Lynton S. Land

The Origin of Massive Dolomite

Most dolomite forms as a calcium-rich and/or poorly ordered metastable phase when seawater is actively circulated through carbonate sediments. Modification of seawater by evaporation, mixing with m...

Mg/Ca Ratio and Salinity: Two Controls over Crystallization of Dolomite

Carbonate precipitation is governed by both thermodynamic and kinetic considerations. The concentration and ionic composition of natural solutions influence not only the mineralogy of the precipitate, but also the crystal size and habit of both metastable and stable phases. Under marine conditions sparry calcite does not precipitate because of poisoning of sideward crystal growth by Mg, which forces Mg-calcite and aragonite to assume a micritic, steep-rhombic, or fibrous habit. Blocky calcite can form only in low Mg/Ca waters, either from primary meteoric solutions, or where Mg has been removed from marine water by dolomite or clays. In hypersaline environments precipitation is commonly rapid and, together with the high concentrations of foreign ions, it is difficult for dolomite to form because of the precise Ca-Mg ordering required. Instead, aragonite or Mg-calcite crystallizes. Dolomite can form only if the Mg/Ca ratio exceeds 5-10:1, and even then it is aphanitic and poorly ordered. At progressively reduced salinities dolomite is able to nucleate at still lower Mg/Ca ratios, approaching 1:1 in meteoric waters. The absence of foreign ions and slow crystallization, commonly under phreatic conditions, cause both dolomite and calcite to form exquisitely limpid, euhedral rhombs. Thus, dolomite probably forms most readily by a reduction in salinity, particularly in a schizohaline environment (alternating between hypersaline and near-fresh conditions, as in a floodable sabkha or a phreatic mixing zone). Flushing marine saline waters with fresh water lowers salinity but maintains a high Mg/Ca ratio; crystallization is slower and the interfering effect of foreign ions is reduced.

Pleistocene History of Bermuda

The Pleistocene rocks of Bermuda consist of shallow-water, beach, and intertidal marine biocalcarenites; eolianites; and red soils, displaying complex facies relationships. Eolianites grade laterally into marine biocalcarenites, indicating deposition during times of high sea level, or interglacial episodes. During glacial times, red soils formed over the present area of Bermuda. Because no evidence for Pleistocene tectonism has been discerned, the Bermuda Islands may represent a good “tide gauge” for the assessment of Pleistocene eustasy. The determination of Bermudian stratigraphy requires recognition of environments of deposition and interpretation of the subsequent diagenetic history of the various units. Bermudian marine biocalcarenites can be distinguished from eolianites. The two red soils represent solutional unconformities, during which time fresh-water percolation induced diagenesis in the underlying rocks. Bermudian calcarenites can be divided into five diagenetic grades, representing increasing intensity of fresh-water alteration. Fossil land snails provide some criteria for the recognition of units. Radiochemical dates are available for most formations and allow a quantitative assessment of Pleistocene eustasy for the last 200,000 years. Bermudian stratigraphy records sea-level fluctuations and can only be interpreted rationally as the result of Pleistocene eustasy.

Diagenesis of Skeletal Carbonates

ABSTRACT Stabilization of metastable aragonite and Mg-calcites is an inevitable process and constitutes a large and important part of carbonate diagenesis. Stabilization can occur in only a limited number of ways. Solid state stabilization takes place by inversion or exsolution for aragonite and Mg-calcite, respectively. Stabilization can take place by total dissolution and reprecipitation, or in the case of the Mg-calcites, by incongruent dissolution yielding a replacement product of calcite plus a solution enriched in magnesium. Lastly, stabilization can take place by replacement by either dolomite or a non-carbonate phase. The processes of stabilization which skeletal reactants undergo have been investigated by hydrothermal techniques, and the products of the reactions studied in thin sections. Aragonite inversion produces coarse-grained equigranular mosaics, and the rate of inversion of skeletal material seems to be only temperature-dependent near 300°C. Non-skeletal aragonite is less reactive. Mg-calcite exsolution is very slow compared to aragonite inversion, and the phase which exsolves is a very calcium-rich disordered protodolomite. The kinetics of dolomitization have been studied semiquantitatively near 300°C in aqueous solution. Dolomitization is speeded by: increased instability of the reactant,increased calcium plus magnesium concentration of the dolomitizing solution,increased magnesium/calcium ratio of the dolomitizing solution, d. increased solution/solid ratio, andincreased temperatureProtodolomite, rather than dolomite, forms as the system becomes more dilute in magnesium. Analytical steady state solubilities of skeletal reactants in distilled water and in sea water are presented. Long term experiments suggest that Mg-calcites undergo an incongruent dissolution to produce calcite plus a solution enriched in magnesium. The different stabilities of the various kinds of skeletal reactants permit different mechanisms of diagenesis to operate, and allow a variety of products to form. Stabilization seems, in many cases to be an early process in the history of carbonate rocks, and therefore preferential reactions of metastable phases indicate an early age for the reactions. We are not yet sufficiently astute to interpret all the various products.

Holocene Dolomitization of Supratidal Sediments by Active Tidal Pumping, Sugarloaf Key, Florida

ABSTRACT Sugarloaf Key is an area where Holocene dolomitization is occurring Calcium-rich dolomite is found in a 0.25- to 10-cm-thick surface crust which transgresses a thin layer of carbonate mud overlying the karsted Pleistocene Miami oolite. Radiocarbon ages of the crust range from 160 years B.P. to 1420 years B.P., with a corresponding increase in dolomite content from 0% to 80%. The relatively high permeability of the underlying Pleistocene oolite and low permeability of the Holocene carbonate mud results in a tidal lag between surface water and the partly confined aquifer. Consequently, seawater is pumped upward and downward through the Holocene sediment during spring tides. The highest concentration of dolomite is found in areas where the sediment layer is thinnest and tidal pumping is most effective. Limited analyses of surface and subsurface water taken at intervals throughout the pumping cycle suggest that the dolomitizing fluid is essentially Florida Bay water. The earliest diagenesis is caused by precipitation of dolomite cement, which occurs as 0.1- to 0.3-micron, subrounded crystallites that show no distinct crystal form. During further cementation, and somewhat later, during replacement of preexisting crystallites, poorly ordered dolomite forms as 1- to 5-micron euhedral rhombs. X-ray diffraction data indicate that the crystallized dolomite is better ordered and less calcium-rich than the dolomite composed of crystallites.

The fate of reef-derived sediment on the north Jamaican island slope

Abstract The Island slope north of Discovery Bay, Jamaica, consists of a steep, 5 km high, normal faulted monocline of Miocene chalk, dissected by submarine canyons, and partially mantled by pelagic brown clay—calcareous ooze. Beds of Holocene sand-sized reef debris are absent on the Island slope and in the adjacent abyssal basins of the Cayman trench. A few thin bends of sand-sized reef-crest sand are locally present less than one meter below the sediment—water interface on the upper Island slope, and resulted from seaward transport during a period of glacially lowered sea level. Minor amounts of sand, silt, and clay-sized reef debris are admixed with pelagic components in the late Pleistocene and Holocene sediments from the upper Island slope and abyssal basins adjacent to the Island slope. Reflection seismic profiles between Jamaica and Cuba suggest the Cayman trench to be a zone of north—south extension as a result of late Miocene normal faulting which created several kilometers of vertical relief. Eocene sediments may outcrop on horsts in the trench which bound the kilometer thick abyssal basins. Mass balance considerations suggest that half of modern carbonate reef productivity dissolves in adjacent deep water.

History of burial diagenesis determined from isotopic geochemistry, Frio Formation, Brazoria County, Texas

Progressive burial diagenesis of the Oligocene Frio Formation in Brazoria County, Texas, has resulted in extensive reaction between pore fluids and sediment in a major shift toward water/rock equilibrium. Carbon and oxygen isotopic data, combined with fluid isotopic data from the literature, indicate that quartz cement formed at 75 to 80°C and kaolinite at approximately 100°C. The zone of most rapid albitization is near 150°C. Authigenic carbonates formed over a wide range of temperatures, and those within the peak zone of hydrocarbon generation are depleted in 13C. At depths shallower than approximately 2,600 m, quartz and carbonate cementation in primary intergranular pore spaces (passive diagenesis) dominated. Below 2,600 m, within the geopres ured zone, reaction of detrital components (active diagenesis) is the major process. Organic maturation, albitization, and the transition of smectite to illite are the processes that contribute most of the components required for precipitation of cements. Quartz cementation occurred quite early in the burial history of the Frio (beginning at approximately 1,500 m of burial), when rates of fluid expulsion were at a maximum and when little of the Frio sandstone section had reached the zone of albitization.

Modern marine dolomite cement in a north Jamaican fringing reef

Minor quantities of ordered dolomite (..delta../sup 18/O = +2.0 per thousand PDB; composition = Ca/sub 1.22/Mg/sub 0.78/ (CO/sub 3/)/sub 2/) have formed from near-normal seawater in a subtidal hardground as part of a modern fringing coral reef. Crystals 5 ..mu..m in diameter precipitated within the past 1.8 ka in the form of syntaxial fringes on Mg-calcite marine cements and skeletal allochems. The crystals have a fine modulated microstructure and c reflections, both apparently formed during crystal growth

Meteoric Burial Diagenesis of Pennsylvanian Arkosic Sandstones, Southwestern Anadarko Basin, Texas

Pennsylvanian arkosic sandstones at Mobeetie field were deposited in fan deltas that prograded onto a shallow shelf in the southern Anadarko basin. The detrital minerals of the sandstones reflect the composition of the Precambrian granites and granodiorites that were exposed in the nearby Amarillo uplift. Distal margins of some fan-delta lobes were reworked by marine processes, and carbonate fossil fragments and oolites were mixed with terrigenous clastics. The diagenetic history and, hence, reservoir quality of the distal, marine-reworked sandstones differ from those of the more proximal, nonreworked sandstones. The earliest diagenetic events--formation of chlorite ooids and precipitation of fibrous submarine Mg-calcite cement--took place in the depositional environment of the marine-reworked sandstones. On the basis of isotopic data, the remainder of the diagenesis evidently occurred in fluids of meteoric origin. Shallow, early meteoric diagenesis induced precipitation of iron-poor calcite spar in reworked sandstones; nonreworked sandstones lack calcite spar but contain early pore-lining chlorite cement. Dissolution of aragonite by fresh meteoric ground water created moldic porosity within reworked sandstones. As burial increased, porosity in both the reworked and nonreworked fan-delta sandstones was reduced by precipitation of authigenic quartz, feldspar, kaolinite, Fe-calcite, and ankerite. Oxygen isotope data suggest that these cements also precipitated from fluids of meteoric origin that became increasingly hot but remained relatively constant in isotopic composition. The last diagenetic events were influenced by the overlying Permian evaporites. Sodium-rich fluids caused partial albitization of detrital plagioclase, and sulfate derived from the evaporites precipitated as anhydrite and celestite.

Early Dissolution of Sponge Spicules from Reef Sediments, North Jamaica

ABSTRACT Opaline sponge spicules are being preferentially removed from reef as opposed to back-reef sediments from North Jamaica. In contrast to presumed chemical solution of silica in the deep sea, rapid removal of opal from reef sediments may be taking place as a result of biologic agents.