Abstracts of Selected Citations in the 1980's

CARBON ISOTOPE FLUCTUATIONS IN CRETACEOUS PELAGIC LIMESTONES: POTENTIAL STRATIGRAPHIC AND PETROLEUM EXPLORATION TOOL

Scholle, P. A. and Arthur, M. A.

American Association of Petroleum Geologists Bulletin, v. 64, p. 67-87, 1980.

Abstract: Significant short-term carbon isotope fluctuations are present in Cretaceous pelagic limestones from widely distributed onshore sections in the Circum-AtlantIc-western Tethyan region. More than 1,000 closely spaced samples were analyzed during this study. At least seven major del ¹³C excursions can be correlated from section to section. The most important "heavy events" occur near the Aptian-Albian and Cenomanian-Turonian boundaries, whereas "light events" are near the Jurassic-Cretaceous, Albian-Cenomanian, Turonian-Coniacian, and Cretaceous-Tertiary boundaries. The association of "events" with stage boundaries and the consistent correlation of "events" between stratigraphic sections provides a significant new tool for time-rock correlation independent of standard biostratigraphic techniques.

The temporal association of these carbon isotope events with stage boundaries (faunal and floral events), global eustatic sea-level variations, and oceanic "anoxic events" demonstrates the potential usefulness of carbon isotope studies in interpreting variations in paleo-oceanic circulation. Furthermore, the association of carbon isotope variations with anoxic events is potentially useful for evaluation of the precise timing and the magnitude of preservation of organic matter in deep-sea and continental-margin sediments. Thus, isotopic studies may aid in estimating potential hydrocarbon resources in largely unexplored oceanic basins or along continental margins.

CARBON ISOTOPES AND RATES OF OCEANIC CONVECTION, FERTILITY, AND CARBON CYCLING

Scholle, P. A. and Arthur, M. A.

Congrès géologique international (26e, Paris), Résumés (Abstracts), v. 1 (Sections 1-5), p. 285, 1980.

Abstract: Variations of the carbon isotope ratios in pelagic limestones and in their calcareous microfossils reflect changes in cycling of carbon through the ocean system. Although the implications of del ¹³C data are not fully understood, time series analyses of del ¹³C series values from a number of localities show systematic, correlative excursions of greater than 1-2 m.y. duration during the Cretaceous and Cenozoic. The fluctuations in del ¹³C values may be up to 3.0 per mil about an average of +1.5 per mil; more rapid changes can be superimposed on longer positive or negative trends. Two major excursions to positive del ¹³C values in Cretaceous pelagic limestones from the circum-Atlantic and Tethyan regions correlate with "Oceanic Anoxic Events." The Aptian-Albian anoxic event, characterized by long-term widespread oceanic deposition of carbonaceous sediment, coincides with a gradual increase in del ¹³C from about +1 per mil in the Early Cretaceous to about +3 per mil in the late Aptian-early Albian. Del ¹³C values generally decrease in the late Albian to early Cenomanian. The Cenomanian-Turonian "anoxic event" manifested as a thin (and brief, ca. 1-2 m.y.) carbonaceous interval in many slope and shelf sequences coincides with a sharp and rapid excursion of del ¹³C values to >+3.5 per mil in the late Cenomanian-early Turonian and a rapid declipe to more negative values in the late Turonian. This close correspondence between more positive del ¹³C ,values and times of enhanced preservation of organic carbon in Cretaceous marine sediments suggests that more rapid rates of burial of marine or terrestrial organic carbon, due to either an increase or decrease in rates of oceanic overturn, disturbs the carbon mass and isotopic balances of the oceans, driving the overall ocean-atmosphere system towards "heavy" del ¹³C values. More negative del ¹³C values imply more efficient recycling of nutrients and better oxygenation of deep water.

GEOCHEMICAL AND SEDIMENTOLOGIC INDICATORS OF DEPOSITIONAL ENVIRONMENTS AND DIAGENETIC TRENDS IN THE NIOBRARA FORMATION (UPPER CRETACEOUS), COLORADO AND KANSAS [abs.]:

Arthur, M. A., Scholle, P. A., Pollastro, R. M., Barker, C. E. and Claypool, G. E.

Geological Society of America, Abstracts with Programs, v. 13, p. 398-399, 1981.

Abstract: The Niobrara Formation (Coniacian-Santonian) in Colorado and Kansas consists of limestone, argillaceous limestone and chalky shale (40-95% CaCO₃) deposited in a relatively shallow epicontinental seaway during an overall transgressive episode. The basal Fort Hays Limestone member exhibits highly bioturbated textures, low organic carbon content (0.35%), highly oxidized organic matter, which, with the whole rock del ¹⁸O and del ¹⁸O of Inoceramids, all reflect more normal salinities than those from the Smoky Hill Chalk above. The major part of the Niobrara Formation, represented by the Smoky Hill Chalk, was apparently deposited under predominantly poorly-oxygenated conditions. Organic carbon (0.5-5.7%; average 2.2%) and sulfur (0.05-2.5%) contents are high, organic matter is well preserved and of largely marine origin, and textures range from moderately bioturbated to finely-laminated. Whole rock del ¹⁸O values, and those from Inoceramids, are anomalously negative, and suggest periodic low-salinity conditions in surface waters. Salinity stratification as indicated by heavy del ¹³C values in limestones may have been responsible for periodic dysaerobic to anaerobic conditions in bottom waters of the seaway.

Degree of clay mineral diagenesis (smectite - mixed-layer smectite - illite) correlates well with depth of burial of the Niobrara, and whole rock del ¹⁸O trends due to carbonate diagenesis. Vltrinite reflectance values and pyrolysis of organic matter also indicate generally increasing maturation with depth of burial. However, anomalously low reflectance values (< 0.4 Ro) from the previously deepest buried portion of the Niobrara Formation strata suggest that rapid burial, intense compaction, and closed system diagenesis may retard organic maturation in these fine-grained limestones.

STRATIGRAPHIC AND PALEOCEANOGRAPHIC SETTING OF ORGANIC CARBON-RICH STRATA DEPOSITED DURING THE CENOMANIAN-TURONIAN "OCEANIC ANOXIC EVENT" [abs.]

Schlanger, S. O., Arthur, M. A., Jenkyns, H. C. and Scholle, P. A.

Association of Petroleum Geologists Bulletin, v. 67, p. 545, 1983.

Abstract: At, or very close to, the Cenomanian-Turonian boundary, strata from several basins bear the imprint of a global, short-lived "oceanic anoxic event" during which large amounts of organic carbon were sequestered in marine sediments. These strata are characterized by one or more of the following features. (1) The presence of a layer, up to 1 m (3.3 ft) thick, of black, laminated shale with total organic carbon contents of up to 23%. The general lack of bioturbation in these shales indicates an absence of benthic metazoan in fauna; the organic carbon is largely of marine planktonic origin. (2) The limestones, with or without an associated black shale horizon, at the Cenomanian-Turonian boundary level, have del¹³C values of +4.0 to +4.3 permil as contrasted to del¹³C values of +2.0 to +3.0 permil exhibited by limestones immediately above and below the boundary horizon. (3) Benthic foraminiferal faunas are lacking or consist of depauperate agglutinate faunas whereas radiolarians are locally very abundant as are diverse planktonic foraminiferal faunas.

These features are interpreted as indicating deposition in many areas within a water mass that was essentially depleted of oxygen. The high del¹³C values are taken to indicate enrichment of the global ocean in del¹³C as a result of the preferential extraction of ¹²C by marine plankton whose organic components were not recycled into the oceanic waters.

The basal and upper contacts between the black shales and the enclosing limestones are generally sharp or gradational over a distance of several centimeters indicating a rapid onset and equally rapid disappearance of deoxygenated waters. Sedimentation rate arguments lead to the conclusion that the Cenomanian-Turonian "oceanic anoxic event" occurred over a time span of approximately 350,000 to 700,000 years. Paleobathymetric interpretation of strata from European and African shelf sequences and sections in the U.S. Western Interior basin show that shallow embayments, flooded by the rapid Cenomanian-Turonian transgression were particularly hospitable to deposition of anoxic sediments as were the neighboring shelves and cratonic shallow seaways. The distribution of the black shale unit indicates that the upper surface of the Cenomanian-Turonian oceanic oxygen-minimum zone was 200 to 300 m (650 to 985 ft) below the sea surface analogous to that of today. The widespread distribution of anoxic sediments deposited synchronously during such a short-lived event indicates that such sediments were not the product of local climatic or local basinal water mass characteristics but were the product of a global expansion and intensification of the Cenomanian-Turonian oxygen-minimum zone. In some regions this was accompanied by increased biological productivity in surface waters.

RHYTHMIC BEDDING IN MESOZOIC-CENOZOIC PELAGIC CARBONATE SEQUENCES: THE PRIMARY AND DIAGENETIC ORIGIN OF MILANKOVITCH-LIKE CYCLES

Arthur, M. A., Dean, W. E., Bottjer, D. and Scholle, P. A.

in A. Berger, J. Imbrie, J. Hays, G. Kukla and B. Saltzman eds., Milankovitch and Climate, Part 1 [NATO ASI Series C, Vol. 126]: Dordrecht (Netherlands), D. Reidel Publishing Co., p. 191-222, 1984.

Abstract: Rhythmicity is a pronounced characteristic of nearly all Cretaceous and Cenozoic pelagic carbonate sequences whether deposited in shallow or deep-water environments. Intercalation of carbonate-rich and carbonate-poor beds, on the order of tens of centimeters thick, are particularly common in environments where there is also an abundant supply of clay. In anoxic or marginally oxic depositional environments, intervals of oxygen-depletion are periodic as well, and enrichment of organic carbon occurs in the relatively clay-rich intervals. In highly compacted pure chalks, the rhythmicity is reflected by decimeter-thick limestone beds separated by thin stylolitic seams. The estimated average periodicities of all most obvious types of cycles in Cretaceous-Cenozoic pelagic strata usually are either about 20 kyr or 40 kyr; groups of beds ("bundles") often form less obvious cycles with periods of 100 kyr. This is in contrast to the dominant 100 kyr periodicity in some Quaternary pelagic carbonate sediments. However, because of presently inadequate absolute time-scales for the Mesozoic, the exact periodicities are difficult to establish.

Glacial-interglacial changes in rates of deep-water production in high latitudes and consequent changes in carbonate dissolution probably cannot be called upon to explain the cyclicity in Cretaceous strata deposited during a period of globally warm climate. We propose, however, that periodic changes in insolation, evaporation, wind stress, and/or rainfall in a wide variety of environments caused changes in input of terrigenous detritus, water mass stratification, surface productivity, deepwater oxygen content, and rates of carbonate dissolution. A number of lines of evidence, including stable isotope and geochemical profiles across individual cycles support these inferences. The importance of different climate-related forcing mechanisms may have varied between depositional environments (e.g., different deep ocean basins or shallow epicontinental seas) and between paleolatitudes, but the overall periodicities are the same, and the manifestations in lithologies are similar regardless of the mechanism that caused the cycles. Cycles in pelagic carbonate sequences with Milankovitch-like periodicities are typical of all periods of time from at least the Late Jurassic to the present. Unfortunately, diagenesis commonly renders the cycles unsuitable for study of primary environmental signals, and progress in understanding the causes of such cyclicity will be slow.

BURIAL DIAGENESIS: OUT OF SIGHT, OUT OF MIND!

Scholle, P. A. and Halley, R. B.

in N. Schneidermann and P. M. Harris eds., Carbonate Cements: Tulsa, OK, SEPM Special Publication No. 36, p. 309-334, 1985.

Abstract: Examination of porosity data from Holocene and Pleistocene carbonate strata indicates that there generally is little or no porosity loss in the zone of near-surface water circulation [that is, in the vadose, meteoric-phreatic, or mixing zone(s)]. Thus, the transition from very porous carbonate sediments to well-cemented, low-porosity carbonate rocks is a dominantly subsurface process. Indeed, both shallow- and deep-marine carbonate strata show a continuous loss of porosity with depth, indicating that porosity-reducing processes act continuously fforn the surface to depths in excess of 4 km.

Experimental, observational, and geochemical data show that porosity loss through burial diagenesis results from both physical and chemical compaction and from cementation. In near-surface sections, dewatering, grain reorientation, grain breakage, and other mechanical processes lead to sediment/rock porosities as low as 30 percent. Continued porosity loss requires mechanical compaction, chemical dissolution at grain contacts and along solution seams or stylolites, and/or reprecipitation of dissolved calcite as intergranular cement. Calcareous shales or marl seams (donor beds) can act as significant sources of dissolved carbonate which is precipitated as cement in adjacent limestones (recipient beds). Quantitantive studies of stylolites and solution seams commonly underestimate the total magnitude of pressure solution because they ignore contributions from associated thick calcareous shale sections and from thinner, regularly and irregularly distributed marl interbeds. Through these mechanisms, carbonate rock porosity may be reduced to values near zero in "semi-closed" systems without significant introduction of allochthonous cementing material.

In many young, subsiding basins, patterns of porosity loss with depth are crudely predictable. These patterns provide standards against which individual case studies of diagenesis may be compared and provide predictive tools for estimating porosity prior to drilling. In other areas, the standards allow identification of anomalously high porosity and focus attention on specific mechanisms which would act to preserve primary (or early diagenetic) porosity or to create secondary porosity at depth. Comparisons of oil field porosities with standard curves will allow further refinement of our understanding of diagenetic processes.

Predictive models are still in their infancy, however. There is a critical need to independently assess how rates of porosity loss with depth are affected by time, temperature, depositional setting, early diagenetic history, maturation history of organic matter, and other factors. In addition, overpressuring, early oil migration, dolomitization, and hydrothermal alteration are known to affect porosity-depth relationships. Refining our understanding of these factors may help geologists use their general knowledge of basin history to make valid predictions of carbonate reservoir quality in frontier areas.

COMPARATIVE GEOCHEMICAL AND MINERALOGICAL STUDIES OF TWO CYCLIC TRANSGRESSIVE PELAGIC LIMESTONE UNITS, CRETACEOUS WESTERN INTERIOR BASIN

Arthur, M. A., Dean, W. E., Pollastro, R. M. and Scholle, P. A.

in L. M. Pratt, E. G. Kauffman and F. B. Zelt eds., Fine-Grained Deposits and Biofacies of the Cretaceous Western Interior Seaway: Evidence of Cyclic Sedimentary Processes: Tulsa, OK, SEPM Field Trip Guidebook No. 4, p. 16-27, 1985.

Abstract: Pelagic limestone units were deposited in the North American Western Interior seaway during two major Cretaceous transgressive episodes. The Bridge Creek Limestone Member of the Greenhorn Formation, deposited during the Late Cenomanian-Early Turonian transgression, and the Smoky Hill Member of the Niobrara Formation, deposited during the overall Early Coniacian-Early Campanian transgression, are both enriched in organic-carbon and exhibit smallscale carbonate cycles representing periodicities in the range 20 to 40 ky. The distinct periodicity and overall unusual depositional milieu of both units are reflected in their sedimentary structures, mineralogy, and geochemistry.

The Bridge Creek Limestone at Pueblo, Colorado, averages 78% CaCO₃ and 1.75% organic carbon with ranges of 42-96% and 0.06-6.97%, respectively, across small-scale cycles. High concentrations of Al, Fe, Mg, K, Ti, Na, Cr, Ni, V; higher Sr/Ca and lower Si/Al ratios; and lighter del ¹⁸O in CaCO₃ in dark-colored clay-rich beds all suggest periodic influx of terrestrial clay minerals during times of peak fresh water runoff from uplifted highlands to the west. Higher Sr/Ca ratios in marlstone beds than in limestone beds suggest that the marlstone beds have undergone less diagenetic removal of Sr. Higher concentrations of organic carbon, hydrogen, and sulfur, and preservation of some lamination in the clay-rich beds also suggest that the times of enhanced runoff may have induced stable salinity stratification in the water column, which led to gradual depletion of dissolved oxygen in the bottom waters and enhanced preservation of organic carbon in sediments. The geochemistry also suggests that a significant change in sedimentation occurred at the Cenomanian-Turonian boundary.

The geochemical characteristics of the Niobrara Formation near Fort collins, Colorado, are very similar to those of the Bridge Creek Limestone at Pueblo, suggesting similar depositional conditions and source of clastic materials. However, the small scale cycles are present but more subdued in the Niobrara Formation than in the Bridge Creek Limestone, and the Niobrara Formation in the Fort Collins area has not been as altered by diagenesis.

RADIAXIAL FIBROUS CALCITE AS EARLY-BURIAL, OPEN-SYSTEM CEMENT: ISOTOPIC EVIDENCE FROM PERMIAN OF CHINA [abs.]

Halley, R. B. and Scholle, P. A.

American Association of Petroleum Geologists Bulletin, v. 69, p. 261, 1985.

Abstract: The Nanpanjiang basin of south China occupies about 100,000 km² in southern Guizhou and eastern Yunnan Provines and northwest Guangxi Autonomous Region., The basin contains a thick Paleozoic carbonate sequence overlain by about 3,000 m of Triassic basinal deposits. Permian carbonate rocks comprise a large portion-of the Paleozoic strata and form several platforms separated by basins containing dark, thin-bedded limestones, siliceous shales, and cherts. The platform margins are rimmed by sponge/algal reefs.

Radiaxial fibrous calcite (RFC) is the most abundant cement in very coarse, sponge/algal debris of Upper Permian reef and fore-reef sediments exposed along the western margin of the Nanpanjiang basin. Small volumes of syndepositional cements, interpreted to have been fibrous calcites and botryoidal. aragonite, predate RFC. Coarse, blocky burial calcite postdates RFC. Evidence that RFC was precipitated during sediment deposition was not found. RFC occurs as isopachous layers up to 15 mm thick and exhibits white, gray and black bands about 1 mm wide. The presence of microdolomite inclusions in these cements iIndicates that they were originally magnesian calcites. Del ¹⁸O of RFC cements are more positive than any of the pre- or post-RFC components of the reef and fore-reef facies. Analyses of successive bands reveals the most positive del ¹⁸O near the center of the isopachous layers. Del ¹³C of successive bands reveals generally more negative values toward the centers of layers.

RFC layers are interpreted to have precipitated during early burial of the platform margin while reef and fore-reef sediments were in communication with sea water. Cement layers recorded isotopic characteristics of sea water as platform edge sediments subsided through the water column at the basin margin. Del ¹⁸O of successive bands records cooler water at depth in the basin followed by geothermal warming. Del ¹³C records increased incorporation of light carbon as the platform subsided through the oxygen minimum zone, followed by a return to normal values at depth. These data and interpretations suggest RFC layers precipitated very slowly during time spans commensurate with those of subsiding platforms (millions of years). Isotopic characteristics of RFC may not reflect shallow sea water. Rather, they may reflect burial environments (100's-1,000's m) where del ¹⁸O is affected by cooler water and del ¹³C is affected by biologic activity.

DIAGENETIC RELATIONSHIPS IN A HYDROCARBON-PRODUCTIVE CHALK — THE CRETACEOUS NIOBRARA FORMATION

Pollastro, R. M. and Scholle, P. A.

in F. A. Mumpton ed., Studies in Diagenesis: Washington, D.C., U. S. Geological Survey Bulletin 1578, p. 219-236, 1986.

Abstract: An understanding of diagenesis is critical in predicting reservoir or source-rock potential of the Niobrara Formation because the primary properties of the chalks and calcareous shales have been greatly modified by burial. Porosity and isotope composition of the chalks are consistently related to burial depth (or paleoburial depth) and thermal history. Initial porosities of 60 percent or more have been reduced by progressive cementation to about 25 percent with I km of burial and to less than 10 percent with burial depth of about 2 km or more. The oxygen isotope composition of the whole rock reflects these changes with shifts to progressively more negative del ¹⁸O values with greater burial.

The clay mineral composition and degree of ordering of mixed-layer illite/smectite (I/S) clay in bentonite beds and insoluble residues from the chalks can also be related to porosity loss due to increasing temperatures with burial depth. Although the starting composition of I/S clay originally deposited in chalk residues was highly variable, the expanclability and range of expandability of the I/S became increasingly more limited with greater burial. A systematic increase in the percentage of illite-like layers in the I/S of bentonites as well as a progressive increase in the degree of ordering of I/S was noted in more deeply buried samples. Regularly interstratified chlorite/smectite (C/S) was also formed in aluminous bentonites at elevated temperatures and pressures during deep burial (>100^o C). The composition and degree of ordering of I/S and the occurrence of C/S in bentonites are in good agreement with the level of maturation of kerogen and hydrocarbons within the Niobrara Formation, Denver basin. The clay minerals can, therefore, be used as geothermometers for predicting indigenous biogenic gas and thermogenic oil from the Niobrara. These depth-dependent relationships also suggest that parts of the Denver basin were more deeply buried in the past than formerly thought and (or) had paleogeothermal gradients higher than those of today.

RHYTHMIC BEDDING IN UPPER CRETACEOUS PELAGIC CARBONATE SEQUENCES: VARYING SEDIMENTARY RESPONSE TO CLIMATIC FORCING

Research on Cretaceous Cycles (R.O.C.C.) Group

Geology, v. 14, p. 153-156, 1986.

Abstract: Rhythmic bedding is a prominent feature of North American and European Upper Cretaceous pelagic carbonate sequences deposited in epicontinental and continental-edge settings. Such bedding rhythms can result from variations in carbonate productivity, terrigenous dilution, redox conditions, or bottom currents. Each type of bedding cycle is expressed differently in the stratigraphic record but probably was caused by climatic cycles that are linked to variations in the Earth's orbital characteristics (Milankovitch cycles). Thus, pelagic carbonates of Cretaceous age acted as particularly sensitive recorders of orbitally induced changes in climate. Documentation of these bedding rhythms will permit detailed chronostratigraphic and lithostratigraphic correlations and will further illuminate depositional processes in Upper Cretaceous carbonate sequences.

THE PERMIAN OF THE WESTERN MARGIN OF THE GREENLAND SEA — A FUTURE EXPLORATION TARGET

Surlyk, F., Hurst, J. M., Piasecki, S., Rolle, F., Scholle, P. A., Stemmerik, L. and Thomsen, E.

in M. T. Halbouty ed. Future Petroleum Provinces of the World: Tulsa, OK, American Association of Petroleum Geologists Memoir 40, p. 629-659, 1986.

Abstract: The Upper Permian of central East Greenland contains a relatively thick, widely distributed, oil-prone source rock. It occurs directly adjacent to large, carbonate buildups that constitute the main potential reservoirs. It is immediately overlain by thick, coarse-grained sandstones that are also a potential reservoir. The whole sequence is draped by Upper Permian and Lower Triassic shales that may act as a seal.

This has implications for the Norwegian shelf north of lat. 62°N. There are good reasons to believe that Upper Permian rocks of the same types as in East Greenland exist, deeply buried in this region. The Upper Jurassic shales are immature over large areas of the Norwegian shelf, and the presence of deeper buried Upper Permian source rock thus adds considerably to the petroleum potential of the area.

EXPLORATION AND DEVELOPMENT OF HYDROCARBONS FROM LOW-PERMEABILITY CHALKS -- AN EXAMPLE FROM THE UPPER CRETACEOUS NIOBRARA FORMATION, ROCKY MOUNTAIN REGION

Pollastro, R. M. and Scholle, P. A.

in C. W. Spencer and R. F. Mast eds., Geology of Tight Gas Reservoirs: Tulsa, OK, American Association of Petroleum Geologists Studies in Geology No. 24, p. 129-141, 1987.

Abstract: Chalk beds of the Upper Cretaceous (Coniacian-Campanian) Niobrara Formation were deposited in a shallow epicontinental seaway in the Western Interior of the United States during a major global sea level rise. Biogenic gas is produced from the thermally, immature, organic-rich chalk beds of the Niobrara in the eastern part of the Denver basin in eastern Colorado, northwestern Kansas, and southwestern Nebraska. These chalks have high porosity and low permeability. Accumulations of shallow gas are not controlled by major structural closures but by local, faulted, low-relief domal structures, or noses. Fracture stimulation, primarily with the use of foam treatments, is necessary to make gas production from these wells economically feasible.

Westward and at greater depth in the basin, however, oil is produced from much tighter, naturally fractured chalk beds that are thermally mature and capable of thermogenic oil generation.

The reservoir properties (mainly porosity and permeability) and types of hydrocarbons produced in the chalk of the Niobrara Formation from a given location within the basin are primarily controlled by diagenetic processes. Maximum burial depth (with associated differential pressure and temperature history) is the main controlling factor in reservoir quality. Thus, reservoir characteristics and source-rock potential of the Niobrara can be predicted from an understanding of the post-Niobrara depositional and thermal history of the region coupled with research that has identified systematic diagenetic changes.

SUBMARINE CEMENTED BRYOZOAN MOUNDS, UPPER PERMIAN, DEVONDAL, EAST GREENLAND

Hurst, J. M., Scholle, P. A. and Stemmerik, L.

in H. J. Geldsetzer, N. P. James and G. E. Tebbutt eds., Reefs, Canada and Adjacent Areas: Calgary, Canada, Canadian Society of Petroleum Geologists Memoir 13, p. 672-676, 1988.

Summary table:

General Location: Wegener Halvø, East Greenland

Age: Late Permian

Reef Type: reef mound, with dominant submarine cement

Dimensions: 150 m high, laterally exposed for approximately I km

Depositional Setting: drowning isolated carbonate platform

Tectonic Region: East Greenland Rift Basin

Crustal Position: cratonic margin, early rift basin

Foundation below Reef: karstified surface on top of hypersaline carbonate platform of the Karstryggen Formation

Bathymetric Range: subtidal, below photic zone, possibly 50 m to mound crest

Reef-forming Process: biological/early cementation

Dominant Organism(s): bryozoans, brachiopods, crinoids

Diagnostic Aspect(s): structureless cemented core; monospecific (brachiopods) flank beds

EVAPORITES AND DOLOMITES IN PERMIAN (GUADALUPIAN) CAPITAN FORE-REEF CARBONATES, DELAWARE BASIN MARGIN, WEST TEXAS AND NEW MEXICO [abs.]

Scholle, P. A. and Melim, L. A.

Geological Society of America, Abstracts with Programs, v. 20 (7), p. A211, 1988.

Abstract: Comparison of fore-reef facies from the Guadalupe Mountains with equivalent subsurface cores from the northern and eastern margins of the Delaware Basin indicates that extensive evaporite diagenesis has occured in both areas. The subsurface Capitan strata are extensively dolomitized and virtually all primary and early secondary porosity has been plugged with anhydrite and/or gypsum. Outcrop sections also show widespread (if localized) dolomitization, but most traces of original evaporite cementation have been obliterated by subsequent leaching and calcitization. Only minor pseudomorphs, breccia zones, and coarsely poikilotopic calcite remain to indicate the former presence of evaporite minerals. Solution-enlarged fractures and related karstic features apparently provided conduits for brines to pass from evaporitic shelf facies through the impermeable (penecontemporaneously cemented) reef and into the underlying fore-reef debris facies. Dolomitization and evaporite cementation were largely confined to such vertical conduits until brines encountered more permeable debris-flow, grain-flow, and turbidite beds of the fore-reef facies. The percolating brines then moved laterally in the more permeable units and produced both dolomite replacement and virtually complete evaporite cementation of the slope facies. The evaporite plugging precluded significant compaction during subsequent burial but also greatly reduced the reservoir potential of this facies. Evaporites presumably were preserved until mid-Tertiary block faulting and uplift led to breaching of regional hydrologic seals and renewed ground water circulation. This initially led to transformation of anhydrite to gypsum, and subsequently resulted in calcitization of gypsum or wholesale removal of evaporite minerals. Most of the coarse, sparry calcite from the Capitan fore-reef (generally described as "burial "- cement) is actually telogenetic-stage calcitized evaporite. These processes continue to the present-day in subsurface areas.

THE EXPRESSION OF SYNDEPOSITIONAL TECTONIC UPLIFT IN THE PERMIAN GOOSE EGG FM. (PHOSPHORIA EQUIVALENT) CARBONATES AND RED BEDS OF THE SHEEP MOUNTAIN ANTICLINE, BIGHORN BASIN, WY [abs.]

Simmons, S. P., Ulmer, D. S. and Scholle, P. A.

American Association of Petroleum Geologists Bulletin, v. 73, p. 413, 1989.

Abstract: Based on detailed field observations at Sheep Mountain, a doubly plunging anticline in the northeastern Bighorn Basin in Wyoming, there appears to have been active tectonic uplift at this site contemporaneous with Pennsylvanian and Permian sedimentation (personal communication, R. Inden).

The Permian (Leonardian to Guadalupian) Goose Egg Fm. at Sheep Mountain consists of 25-60m of silty red beds (including minor carbonate and evaporite units) by 15-30 m of dominantly intertidal carbonates (the Ervay member). A strong lateral variation of facies normal to the trend of the anticline is found within the red bed sequence: carbonate horizons on the anticline flanks are transitional with a gypsum/anhydrite along the crest. Similarly, shales on the anticline limbs grade into sands near the fold axis, indicating a paleohigh roughly coincidental with the present day anticline crest.

Ervay time (late Guadalupian) was marked by a more extensive uplifted structure in a marginal marine setting. On Sheep Mountain the unit is typified by intertidal fenestral carbonates, whereas outcrops to the east suggest a restricted marine facies, while outcrops to the west reflect a more open marine environaent. Thin sand lenses present in the Ervay are thought to represent terriginous sediments blown onto the sometimes emergent bank which were then captured through adhesion and cementation.

Anticlinal features similar to Sheep Mountain are common along the eastern margin of the Bighorn Basin. When found in the subsurface, these structures are often associated with hydrocarbon production from the Ervay member. Tectonic uplift contemporaneous with deposition of this unit may explain the localization of the productive fenestral facies on the present day anticlines.

SUBAERIAL EXPOSURE AND EROSION IN THE CAPITAN REEF (PERMIAN), GUADALUPE MOUNTAINS, NEW MEXICO [abs.]

Melim, L. A. and Scholle, P. A.

American Association of Petroleum Geologists Bulletin, v. 73, p. 390, 1989.

Abstract: A prominent subaerial exposure surface has been identified within the Capitan reef in Rattlesnake Canyon. Equivalent to the middle Yates interval in the back reef, the exposure surface records planation of the reef during a sea level drop to below the shelf edge. This is the first exposure surface recorded within the reef, although several are known within the back reef facies.

The surface is quite planar and can be traced shelfward for nearly 1 km before it is lost in near-back reef grainstones. The surface does not appear to correlate directly with one of the Yates sandstone beds. Channels cut into the surface have a maximum relief of nearly 2 m. Gypsum molds (now calcite filled) and mud cracks are found in the channel-filling sands. In nonchannel areas, a thin (1 to 5-cm) zone of reddened pebbles is sometimes present.

Although the exposure horizon has only a thin sediment veneer, the extensive truncation of the underlying beds implies significant sediment transport across the surface. This is presumably reflected in one of several prominent clastic lowstand wedges found in the Bell Canyon Formation of the Delaware basin.

The back reef facies prograded out over this surface during the upper Yates and lower Tansill intervals. During this period of progradation, the reef and forereef facies were minor or completely absent. Water depths over the surface reached approximately 70 m before the surface was completely buried during Tansill deposition.