Marine Carbonate Rocks Research Articles

Middle Paleozoic magmatism and orogenesis in the Brooks Range, Alaska

Felsic metaigneous rocks from the southern Brooks Range formerly thought to be of Cretaceous age have yielded Devonian and late Proterozoic U-Pb and Pb-Pb zircon ages. Eight slightly discordant U-Pb ages on euhedral zircons from south-central Brooks Range plutons and metavolcanic rocks define a chord that intersects concordia at 365 ±15 m.y. and 6 ± 120 m.y. Assuming episodic lead loss, the upper intercept is interpreted as the time of magmatic crystallization and the lower intercept as the time of cessation of lead loss due to metamorphism. The Middle Devonian crystallization date is supported by a Rb-Sr whole-rock isochron of 373 ± 25 m.y. Six zircon separates from the Ernie Lake and Sixtymile plutons of the south-central Brooks Range gave various late Proterozoic U-Pb and Pb-Pb ages. Because some of the zircons from these two plutons have rounded cores inherited from zircons in either the source rocks or the country rocks of the plutons, we are uncertain if the two plutons first crystallized during Proterozoic or Paleozoic time. From these data and other geologic criteria we hypothesize that (1) many Brooks Range plutons and some volcanic rocks were formed during middle Paleozoic time; (2) younger K-Ar mineral dates from these metaigneous rocks reflect a Mesozoic metamorphic event; (3) middle Paleozoic magmatic rocks together with adjacent coeval marine carbonate and clastic rocks and syngenetic ore deposits constituted an ensialic island arc or submerged continental margin magmatic belt; and (4) amphibolite-facies rocks around the plutons include early Paleozoic and late Proterozoic(?) crystalline basement to the middle Paleozoic magmatic belt.

Stratigraphic Relations of Permian Formations in Parts of Colorado and Utah: ABSTRACT

This study suggests that the Permian rocks in northwestern Colorado and northeastern Utah were deposited in shallow-marine shelf, transitional, and terrestrial environments. The Wolfcampanian and early Leonardian upper layers of the Weber formation are predominantly eolian quartz sandstones deposited in a broad coastal area of low relief. This coastal area was at the northwestern end of the ancestral Uncompahgre uplift, and was intermittently covered by seawater as indicated by the few thin marine carbonate rocks. The upper Weber intertongues with the Grandeur Member of the Park City Formation in the study area. The carbonates and cherts of the Grandeur were deposited in shallow-marine waters during a transgressive cycle which was probably caused by a crustal downwarping f the shelf. The Meade Peak Member of the Phosphoria Formation was deposited on top of the Grandeur by cold, phosphorous-rich, upwelling water as a result of continued Early Permian transgression. Landward from the phosphorites, carbonates were contemporaneously deposited, and further landward, siltstones. Regression near the end of Leonardian shifted the depositional environment belts westward and resulted in deposition of the Franson Member carbonates and cherts on top of the phosphorites. Maximum regression during the Guadalupian produced very shallow and highly saline waters in the area and subaerial exposure for long periods, combined with a significant increase of terrestrial, fine-grained sediment supply. These conditions led to the deposition of interstratified gypsum, silt, and shale of the Mackentire Tongue redbeds. The eastern half of the study area is characterized by greenish-gray and tawny beds which are partly time-equivalent of the Meade Peak, Franson, and Mackentire. The environments of deposition are interpreted to be those of a reducing, restricted marine embayment. These beds are more closely related to the Goose Egg Formation in central Wyoming than to any other formation in the area and are so designated. Extensive regression beginning in late Guadalupian continued into the Triassic and caused the deposition of the Moenkopi red beds. End_of_Article - Last_Page 728------------

Marine Permian Rocks of Tunisia: ABSTRACT

The only marine Permian rocks in northern Africa are exposed in the Djebel Tebaga area of southern Tunisia. Fault-duplicated sections of approximately 850 m of marine shale, limestone, and dolomite, are exposed over a distance of approximately 15 km, in the mountains 25 km northwest of Medenine. The sections grade upward from moderately deep marine shale and carbonate rocks, through a dolomitic, shallow marine section, to interbedded red and green terrigenous clastic and minor carbonate beds deposited in a littoral environment. The upper section is red beds, at and above the Permian-Triassic boundary. The apparently conformable sequence of Permian and Triassic rocks document a quiet termination of Tethyan marine deposition and suggest that the northern margin of the African plate was not involved in significant forward motion. Marine Permian rocks near Medenine represent reef complexes that accumulated in relatively low-energy environments, interrupted only by short high-energy pulses of deposition. Reef talus is rare. The reefs are composed principally of algae with some sponges and limited bryozoans and brachiopods. Sponges are particularly abundant around the shaly margins of the carbonate masses. Echinoderms, gastropods, brachiopods, and bivalves are relatively minor interbioherm elements and are scattered through the abundant sponges. Fusulinaceans of the Neoschwagerina-Yabeina assemblage occur throughout the marine part of the sequence. Post-Jurassic faulting and possible intra-Permian folding interrupt the section. End_of_Article - Last_Page 516------------

Sedimentologic Studies in Jurassic of Tunisia: ABSTRACT

Modern sedimentologic interpretation of the Tunisian Jurassic is based on excellent field studies by Tunisian and French geologists from 1955 to 1972. Jurassic facies north to south across Tunisia were formed where the African craton sloped into the southern edge of the Tethyan Sea and to the south in a restricted marine and evaporitic basin on the African shield itself. Thickness of the system varies abruptly off banks and into starved basins between 300 and 1,000 m. The Lias of central Tunisia forms a broad and typical carbonate platform separating the pelagic facies of the north from the major interior evaporite basin. A great north-south escarpment through southern Tunisia beautifully exposes these evenly bedded, restricted marine carbonate rocks and gypsum. Through the Middle and Late Jurassic the northern starved basin and slope facies (respectively radiolarian shales and Ammonitico Rosso) expanded into central Tunisia. Carbonate banks and patch reefs developed along the north-south axis west of the Sahel and its extension in the Jurassic ranges from Zaghouan to Bou Kornine uplifts. These probably rimmed the western margin of an ancestral Pelagian block. Bathonian slope deposits here consist of debris flows near Tunis, and the Kimmeridgian of Jebel Zaghouan shows a local reefy facies grading abruptly into turbidites and pelagic limestones to the north and west. These abruptly changing facies indicate a moderately unstable (rifting?) margin with intermittent reef growth. The Bathonian debris flows record a tectonic pulse which can be correlated with marked changes in thickening and elimination of strata along the Tebaga-Djeffara lineament, an important structural feature separating the southern evaporite basin from the northern unstable platform and basinal area. End_of_Article - Last_Page 553------------

Isotopic Composition and Sources of Strontium in Sandstone Cements in High Plains Sequence of Wyoming and Nebraska: ABSTRACT

The 87Sr/86Sr ratios of sandstone cements reflect the isotopic composition of strontium released into the pore fluid by different rock and mineral constituents. However, little is known about the extent to which the isotopic compositions of strontium in cements reflect local or regional variations in sandstone compositions. The 87Sr/86Sr ratios of sandstone cements permit identification of the major sources of strontium in the pore fluid and indicate the dimensions of the aquifer system within which the pore fluid was isotopically homogenized. Since the abundances of radiogenic 87Sr is continually increasing by decay of 87Rb, isotopic compositions of strontium may also suggest the sequence and time of cemen formation in sandstones that contain detrital mineral and rock grains having high Rb/Sr ratios. After removing the calcite, montmorillonite, or zeolite cements, sandstones from the Arikaree and Ogallala Groups of the High Plains sequence (Oligocene to Pliocene) have 87Sr/86Sr ratios ranging from 0.7065 for plagioclase arenite to 0.7491 for arkosic arenite; rhyolitic vitric ash samples have intermediate ratios of 0.7093 and 0.7133. In contrast to the detrital fractions of the sandstones, the cements contain strontium that is isotopically homogeneous over distances of 70 km or more. Calcite and montmorillonite cements from the Arikaree Group (Oligocene-Miocene) have an 87Sr/86Sr ratio of 0.7103, whereas calcite and clinoptilolite from the Ogallala Group (Miocene-Pliocene) yield 0.7112. The ratio of the cements suggests that the pore wate s were homogeneous on a regional basis and were not locally controlled. The slight difference in the isotopic composition of strontium in the cements of the Arikaree and Ogallala Groups may have resulted either from decay of 87Rb during the time interval of about 20 m.y. between lithogenesis of the Arikaree and Ogallala Groups or from differences in their mineral compositions. A quantitative model for mixing of different isotopic varieties of strontium indicates that Precambrian plagioclase, Paleozoic marine carbonate rocks, and Tertiary volcanic ash were the dominant sources of strontium in the pore solution and that Precambrian K-feldspar was the principal contributor of radiogenic 87Sr. End_of_Article - Last_Page 533------------

Strontium isotope composition of marine carbonates of Middle Triassic to Early Jurassic age, Lombardic Alps, Italy*

ABSTRACTThe 87Sr/86Sr ratios and strontium concentrations for thirty‐three samples of marine carbonate rocks of Middle Triassic to Early Jurassic age have been determined. The samples were collected from four measured sections in the areas of Val Camonica in northern Italy. The strontium concentrations vary from 40 to 7000 ppm. Most of the samples are calcitic limestones containing less than 10% of non‐carbonate residues. Dolomitic samples and those containing appreciable non‐carbonate residues have significantly diminished strontium concentrations. 87Sr/86Sr ratios of the carbonate phases of these rocks appear to be unaffected by dolomitization and by the presence of non‐carbonate minerals. The average 87Sr/86Sr ratios of the formations vary systematically in a stratigraphic sense. The ratio increased from Early Anisian to Early‐Middle Ladinian, declined during Late Ladinian and Carnian, rose again during the Norian and then declined throughout the Late Norian (Rhaetian), Hettangian, Sinemurian and Pliens‐bachian ages. The average 87Sr/86Sr ratios, relative to 0.7080 for the Eimer and Amend standard, are: Anisian: 0.70805 ± 00019; Early Ladinian: 0.7085 ± 0.00038; Late Ladinian: 0.70791 ± 0.00013; Carnian: 0.70776 ± 0.00015; Norian and Rhaetian: 0.70791 ± 0.00014; Hettangian: 0.70762 ± 0.00021; Sinemurian: 0.7070 ± 0.00038; Pliensbachian: 0.7070 ± 0.00015. These variations reflect changes in the isotopic composition of Sr entering the oceans in early Mesozoic time due to varying rates of weathering and erosion of young volcanic rocks (low 87Sr/86Sr) and old granitic rocks (high 87Sr/86Sr). The data presented in this report contribute to a growing body of information regarding the changes that have occurred in the 87Sr/86Sr ratio of the oceans in Phanerozoic time.

Isotopic Evidence for Microbiologic Role in Genesis of Crude Oil from Barrow Island, Western Australia

Oxidation of organic matter to carbon dioxide by microorganisms in the sedimentary environment is a process which alters the chemical composition of crude oils by sequential removal of hydrocarbon types. Differences in the chemical composition of Early Cretaceous Windalia crude oil (aromatic-naphthenic) and Late Jurassic crude oil (high wax) from the Barrow Island field of the Northwest Shelf, Western Australia, are consistent with the removal of n-alkanes from the former by microbiologic oxidation. However, the carbon dioxide so formed could combine with metal ions in solution to produce 12C-enriched secondary biogenic carbonate rocks in which the 13C/12C ratio of the parent organic matter would be preserved. Such carbonate rocks would di fer markedly in isotopic composition from those normal primary marine carbonate rocks in which the 13C/12C ratio averages zero PDB (Pee Dee Belemnite) and which result from equilibration with atmospheric carbon dioxide. A method for differentiation between the two carbonate rocks is thus provided. The concentration and isotopic composition of total reactive carbonate material in core and ditch-cutting samples from nine exploration wells in the Barrow subbasin were determined. Extensive sedimentary secondary biogenic carbonate rocks having 13C/12C ratios as low as -27 parts per thousand PDB were found in association with the main oil-producing Windalia zone (Early Cretaceous). The deeper, Late Jurassic high-wax oil-producing beds contained relatively minor amounts of secondary biogenic carbonate material. Carbonate rocks from other than these two oil-producing intervals were entirely or largely of primary marine type. Evidently normal diagenetic or geothermal maturation processes are not responsible for the high concentration of secondary biogenic carbonate rocks associated with the upper oil-rich zone; otherwise similar or greater contents of such biogenic carbonate material might be expected to accompany the oil found at greater depth. The extensive occurrence of significant secondary biogenic carbonate material in the Windalia zone and the virtual absence of n-alkanes in that interval suggest that the Windalia oil has been microbiologically modified. Possibly the traces of paraffinic-naphthenic oil which occur in the Windalia zone at the Flag 1 well, which are similar in composition to the Jurassic crude oils, are representative of the bacterially unaltered parent crude.

Petrology of Carbonate Rocks of the Green River Formation (Eocene)

ABSTRACT Lacustrine carbonate rocks of the Green River Formation contain a diversity of lithologic components and sedimentary features that are comparable to carbonate formations of marine origin. Fossils (algal plates, ostracodes, molluscs), coated grains (ooliths, pisoliths, circumcrusts), and polygenetic microcrystalline carbonate aggregates (intraclasts, peloids) are the main rock-forming carbonate grains. No genetic difference between ooliths and pisoliths was found. Both grain types display probable algal cell molds and are believed to be algal precipitates. Microcrystalline carbonate is more abundant than sparry carbonate in most rocks, although well-washed, grain-supported sparites are not uncommon. Dolomicrite is ubiquitous in the formation and petrographic evidence indicates that it ormed as a selective replacement product of calcium carbonate mud during early diagenesis prior to lithification. Admixed terrigenous constituents are an important component in most of the carbonate rocks; siltstone and sandstone are transitional with silty and sandy carbonate rocks. The similarity of lacustrine and marine carbonate rocks indicates that the two types can not be differentiated solely on the basis of petrographic relations. Sedimentary structures and stratification types are especially well developed in many of the carbonate rocks and greatly aid in the interpretation of subenvironments of deposition. The use of structures and stratification types in conjunction with lithologic variations and associations permits the recognition of several lacustrine subenvironments: mudflat, beachbar, lagoonal, shoal, transitional and offshore. Algal stromatolite bioherms locally are present in rocks of the shoal facies.

Metamorphic Alteration of Carbon Isotopic Composition in Ancient Sedimentary Organic Matter: New Evidence from Australia and South Africa

Insoluble organic matter (kerogen) was isolated from a suite of 23 marine shales and carbonate rocks of early Proterozoic to Middle Cambrian age from Australia. The stable carbon isotopic composition, δC 13 PDB , of the organic matter appears to vary with late diagenetic or incipient metamorphic rank as given by the hydrogen-to-carbon atomic ratio. Geochemical maturation of the kerogen beyond a rank equivalent to 91 to 93 percent carbon results in marked enrichment of the residual organic matter in C 13 . Hence, low-rank (unmetamorphosed) organic matter is isotopically lighter (δC 13 PDB range = −31.3 to −18.6 per mil, mean = −26.9 per mil) than high-rank (metamorphosed) material (δC 13 PDB range = −25.8 to −10.8 per mil, mean = −17.2 per mil). Kerogens from two Precambrian evaporitic sedimentary rocks are isotopically heavier than normal marine organic matter of similar rank. New isotopic and rank data from 11 carbonaceous, chert samples from the Onverwacht Group, Swaziland Sequence, South Africa, indicate that postdepositional thermal alteration may account for the anomalously heavy reduced carbon in the Theespruit Formation.

Stratigraphic Relations and Petroleum Potential of Smackover-Buckner Sequence (Jurassic), Clarke County, Mississippi: ABSTRACT

In eastern Mississippi, the upper member of the Smackover Formation and the lower member of the Buckner Formation represent a major regression of the Jurassic shoreline. The upper Smackover carbonate strata were deposited under shallow open-marine conditions, and the vertical sequence indicates progressive shoaling and basinward progradation of environments. The overlying lower member of the Buckner Formation consists of thick units of nodular anhydrite with interbedded dolomite, and is inferred to be the supratidal equivalent of the upper Smackover marine carbonate rocks. The principal reservoir rocks in the Smackover Formation are oolitic grainstones with primary depositional interparticle porosity. The high-energy oolitic deposits formed by tidal action on shoals which were aligned roughly parallel with the coastline. An understanding of the structural and sedimentologic factors which controlled the location of the oolite shoals is critical to explorations in adjacent areas. The oolitic deposits appear to have build up as a consequence of the intersection of wave base and a gently sloping sea floor, as there is no indication of a controlling shelf break. Although there is strong evidence of penecontemporaneous growth of salt-cored anticlines during deposition of the upper Smackover sediments, these structures are not thought to be the principal factor in controlling the location of the high-energy shoals. End_of_Article - Last_Page 2206------------

Carbonate Petrology of Green River Formation (Eocene), Uinta Basin, Utah: ABSTRACT

The Green River Formation contains a diverse suite of lacustrine carbonate rocks comparable to that of carbonate formations of marine origin. Fossils (calcareous algae, ostracodes, gastropods, pelecypods), coated grains, microcrystalline carbonate aggregates, sparry carbonate, microcrystalline carbonate, and terrigenous grains are the main rock-forming components of the lacustrine carbonates. The most abundant allo-chemical constituents are polygenetic microcrystalline carbonate aggregates (intraclasts, pelletoids) and fragmental algal plates. Coated grains (ooliths, pisoliths, circumcrusts) are less common and probably are biochemical (algal?) precipitates. Microcrystalline carbonate is the most abundant orthochemical constituent, but neomorphic and pore-filling sparry calcite are present. Dolomicrite is ubiquitous and probably formed as a replacement product of calcium carbonate before lithification. Terrigenous constituents are present in nearly all carbonate rocks; they constitute as much as 50% of some carbonates. The similarity of lacustrine and marine carbonate rocks indicates that the 2 types can not be differentiated solely on the basis of petrographic relations. Sedimentary structures, stratification, color, and lithologic associations and variations within the Green River Formation indicate that the carbonates were deposited in a wide range of lacustrine environments. Recognized depositional environments include mudflat, lagoonal, shoal, reef, and offshore. Preliminary ^dgr 18O and ^dgr 13C analyses of carbonate rocks from the Green River Formation indicate a biogenic fractionation for the microcrystalline carbonate of ooliths, pisoliths, and certain microcrystalline carbonate aggregates, and an early diagenetic replacement origin for dolomicrite. End_of_Article - Last_Page 812------------

Tectonic Control of Late Paleozoic and Early Mesozoic Sedimentation Near Hinge Line of Cordilleran Miogeocline: ABSTRACT

Pennsylvanian, Permian, and Lower Triassic strata in the eastern Great Basin aggregate about 35,000 ft of dominantly marine clastic and carbonate rocks that accumulated in the eastern part of the Cordilleran miogeocline. Subsidence and hypersubsidence created the Sublett, Oquirrh, Arcturus, Park City, Bird Springs, and other sedimentary basins within this major downwarp of the earth's crust. Late Paleozoic and early Mesozoic tectonism within and adjacent to the miogeocline controlled contrasting realms of clastic and carbonate sedimentation. Highlands in western and northwestern Utah, in eastern and northeastern Nevada, and in southern Nevada, certain orogenic belts (e.g., Sonoma) were stripped, in places to their Precambrian cores, and provided abnormal thicknesses of se iment to adjacent mobile negative depocenters. The craton east of the tectonic hinge line provided additional sediment. The Cordilleran miogeocline did not close its doors in chaotic orogenic spasms at the end of the Paleozoic. Contrarily, the changeover from Permian to Early Triassic was not a major diastrophic event. A paraconformity typifies the boundary at many places, with disconformities elsewhere. Up to 4,000 ft of Early Triassic sediments accumulated; by Middle Triassic time, a major reversal occurred: what had been a major negative mobile belt since late Precambrian time was now uplifted, although the region east of the hinge line became negative, and the hinge line was a fulcrum. The Cordilleran miogeocline was destroyed, and some of its sediments were stripped away, only to be recycled and deposited in the Rocky Mountain geosyncline on the east. End_of_Article - Last_Page 770------------

Sedimentology of Gachsaran Formation (Lower Fars Series), Southwest Iran

The Gachsaran Formation, formerly known as the Lower Fars Series, is exposed extensively in the foothills of the Zagros Mountains in southwest Iran, and forms the caprock at many important oil fields. During recent visits to this area, we were impressed by the almost universal nodular habit of the gypsum-anhydrite rocks, and by the conspicuous cyclic nature of the sediments. This paper presents two field sections which we suggest as typical of the marginal and the central-basin facies. The former was measured near the northern margin of the basin, where the beds at the top of each cycle are red marl, siltstone, or even sandstone, clearly of continental aspect; the latter was measured near the center of the depositional basin, where the evaporitic part of the cycle is dominant. In both sections, the base of each cycle is a marine carbonate rock of demonstrably shallow-water association, which is overlain by nodular gypsum-anhydrite rock, and at the top by dominantly red or variegated marl. The entire sequence is remarkably similar to the shallow-water, marine, supratidal sabkha, and terrestrial environments of the Trucial Oman Coast first described by Shea man in 1963. The Gachsaran Formation records a succession of minor cycles of sedimentation between the fully marine development of the Asmari limestone and the continental redbeds of the Agha Jari Formation.

Queen Formation (Guadalupian, Permian) Outcrops of Eddy County, New Mexico, and Their Bearing on Recently Proposed Depositional Models: GEOLOGICAL NOTES

The Queen, like several other formations of the Permian basin, contains a widespread quartz sandstone-dolomite unit in its uppermost part. The Shattuck Member is a widespread surface-subsurface marker unit. In the outcrop area investigated, the Shattuck Member of the Queen Formation undergoes facies change up depositional dip into marginal-marine evaporite (gypsum on outcrop, anhydrite in the subsurface) and intertongues south-southeastward with marine carbonate rocks. Prior to development of the Seven Rivers embayment as a topographic feature and before erosion of the upper part of the Queen from the Queen Mesa area, the updip facies change to gypsum is believed to have covered a sizable area, probably at least several hundred square miles. The Shattuck Member at outcrops in the Guadalupe Mountains is largely of marine origin, although it may have passed through an eolian stage prior to deposition in a marginal-marine lagoon. It was not deposited by southward progradation of dune sand updip from, and eventually over, an evaporite environment. Subsurface areas of largely, if not wholly, continental eolian Shattuck probably exist. Origin of a unit as widespread as the Shattuck should not be conceived as the product of one, or even mostly one, environment. The top of the Shattuck is neither an unconformity nor a terminus of the nonmarine phase of a cycle throughout its extent. Rather, this formational contact is a broadly subtle one intertonguing with marine to marginal-marine dolomites and dolomitic limestones of the overlying Seven Rivers Formation. Our evidence shows north-northwestward-sloping depositional topography flanking carbonate-sand barrier islands north of the Goat Seep bank. This slope reversed itself farther north to form the narrow-to-broad lagoon. The current regime was such as to allow quartz- and carbonate-sand transportation parallel with the barriers along the south side of the lagoon. Such a setting would foster quartz sand deposition in facies relation with marine carbonates flanking the islands on the south-southeast and marginal-marine evaporites on the north-northwest. Quartz sand could have reached the lagoon elsewhere along the coast by eolian or deltaic processes, or both.

Petrology and Stratigraphy of Earp Formation, Pima and Cochise Counties, Arizona: ABSTRACT

Stratigraphic and petrologic studies show that the Earp Formation of Pennsylvanian and Permian ages in southeastern Arizona and southwestern New Mexico consists of fluvial clastic rocks that interfinger with supratidal, intertidal, and subtidal marine carbonate rocks. The clastics encroached initially from the northwest into the shallow Earp sea during Missourian time and continued migrating eastward during Virgilian and Wolfcampian time. Clastic-ratio and isopach maps, a convex-upward geometry shown by a thinning of the overlying Colina Limestone, and primary sedimentary structures indicate that the clastic-dominated part of the Earp Formation was deposited as a delta. In the carbonate lithologies, 3 environments of deposition are recognizable: supratidal, intertidal, and subtidal. The supratidal environment is characterized by pelmicrites filled with sparry calcite surrounded by a micrite envelope; desiccated, interlaminated carbonates of algal origin (biomicrites); the absence of fossils; evidence of dedolomitization; and bird's-eye structure (dismicrite). The intertidal environment is composed of pelmicrites, biomicrites, micrites, and biosparites. Fusulinid biomicrites and biomicrites that have delicate tests preserved are characteristic of the subtidal environment. End_of_Article - Last_Page 349------------

Sedimentary Facies and Distribution of Biota in Coupon Bight, Lower Florida Keys

Coupon Bight is a 9-sq-km bay separated from an offshore Inner Reef Tract by low-lying islands and a bay-mouth bar. Coupon Bight has less wave activity and less complete tidal exchange than Inner Reef Tract waters, and is no more than 250 cm deep. Temperatures and salinities in Coupon Bight are more variable and usually higher than those in waters of the Inner Reef Tract. The harsher environmental conditions of Coupon Bight are reflected in its less diverse biota, compared to that of similar areas in the Inner Reef Tract. Environmental gradients persisting between the various parts of Coupon Bight produce more pronounced faunal variations than are found in the more homogenous Inner Reef Tract. Physical and chemical conditions, sedimentary facies, and biofacies of the bight are closely related. Several major environments can be recognized in Coupon Bight, each characterized by a particular assemblage of organisms. Open bay, nearshore, restricted bay, baymouth bank, and tidal channel environments are present. The unconsolidated sediment of Coupon Bight is mainly autochthonous skeletal debris, with minor contributions washed in from the Inner Reef Tract and from eroded limestone bedrock. The texture of bight sediments ranges from sand to mud, with most of the mud facies found in bank-like deposits associated with thick growths of the marine grass Thalassia. Most of the fine-grained sediment in Coupon Bight is the product of disintegration of coarse shell debris. Fine materials are winnowed out by waves and currents and either trapped on Thalassia covered mud banks or carried out of the bight. The variability of and relations between environments, sedimentary facies, and biofacies that can occur in a small area such as Coupon Bight are clarified by the data collected during this study. As a result, similar variability in shallow-water marine carbonate rocks can be better interpreted.

Carbonate Facies and Coral Zonation, Mississippian, Kogruk Formation, Lisburne Group, Delong Mountains, Brooks Range, Northwestern Alaska: ABSTRACT

The Kogruk Formation, 1,500 to 2,000 ft thick, is composed of marine carbonate rocks deposited in normal-marine to shoal-water environments. Carbonate rock types typically are bryozoan, echinoderm packstone and wackestone, and lesser amounts of calcareous mudstone, and ooid grainstone and packstone. Dolomite and other carbonate sedimentary rocks characteristic of intertidal and supratidal environments are absent in the sections studied. The Kogruk Formation was deposited in an open-marine environment on a subsiding shelf on which carbonate deposition and subsidence were near equilibrium. Only minor oscillations in environments of deposition are seen in a typical section. Lithostrotionid coral faunas are best developed adjacent to the shoal-water facies. Two major coral faunas are recognized. The older, 600-800 ft above the base of the Kogruk Formation, consists of Lithostrotion (Siphonodendron) sinuosum (Kelly), L. (S.) warreni (Nelson), Lithostrotionella mclareni (Sutherland), Thysanophyllum astraeiforme (Warren), Thysanophyllum orientale Thomson, and Sciophyllum lambarti Harker and McLaren. This fauna is of middle Meramecian age. The younger coral fauna is in the highest 600-800 ft of the Kogruk Formation and contains many of the species of lithostrotionids which are present in the lower beds, plus Lithostrotionella aff. L. macouni (Lambe), L. banffensis (Warren), a new species each of Lithostrotionella and Sciophyllum, and two species of Faberophyllum. This fauna is of late Meramecian age. End_of_Article - Last_Page 704------------

Revised Stratigraphic Nomenclature of Pennsylvanian System, Paradox Basin

Pennsylvanian rocks of the Paradox basin provide an excellent example of rock-stratigraphic units occurring in a clear-cut relation with time-stratigraphic units. These rocks have had many names applied to them; some are confusing and contradictory. A system of simple, formal rock-stratigraphic and time-stratigraphic terms is proposed that is usable even in difficult transition areas where evaporite facies grade into shelf carbonate and terrigenous clastic rocks. The Hermosa Group includes rocks that overlie the Molas Formation of Late Mississippian and Early Pennsylvanian age and underlie the Permian Cutler Group, except in the northwestern part of the basin where the Elephant Canyon Formation of Early Permian age disconformably overlies lithologically similar Pennsylvanian rocks. The Hermosa Group consists of three distinctive formations. The oldest is the Pinkerton Trail Formation consisting mainly of marine carbonate rocks and shale. The overlying unit is the Paradox Formation, a sequence of evaporites, carbonate rocks of restricted marine origin, and dark-colored shale. The Paradox Formation is restricted to strata containing evaporites; time-equivalent rocks containing no evaporites are excluded from the formation. Outside the area of ev porite deposition, the Hermosa Group is called the Hermosa Formation undifferentiated because the individual units become indistinguishable. The youngest formation of the group is the Honaker Trail Formation consisting of carbonate, shale, and sandstone. The Honaker Trail Formation disconformably underlies the Permian rocks in the greater part of the Paradox basin. The group contains rocks of Morrowan(?), Atokan, Desmoinesian, Missourian, and Virgilian Series, identified for the most part by abundant fusulinids. A formal time-stratigraphic nomenclature is proposed for further subdivision of the Paradox and Honaker Trail Formations. This system uses the pay zones of the petroleum geologists. These units are bounded at top and base by dark-colored shales of regional distribution which are approximate time-marker beds according to fusulinid determinations. These time-stratigraphic shale units can be traced through many thousands of square miles, even from the evaporites into non-evaporitic facies of the Paradox basin. The shale markers are 10-25 feet thick in most of the basin, but are thicker toward the east. A formal time-stratigraphic unit named the Four Corners Stage is proposed as a subdivision of the Desmoinesian Series. The Ismay, Desert Creek, Akah, and Barker Creek Substages are included in the new stage. It is suggested that these units (formerly the pay zones) be called substages when used as time-stratigraphic units in the Paradox basin. Alternate type sections are proposed for some Pennsylvanian formations: (1) an alternate (subsurface) type section for the Pinkerton Trail Formation, (2) a subsurface type section for the Paradox Formation, and (3) an alternate type section for the Honaker Trail Formation.

Clay and Other Non-Carbonate Minerals in Some Limestones

The minerals in acid-insoluble residues, 2 microns and less in diameter, from 27 limestones and dolomites, ranging in age from Cambrian to Recent, both marine and non-marine in origin were determined qualitatively. Quartz is widespread and abundant. Illite is the dominant clay mineral in both marine and non-marine carbonate rocks. Kaolinite is most likely to occur in Mississippian and Pennsylvanian limestones, and may be more common in non-marine than in marine rocks. Montmorillonite was found to be the prominent clay mineral in the Bethany Falls (Pennsylvanian) and in one bed of the Morrison limestones.