenetic products in sedimentary basins, the nature of subsurface fluid-rock interactions, temporal and spatial distribution of the geochemistry of basinal fluids, and factors controlling heat flow in sedimentary basins.

Sequence Stratigraphy and Depositional Facies of Lower Ordovician Cyclic Carbonate Rocks, Southern Missouri, U.S.A.

ABSTRACT Lower Ordovician cyclic carbonate strata of southern Missouri were deposited in a warm, shallow, epeiric sea on a fully aggraded carbonate platform. Sedimentological characteristics distinguish the Jefferson City and Cotter dolomites from the underlying Gasconade and Roubidoux formations. Mixed carbonate-siliciclastic sedimentation characterizes the Roubidoux Formation, with sandstones accounting for up to 60% of sedimentation. The Gasconade, Jefferson City, and Cotter dolomites exhibit an increased occurrence of chalcedonic chert nodules in very similar shape and texture to the gypsum and anhydrite nodules common on modern sabkha supratidal flats. Casts of halite and ghosts of gypsum laths also exist in the Jefferson City and Cotter strata but are rarely found in the underlying units. Facies analysis from drill cores and outcrop sections provides the basis for identifying two major meter-scale cycle types. Type I cycles consist of algal stromatolites, tidal-flat laminites (mechanical and algal), ooid grainstones, wavy peloidal wackestones, and quartz sandstones interpreted as peritidal facies. They are the dominant components of the Roubidoux Formation, Jefferson City Dolomite, and Cotter Dolomite. Type II cycles consist mostly of subtidal facies such as strongly burrowed mudstone, thrombolite boundstone, and stromatolites. Type I cycles are thinner and represent highstand systems tracts, whereas the thicker type II cycles represent transgressive systems tracts and are dominant in the Gasconade Dolomite. The cycle stacking patterns, facies changes, and the intrabasinal correlatability of Fischer plots made from the widely spaced sections argue for a eustatic control on sea-level fluctuation on the platform. Interbasinal correlation with other North American basins is possible using biostratigraphic information and comparison of Fischer plots. Five Missouri sequences correlate with those described for other regions. The continent-wide uniformity in cycle stacking patterns indicates a primarily eustatic control on Lower Ordovician meter-scale cycle development. Regional tectonic and autocyclic controls probably account for general differences in sedimentation pattern among the correlated basins.