Skeletal Wackestone Research Articles

Carbonate Petrology of Arun Limestone, Arun Field, Sumatra, Indonesia: ABSTRACT

End_Page 524------------------------------The Arun gas and gas distillate field (estimated 13.7 tcf hydrocarbon gas in place) is a large Miocene coral-algal reef complex located on an intrabasin high in the North Sumatra basin. It was discovered in 1971 by Pertamina/Mobil Oil Indonesia following definition of a reflection seismic anomaly. The field is a large, asymmetric stratigraphic trap 18.5 km (11 mi) long and 5 km (3 mi) wide. The Arun Limestone which forms the reservoir is overlain, underlain, and possibly surrounded by shale. The limestone ranges in thickness from zero west of the field to about 1,200 ft (365 m) in well A-10. Closure on the Arun Limestone is at least 1,200 ft (365 m). The reservoir rock is made up of several carbonate rock types, including coral-algal boundstones, foraminiferal packstones and wackestones, mixed-skeletal wackestones and packstones, and dolomite. Interstitial fill of the reef consists of lime and reef detritus (i.e., skeletal wackestones and packstones); grainstone fabrics are notably absent in the Arun reef and related facies. Diagenesis has had a strong effect on the original sediments; (1) the lower part of the reservoir is completely dolomitized; (2) patches of limestone throughout the Arun are recrystalized to sparry calcite; and (3) much, if not most, of the reservoir is strongly micritized. The most pronounced effect of diagenesis has been the formation of secondary moldic and vuggy porosity formed by the dissolution of aragonite fossils (mainly branching corals, mollusks, and foraminifera). Had there been no diagenesis of the original limestone, it is doubtful that a hydrocarbon reservoir would be present on the Arun structure. End_of_Article - Last_Page 525------------

Corals of Madison Group (Mississippian), Williston Basin, North Dakota: ABSTRACT

Coral faunas studied from subsurface cores of the Mississippian Madison Group in the Williston basin of North Dakota indicate that Sando's coral zones for outcrops in western North America can be extended into the subsurface of North Dakota. Coral zones II and III are recognized as corresponding roughly to lower and upper Mission Canyon strata, respectively. These data were obtained from 12 wells along the northern border of North Dakota in Divide, Burke, Renville, and Bottineau Counties, and two wells near the center of Williston basin in Dunn and McKenzie Counties. Coral faunas appear to show a relatively low diversity of 13 species distributed among the following genera: Syringopora, Vesiculophyllum, Sychnoelasma, Amplexizaphrentis, Lophophyllum, Cyathaxonia, Lithostrotion (Siphonodendron), Diphyphyllum, Michelinia, and Stelechophyllum. Vesiculophyllum, Sychnoelasma, and Syringopora are the most abundant genera of the Madison Group in North Dakota. Corals found in dark argillaceous crinoid-skeletal wackestones representing deeper waters are robust, and this may infer a hospitable environment for their growth. However, evidence from the coral and lithologic associations refute the pervading dogma that the occurrence of corals is strictly facies controlled. Abundant smaller corals have been found from buff-colored skeletal wackestones and algal mudstones which alternate with subaqueous anhydrites representing a marginal marine environment. In addition, corals have been found in buff-colored skeletal and peloidal grainstones of adjacent shoals and in brown pisolitic-oolitic packstones-wackestones of possible tidal ponds. These latter deposits may represent allochthonous accumulations, but the amount of time involved in transport f corals would not invalidate their usefulness as biostratigraphic tools. End_of_Article - Last_Page 1360------------

Stratigraphy and Sedimentology of Upper Helderberg Group (Lower Devonian), New York: ABSTRACT

Field and petrologic study of the upper Helderberg Group has elucidated facies relationships. A tongue of the Kalkberg formation, recognized locally by Rickard (1962), at the top of the lower Helderberg cycle, extends southeastward to Catskill Creek where it thickens slightly, changes lithologic character, and passes laterally into the basal Becraft formation of the upper cycle. The Becraft represents a progradational phase within the general Lower Devonian transgression, and is seen as correlative with parts of both cycles. Thus, sedimentation was continuous between the two transgressive cycles. These revisions only slightly alter lower-cycle stratigraphy and preserve the integrity of Rickard's time lines. The Becraft formation is divisible into three units. (1) (Base) High-angle, cross-stratified, crinoidal grainstones interbedded with shale, cross-laminated, silty, peloidal grainstones, and packstones. (2) Trough cross-stratified, gypidulid-bearing, crinoidal grainstones with rare channels. (3) (Top) Finer grained, bioturbated to vaguely laminated grainstones containing gypidulids, and in-situ, rootlike crinoid holdfasts, but less crinoidal debris. Units 1 and 2 represent progradation of a tidally influenced, skeletal-sand shoal over shoal-margin and deeper subtidal environments. Unit 3 is transitional to the superjacent Alsen formation. The Alsen formation consists of irregularly bedded, bioturbated, skeletal, and peloidal grainstones. Bryozoans, brachiopods, and End_Page 1167------------------------------ micritized skeletal debris are the dominant particles. Peloids are abundant in the middle of the formation. The Alsen represents slightly deeper water and less current activity than the equivalent upper Becraft. Port Ewen lithologies include irregular and nodular, silty, skeletal wackestones and packstones, silty, peloidal packstones, and interbedded shale. Ostracodes, bryozoans, brachiopods, and micritized skeletal grains represent the sparse fauna. Bioturbation is ubiquitous; Chondrites, Planolites, and Zoophycos lebensspuren are common. The Port Ewen formation records the deepest, least-oxygenated environment in the Helderberg Group. End_of_Article - Last_Page 1168------------

Anatomy of the Dolomitized Carbonate Reservoir, Mission Canyon Formation, Little Knife Field, North Dakota: ABSTRACT

The Mission Canyon Formation is a regressive, shoaling-upward carbonate to anhydrite sequence deposited in a slowly shrinking epeiric sea. From its base upsection, the formation is mostly subtidal in origin and emergent at its top, and consists of (1) deeper water carbonates, (2) major cycles of open shallow-marine mudstones grading up into skeletal packstone or grainstone, (3) minor cycles of dolomitized transitional open to restricted marine mudstone grading up into skeletal wackestone, (4) dolomitized restricted marine pelletal wackestone or packstone, (5) partially dolomitized marginal marine skeletal wackestone, (6) slightly skeletal, oolitic-pisolitic wackestone or grainstone barrier-island buildups with storm washover aprons, (7) thin lagoonal limestones, (8) tidal flat anhydrite, and (9) sabkha anhydrite. The oil is structurally trapped on the north, east, and west, within the northward plunging Little Knife anticline. Facies changes entrap the oil southward; the vertical seal is the overlying anhydrite beds. Closure is less than 100 ft (30 m). Porous, hydrocarbon-bearing beds were deposited as transitional open-to-restricted marine, restricted marine, and marginal marine lime muds. These became porous dolomitic reservoir rock by undergoing three diagenetic changes: (1) anhydrite replacement of skeletal fragments, (2) dolomitization of the muddy matrix, and (3) later, leaching of the anhydrite to create moldic porosity. The reservoir's pore system is composed of moldic pores and three types of dolomite intercrystalline pores--polyhedral, tetrahedr l, and interboundary-sheet pores. Pore throats in productive beds are of two general sizes (1.2-1.6 µ and 5.2 µ). End_of_Article - Last_Page 854------------

Lithofacies and Depositional Environments of Middle Ordovician Shelf Carbonates in Southern Appalachians: ABSTRACT

Detailed field and laboratory observations are the basis for the description of 13 lithofacies in the Middle Ordovician Chickamauga Group in Jefferson County, Alabama. These lithofacies, including stromatolitic, fenestral and bioturbated mudstones, and varieties of skeletal and peloidal wackestones, packstones and grainstones, represent deposition in seven major environments: high intertidal, low intertidal, tidal channel, subtidal wave baffle, subtidal bar, subtidal bank, and subtidal level bottom. Middle Ordovician deposition began on the extremely irregular karstic surface of the Cambro-Ordovician Knox Dolomite. A lag deposit of reworked dolomite and chert clasts (Attalla Member) is overlain by intertidal deposits. High and low intertidal lithologies vary laterally in hickness in response to deposition over highs present on the irregular Knox surface. With continued transgression, shallow-water facies were succeeded by a suite of subtidal deposits composed dominantly of level-bottom lithologies deposited below wave base. Changes in sedimentation and/or subsidence led to a gradual shallowing and deposition above wave base. Coarse-grained, high-energy tidal bars and skeletal bank deposits dominate this part of the sequence. In the southern part of the study area, subtidal level-bottom lithologies dominate the upper part of the Middle Ordovician sequence. To the north, these lithologies grade laterally into shallower subtidal and intertidal lithologies. The sequence is unconformably overlain by the Upper Ordovician Sequatchie Formation. End_of_Article - Last_Page 1433------------

Chappel (Mississippian) Biohermal Reservoirs in Hardeman Basin, Texas: ABSTRACT

Carbonate mud mounds with flanking crinoidal sands are commonly associated with oil production in the Hardeman basin. These buildups are similar to the Waulsortian facies in Europe and to the Mississippian bioherms in the Sacramento Mountains of New Mexico. Cores and logs from Sun Oil Co.'s Quanah field provided subsurface control, enabling us to delineate six principal lithofacies that portray the depositional history of the Quanah buildup. The facies, in ascending order, are: (1) the mudstone core facies, (2) the skeletal wackestone upper core facies, (3) the skeletal veneer facies, (4) the flanking facies, (5) the intermound facies, and (6) the oolite shoal facies. The facies relations indicate an overall shoaling-upward sequence with two regressive-transgressive cycle superimposed. Early regression subjected parts of the core facies to subaerial exposure. Early dolomitization and solution brecciation resulted in intercrystalline and vuggy porosity. Subsequent marine sedimentation blanketed the karst surface. The dolomitized, vuggy core facies was subjected to several episodes of fracturing in the subsurface. One early fracture set was partly filled with large saddle dolomite crystals. Equivalent limestone strata were not comparably fractured and do not contain the saddle dolomite. The Quanah biohermal reservoir exhibits three kinds of porosity: moldic-vuggy, intercrystalline, and fractures. However, the porous zones are only marginally productive unless they are fractured because the vuggy porosity has low permeability. End_of_Article - Last_Page 1423------------

Facies Succession of Pliocene-Pleistocene Carbonates, Northwestern Great Bahama Bank: REPLY

Preliminary study of nine core borings from the northwestern Great Bahama Bank has yielded the first regional information on depositional facies and stratigraphy of pre-Holocene carbonates to depths of 75 m below sea level. The cross section of borings extends 120 km from Tongue of the Ocean across Andros Island and the shallow bank to Orange Cay on the western bank margin facing the Straits of Florida. The depositional facies present an atoll-like cross section with thick marginal accumulations of coral-coralline algal limestones at least 5 km wide both on the windward (east) and the leeward (west) margins. The discovery of these reef rims confirms earlier inferences that the Bahama Banks are Tertiary-Quaternary atolls. Although both margins have reef rims, there is a distinct asymmetry of facies within the bank: grainstones are more abundant on the windward margin and wackestone-mudstones are widespread over the interior and leeward part of the bank. The succession in borings from the interior of the bank can be subdivided into three intervals: the upper two intervals whose aggregate thickness averages 43 m are irregularly cemented, unstratified packstones and wackestones in which peloids and ooids are the predominant grains. The third, lowermost interval is predominantly stratified skeletal packstone with subordinate amounts of skeletal wackestone; it has an abundant fauna of bivalves, Foraminifera, and, commonly, branches of the finger-sized coral Stylophora spp. As the change from nonskeletal to skeletal limestones is relatively rapid and traceable for 40 km, it is used to define the base of a new formation, the Lucayan Limestone. The upper boundary of this formation is the limestone surface exposed on the Bahama Islands or cov red by soft Holocene sediments on the submerged banks. The Lucayan Limestone is dated as late Pliocene-Pleistocene on the basis of the disappearance of Stylophora spp. The known uppermost range of Stylophora spp. in the western Atlantic is through the upper Pliocene. Additional support for this dating comes from the distribution of discontinuity surfaces that are considered to be the record of subaerial exposure of the marine deposits. In the Lucayan Limestone, the frequency of discontinuity surfaces per meter of core is twice that of the pre-Lucayan unit; this increased frequency of subaerial exposure is consistent with the assignment of the Lucayan Limestone to the late Pliocene and Pleistocene. The major change in facies from stratified, skeletal, pre-Lucayan limestones to the unstratified, nonskeletal Lucayan Limestone is interpreted as the result of apparent shallowing of the bank triggered by the more frequent fluctuations of sea level and possibly by changes in the elevation of high stands; both began in the late Pliocene and continued throughout the Pleistocene.

Holocene Facies Succession and Depositional Environments of Semi-Enclosed Windward Lagoon off Great Abaco Island, Bahamas: ABSTRACT

The Abaco windward lagoon, bounded bankward by Great Abaco Island and seaward by the discontinuous Abaco Cays, exhibits up to 6 m or more of lagoonal, non-reef derived sediments, 5 km from the marginal escarpment of northeastern Little Bahama Bank. Six sedimentary facies, distinguished by texture, composition, and molluscan fauna, record the changing depositional environments as water depth and circulation increased during the Holocene transgression. A skeletal grainstone and mixed (skeletal-ooid-aggregate) grainstone are limited to inter-island channel areas. Away from these channel energy windows, a maximum sequence of 4 depositional facies is observed. The bulk of sediments are a normal marine (N.M.) skeletal wackestone or mixed (pellet-skeletal) packstone. The N.M. wa kestone dominates the western half of Abaco lagoon while the packstone occurrence End_Page 949------------------------------ characterizes eastern Abaco lagoon where it overlies the wackestone facies, accounting for 50% of the normal marine section. The N.M. wackestone typically grades downward into a dark gray restricted marine (brackish to hypersaline) skeletal wackestone, ^eqles70 cm thick, and then to a dark brown nonmarine soil zone, <=16 cm thick, above bedrock. Radiocarbon dates indicate flooding of Abaco lagoon at least by 7,446 YBP at - 10 m, followed by the transition from restricted to normal marine conditions as early as 4,716 YBP. Sedimentation rates increase from 16 cm/1,000 years for the restricted marine wackestone, to 58-104 cm/1,000 years and 216 cm/1,000 years for the N.M. wackestone and packstone facies, respectively. The windward lagoon setting illustrates the caution required in pre iction of facies continuity perpendicular to carbonate bank margins. Recognition of an ancient windward lagoon sequence may have important implications regarding sea level history and paleogeographic reconstruction. End_of_Article - Last_Page 950------------

Carbonate Depocenters and Facies Distribution on Passive Cambrian Shelf and Middle Ordovician Foreland Basin, Appalachian Orogene, Virginia: ABSTRACT

Depocenters in the southern and central Appalachians in Virginia appear to have been a major influence on thickness and carbonate facies distribution on the passive Cambrian-Ordovician shelf and in the Middle Ordovician foreland basin. The Upper Cambrian Nolichucky Formation illustrates sedimentation patterns associated with a shelf embayment (located above the southern Appalachian depocenter) on the passive Cambrian shelf. The Noli-chucky Formation is an onlap-offlap, shallow ramp to deep shelf sequence that consists of peritidal stromatolitic carbonate rocks, ooid grainstones, deeper ramp ribbon carbonate rocks, and embayment plain shale/siltstone/flat-pebble conglomerate facies. Facies bands are normal to the regional shelf edge and parallel the outline of the shelf embayment. The Middle Ordovician sequence illustrates the influence of depocenters on foreland basin evolution during a time of profound tectonism, when the shelf edge was uplifted, deformed, and subjected to erosion. The Middle Ordovician ramp-to-basin sequence is an onlap-of-flap package, that consists of peritidal fenestral lime mudstones, shallow subtidal cherty wackestones, ramp and downslope skeletal buildups, deeper ramp shaly skeletal wackestones, and basinal black limestones and shales. Widespread ramp and basin deposition commenced in southwest Virginia during south to north transgression from the southern depocenter. Widespread downwarping extended the basin into northern Virginia. Rapid clastic influx coupled with progradation of the carbonate ramp caused the southern basin to fill. Recognition of depocenters associated with carbonate sequences in orogenic belts is important if controls on direction of transgression, facies distributions, and thicknesses of units are to be better understood. Furthermore, the carbonate depocenters appear to localize development of subsequent clastic wedges. End_of_Article - Last_Page 770------------

Facies Succession of Pliocene-Pleistocene Carbonates, Northwestern Great Bahama Bank

Preliminary study of nine core borings from the northwestern Great Bahama Bank has yielded the first regional information on depositional facies and stratigraphy of pre-Holocene carbonates to depths of 75 m below sea level. The cross section of borings extends 120 km from Tongue of the Ocean across Andros Island and the shallow bank to Orange Cay on the western bank margin facing the Straits of Florida. The depositional facies present an atoll-like cross section with thick marginal accumulations of coral-coralline algal limestones at least 5 km wide both on the windward (east) and the leeward (west) margins. The discovery of these reef rims confirms earlier inferences that the Bahama Banks are Tertiary-Quaternary atolls. Although both margins have reef rims, there is a distinct asymmetry of facies within the bank: grainstones are more abundant on the windward margin and wackestone-mudstones are widespread over the interior and leeward part of the bank. The succession in borings from the interior of the bank can be subdivided into three intervals: the upper two intervals whose aggregate thickness averages 43 m are irregularly cemented, unstratified packstones and wackestones in which peloids and ooids are the predominant grains. The third, lowermost interval is predominantly stratified skeletal packstone with subordinate amounts of skeletal wackestone; it has an abundant fauna of bivalves, Foraminifera, and, commonly, branches of the finger-sized coral Stylophora spp. As the change from nonskeletal to skeletal limestones is relatively rapid and traceable for 40 km, it is used to define the base of a new formation, the Lucayan Limestone. The upper boundary of this formation is the limestone surface exposed on the Bahama Islands or cov red by soft Holocene sediments on the submerged banks. The Lucayan Limestone is dated as late Pliocene-Pleistocene on the basis of the disappearance of Stylophora spp. The known uppermost range of Stylophora spp. in the western Atlantic is through the upper Pliocene. Additional support for this dating comes from the distribution of discontinuity surfaces that are considered to be the record of subaerial exposure of the marine deposits. In the Lucayan Limestone, the frequency of discontinuity surfaces per meter of core is twice that of the pre-Lucayan unit; this increased frequency of subaerial exposure is consistent with the assignment of the Lucayan Limestone to the late Pliocene and Pleistocene. The major change in facies from stratified, skeletal, pre-Lucayan limestones to the unstratified, nonskeletal Lucayan Limestone is interpreted as the result of apparent shallowing of the bank triggered by the more frequent fluctuations of sea level and possibly by changes in the elevation of high stands; both began in the late Pliocene and continued throughout the Pleistocene.

Porosity Examples from IAB Field, Coke County, Texas: ABSTRACT

IAB field produces oil from an Upper Pennsylvanian (Cisco-Canyon) carbonate buildup situated on the western edge of the Eastern shelf. The reservoir is approximately 6 mi (9 km) long, 2 mi (3 km) wide, and 300 to 1,000 ft (100 to 300 m) thick. The upper surface of this buildup is irregular with several pinnacles, probably caused by subaerial erosion that occurred during drops in sea level. From examination of cores, 10 lithofacies have been identified and divided according to texture, sediment types, fossils, and mineralogic composition. Facies recognized are subtidal normal-marine deposits, ranging from relatively quiet-water skeletal wackestones to relatively high-energy, shallow-water oolitic grainstones. Porosity consists of both primary and secondary types and is controlled by depositional fabric, dolomitization, and freshwater diagenesis. Five main porosity types are (1) primary interparticle and (2) primary intraparticle porosity, both in grain carbonate rocks, (3) secondary leached grain and moldic porosity in grain carbonate rocks, created during a drop in sea level, (4) secondary vug porosity in muddy or very fine-grained carbonate rocks, also created during a drop in sea level, and (5) secondary intercrystalline and vug porosity in dolomites. Muddy carbonate beds had low initial effective porosities because of poor sorting, whereas grain-rich carbonate beds had high initial effective porosities owing to good sorting. Following deposition, sea level probably dropped, and the buildup was exposed to freshwater vadose and phreatic diagenesis. Solution of various grains and simultaneous precipitation of equant calcite cements produced secondary leached porosity and reduced primary interparticle and intraparticle porosity. Porosity preserved during this period of subaerial exposure and freshwater diagenesis was probably later reduced during deeper burial by additional cementation and compaction. End_of_Article - Last_Page 457------------

Fenestral and Associated Vadose Diagenetic Fabrics of Tidal Flat Carbonates, Middle Ordovician New Market Limestone, Southwestern Virginia

ABSTRACT Tidal-flat facies of the Middle Ordovician (Chazyan) New Market Limestone (0-67 m), southwestern Virginia. are fenestral. pellet-intraclast packstone/wackestone and lime mudstone. The unit unconformably overlies Lower Ordovician dolomites (Knox Group), interfingers with laterally equivalent skeletal limestone and is overlain by skeletal wackestone (Lincolnshire Limestone). Fenestral fabrics consist of: (1) tubular fenestrae (0.1-2 mm dia.), subvertical, cylindrical and straight to curved; (2) laminoid fenestrae (0.1-1 mm high), parallel to curved and less than 15 mm long; (3) irregular fenestrae (0.5-5 mm dia.), equidimensional to irregular. Counts of the fenestrae in two measured sections indicates that all three fenestral types occur together forming complex fabrics; the fabrics are not cyclically repeated but are influenced by depositional fabric. Tubular fenestrae probably formed by burrowing organisms in soft, intertidal and/or shallow subtidal sediments. Irregular and laminoid fenestrae formed by desiccation/lithification in more frequently exposed sediments resulting in hard, indurated surfaces in tidal flats, which inhibited further burrowin . New Market fenestral fabrics suggest that tidal flat sedimentation kept pace with subsidence or sea level rise so that the depositional surface remained in tidal/supratidal zones for long periods, causing complex overprinting of fabrics. General absence of cryptalgalaminates reflects intense browsing/burrowing probably related to non-hypersaline tidal waters. Associated diagenetic features (abundant vadose silt; commonly pendant equant calcite cements, equant calcite cements which predate vadose silt; molds of shells; and solution-enlarged molds of shells) indicate early lithification and widespread vadose-meteoric diagenesis. Rare beds containing columnar (fibrous) calcite cements suggest precipitation of cements from more saline waters. Intra-and interformational erosional surfaces that truncate grains and diagenetic fabrics are further evidence of early lithification. Fenestral and associated vadose diagenetic fabrics, and the lack of dolomites, evaporites, intraclast breccias and cryptalgal sediments indicate that New Market sediments formed under the influence of non-hypersaline (normal marine to meteoric) tidal and ground waters and wet climatic conditions. Such fabrics may characterize other carbonate tidal deposits formed under similar conditions.

Nearshore to offshore facies and depositional history of the Ordovician Daylesford Limestone New South Wales

Abstract The Daylesford Limestone is the basal formation of the Ordovician Bowan Park Group of central western New South Wales. The formation contains four main limestone types and minor intercalated terrigenous beds. Limestones are: (1) grain‐stone, (2) grey skeletal wackestone and packstone, (3) dark grey burrowed wackestone and packstone, and (4) dark grey burrowed lime mudstone. Grainstone and grey skeletal wackstone and packstone are dominant in eastern sections; they are laterally equivalent to, and interfinger with, dark grey muddy limestones that dominate western sections. Lithoclasts are abundant in the grainstone but are absent from muddy sections to the west except in thin beds above disconformities. The rock types of the Daylesford Limestone also tend to occur sequentially above some disconformities; the full sequence is: grainstone (or grey skeletal wackestone and packstone) grading up into dark grey burrowed wackestone and packstone and thence into dark grey burrowed lime mudstone. Each sequ...

Eustatic Low Stand of Sea Level between 125,000 and 105,000 B.P.: Evidence from the Subsurface of Barbados, West Indies

Brown micritic crusts occur at an elevation of 32 m below sea level in a borehole on the central west coast of Barbados, West Indies. The crusts are developed throughout a 2- to 3-m-thick interval of fore-reef skeletal wackestone that formed as part of a reef complex 125,000 B.P. Overlying the crust-bearing section is quartz-bearing skeletal grainstone containing clasts of the crust. The crusts formed in a subaerial environment during a low stand of sea level after deposition of the 125,000-B.P. reef. The overlying quartz-bearing grainstone is an ancient beach deposit that formed during the initial stages of the rise in sea level that culminated in the known 105,000-B.P. high stand. We estimate that the elevation of the low-stand sea level between 125,000 and 105,000 B.P. was −71 ± 11 m relative to present sea level. A sea-level fluctuation of this magnitude must be associated with an intense glacial event.

Algal Mudstone Mounds in Morrowan Stage (Lower Pennsylvanian) in Northeastern Oklahoma: ABSTRACT

The middle of the Morrowan Stage is marked by the development of an algal carbonate mudstone. The area of exposure covers 700 sq mi in northeastern Oklahoma with a maximum development of 60 ft in the southwestern 30 sq mi of the outcrop. A broad algal bank and smaller algal mounds developed within the area of maximum thickness, on the southwestern margin of the Ozark uplift. This area has high faunal diversity and the bank is cut irregularly by channels of skeletal sandstone. Northeastward, the faunal diversity of the unit is low, and the mudstone thins owing to replacement by skeletal grainstones and shale. The algal mounds are up to 6 ft high and 10 ft across. The core material consists of Archaeolithophylum and Cuneiphycus mudstone and boundstone, whereas the flank and intermound material consists of coarse skeletal packstone and wackestone. Influx of fine terrigenous clastics occurred during formation of this unit, as shale is present in small thin streaks and pockets. Locally, oolitic packstones and beds of algal oncoliths are found in the top. The mudstone is encompassed between skeletal grainstone throughout the areas of exposure. The overall dearth of skeletal debris, abundance of algae, occurrence of stromatolite-type boundstone, burrowing, and occurrence of dolomite indicate that the mudstone was formed in a shallow subtidal or tidal-flat environment. Abundant recrystallization of matrix mud to microspar and pseudospar has taken place, and dolomite, ferroan dolomite, and siderite are present locally as replacement of skeletal debris and mud matrix. End_of_Article - Last_Page 362------------

Upper Devonian (Frasnian) algae and foraminifera from the Ancient Wall carbonate complex, Jasper National Park, Alberta, Canada

Marine algae and foraminifers occur in the Ancient Wall carbonate complex of Upper Devonian (Frasnian) age in Jasper National Park, Alberta, Canada. This microbiota is the most diverse and abundant yet described and illustrated from North America; it closely resembles other microbiotas from the Upper Devonian of Western Australia and the Volga–Urals region of the Soviet Union.Strata of the Ancient Wall complex comprise two formations each with two members. They are: the Cairn Formation with a lower Flume Member and an upper member and the overlying Southesk Formation with a lower member and an upper Simla Member. A biota of 15 genera and 21 species is recorded from these rocks. The forms grouped under the algae are: Vermiporella sp., Girvanella sp., Sphaerocodium sp., Renalcis sp. cf. R. turbitus Wray, R. sp., radiosphaerid calcispheres, and algal? borings. The calcareous foraminifers are: Parathurammina sp. cf. P. spinosa Lipina, P. sp., Bisphaera sp., lagenid-like form, Paratikhinella sp. cf. P. cannula (Bykova), P. sp., Tikhinella sp. cf. T. measpis Bykova, T. sp., Evlania? sp., Eonodosaria sp. cf. E. evlanensis Bykova, E. sp., Multiseptida sp. cf. M. corallina Bykova, M. sp., and Nanicella sp.Calcareous foraminifers, dominantly parathuramminids, comprise about two-thirds of the total biota. The remaining one-third consists of algae, amongst which radiosphaerid calcispheres are the most abundant. The relatively primitive parathuramminids are more numerous in the Cairn Formation, whereas more advanced foraminifers such as tikhinellids, eonodosarids, and multiseptids occur more frequently in the overlying Southesk Formation. There is also an appreciable increase in foraminiferal diversity between the basal Flume Member and upper member of the Cairn Formation. Radiosphaerid calcispheres and parathuramminids are most numerous in skeletal wackestones and tikhinellids, eonodosarids, and multiseptids in less muddy packstones and grainstones.Study of this biota substantiates previous interpretations of the depositional environment based on stratigraphic and petrographic evidence. In general, these indicate shallow-water environments for both the Cairn and Southesk Formations, with the packstones and grainstones, which occur more frequently in the Southesk Formation, indicative of more agitated and presumably shallower waters.

Silurian Reef in Michigan Basin--Stratigraphic-, Facial-, and Reservoir-Properties Analysis: ABSTRACT

The Belle River Mills gas field (China Twp., St. Clair Co.), discovered in 1961 and since 1965 utilized as an underground gas-storage reservoir, is one of 42 known reefs of Niagaran (Middle Silurian) age in southeastern Michigan, 36 of which are oil or gas producers. The reef is an elongate pinnacle structure, about 2 mi long and 0.75 mi wide. It attains a maximum relief of 420 ft with slopes ranging from 10 to 30°. Three major growth phases are recognized: (1) biohermal, consisting of skeletal (crinoid, bryozoan, coral, and tabular stromatoporoid) wackestone and packstone rudites and arenites; (2) organic reef, consisting of a reef core (massive stromatoporoids, corals, and algae?) and associated interbedded and interfingering lithofacies of backreef skeletal wackestone rudite, burrowed mudstone and laminite, and coarse skeletal forereef talus; and (3) supratidal cover complex, composed of stratified algal stromatolites, flat pebble conglomerates, oncolites, and burrowed pelletal mud. The mound developed to a height of 150 ft in quiet, relatively deep water. The reef grew in turbulent water, attaining a height of 300 ft above the surrounding sea floor. Reef growth stopped as a result of shallowing and increased salinity, which led to the deposition of the supratidal complex. A conglomerate composed of algal stromatolite pebbles and boulders derived from the supratidal complex phase is present 400 ft below the reef crest at the base of the offreef Salina Group. It is covered by the A-1 anhydrite (15 ft) and the A-1 carbonate (120 ft) which both wedge out toward the reef walls. The A-1 carbonate is finely laminated, finely crystalline dolomite devoid of fossils, in places pelletal with algal mats, birdseye, and desiccation features and having an abundance of various textural forms of anhydrite. It is indicative of a very shallow-marine, partly supratidal environment. The conglomerate and the environmental interpretation of the A-1 carbonate demonstrate that the offreef sequence is post-reef in age and that the reef was subaerially exposed during the deposition of the overlying units. Dolomitization and diagenetic processes of leaching and reprecipitation associated End_Page 848------------------------------ with this exposure period destroyed much of the original depositional texture. A statistical data-reduction method was developed and used to construct a representative picture of the spatial distribution of reservoir properties. There is a correlation between stratigraphic and reservoir facies, markedly modified by diagenesis. End_of_Article - Last_Page 849------------

Bank to Basin Transition in Permian (Leonardian) Carbonates, Guadalupe Mountains, Texas: ABSTRACT

Light-colored, parallel-stratified, non-reef marine limestone and dolomite (Victorio Peak Formation) grade southeastward abruptly into dark cherty limestone (Bone Spring) along the northwestern margin of the Delaware basin (King, 1948). Study of the well-exposed transition zone in the Guadalupe Mountains suggests the presence of three major contemporaneous environments along a gentle basinward slope: bank, and bank margin (0.5-1 mile wide), both Victorio Peak; and euxinic basin, Bone Spring. Basinward regression of transition facies was 2-3 miles during accumulation of 1,500 feet of section. Middle Permian erosion truncated the transition strata creating basin-sloping unconformities on which limestone, megabreccia, and sandstone (Cutoff and Brushy Canyon Formations) were eposited. Principal lithologic facies of the transition zone are successively: carbonate grainstone and dolomite packstone (bank); dolomite wackestone and mudstone (bank margin); and carbonate mudstone (basin). Paucity of desiccation or solution features, algal laminates, oolites, and coated or composite grains suggests prevalence of sub-tidal bank environments. Allochthonous channel fills and sheet deposits of skeletal packstone and wackestone occur in bank-margin and basin-edge facies. Sand-size skeletal grains and carbonate silt are dominant constituents in bank and bank-margin rocks. Carbonate silt predominates in basin facies. Characteristics of bank relative to basin strata are lighter color, dominance of grain-supported rocks, coarser grain size, normal-marine fauna, more dolomitization, less chert (but larger, more rounded nodules), and thicker, more massive beds. Undulatory bedding and disturbed laminations characterize many basin strata adjacent to the bank margin. Two major controls on sedimentation and subsequent diagenesis were diminished turbulence with depth, and an abrupt change along the depositional slope from normal-marine to euxinic water at inferred depths of a few hundred feet. Bank proximity and finer particle size favored dolomitization. Low permeability of organic-rich basin facies apparently inhibited downdip dolomitization. Cementation obliterated most depositional porosity. End_of_Article - Last_Page 474------------

ENVIRONMENT RECONSTRUCTION FOR A PART OF THE GLEN ROSE LIMESTONE, CENTRAL TEXAS

SUMMARYAn approximately 50 ft. stratigraphic section of the Lower Cretaceous Glen Rose Limestone was sampled at 24 separate localities in central Texas from the standpoint of reconstructing the depositional environment. Among these samples 199 representative specimens were selected and subjected to point‐count analyses, X‐ray analyses, and insoluble residue determinations. Statistical analysis of the accumulated data with an IBM 7090 computer yielded the following facies: Corbula facies—characterized by a relative abundance of thick‐shelled ostracods, thin‐walled miliolid foraminifers, and the small pelecypod Corbula martinae; steinkern facies—characterized by large mollusc steinkerns, cellular mollusc shells and the foraminifer Orbitolina; mudstone facies—typified by less than 10% sand‐sized grains; and mixed particle facies—characterized by worn skeletal and nonskeletal carbonate grains. The mud‐stone facies subsequently was divided into two subfacies: (1) lime mudstones—characterized by delicate skeletal constituents and lime mud, and (2) marly mud‐stones—consisting of a mixture of lime mud and terrigenous clay‐sized material. Similarly the mixed particle facies was divided into four subfacies: skeletal calcarenites—characterized by sand‐sized skeletal debris; skeletal wackestones—consisting of sand‐sized skeletal particles floating in a mud matrix; nonskeletal calcarenites—characterized by nonskeletal carbonate grains; and nonskeletal wackestones—consisting of nonskeletal carbonate grains floating in a mud matrix. In addition limey sandstone, dolomite, and stromatolite facies were distinguished on the basis of relatively obvious textures and compositions.The attributes of these facies as evidenced by the 199 statistically analyzed specimens were then used to assign each of the additional samples (350) to a particular facies and to identify the distribution of these facies in the field. A reconstruction of the depositional environment was made for each facies, and the following depositional history was interpreted from the resulting facies pattern.The lowermost beds of the unit, consisting largely of stromatolites and nonskeletal calcarenites, are interpreted as representing deposition in very shallow, probably intertidal, waters, Following the deposition of these beds, the depth of water increased