Reflux Dolomitization Research Articles

Origin of Dolostone Reservoir Rocks, Smackover Formation (Oxfordian), Northeastern Gulf Coast, U.S.A. (1)

Formation of regionally extensive dolostone reservoir rocks in the Smackover can be understood despite the possible effects of recrystallization. Geochemical and petrographic data suggest that dolomitization took place in (1) seawater-seepage, (2) reflux, (3) near-surface mixed-water, (4) shallow-burial mixed-water, and (5) deeper burial environments, which overlapped in time and space to form a platform-scale dolostone body composed of a complex mixture of dolomites. Seawater-seepage and reflux dolomitization occurred in the near surface penecontemporaneously with deposition of the Smackover and overlying Haynesville Formations. Dolomitization by seawater seepage occurred within an oolite grainstone sill which separated an intraplatform salt basin from the open sea. Seawater flowed landward through the sill in response to evaporitic drawdown of brines in the isolated intraplatform basin. Isolation of the salt basin occurred during the Oxfordian when the shoreline retreated from the Conecuh embayment. Dolomite located at the top of the Smackover enriched in {18}O suggests additional dolomitization by reflux of hypersaline brines. Reflux occurred as Buckner coastal sabkhas prograded over Smackover oolite grainstone shoreface deposits. Vugs lined with shallow-burial calcite and dolomite cements indicate flushing of the Smackover grainstone aquifer with fresh water. Freshwater intrusion probably occurred following sea level lowstands during the Late Jurassic and Early Cretaceous. Leaching in the proximal portion of the freshwater aquifer produced excellent limestone reservoir rocks in the updip Smackover. Dolomitization in the contemporaneous downdip mixed connate/freshwater zone formed dolostone reservoir rocks with depleted isotopic compositions consistent with a shallow-burial mixed-water origin.

Sedimentary and Faunal Associations of Protected Carbonate Shelves and Platforms: ABSTRACT

Protected carbonate shelves and platforms exhibit faunal and lithologic uniformity through time and space. Organic reefs of varied origin (stromatoporoid, rudist, coral) have sedimentologically and faunally similar backreef and lagoonal deposits. Backreef deposits, by definition, are dependent on organic reefs for their origin. Lagoonal deposits, however, primarily reflect restriction of circulation and are independent of the type of circulation barrier (reefs, islands, shoaling water). Thus, backreef deposits indicate presence of reefs, but lagoonal deposits reveal only the environmental character of the shelf or platform. Backreef deposits--skeletal sands and gravels originating through breakdown and transport of reef organisms--reflect relatively high-energy nearreef environments. Though generally characterized simply as debris deposits, these materials form well-defined sedimentary bodies. Transport by sheet flow across the reef or by tidal currents in interreef channels results in formation of skeletal sand banks, islands, downwind offreef drape, coarse rubble piles, and tidal deltas. Reef migration and lagoonward expansion of the reef flat may incorporate these deposits in the reef mass, accounting in part for the large amount of loose debris in reefs. More detailed investigation of lateral and vertical relations in modern environments should lead to recognition of these sedimentary bodies in ancie t rocks. Lagoonal deposits reflect restriction of circulation, and lagoons commonly are characterized as sediment traps for fine-grained carbonates. Geometry, lithology, and faunal content of lagoonal deposits reflect depth, lagoonal circulation, and ecology of lagoonal organisms. Facies belts paralleling bathymetric contours in atoll lagoons, and paralleling strike of carbonate-shelf lagoons reflect the effect of depth. The generally simple pattern of lagoonal deposition is modified by sedimentary accumulations such as mud mounds, islands, and their intertidal and supratidal facies, tidal deltas, and oolite bars. Climate and tectonics influence relative abundance of different types of lagoonal sediments. Increased restriction, arid climate, and low coastal relief lead to formation of vast sup atidal sabkhas, lagoonal evaporites, and reflux dolomitization. Exchange with the open sea promotes organic productivity and formation of skeletal sediments. Terrigenous influx results in complex terrigenous-carbonate facies. Sparse paleontologic data reveal consistent faunal associations in lagoonal and backreef deposits of diverse origins and ages. Foraminifera, mollusks, and algae dominate these faunas. Ostracods also are characteristic. Devonian reefs are unique in that lagoonal deposits are dominated by abundant fragile branching stromatoporoids. The striking similarity of lagoonal faunas through time should provide a superb framework for ecologic and evolutionary studies. Detailed resolution of carbonate subenvironments has been attempted most commonly in relatively thin, shelf-carbonate sequences. More detailed resolution of backreef and lagoonal subenvironments and sediment bodies is obtainable by more thorough definition and by application of available recent models. Investigation of vertical and lateral variation in lagoonal carbonates should provide useful data on shelf history. Effects of sea-level change, climatic variation, growth and destruction of barriers, and variation of sediment sources may be revealed more explicitly in these relatively uncomplicated sediments than they would in the complexly varying facies of the shelf or platform edge. Consideration of recent models suggests possible reinterpretation of development in several ancient carbonate complexes. End_of_Article - Last_Page 872------------

Mississippian Dolomites from Lisburne Group, Killik River, Mount Bupto Region, Brooks Range, Alaska

Allochthonous Osagian and Meramecian shelf carbonate rocks of the Lisburne Group that crop out from the Killik River west to Mount Bupto are extensively dolomitized and silicified. The carbonates were deposited as open-marine echinoderm-bryozoan wackestone, packstone, and grainstone with a thick sequence of interbedded carbonate mudstone in a shallow, restricted, marine to supratidal environment. Sequence of major diagenetic events in the dolomite is silicification of limestone to form chert nodules, followed by dolomitization of remaining limestone. Petrographic studies of the dolomites indicate that they have (1) dolomite rhombs with cloudy centers and clear rims, (2) voids formed by the partial dissolution of monocrystalline crinoid fragments, and (3) preserved organic skeletal voids. These features indicate dolomitization of limestone by hypersaline brines deficient in CO2. Field and stratigraphic relations suggest reflux dolomitization by hypersaline brines formed in a supratidal environment. The dolomite porosity, apparently not related to an ancient erosion surface or recent weathering processes, is thought to have been caused by the dolomitization process. Some pores contain anthroaxolite. Dolomite reservoir porosity may be present in the subsurface of the Brooks Range foothills and the North Slope from the Killik River to the Kiligwa River.

Recognition of Evaporite-Carbonate Shoreline Sedimentation: ABSTRACT

Evaporitic-carbonate shoreline sediments are deposited in an arid or semiarid climate by tidal currents which transport the sediment from the marine environment to the shore. The sediments accumulate primarily as tidal-flat deposits which prograde seaward producing a vertical sedimentary sequence from marine to supratidal. The supratidal sediments are the most easily recognized. They are characterized by irregular laminations, desiccation features, lithoclasts, and a general lack of fossil material. The intertidal sediments are more difficult to identify. They commonly are pelleted carbonate mud with burrows and a restricted fossil assemblage. Gastropods are dominant in many places. The vertical sedimentary sequence is the most useful tool to identify intertidal sediments The sediments just below the supratidal are generally intertidal sediments. The arid or semiarid climate allows seawater to evaporate and become saturated with respect to gypsum or anhydrite when the water circulation is sufficiently restricted from the open ocean. Primary bedded gypsum is precipitated in shallow lakes in the tidal-flat environment. In order for gypsum to be deposited in lagoons connected to the open sea by a channel, the ratio of the surface area of the lagoon to the cross sectional area of the channel must be about 106. Most gypsum or anhydrite associated with shoreline sediments is present as nodular, replacement, or porefilling crystals. This type of evaporite is secondary and crystallized from hypersaline interstitial water. The hypersaline interstitial water is the result of evaporation at the sediment-air interface. Bedded g psum or anhydrite, then, indicates the existence of a hypersaline lake or lagoon environment whereas nodular, replacement, or pore-filling gypsum or anhydrite is secondary and is found in marine, intertidal, and supratidal sediments. The precipitation of gypsum or anhydrite from seawater produces a dolomitizing fluid according to the reflux dolomitization theory. This type of dolomitization starts in the supratidal sediments and spreads into the underlying sediments. Therefore, extensive dolomitization associated with tidal-flat sedimentation indicates evaporitic shoreline sedimentation. Evaporites are removed easily by shallow groundwater. They are not well preserved in outcrops, and most commonly they are absent. Comparisons of subsurface anhydrite and outcropping shoreline sediments show that, in many places, the anhydrite or gypsum has been leached from the outcrop samples leaving molds and in places producing solution collapse breccias. Calcification of anhydrite or gypsum and dolomite is common. The recognition of evaporitic shoreline sedimentation from outcrop samples commonly makes it necessary to establish whether dissolution of evaporites End_Page 729------------------------------ and calcification of dolomite and anhydrite or gypsum have occurred. End_of_Article - Last_Page 730------------