Algal Boundstone Research Articles

Cambrian platform - outer shelf relationships in the Nordenskiöld Fjord region, central North Greenland

Cambrian sequences around the head of Nordenskiöld Fjord, central North Greenland, preserve the transition from platform interior carbonates of the Ryder Gletscher Group (redefined) to outer shelf-slope and platform margin sequences assigned to the Brønlund Fjord and Tavsens Iskappe Groups. Study of this transition has allowed redefinition and integration of these two stratigraphic schemes. Shallow subtidal to intertidal, well-bedded carbonates of the platform interior grade north-eastwards into high-energy grainstones and algal boundstones of the platform margin complex. Platform foreslope facies show well-developed clinoform bedding and wedge out into dark carbonates and clastics of the outer shelf sequence.

Lithofacies and Diagenetic Controls on Reservoir Development and Production, Goose Lake Field, Eastern Montana: ABSTRACT

Goose Lake field is located on a north-northwest-oriented structural nose on the western side of the Williston basin and produces from carbonate island facies in the Mississippian Ratcliffe zone. Island facies are concentrated on the structural nose and include low-energy, fenestral, pisolite-algal lime boundstones (supratidal) and high-energy, evenly laminated, pellet-oolite lime grainstones (beach and storm deposits). These beach grainstones occur preferentially along a narrow zone on the western side of the field, suggesting that major wave and wind approach was from the west. Development of distinct porosity systems within these rocks and in encasing offshore facies accounts for variations in permeability, oil and water production ratios, and overall cumulative production. Both reservoir facies contain porosity as high as 20%, whereas permeability is highly variable. In wells with high cumulative production (i.e., above 200,000 bbl), permeability is normally 10-20 md. Grainstones are typified by primary intergranular, oomoldic, and microvuggy porosity, whereas algal boundstones have micro-intercrystalline, fenestral, and microvuggy to mesovuggy porosity. Porosity occlusion in these facies is due to a variety of early calcite cements, late-stage anhydrite, and baroque dolomite. The encasing offshore marine facies may contain high micro-intercrystalline poro ity but lack effective permeability and serve as the updip seal. Porosity in these fine-grained facies and supratidal units is probably due to an early freshwater leaching episode that affected islands situated on paleohighs during rising sea level and less arid climatic conditions. End_of_Article - Last_Page 848------------

Provenance of Conglomerate Clasts from Upper Cretaceous Kuskokwim Group, Southwest Alaska: ABSTRACT

The predominantly Upper Cretaceous (Albian to Coniacian) Kuskokwim Group consists of marine turbidites and subordinate fluvial and shallow marine strata, deposited in an elongate southwest-trending basin covering over 70,000 km2 in southwestern Alaska. In the Sparrevohn and Cairn Mountain areas of the Lime Hills A-7 and A-8 quadrangles, fluvial, inner fan, middle fan, and outer fan facies are stacked with distal facies over proximal facies by northwest vergent thrust faults. Inner fan pebble-to-cobble conglomerate and pebbly sandstone were deposited as submarine grain flows up to 10 m thick containing reversely graded bases and normally graded tops. Clasts from these conglomeratic deposits are predominantly sedimentary rock fragments--particularly sandstone, si tstone, and argillite--originally thought to have been derived from the nearby Jurassic and Lower Cretaceous flysch (Kihiltna terrane). Detailed examination of these clasts, however, indicate that they contain as minor constituents Paleozoic coral, oolitic limestone, algal boundstone, radiolarian chert, and mafic, intermediate, and felsic volcanic rocks, most likely derived from the adjacent Nixon Fork, Dillinger, and Mystic terranes. Sandstone clasts are arkosic (Q25-F37-L38, n = 7) and contain subequal amounts of K-feldspar and plagioclase. Sand grains within these clasts are moderately sorted, subrounded, and have presolved contacts. Similar arkosic rocks have been described from the Dillinger terrane of the McGrath quadrangle and are the most likely source for the pebbles and cobbles within the Kuskokwim Group. End_of_Article - Last_Page 660------------

Calcified Algae as Sediment Contributors to Early Paleozoic Limestones: Evidence from Deep-Water Sediments of the Cow Head Group, Western Newfoundland

ABSTRACT Calcified algae are important elements of deep-water carbonate sediments in the Cow Head Group. Large algal boundstone boulders in debris-flow conglomerates consist of Girvanella sheets and arborescent clusters of Epiphyton. Silt- and sand-sized particles in fine-grained, thin-bedded slope limestones include abundant Girvanella, as single tubules, rafts of intertwined tubules, oncolites, and intraclasts. Integration of petrographic information from the boulders and fine-grained limestones indicates that many micritic peloids of silt to very fine sand size, the single most abundant particle type in the Cow Head Group, are a combination of comminuted Girvanella tubules and broken Epiphyton rods. Much of the structure grumeleuse in intraclasts is also diagenetically modified calcified algae. These findings suggest that many peloids and intraclasts in Early to Middle Paleozoic carbonates, in both shallow and deep water, may have been derived from calcified algae. Calcified algae may have been just as important in producing carbonate sediment during this time as benthic calcified algae are in the modern ocean. The difficulty in recognizing the algal contribution in most ancient sediments is probably caused by the simple microfabric of the resultant grains, filling and obliteration of tubules with micrite cement, or micritization.

Petroleum Potential and Stratigraphy of Holitna Basin, Alaska: ABSTRACT

The Holitna basin, an interior Alaskan basin, is flanked by Cambrian to Cretaceous sedimentary rocks that have been highly folded and faulted. Gravity mapping and modeling indicate up to 15,000 ft of sedimentary section is present within the basin. Cambrian rocks consist of trilobite-hash lime mudstone, red siltstone, and basinal chert. Ordovician through Devonian basinal facies rocks consist of platy limestone to graptolitic shale with minor interbeds of limestone turbidites and turbidite-channel debris flows. Ordovician through Middle Devonian shallow-water platform carbonate rocks prograde over the basinal rocks and are composed of algal boundstone buildups with associated inboard lagoonal, oolitic shoal, and tidal-channel deposits. Toward the interior of the platform, restricted subtidal and intertidal to supratidal carbonate rocks were deposited. Upper Devonian to Permian platform carbonate deposition occurred to the east of the older platform rocks and conformably(?) over basinal rocks. Minor mixed carbonate-clastic deposi ion occurred into Triassic time. The Cretaceous Kuskokwim Group is in fault contact with and/or unconformably overlies the Paleozoic carbonates. This unit varies from lithic-rich conglomerates to marine turbidite deposits. Maturation values for Cretaceous and Paleozoic rocks are within the oil window, with most of the shales showing a thermal alteration index (TAI) from 2 to 3. Organic carbon content exceeds 3% in some samples, however, deep surface weathering has resulted in low hydrocarbon values. The lithic-rich Cretaceous sandstones are well indurated, poorly sorted, and are considered to have low reservoir potential. Partly dolomitized, shallow-water Ordovician to Devonian carbonate rocks are the best potential reservoir rocks exhibiting vuggy porosities greater than 10% and good intergranular porosity. End_of_Article - Last_Page 308------------

Reef to Back-Reef Microfacies and Diagenesis of Permian (Guadalupian) Tansill-Capitan Transition, Dark Canyon, Guadalupe Mountains, New Mexico: ABSTRACT

In Dark Canyon, the transition from Capitan reef facies to Tansill back-reef facies occurs across a distance of 150 m (500 ft). Detailed 3-dimensional sampling of the transition reveals facies based on biota content and textural changes. Also revealed are postdepositional modifications owing to diagenesis. Three microfacies (A, B, and C), defined by the presence of a few diagnostic biota, occur between the Capitan reef and Tansill back-reef deposits. Several constituent biota occur throughout the zone, including calcareous sponges, gastropods, ostracods, problematic Tubiphytes, and foraminifera. Grading from Tansill shelf deposits to the Capitan reef, the facies are: (1) Tansill sensu stricto: dasyclad alga Mizzia, large gastropods, and brachiopods; (2) facies A; massive colonies of Collenia, bivalves, and red alga Parachaetetes; (3) facies B: large gastropods, bivalves, brachiopods, and bryozoans; (4) facies C: heads of Archaeolithoporella, and Mizzia; (5) Capitan reef: Archaeolithoporella, crinoids, and Mizzia. the reef proper is an algal boundstone, while backreef facies are packston s-grainstones. Submarine cements that have been modified diagenetically are dominant. The majority of the biota has been micritized. Large voids are filled by fibrous aragonite which has been replaced by botryoidal radial fibrous calcite. Many reef and near-reef limestones are extremely recrystallized. Back-reef areas have undergone several stages of dolomitization, whereas the actual reef is limestone. Some anhydrite moldic porosity is occluded End_Page 454------------------------------ by sparry calcite. More recent episodes of cementation have left some gravitational voids. End_of_Article - Last_Page 455------------

Zuloaga Formation (Upper Jurassic) Shoal Complex, Sierra de Enfrente, Coahuila, Northeast Mexico: ABSTRACT

The Zuloaga Formation at Sierra de Enfrente may be divided into 7 interbedded carbonate facies. In order of abundance, they are: pell-ooid grainstone-packstone, lime mudstone, pelletal packstone-grainstone, ooid grainstone, pelletal wackestone, skeletal wackestone, and algal boundstone. Three measured sections located on an east-west trending, overturned anticline (Sierra de Enfrente) were described and sampled. The sections are oriented along depositional strike and are roughly 395 m (1,300 ft) thick. Approximately 20 shoaling-upward that display an upsection trend from mudstone-wackestone lithofacies to packstone-grainstone-boundstone lithofacies are readily distinguished in each section. The cycles vary in thickness with thick cycles correlating well from section to section along depositional strike. Within any cycle, interpreted subenvironments may include, from base to top: subtidal, outer shoal, inner shoal, and inter-tidal-supratidal. The Zuloaga Formation sediments were deposited on a broad carbonate ramp dipping southward off the emergent Coahuila peninsula. The study area is located between Zuloaga-equivalent, near-shore, siliciclastic deposits to the north and outer-ramp Zuloaga lime mudstone facies to the south. This intermediate position was ideally situated for the development of high-energy shoals. Time-equivalent shoal facies that rim the Gulf of Mexico are prolific hydrocarbon producers (Smackover Formation, USA; San Andres Formation, Mexico). End_of_Article - Last_Page 476------------

Pedogenetic and diagenetic fabrics in the Upper Proterozoic Sarnyëré formation (Gourma, Mali)

Abstract Several diagenetic fabrics are described in the uppermost member of the Sarnyere formation, an Upper Proterozoic dolomite (700-600 Ma). Micronodular micrite, micro-sparite showing nodulisation processes, orbicular crusts and oncoids are thought to be due to the pedogenetic alteration of an algal (Porostromata) boundstone. Further evolution in a vadose zone creates a network of cavities which are later filled by “cave stromatolites”, micropopcorn micrite and multilayered cements. A rhythmical repetition of facies evidences a polyphased pedogenetic history, leading to a 30 m thick highly complex section. Such numerous and diversified structures are rarely preserved, especially in Proterozoic rocks, but each of them have equivalents in recent examples of caliche soil profiles.

Paleotectonic Control of Depositional Facies (Mississippian), Southwest Montana: ABSTRACT

Mission Canyon deposition occurred in southwest Montana on a shallow carbonate platform that extended to the Antler orogenic foreland basin, the eastern margin of which is near the Idaho-Montana border. Deposition on this platform did not take place in regular facies belts paralleling depositional strike, but instead occurred on fairly regular shelves (Alberta and Beartooth) in the east, and along shoals, emergent islands, and deeper water channels throughout most of the area. The facies that were deposited in these zones are directly related to the northeast-southwest and northwest-southeast structural trend imposed on the area during late Precambrian time, and the movement of paleostructural elements during Mission Canyon and pre-Big Snowy time. The thickest sequences i the Mission Canyon are those comprised of low-energy mudstones and wackestones, which were deposited in trough areas such as the Ruby and Centennial troughs, and the high to moderate-energy grainstone and packstone sections that typify the far western shelf margin sequences. High-energy shoal (lime grainstone) and low-energy island (algal boundstone) deposits are concentrated both on and around fault-bounded paleohighs (e.g., Pioneer Mountains in the Belt Island Complex) present during early Mission Canyon time; they are represented by thins on the Mission Canyon isopach map. Paleolows, such as the Ruby-Crazy Mountain and Centennial troughs, developed on east-northeast-trending, downdropped, fault-bounded basement blocks, and were filled with thick sequences of mostly restricted marine dolomite mudstones and wackestones. These restricted marine lithologies also occur as relatively thin zones in the upper portions of the fining upward high-energy shoal sequences in the lower Mission Canyon. However, they are most extensively developed in the upper Mission anyon, where they contain regionally correlative evaporite units that are represented on outcrop by solution collapse breccias. Only minor amounts of low energy, normal marine rocks occur in the Mission Canyon Formation, and most of these were deposited near the far western margins of the carbonate platform. A major lowering of sea level took place in middle Mission Canyon time. This resulted in the deposition of a regionally correlative supratidal sequence. The ensuing regional transgression resulted in the development of a new shelf margin farther to the west, and deposition of the restricted marine and evaporite units, mentioned above, behind it. Deposition of the Mission Canyon Formation ended with the total withdrawal of the sea from southwest Montana, regional exposure, and the formation of an extensive karst system and widespread solution collapse breccias. End_of_Article - Last_Page 1343------------

Depositional Framework and Reservoir Potential of an Upper Cotton Valley (Knowles Limestone) Patch Reef, Milam County, Texas: ABSTRACT

ABSTRACT The Knowles Limestone is an upper unit of the Cotton Valley Group, and in Milam County, Texas, is approximately 350 feet thick and consists of shales, terrigenous dolomitic limestones, grainy limestones, and algal boundstones with stromatoporoids and corals. The boundstones represent an elongate, wave resistant, encrusted skeletal patch reef which probably developed on a subtle salt-generated topographic high. The reef appears to be slightly more than 1 mile across in its narrowest lateral dimension as determined by facies correlations of three cored wells in the study area. Principal reef framebuilders included massive and columnar stromatoporoids, solitary or dendroid corals, and encrusting Tubiphytes and Lithocodium algae. Dendroid stromatoporoids, digitate corals, Tubiphytes oncoids, echinoids and various mollusks were the principal contributors to the detrital infill of the reef. Increased dip and dip reversal of draping beds suggest that the Knowles reef achieved relief above the sea floor. Evaluation of dipmeter logs indicates that the beds dip 5° northwest on the updip reef flank and 10° southeast on the downdip reef flank. Reef core boundstones and reef talus were consistently present downdip, and lagoonal to tidal flat facies were common updip throughout Knowles deposition. The reef organisms eventually became overwhelmed with terrigenous sediment transported downdip as the tidal flat environment prograded over the lagoonal, reef talus and reef core facies at the end of Knowles deposition. Early dolomitization of tidal flat and lagoonal facies has created local porous zones in some of these rocks. However, the reef, per se, is cemented by sparry calcite and is not a potential reservoir facies.

Oxygen Isotopes as Index for Paleosalinity During Deposition of Pennsylvanian Marble Falls Limestone of Central Texas: ABSTRACT

One major goal of a depositional model is to portray variations in salinity across an ancient facies tract. Qualitative determination of paleosalinity gradients with whole-rock, oxygen isotope analyses of limestones is usually hampered by the effects of cementation and reequilibration with formation water at depth. Numerous characteristics of the Marble Falls Limestone make it ideal for facies analysis using isotopes. Burial did not exceed 1 km and the low vitrinite reflectance, averaging 0.30%, indicates cool temperatures. A closed system during stabilization is manifested in high strontium levels in calcite (2,000 to 5,000 ppm). The Marble Falls was deposited on the Llano platform which was bordered on the east by the Fort Worth basin. An east-west sediment profile consisted of black spiculite, algal boundstone, oolitic grainstone, spiculitic wackestone, and calcareous shale. Carbonate mud within spiculite and boundstone of the foreslope environment is isotopically (^dgr18O = -2.52 + 0.55 per mil PDB, n = 12). The whole-rock composition of grainstone (^dgr18O = -4.00 ± 0.56, n = 10) is in accord with a two-component mixture consisting of marine ooids and meteoric cement, supporting the contention that the platform rim was exposed. The high standard deviation of the del values for spiculitic wackestone from the platform interior (^dgr18O = -3.76 ± 1.24, n = 2 ) is due to changing amounts of runoff from the Concho arch. Calcite within the shale, a marsh deposit, is depleted (^dgr18O = -5.10 ± 2.26, n = 2). It is rare to find limestone sequences with facies-specific oxygen isotopes because most have undergone complex cementation histories or burial-related recrystallization. In limestones known to have escaped burial, oxygen isotopes should be of great utility for paleosalinity determinations, when combined with other types of data. End_of_Article - Last_Page 642------------

Facies Analysis of Pinnacle Reefs of Guelph Formation (Middle Silurian), Northern Michigan: ABSTRACT

Five facies were defined on the basis of a Q-mode cluster analysis of petrographic data derived from well core samples of five Guelph Formation pinnacle reefs. The five statistically significant facies are crinozoan facies, bryozoan facies, mixed skeletal facies, stromatoporoid-coral facies, and brachiopod facies. Additionally, empirical data were used to define an algal facies. Pinnacle reefs initially formed as carbonate mounds when bryozoan and crinozoan mud dwellers baffled and trapped sediment. This mechanism allowed the mounds to grow upward into a zone near wave base. As mud-mound buildup reached wave base, an ecological regime suitable for the development of a stromatoporoid-coral reef was established. Guelph Formation pinnacle reef growth ended after the deposition of an algal boundstone lithofacies. Crests of pinnacle reefs were at or close to sea level prior to a major sea regression marking the end of the Niagaran Series. End_of_Article - Last_Page 573------------

The Hydrological Significance of Holocene Tepees, Stromatolites, and Boxwork Limestones in Coastal Salinas in South Australia

ABSTRACT Holocene groundwater lakes (salinas) are common in the coastal zone of South Australia. They usually contain an evaporitic carbonate sequence that surrounds a central gypsum unit to form a bull's-eye pattern of sediment distribution in the salina. The evaporitic carbonates can be subdivided into three units: a boxwork boundstone unit, a veneer boundstone unit, and an algal boundstone unit. These units contain many interesting structures including tepees, stromatolites, and mound springs, all of which show a megapolygonal outcrop pattern. The salina carbonates have formed under a hydrological regime that has changed little since its inception some 6000 years ago. Consequently it has been possible to interpret the hydrological significance of the salina carbonates. The salinas are fil ed by schizohaline surface waters whose water levels and salinities fluctuate seasonally. All of the salina carbonates form where marine-derived groundwater resurges from a surrounding dune aquifer into the margin of the salina. Tepee structures form in the veneer boundstone as a response to groundwater-induced seasonal changes in the pore pressures of the underlying boxwork sediments. The recognition of such groundwater tepees or other groundwater-controlled structures in ancient carbonate sequences could aid paleohydrological interpretation of the ancient basin.

Sedimentology of a Middle Cambrian Outer Shelf Margin with Evidence for Syndepositional Faulting, Eastern California and Western Nevada

ABSTRACT The Middle to Upper Cambrian Emigrant Formation at Horse Thief Canyon, California, is a transitional section between the outer shelf Bonanza King Formation and the basin facies represented by the Emigrant Formation in Nevada. At Horse Thief Canyon the Emigrant is subdivided into four members: the Limestone-Siltstone Member, the Papoose Lake Tongue, the Megabreccia Member, and the Limestone-Chert Member. For a portion of Middle Cambrian time the shelf structure was that of a ramp. Bedded and burrowed pelletal dolostones were deposited in the deeper waters of the outer shelf. Sediment for the outer shelf may have been derived from a mid-shelf shallow subtidal to peritidal zone. Cycle patterns of burrowed and bedded dolostones are probably related to periods of little sediment input accompanied by intense burrowing of the bottom followed by periods of heightened current activity and bedded dolostone deposition. Later in the Middle Cambrian the shelf structure changed to a platform. Megabreccia and breccia beds representing debris flow deposits and interbedded allodapic calcarenite and calcisiltite are evidence for the existence of a significant slope between shelf and basin. This slope may have been caused by syndepositional faulting and collapse of a portion of the outer shelf. The fault hypothesis is based on a rust-colored marker unit recognized in the Emigrant at Horse Thief Canyon and in the outer shelf Bonanze King section 13 km to the south along with the presence of distinctive algal boundstone clasts in the Megabreccia Member. The megabreccia beds lie above the marker unit but all known source rocks for the algal boundstone occur stratigraphically well below the marker. Uncertainty egarding the syndepositional fault hypothesis centers on whether the Horse Thief and Last Chance Range sections are in the same or different allocthons.

Late Jurassic Reefs of Smackover Formation--Preliminary Report: ABSTRACT

Algal and coral reefs are recognized in conventional cores of the upper Smackover Formation from southwestern Arkansas eastward into the panhandle of Florida. Although only one known reef has produced commercial hydrocarbons, attractive porosities and permeabilities (mean ^phgr of 15%, mean K of 20 md) result from freshwater leaching, fracturing, or dolomitization. In addition, the reefs may have provided a positive structural aspect to localized areas during later Smackover deposition and diagenesis. Smackover reefs formed in the Late Jurassic during periods of maximum marine transgression (good circulation, clear water, normal marine salinity) in three major paleogeographic settings: (1) the margins of Paleozoic highs protruding into the Smackover basin, i.e., Vocation field in Alabama; (2) upthrown basement fault blocks, i.e., Melvin field in Alabama; and (3) the seaward edges of upthrown salt-cored fault blocks, i.e., Walker Creek field in Arkansas, Hico Knowles and North Haynesville fields in Louisiana, and West Paulding field in Mississippi. The buildups are commonly elongate, 3 to 40 m thick and generally cover an area of several square kilometers. The reefs appear higher (younger) in the stratigraphic section downdip. Also, the reefs are younger and have a more diverse biot in Arkansas and Louisiana than they do in Alabama and Florida. Smackover reefs in Alabama and Florida were constructed by algae. Vertical relief on the reef surface during growth may have been a few meters. Similar reefs in southern Arkansas and northern Louisiana exhibit a vertical zonation suggesting an evolving reef community. These buildups are Tubiphytes-stromatolitic algal boundstones containing scattered corals toward the base; diversity increases upward with the addition of abundant corals (Actinastrea, Complexastrea, Thamnasteria, and others), sponges, skeletal algae, and byrozoans. The reefs are commonly underlain and overlain by subtidal peloidal lime packstones containing oncolites and scattered fossils, and they can develop in close proximity to subtidal quartz sands. End_of_Article - Last_Page 1675------------

Facies, Diagenesis, and Porosity Development in Lower Cretaceous Bank Complex, Edwards Limestone, North-Central Texas: ABSTRACT

An Edwards carbonate bank complex developed on the structurally positive Belton high in north central Texas. Mobile carbonate sand bars, rudist reefs, and beaches of the shallow-water bank differ strikingly from time-equivalent deeper water muds of the Tyler basin to the north, and supratidal dolomites and evaporites of the central Texas platform to the south. Early stages of bank development are well-exposed near Belton, Texas, and exhibit the following progradational vertical facies succession: open marine basin, shallow grain shoal, rudist path reef, exposed beach, restricted lagoon, and intertidal-supratidal mud flats. Paleocaliche, desiccation cracks, and algal boundstone suggest subaerial exposure of the uppermost unit. The major diagenetic changes occurred early, and predate regional exposure of the bank complex at the end of Edwards time. Most calcite cementation is early, and associated with local meteoric water tables, where fine crystalline bladed crusts, syntaxial overgrowths, solution-cavity fill, and medium to coarse crystalline equant calcite were precipitated. Early Mg-calcite bladed crusts also formed in the marine or mixing zone environments. Late-stage calcite cementation was limited to coarse crystalline equant calcite. Most of the dolomite is of multi-stage fresh water mixing origin. Dolomitization of lime muds and calcite cements occurred early, in association with local meteoric water tables, and late, as the result of a meteoric ground water system. Fine crystalline anhedral dolomite in lagoonal and tidal flat facies is of hypersaline origin. The shoreface and foreshore facies of the beach complex have up to 35 percent (thin section estimated) moldic and interparticle porosity as a result of (1) high original interparticle porosity, (2) early cementation to reduce compaction, (3) aragonite allochem dissolution to produce moldic porosity, (4) subaerial exposure to reduce further cementation, and (5) an early seal to retard fluid migration. The lagoonal and tidal flat facies have up to 20 percent (estimated) moldic and intercrystalline porosity due to (1) dissolution of aragonitic allochems, and (2) partial dolomitization and subsequent dissolution of the lime mud. 'Shell Oil Company, Houston, Texas HYDROGEN AND OXYGEN ISOTOPIC COMPOSITION OF CHERT FROM THE EDWARDS GROUP, LOWER CRETACEOUS, CENTRAL TEXAS

Origin of massive banded carbonates with "bluebird structures" in the Cambrian of the eastern Great Basin

Massive, banded, black and light brownish gray lime mudstone and coarsely crystalline dolostone containing millimeter to centimeter size, discrete white carbonate spar features known as bluebird are typical of the Middle Cambrian in the eastern Great Basin. Association of the lime mudstones with grainstones and stromatolitic algal boundstones indicates that the carbonate fines accumulated in calm water areas within an island and shoal complex and to some extent on tidal flats. Banding appears to be a function of the preservation of organic material due to varying redox relationships in poorly circulated waters of intershoal basins and island embayments. Bluebird Structures were formed by the calcitization of scattered patches of coarsely crystalline dolomite. The structures typically show centripetal and centrifugal fabrics which were produced by pore filling and replacement processes. Pore-filling fabrics are associated with the centrifugal replacement of ferroan dolomite; nonferroan dolomite appears to be centrifugally replaced by the growth of internal calcite inclusions.

Stratigraphy and Diagenesis of Niagaran Pinnacle Reefs (Silurian) in Northern Michigan Basin: ABSTRACT

In the Michigan basin, the Middle Silurian Niagaran reef facies directly underlies the Upper Silurian Salina evaporite facies. The basal Salina Formation includes the A1-evaporite, subjacent to the A1-carbonate and the A2-evaporite in successive order. Subsurface study of 40 drill cores disclosed that most of the Niagaran pinnacle reefs in the inner-basin slope environment ceased growth by A1-evaporite time and were regenerated during deposition of the A1-carbonate. The uppermost section of the algal stromatolite and algal boundstone facies at the top of the pinnacle reefs was deposited contemporaneously with the A1-carbonate of the off-reef section. The algal-stromatolite facies of the A1-carbonate overlies the A1-evaporite on the reef flanks and extends laterally to the off-reef facies in the vicinity of the pinnacle reefs. The general lithology of the A1-carbonate in the off-reef sections differs greatly from the lithology of the A1-carbonate reef facies. Some of the pinnacle reefs apparently continued to grow during the early formation of the A2-evaporite. There is no significant erosional contact between the A2-evaporite and the algal stromatolite at the top of the reef. Furthermore, LLH- and SH-type algal stromatolites show repeated stages of growth in situ, within the lowermost few feet of the A2-evaporite section. A vadose zone representing subaerial exposure is present in the pinnacle reefs at the top of the organic reef facies, which overlies the basal biohermal facies. This zone is characterized by a more complex sequence of diagenesis than either the basal biohermal facies or the uppermost algal stromatolite facies. End_of_Article - Last_Page 785------------

Paleoenvironmental Patterns in Middle to Lower Upper Cambrian Interval in Eastern Great Basin

In the eastern Great Basin region during the Middle Cambrian and early Late Cambrian a north-trending, near-sea-level shoal separated a deeper water open shelf on the west from a shallow shelf-lagoon. Regression in the late Middle Cambrian (unit K) led to a flooding of terrigenous sand across the continental shelf and the emergence of islands flanked by supratidal and intertidal zones on the shoal. At times preceding and following this event, a lacework of tidal algal mud banks and shallow subtidal basins occupied the shoal. Hydrologic conditions determined the morphology of the stromatolites on the mudbank. Normal supratidal processes were responsible for dolomitization along island shorelines, but the biogenic concentration of magnesium in algal mats, in conjunction with magnesium-enriched brines, accounted for the dolomite in the stromatolites associated with the tidal mudbanks. Dolomitization in the subtidal basins and shelf-lagoon was controlled by bedding and mottled structures of physical and biogenic origin, and was more pervasive in the lagoon. Unit K and its correlatives across the shelf are believed to be time-parallel in that there is little time differential along unit boundaries. On the shoal other algal boundstone units also may be time-parallel and serve as a basis for establishing temporal relations in the eastern Great Basin. Comparison of tidal-subtidal cyclic sequences in the Cambrian of North America and elsewhere may permit the recognition of eustatic and/or epeirogenic events useful in establishing temporal relations on a continental or larger scale.

Observations on Sandstone Cementation: ABSTRACT

Comparative study of Permian and modern caliche, laterite, bauxite, and beach rock have shown that vadose cement, regardless of composition, can be distinguished from cementation below the water table or sediment-water interface. Phreatic and connate cementation which occurs beneath the water table or sediment-water interface is more coarsely crystalline than vadose cement and is interstice-filling rather than grain-coating. Early carbonate cement precipitated from water-filled voids at shallow depths consists of sparry calcite and may push apart and replace quartz and feldspar to produce textures. Silica cement occurs as void-filling overgrowths on quartz grains. After deep burial uncemented sand grains become tightly packed, the depositional texture is modified, and late aqueous cement occupies smaller interstices. The following characteristics and stages of intensity of vadose cementation can be distinguished. 1. Initial stage: The original depositional texture is lost as precipitation of fine-grained evaporitic films of carbonate, iron oxide, or aluminum hydroxide forces the grains apart so that films separate grains at former contact points. Sedimentary structures such as beach laminae and current ripple cross-bedding still may be preserved in this stage. 2. Intermediate stage: Floating textures are produced as grains become pushed apart by evaporite cement and by replacement of quartz and feldspar by carbonate. Coalescence of evaporite films around clusters of grains forms small pisolitic concretions. Sedimentary structures become obliterated or greatly obscured. 3. Late stage: Extreme floating textures and large composite pisolites form by addition of more cement and replacement of quartz and feldspar. Brecciated caliche anticlines (also known as teepee structures) are formed by expansion resulting from the addition of large volumes of carbonate cement. Extreme caution must be employed in interpreting the genetic significance of rocks classified as sandy micrite, wackestone, and boundstone or biolithite. Transformation of quartzose sand into sandy carbonate, consisting of up to 90% fine-grained carbonate (micrite) which commonly is pisolitic (strongly resembling algal boundstone), is well documented by numerous examples. Similarly, accumulations of ooliths, clasts, skeletal fragments, pellets, and calcarenaceous material may be transformed into rocks which could be identified as oomicrite, intramicrite, biomicrite, pelmicrite, wackestone, and boundstone or biolithite (pisolitic) by vadose processes (calichefication). Many so-called oolitic iron ores represent vadose concentrations of iron oxide and hydroxide. End_of_Article - Last_Page 208------------