Abstracts of Selected Citations in the 1990's

ORIGIN OF MAJOR KARST-ASSOCIATED CELESTITE MINERALIZATION IN KARSTRYGGEN, CENTRAL EAST GREENLAND

Scholle, P. A., Stemmerik, L. and Harpøth, O.

Journal of Sedimentary Petrology, v. 60, p. 397-410, 1990.

Abstract: Celestite mineralization is known from an approximately 80 km² area of Karstryggen, on the western margin of Jameson Land in central East Greenland. The Upper Permian Karstryggen and Wegener Halvø Formations, which host the celestite, contain dominantly limestones and subordinate evaporites deposited in shallow subtidal to supratidal hypersaline environments. Celestite occurs both in a lower (3-10 m thick) algally laminated limestone unit and in a thick (at least 50 m), overlying, karstified conglomerate/ breccia sequence.

Celestite replaces calcite, dolomite and gypsum in the algally laminated limestone and constitutes the last diagenetically precipitated mineral phase found in these sediments. However, all other diagenetic products are of very early, near surface origin, weakening the evidence for the timing of the replacement. In the karstified limestone conglomerate/breccia sequence, mineralization occurs as a replacement of algally laminated limestone clasts, as celestite cement in the karst breccias, and as pockets, lenses and veins of celestite-filled karst fractures and caves.

The SrSO₄, content is locally up to 80-90% in the karstified conglomerate sequence, but 50-60% is the average grade for an estimated 25-50 x 10⁶ T of this sequence in a restricted area around Revdal. This places the Karstryggen celestite occurrence among the largest known in the world.

Sr-isotopic values obtained from the celestite show a radiogenic signature for the strontium and preclude a seawater or precursor limestone source of the material. The strontium was apparently delivered by surface and/or subsurface waters derived from the adjacent Caledonian highlands. These waters passed through immediately underlying arkosic redbeds concentrating Sr through the leaching of feldspars. However, in addition to the diagenetic factors, the primary sedimentary history also appears to be important in controlling the location of mineralization. In particular, the localized (tectonically controlled) gypsum basin provided a source of sulfur, whereas early karstilication enhanced porosity of the limestones and provided important permeability conduits for subsequent mineralization.

FACIES MAPPING AND RESERVOIR EVALUATION OF THE UPPER PERMIAN WEGENER HALVØ FORMATION ALONG THE WESTERN MARGIN OF THE JAMESON LAND BASIN, EAST GREENLAND

Stemmerik, L., Scholle, P. A., Henk, F. H., Di Liegro, G., Mantovani, M. and Ulmer, D. S.

Grønlands Geologiske Undersøgelse Rapport No. 148, p. 105-108, 1990.

Abstract: The depositional pattern of the Upper Permian Wegener Halvø Formation is mainly controlled by the karst topography of the surface of the underlying Karstryggen Formation. The area was divided by a NNW-SSE trending depositional and/or erosional high during Late Permian times. Peritidal carbonates and evaporites are dominant on the platform to the west while to the east oolite and biogene grainstone deposits dominate.

Reservoir-quality properties are mainly confined to the grainstone deposits east of the high.

DIAGENESIS AND POROSITY DEVELOPMENT ASSOCIATED WITH MAJOR SEA-LEVEL FLUCTUATIONS, UPPER PERMIAN, JAMESON LAND, EAST GREENLAND [abs.]

Scholle, P. A., Ulmer, D. S. and Stemmerik, L.

American Association of Petroleum Geologists Bulletin, v. 74, p. 759, 1990.

Abstract: The Upper Permian of Jameson Land includes two major carbonate sequences, represented by the Karstryggen and Wegener Halvø Formations. The initial Karstryggen transgression led to the development of a shallow marine platform with structurally controlled evaporite basins (salinas) separated by stromatolitic, peloidal, or micritic carbonate depositional areas. The Wegener Halvø sequence reflects more rapid and extensive transgression with the deposition of three sub-cycles of fully marine, platform or biohermal carbonates containing minorevaporites nearthebasin margins. Bioherms (bryozoan-brach iopod-marine cement mounds) show >100 meters of relief, indicating that large relative sea level changes were involved. Both the Karstryggen and Wegener Halvø cycles were terminated by major regressions which led to deep (up to 100 meter) karstic and fluvial incision of the underlying sequences.

Not surprisingly, carbonate and evaporite diagenesis was greatly affected by these regional or eustatic sea level fluctuations. Evaporites dissolved or were replaced by calcite and celestite under the influence of meteoric waters. Limestones show collapse brecciation, grain leaching, soil development, and characteristic vadose and phreatic cements. Most significantly meteoric flushing led to massive dissolution of botryoidal marine cements (aragonite and probable high-Mg calcite) within biohermal facies on the Wegener Peninsula. This early porosity resurrection led to the preservation of porous bioherm core zones until hydrocarbon migration. Only late (post-hydrocarbon), probably hydrothermal, fluid flow led to cementation of the bioherm cores while expelling most of the reservoired hydrocarbons.

If, as is likely, the sea level changes affecting the Greenlandic Permian are eustatic, then this study may provide significant clues to porosity development throughout the largely unexplored northern Zechstein basin. It also provides some important links to the probably time-equivalent Guadalupian rocks of west Texas and New Mexico. The Greenland carbonates were deposited at more northerly latitudes (ca. 35 degrees) than the equatorial deposits of the Permian Basin, and thus show significant faunal differences. Nevertheless, the two areas probably share a similar history of glacio-eustatic sea level changes. Although the diagenetic effects of such fluctuations are very climate (rainfall) sensitive, the Greenlandic history probably has many parallels in the Capitan reef and age-equivalent strata. The most important of these is the episodic, shelf-derived, exposure-related influx of hypersaline pore fluids which led to widespread dolomitization and evaporite pore-filling in Capitan-equivalent back-reef, reef, fore-reef and even basinal strata.

EVAPORITE REPLACEMENT WITHIN THE MID-PERMIAN (LEONARDIAN-GUADALUPIAN) PARK CITY FORMATION, BIGHORN BASIN, WYOMING [abs.]

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

in R. Rezak and D. Lavoie eds., Carbonate Microfabrics Symposium and Workshop: College Station, TX, Texas A & M University/NOARL, p. 47, 1990.

Abstract: The Park City Formation consists of cyclic subtidal to supratidal carbonates controlled by glacio-eustatic sea level fluctuations and localized tectonic uplift. Subsurface cores show significant preserved interstitial evaporite, but on outcrop this unit has extensive silica and calcite replacement of former gypsum and anhydrite crystals and nodules.

These replacements appear to be a multistage phenomenon. Field and petrographic evidence (matted fabrics in nodules; evaporite inclusions) indicate that silicification involved direct replacement of evaporites and probably occurred during early stages of burial. Calcitization, however, appears to be a much later phenomenon and involved precipitation of coarse crystals within evaporite molds. The replacement calcites have a wide range of isotopic values (oxygen -6.04 to -25.02 per mil, average -18.00 per mil; carbon 2.82 to -25.26 per mil, average -7.01 per mil; all values relative to PDB). The calcites are typically free of evaporite remnants but are laden with hydrocarbon inclusions.

The light carbon and oxygen isotopic values and the presence of oil inclusions within the calcites support replacement during late diagenesis, generally following hydrocarbon emplacement. The extremely broad isotopic range indicates that calcitization occurred over a long period of time possibly related to thermochemical sulfate reduction and progressive uplift and Tertiary-age hydrologic events in the Bighorn Basin. Thus, significant porosity changes have taken place in Park City carbonate strata during "late" diagenesis, and the isotopic variations in the calcitized evaporites provide a history of water and hydrocarbon migration through these units.

DEDOLOMITIZATION AND CALCITIZATION OF GYPSUM WITHIN THE MISSISSIPPIAN ARROYO PEÑASCO GROUP, NORTHERN NEW MEXICO [abs.]

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

Geological Society of America, Abstracts with Programs, v. 22 (7), p. A313, 1990.

Abstract: Mixed carbonate and clastic strata of the Osagean to Chesterian Arroyo Peñasco Group are the oldest Paleozoic strata found in northern New Mexico's Sangre de Cristo Mountains. This thin sequence of supratidal to shallow subtidal carbonates has undergone a complex diagenetic history including extensive dedolomitization as well as calcitization and silicification of evaporites. A thick solution collapse breccia is found throughout most of the study area — it caps the intensely recrystallized carbonates and evaporites and apparently resulted from intrastratal dissolution of evaporite beds.

Both calcitized dolomites and evaporites (gypsum) are typically euhedral and range in size from finely to very coarsely crystalline. Many of the more coarsely crystalline dedolomites appear to preserve remnant dolomite zoning under cathodoluminescence due to thin film dissolution/precipitation processes. The del ¹³C (PDB) isotopic analyses of the dedolomites range from 2.29 to -2.10 per mil; the calcitized evaporites range from 1.95 to -4.97 per mil. The del ¹⁸O (PDB) values of the dedolomites range from -5.04 to -11.11 per mil whereas those from the calcitized evaporites range from -3.50 to -7.66 per mil.

Regional geology, isotopic analyses and petrography support the idea that replacements of evaporites and dolomite were a result of the migration of meteoric fluids through these units, presumably during a period of Late Mississippian or Pennsylvanian subaerial exposure. Meteoric fluids dissolved the evaporites within the section, increasing the fluid's Ca²⁺ content, and increasing dolomite solubility. Ultimately, wholesale replacement occurred only as long as there were evaporites within the section reinforcing the concept that the evaporites associated with carbonates can profoundly affect carbonate diagenesis.

REGIONAL BASINAL SANDSTONE DEPOSITIONAL PATTERNS DURING THE GUADALUPIAN (LATE PERMIAN), DELAWARE BASIN, WEST TEXAS-NEW MEXICO [abs.]:

Geisen, J.H., and Scholle, P.A.

American Association of Petroleum Geologists Bulletin, v. 74, p. 660-661, 1990.

Abstract: Examination of well logs from more than 300 Delaware basin wells penetrating the Bell Canyon and Brushy Canyon formations has allowed definition of regional depositional patterns during the Late Permian (Guadalupian). Characteristic gamma-ray hot-kicks mark thin but widespread calcareous shales or limestones representing starved basin sedimentation during sea level highstands. Correlation of such markers along three strike and ten dip lines permitted isopaching of intervening lowstand clastic wedges.

The low-stand wedges typically thin significantly from basin margin to basin center and are marked by a prominent linearity oriented perpendicular to the margin. These lineations probably represent channelized turbidite and grain-flow deposits. Most intervals show dozens of such lineations indicating multiple input points for terrigenous detritus rather than just a few major point sources of debris. The resulting deposits appear to be more apron-like than fan-like and coalesce into broad, sheet-like deposits toward the basin center. Isopach thicks vary in position through time, but terrigenous sediment transport is predominantly from northerly directions throughout the analyzed interval. Thus, the filling of the Midland basin at the close of Cherry Canyon deposition did not result in major new source of terrigenous debris from the east (Central Basin platform).

The well-sorted nature of the basinal sands, their widely distributed input points, apron-like geometry, and other factors argue for migration of eolian dunes to the shelf margin during sea level lowstands. Transport of these well-sorted, unconsolidated sands into the basin was not, however, mainly by direct eolian processes as has been proposed recently, but must have involved submarine current mechanisms.

DIAGENETIC HISTORY AND HYDROCARBON POTENTIAL OF UPPER PERMIAN CARBONATE BUILDUPS, WEGENER HALVØ AREA, JAMESON LAND BASIN, EAST GREENLAND

Scholle, P. A., Stemmerik, L. and Ulmer, D. S.

American Association of Petroleum Geologists Bulletin, v. 75, p. 701-725, 1991.

Abstract: The Upper Permian of Jameson Land includes two carbonate sequences, the Karstryggen and Wegener Halvø formations. The Karstryggen Formation contains hypersaline carbonates and localized evaporites that were heavily weathered and dissected prior to deposition of the overlying strata. The overlying Wegener Halvø Formation represents an abrupt and extensive marine inundation over the underlying karstified Karstryggen surface. Bryozoan-brachiopod-algal-cement buildups of the Wegener Halvø Formation are localized on karstic highs, and show up to 150 m of depositional relief. In-situ mound-core deposits are flanked by allodapic limestones, which pass laterally into intermound calcareous shales.

The diagenetic histories of the core and flank facies are very different. Core facies porosity was intially obliterated by marine cements, but repeated meteoric exposure altered unstable core facies constituents. This alteration produced extensive secondary porosity through grain and cement leaching with local collapse brecciation. Flank strata, however, underwent little sea-floor diagenesis, and low permeability and mineralogically stable grain composition protected these strata from meteoric alteration.

Early cementation and stabilization of core strata led to minimal burial-diagenetic porosity loss. Uncemented flank beds, however, underwent profound mechanical and chemical compaction during this stage. Thus, at the time of hydrocarbon generation, distal flank beds had less than 2% porosity, coarse upper flank beds had 5-10% remnant primary porosity, and core facies deposits had 8-12% secondary pore space.

Hydrocarbons generated from surrounding marine shales charged many of the bioherms with oil. Subsequent fracturing and hydrothermal fluid flow, however, flushed hydrocarbons and filled pores with ferroan calcite, barite, fluorite, galena, and baroque dolomite. This heating and flushing is thought to have been especially intense in the Wegener Halvø region; thus, more basinal areas may still have reservoirs containing significant oil in equivalent Upper Permian limestones.

If, as is likely, the sea level changes affecting the Greenland Permian were eustatic, then this study may provide significant clues to porosity development throughout the largely unexplored northern Zechstein basin and the Arctic basin of the Barent Sea. This study also provides some important connections to the probably time-equivalent Guadalupian carbonate reservoir rocks of west Texas-New Mexico and Wyoming.

LATE-STAGE CALCITES IN THE PERMIAN CAPITAN FORMATION AND ITS EQUIVALENTS, DELAWARE BASIN MARGIN, WEST TEXAS AND NEW-MEXICO : EVIDENCE FOR REPLACEMENT OF PRECURSOR EVAPORITES

Scholle, P. A., Ulmer, D. S. and Melim, L. A.

Sedimentology, v. 39, p. 207-234, 1992.

Abstract: Comparison of Upper Guadalupian fore-reef, reef and back-reef strata from outcrops in the Guadalupe Mountains with equivalent subsurface cores from the northern and eastern margins of the Delaware Basin indicates that extensive evaporite diagenesis has occurred in both areas. In both surface and subsurface sections, the original sediments were extensively dolomitized and most primary and secondary porosity was filled with anhydrite. These evaporites were emplaced by reflux of evaporitic fluids from shelf settings through solution-enlarged fractures and karstic sink holes into the underlying strata. Outcrop areas today, however, contain no preserved evaporites in reef and fore-reef sections and only partial remnants of evaporites are retained in back-reef settings. In their place, these rocks contain minor silica, very large volumes of coarse sparry calcite and some secondary porosity. The replacement minerals locally form pseudomorphs of their evaporite precursors and, less commonly, contain solid anhydrite inclusions.

Some silicification, dissolution of anhydrite and conversion of anhydrite to gypsum have occurred in these strata where they are still buried at depths in excess of 1 km; however, no calcite replacements were noted from any subsurface core samples. Subsurface alteration has also led to the widespread, late-stage development of large- and small-scale dissolution breccias.

The restriction of calcite cements to very near-surface sections, petrographic evidence that the calcites post-date hydrocarbon emplacement, and the highly variable but generally 'light' carbon and oxygen isotopic signatures of the spars all indicate that calcite precipitation is a very late diagenetic (telogenetic) phenomenon. Evaporite dissolution and calcitization reactions have only taken place where Permian strata were flushed with meteoric fluids as a consequence of Tertiary uplift, tilting and breaching of regional hydrological seals. A typical sequence of alteration involves initial corrosion of anhydrite, one or more stages of hydration/dehydration during conversion to gypsum, dissolution of gypsum and precipitation of sparry calcite. Such evaporite dissolution and replacement processes are probably continuing today in near-outcrop as well as deeper settings.

This study emphasizes the potential importance of telogenetic processes in evaporite diagenesis and in the precipitation of carbonate cements. The extensive mineralogical and petrophysical transformations which these strata have undergone during their uplift indicates that considerable caution must be exercised in using surface exposures to interpret subsurface reservoir parameters in evaporitic

ISOTOPE GEOCHEMISTRY OF CALCITIZED EVAPORITES AND DOLOMITES WITHIN THE PERMIAN WEGENER HALVØ FORMATION, KARSTRYGGEN PLATEAU, EAST GREENLAND [abs.]

Ulmer, D. S., Scholle, P. A. and Stemmerik, L.

American Association of Petroleum Geologists 1992 Annual Convention Program, v. 1, p. 132, 1992.

The Late Permian Wegener Halvø Formation of Karstryggen Plateau, East Greenland consists of a series of subtidal stromatolites, oolitic shoals and carbonate turbidites which lay seaward of areas of supratidal carbonate and evaporite deposition. This diverse assemblage of facies was deposited on top of a highly karsted substrate, the Karstryggen Formation. Pre-existing topographic relief, syn-sedimentary sea level fluctuations and localized uplift controlled Wegener Halvø deposition and also affected its diagenectic alteration. Amoung the most significant diagenetic changes are the extensive calcitization of evaporites and dolomites. These calcitized evaporite units are preserved as topographic "mounds" of white neomorphic calcite spar.

Evaporite and dolomite pseudomorphs are commonly found, and solution collapse breccias are locally present. The del ¹³C (PDB) isotopic analyses of the calcitized evaporites range from 6.57 to -1.29 per mil (average = 3.48 per mil); the dedolomites range from 6.59 to 1.83 per mil (average = 4.75 per mil). The del ¹⁸O (PDB) values of the calcitized evaporites range from -7.14 to -22.66 per mil (average = -12.06 per mil) and the dedolomites range from -5.43 to -14.81 per mil (average = -9.97 per mil). The isotopic values of the calcites are depleted in both carbon and oxygen relative to Permian seawater, but show a meteoric signature analogous to recrystallized aragonite cements within the more marine facies of the Wegener Halvų Formation.

Field evidence and isotopic data both indicate that calcitization of evaporites and dolomites occurred during Permian meteoric exposure events. At least three exposure horizons have been identified within the Wegener Halvø Formation and are associated with red "soils", solution collapse breccias, and the calcitized evaporities and dolomites. Calcitization appears to have continued during shallow burial leading, in some instances, to intrastratal dissolution and further collapse brecciation. Considerable porosity was generated by these near-surface diagenetic reactions providing localized reservoir-quality units throughout the Karstryggen Region.

SILICIFICATION OF EVAPORITES IN PERMIAN (GUADALUPIAN) BACK-REEF CARBONATES OF THE DELAWARE BASIN, WEST TEXAS AND NEW MEXICO

Ulmer-Scholle, D. S., Scholle, P. A. and Brady, P. V.

Journal of Sedimentary Petrology, v. 63, p. 955-965, 1993

Abstract: Outcrops of the Seven Rivers, Yates, and Tansill Formations contain widespread evaporites replaced by quartz and calcite. The original evaporites consisted of discrete horizons, scattered nodules, enterolithic layers, and individual crystals or crystal fragments of gypsum and/or anhydrite within a finely crystalline dolomite matrix. Based on field and petrographic observations, silicification of evaporites proceeded from the exterior to the interior of the nodules. The earliest replacement was by euhedral, black megaquartz that contains abundant fluid inclusions (water and hydrocarbons) and solid inclusions (mainly anhydrite and dolomite). The siliceous replacements were followed by precipitation of equant, blocky calcite spar, which filled pores left by late-stage dissolution of evaporites.

The fluid inclusions in the replacive megaquartz are primary, and many contain both hydrocarbons and water. Daughter minerals of halite, gypsum, or possibly antarcticite (CaCl₂^.6H₂O) are also found within the aqueous inclusions. Homogenization-temperature data for hydrocarbon and aqueous fluid inclusions average 67.7-degrees-C and 67.1-degrees-C, respectively. Hydrocarbon-bearing and aqueous inclusions are thus interpreted to have formed simultaneously from the same fluids. Eutectic melting and final melting temperatures for aqueous inclusions indicate that the fluids were concentrated brines consisting of CaCl₂ and NaCl. Oxygen-isotope values for the megaquartz replacements averaged 28.4 per mil (SMOW), indicating precipitation from evaporative waters with an isotopic composition of +2.9 per mil (SMOW).

Evaporite silicification was coeval with or slightly postdated hydrocarbon migration. The fluid-inclusion data provide a record of the fluid temperatures and compositions that prevailed during silica precipitation. These data, coupled with regional stratigraphy and published geothermal gradients, suggest a burial depth of approximately 1.3 km during silicification.

The source of the silica for evaporite replacement is problematic. We postulate, however, that silica may have been derived from dissolution of siliciclastics in back-reef units. Organic acids that form from breakdown of hydrocarbons increase the solubility of quartz by bonding with silicic acid. As these products (and associated brines) moved into cooler oxidizing zones, the organic acids broke down, releasing silica into solution. This organically mobilized silica eventually precipitated as megaquartz replacements of preexisting evaporites.

PALEOKARST-INFLUENCED DEPOSITIONAL AND DIAGENETIC PATTERNS IN UPPER-PERMIAN CARBONATES AND EVAPORITES, KARSTRYGGEN AREA, CENTRAL EAST GREENLAND

Scholle, P. A., Stemmerik, L., Ulmer, D. S., Di Liegro, G. and Henk, F. H.

Sedimentology, v. 40, p. 895-918, 1993.

Abstract: The Karstryggen area of eastern Greenland represents the western edge of sedimentation in the Jameson Land Basin, an arm of the northern Zechstein seaway. Upper Permian strata of this area were deposited as two major sequences. The first marine incursion transgressed largely peneplaned Lower Permian strata and deposited thin, paralic conglomerates, sandstones and shales (the Huledal Formation) followed by a thick package of carbonates and evaporites (the Karstryggen Formation). Although the Karstryggen Formation represents the transgressive maximum of this sequence, it contains only marginal or restricted marine strata, including micritic, stromatolitic and peloidal carbonates and thick, but localized, bedded gypsum deposits. These lithofacies indicate that relatively arid climates prevailed in this basin, as in most of the Zechstein region.

A major regression, associated with a change to a more humid climate, terminated Karstryggen sedimentation. Pre-existing evaporites and carbonates underwent diagenetic alteration, including widespread calcitization and dissolution of gypsum. More importantly, topographic relief in excess of 120 m was generated by fluvial drainage systems and karstic sinkholes.

A second marine incursion, accompanied by a return to a semi-arid climate, drowned this high relief topography, producing a complex sequence of strata (the Wegener Halvø Formation) in which sedimentation was greatly influenced by the rugged underlying terrain. Marine cemented algal-molluscan grainstones draped pre-existing palaeotopography during the initial stages of flooding. Continued drowning led to differential sedimentation on 'highs' and in 'lows'. Oolitic and bryozoan-brachiopod grainstones formed as shoals on the crests of most prominences, whereas shales, conglomeratic debris flows, evaporites. or oolitic turbidites were deposited in the lows. More restricted sedimentation took place in the westernmost areas which lay closest to the mainland shoreline and were situated to the west of a palaeotopographic ridge. There, oolitic, stromatolitic and evaporitic strata were deposited under hypersaline conditions indicative of a return to more arid climatic conditions.

Three subcycles mark smaller scale relative changes of sea level that occurred during deposition of the Wegener Halvø Formation; they are delimited by regional surfaces with moderate relief (5-20 m) developed during subaerial exposure. Widespread diagenetic changes, including leaching of aragonitic grains, dissolution/collapse brecciation of evaporites and meteoric calcite cementation, occurred in association with these smaller scale sequence boundaries, again reflecting climatic oscillations.

Relative sea level fluctuations, coupled with regional climate changes, played a dominant role in determining both depositional and diagenetic relations in these strata. These features undoubtedly extend into subsurface parts of this basin as well as into yet unexplored areas of the northern Zechstein Basin and Barents Shelf, and may have economic significance for the localization of hydrocarbons.

PALEO-OCEANOGRAPHIC CYCLES AND EVENTS DURING THE LATE CRETACEOUS IN THE WESTERN INTERIOR SEAWAY OF NORTH AMERICA

Pratt, L. M., Arthur, M. A., Dean, W. A. and Scholle, P. A.

in W. G. E. Caldwell and E. G. Kauffman eds., The Evolution of the Western Interior Basin: St. John's, Newfoundland, Geological Association of Canada Special Paper 39, p. 333-353, 1993.

Abstract: A high-resolution stratigraphic framework based on macrofossils, microfossils, regional marker beds, radiometric age determinations, inorganic geochemistry, and stable isotopic composition of carbonate and organic matter has been developed for the Upper Cretaceous Greenhorn and Niobrara formations, and equivalent units, in the United States and Canada. Use of this framework has allowed characterization of cycles and events in paleoclimatic and paleoceanographic conditions that influenced sedimentation and early diagenesis of fine-grained carbonates in the Western Interior Seaway. Stable carbon and oxygen isotopic values of carbonate change cyclically across limestone-shale bedding couplets and across larger scale bedding cycles in the Greenhorn and Niobrara formations. These systematic changes result from varying proportions of original biogenic carbonate, early diagenetic cement, and deeper-burial cement. Oxygen-isotopic compositions of carbonate in whole-rock samples of limestone are diagenetically shifted to progressively more negative values in proportion to depth of burial, and carbon-isotope compositions are shifted to lighter values indicative of decomposition of organic matter during early diagenesis. In whole-rock samples of shale or marlstone, oxygen-isotopic compositions are shifted relatively less than values for limestones with increasing depth of maximum burial, and carbon isotopic compositions are close to original biogenic values regardless of burial depth. Organic and inorganic geochemical data and stable-isotopic data on closely spaced samples of the Greenhorn and Niobrara formations reveal excursions from background values that reflect short-term perturbations of the global carbon cycle. The most pronounced of these global events occurs in association with faunal extinctions at the CenomanianTuronian boundary. A marked positive shift in the carbon-isotopic composition of organic matter and carbonate occurs in sections throughout the western interior during this event. Depletion of ¹²C from total-dissolved carbon in oceanic surface waters and from atmosphere C0₂ probably was caused, in part, by enhanced preservation of organic matter and restricted bioturbation of sediment resulting from low oxygen contents in bottom water. The relative importance of such factors as reduced rates of replenishment of deep-ocean water masses or higher primary productivity during initiation of the Cenomanian-Turonian event remains uncertain.

Regionally correlative, decimetre-scale, bedding couplets (limes tone- marlstone and limestone-shale) are a characteristic feature of the Greenhorn and Niobrara formations in outcrops and cores. Estimated Periodicities of bedding couplets in the Bridge Creek Limestone Member of the Greenhorn Formation are about 41 k.y., compared to estimated periodicities of about 18-19 k.y. for bedding couplets in the Fort Hays and Smoky Hill Members of the Niobrara Formation. Larger scale cycles in the Niobrara are defined on the basis of geochemical and gamma-ray logs and have estimated periodicities of 100 k.y., 280 k.y. and 1.7 m.y. The periodicity of the Greenhorn bedding couplets suggests correspondence with the cycle of Earth's axial obliquity, and that of the Niobrara bedding couplets suggests correspondence with the precessional cycles of the Earth's orbit. The 100-k.y. Niobrara cycles appear to be related to orbital eccentricity. Deposition of pelagic and hemipelagic carbonates in the Western Interior Seaway was unusually sensitive to solar-terrestrial cycles during the Late Cretaceous. The north-south elongation of the seaway, restricted oceanic connections, and western catchment area in an orogenic highland provided a tight linkage between regional climate, water-column structure, sediment supply, and level of oxygen in the bottom water.

SEDIMENTOLOGY AND DIAGENESIS OF THE UPPER PERMIAN WEGENER HALVØ FORMATION CARBONATES ALONG THE MARGINS OF THE JAMESON LAND BASIN, EAST GREENLAND

Stemmerik, L., Scholle, P. A., Henk, F. H., Di Liegro, G. and Ulmer, D. S.

in T. O. Vorren, E. Bergsager, Ø. A. Dahl-Stammes, E. Holter, B. Johansen, E. Lie and T. B. Lund eds., Arctic Geology and Petroleum Potential [Norwegian Petroleum Society (NPF) Special Publication 2]: Amsterdam, Elsevier, p. 107-119, 1993.

Abstract: The facies distribution in the Wegener Halvø Formation carbonate platforms along the margins of the Jameson Land Basin is controlled by the topography of an underlying karst surface. Shallow water facies are confined to palaeo-topographic highs; they include algal stromatolites, algal cementstone, oolitic grainstone, bivalve grainstone, bryozoan cementstone, bryozoan-foraminifer boundstone and calcitized evaporite. Deeper water facies were deposited in palaeo-topographic lows like fluvial incised valleys and karst sink holes, and in basinal areas; they include various types of resedimented carbonate, bedded evaporite and black shale.

Early diagenesis involved pervasive marine cementation by aragonite and high-Mg calcite followed by extensive fresh water dissolution during subacrial exposure. Late diagenesis is dominated by ferroan calcite associated with locally abundant baryte, quartz, galena and fluorite. It post-dates hydrocarbon migration.

REPLACEMENT OF EVAPORITES WITHIN THE PERMIAN PARK CITY FORMATION, BIGHORN BASIN, WYOMING, USA

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

Sedimentology, v. 41, p. 1203-1222, 1994

Abstract: The Permian Park City Formation consists of cyclically bedded subtidal to supratidal carbonates, cherts and siltstones. Early diagenesis of Park City Formation carbonates occurred under the influence of waters ranging from evaporative brines to dilute meteoric solutions and resulted in evaporite emplacement (syndepositional nodules and cements), as well as dolomitization, silicification and leaching of carbonate gains.

Major differences are seen, however, in the diagenetic patterns of subsurface and surface sections of Park City Formation rocks. Subsurface samples are characterized by extensively preserved evaporite crystals and nodules, and preserve evidence of significant silicification (chert, chalcedony and megaquartz) and minor calcitization of evaporites. In outcrop sections, the evaporites are more poorly preserved, and have been replaced by silica and calcite and also leached. The resultant mouldic porosity is filed with widespread, very coarse, blocky calcite spar.

These replacements appear to be multistage phenomena. Field and petrographic evidence indicates that silicification involved direct replacement of evaporites and occurred during the early stages of burial prior to hydrocarbon migration. Siliceous sponge spicules provided a major source of silica, and the fluids involved in replacement were probably a mixture of marine and meteoric waters. A second period of replacement and minor calcitization is inferred to have occurred during deep burial (under the influence of thermochemical sulphate reduction), although the presence of hydrocarbons probably retarded most other diagenetic reactions during this time interval. The major period of evaporite diagenesis, however, occurred during late stage uplift. The late stage replacement: and pore-filling calcites have del ¹³C values ranging from 0.5 to -25.3 per mil and del ¹⁸O values of -16.1 to -24.3 per mil (PDB), reflecting extensive modification by meteoric water. Vigorous groundwater flow, associated with mid-Tertiary block faulting, led to migration of meteoric fluids through the porous carbonates to depths of several kilometres. These waters reacted with the in situ hydrocarbon-rich pore fluids and evaporite minerals, and precipitated calcite cements.

The Tosi Chert appears to have been an even more open system to fluid migration during its burial and has undergone a much more complex diagenetic history, as evidenced by multiple episodes of silicification, calcitization (ferroan and non-ferroan), and hydrocarbon emplacement.

The multistage replacement processes described here do not appear to be restricted to the Permian of Wyoming. Similarly complex patterns of alteration have been noted in the Permian of west Texas, New Mexico, Greenland and other areas, as well as in strata of other ages. Thus, multistage evaporite dissolution and replacement may well be the norm rather than the exception in the geological record.

QUANTITATIVE AND GEOLOGIC CONSTRAINTS FOR REGIONAL-SCALE, SHALLOW-BURIAL SEEPAGE REFLUX DOLOMITIZATION, LATE DEVONIAN, WESTERN CANADA SEDIMENTARY BASIN [abs.]

Shields, M. J., Scholle, P. A. and Brady, P. V.

American Association of Petroleum Geologists 1994 Annual Convention Program, v. 3, p. 258, 1994.

Abstract: It is generally agreed that most Late Devonian replacement dolomites in the Western Canada Sedimentary basin formed in a burial environment after significant lithification of host strata. However, the origin of the Mg, and the hydrologic mechanism of water circulation, remain controversial. Basin-scale mass balance analysis strongly suggests that residual seawater brines, associated with Late Devonian evaporite deposits, provided the Mg; hydrologic modeling indicates that seepage reflux of brines could have provided the transport mechanism.

Basinwide distribution of comparable dolomites indicates that Mg-rich parent waters flowed laterally for tens to hundreds of kilometers. The assumption that brines were derived only from Devonian evaporites artificially constrains the duration of the modeled reflux system to 16 Ma or less. Late Devonian evaporites are composed mainly of anhydrite, indicating that residual brines had densities of between 1120 and 1200 kg/cu m and, therefore. differential heads between 9 and 17% greater than seawater. Basin-fill relationships indicate that platform-to-basin relief ranged from several tens to a few hundred meters, thus constraining the base level for reflux. Assuming that Mg supply was the limiting factor for dolomitization, a conservative volumetric flow model, based on the above constr ints, indicates that regional-scale reflux could have supplied the requisite Mg during the Late Devonian. High permeability zones, such as platform margins, fractures and karst systems, could have acted as conduits for significant volumes of brine moving hundreds of kilometers from recharge areas during this time period. Such long distance lateral flow can explain dolomitization at sites where local evidence for evaporite deposition is lacking. Seepage reflux, thus, is a viable mechanism for regional dolomitization in the Western Canada Sedimentary basin.

THE FOREREEF FACIES OF THE PERMIAN CAPITAN FORMATION : THE ROLE OF SEDIMENT SUPPLY VERSUS SEA-LEVEL CHANGES

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

Journal of Sedimentary Research, Section B: Stratigraphy and Global Studies, v. B65, p. 107-118, 1995.

Abstract: Current models for steep forereef facies give a primary role to talus and rock fall processes during both sea-level highstands and lowstands. Highstand sedimentation is also believed to include abundant basinal turbidites, whereas basin starvation is likely during extensive lowstand talus formation. However, the forereef facies of the Capitan Formation (Permian, west Texas and New Mexico) largely lacks the characteristic matrix-poor carbonate breccias formed by talus processes and provides an alternative model appropriate to carbonate margins that produce a wider mixture of coarse- to fine-grained debris. Such a margin not only has different depositional processes but can also have a significantly different response to sea-level fluctuations.

The Capitan forereef can be divided into an upper, middle, and lower forereef on the basis of primary dip and lithologic variations. The upper forereef has primary dips of approximately 30 degrees and is composed of rudstones with minor grainstones, The middle forereef is distinguished from the upper forereef by a decrease in primary dip (to 15-30 degrees) and a change to a mixture of rudstones and packstones with minor wackestones. Dips decrease to 5-10 degrees in the lower forereef accompanied by an increase in packstones to grainstones, but rudstones are still significant. The lower forereef interfingers with packstones to wackestones and sandstones of the basin-margin facies.

During sea-level highstands, the Capitan forereef was supplied with abundant very fine to coarse debris from the wide reef facies. Most of this was deposited on the upper to middle forereef by debris flows and on the lower forereef to basin by debris flows and high- to low-density turbidity currents, During the deposition of the lower Capitan only, siliciclastic sands were also present, primarily as matrix in debris-flow units. Lowstand deposits, in contrast, are almost entirely basinal siliciclastics because production of carbonate sediment was minimal. Transgressive deposits are composed of minor forereef matrix-poor debris flows and a few major debris flows with a siliciclastic matrix that extend far out into the basin.

Although the large-scale stratal patterns may appear similar, a comparison of the Capitan forereef facies with the Triassic Latemar margin and the Quaternary Tongue of the Ocean reveals contrasting details, with the Capitan debris-flow-dominate and the others rock-fall-dominated. This study emphasizes the role of primary sediment production in determining the depositional processes in forereef and foreslope settings, and, as a result, the stratal patterns commonly used to interpret the history of relative sea level.

CARBON AND SULFUR ISOTOPE STRATIGRAPHY OF THE PERMIAN AND ADJACENT INTERVALS

Scholle, P. A.,

in P. A. Scholle, T. M. Peryt and D. S. Ulmer-Scholle eds., The Permian of Northern Pangea, Vol. 1, Paleogeography, Paleoclimates, Stratigraphy: New York, Springer-Verlag, p. 133-149, 1995.

Discussion and conclusions: The compilations of carbon isotopic data for four types of carbonate materials have served mainly to demonstrate the susceptibility of all these phases to diagenetic or other problems. Difficulties with regional and interregional correlations may also blur smaller-scale variations when data from multiple areas are combined as in these plots. Yet despite these problems, some useful information can be derived from these global secular variation curves. By using the least altered portion of the data set, approximate carbon isotope secular variation curves can be generated. Figure 9 shows a comparison of those curves for the four types of carbonates analyzed. Although there is considerable divergence of these curves in the Middle and Late Carboniferous, there is surprising agreement for most of the subsequent time intervals. Carbon isotopic values for all data types rise from a minimum of +3 to +4.5 per mil at the base of the Carboniferous to maximum of +7 to +8 per mil in the latest Permian and then fall abruptly to between + 2 and + 3 per mil at, or very close to, the Permo-Triassic boundary. These values climb gradually and uniformly through the Triassic and Early Jurassic to about +3.5 per mil by mid-Jurassic time.

The discrepancies between the isotopic signatures of the four carbonate types during the Late Carboniferous may be a consequence of two factors. First, this is a time interval for which relatively little dolomite, limestone, or marine cement data was available so the information used may be statistically unreliable. In addition, this interval was characterized by widespread southern hemisphere glaciation which led to cyclothemic sedimentation and repeated episodes of subaerial exposure and meteoric diagenesis of carbonate strata (Goldstein 1988). Whole-rock limestones and dolomites would have been most affected by such changes and many would show abnormally "light" carbon isotopic signatures, reflecting near-surface meteoric conditions.

For later time intervals, however, the internally consistent overall patterns reaffirm the value of isotopic studies. Most significantly, the profound isotopic shift of ocean water chemistry at the Permo-Triassic boundary is probably related to significant changes in burial of organic carbon during that time interval (Berner and Raiswell 1983; Garrels and Lerman 1984; Holser 1984; Holser et al. 1986b; Berner 1989a; Berner and Lasaga 1989; Kump 1989, 1991). However, the stratigraphic incompleteness of most PermoTriassic boundary sections and the low carbonate carbon and/or high organic carbon contents of strata at or near that boundary in many sections has made detailed understanding of the exact nature of the isotopic shift problematic at best. Likewise, the reported existence of abrupt isotopic boundaries within the Permian interval (Magaritz et al. 1981, 1983) is not supported by these compilations. Such "events", as well as some of the multiple cycles reported at or near the PermoTriassic boundary (Baud et al. 1989; Magaritz et al. 1988), are almost certainly a function of diagenetic alteration (e.g., Fig. 5). When data from organic-rich shales, sandstone cements, marine limestones, and concretionary carbonates are combined in a secular variation curve, it is not surprising that isotopic shifts coincide with formational boundaries. That is where lithologic shifts occur (by definition) and that is also where variations in the degree of diagenetic alteration and thus isotopic shifts occur. It is impossible to differentiate primary from secondary isotopic changes when such data are used. The presence of large-scale, apparently eustatic sea level variations in the Permian (Forney 1975; Ross and Ross 1987), including at the Permo-Triassic boundary, exacerbate these problems, insuring not only correlative lithologic variations but also regionally consistent patterns of subaerial and submarine diagenesis.

The establishment of a secular variation curve which has enough detail and reliability to be used as a chemostratigraphic tool still lies in the future. It will require careful collection of isotopic information from single groups of geochernically unaltered fossils or marine cements. It will also involve laborious collection and isolation of individual components coupled with rigorous discarding of unsuitable materials, in sharp contrast to most prior work. Finally, the curve will have to be constructed either from strata in a single region, or our biostratigraphic techniques will have to improve to the point where inter-regional correlations become significantly more reliable than they are today.

The chemostratigraphic curve for sulfur (Fig. 8) is comparable in many ways to the carbon curve. In most time intervals there is about 5 to 6 per mil range of values, not especially encouraging for detailed chemostratigraphic resolution but also not so great as to obscure major temporal trends. There appears to be a steady decline in average del 34S values through the Carboniferous and Permian to a minimum of about +10 per mil in the Late Permian. This trend reverses at or near the Permo-Triassic boundary and a very abrupt rise to about +30 per mil occurs during the Lower Triassic. This is followed by an equally abrupt drop to a plateau of more typical Triassic-Jurassic values of +15 to 17 per mil.

The striking reciprocal relationship between isotopic geochernistry of carbonate carbon and sulfate sulfur has been noted by many authors and has been used to model worldwide rates of organic carbon burial, the formation of sedimentary carbonates, sulfates, and sulfides, and levels of atmospheric oxygen in the Late Paleozoic and Early Mesozoic (Veizer et al. 1980; Schidlowski and Junge 1981; Berner and Raiswell 1983; Garrels and Lerman 1984; Berner 1987; Kump 1989). In general, most of these modeling results show that in order for atmospheric oxygen to remain within reasonable limits, the burial of large amounts of either terrestrial or marine organic matter must be offset by a corresponding shift from pyrite burial to pyrite weathering. This, in turn, should result in an increase in average del ¹³C values of carbonates (as light organic carbon is removed from seawater) and a decrease in del ³⁴S values of sedimentary sulfates. The fact that the carbon and sulfur reservoirs have different sizes and different response times, however, indicates that significant shifts in atmospheric oxygen content could have occurred during the Late Permian (Berner 1989b; Berner and Canfield 1989).

The rapid shifts in carbon and sulfur isotopic composition (and in associated atmospheric oxygen concentrations) are almost certainly reflective of the faunal crisis at the close of Permian. Holser et al. (1989) argued for the weathering of older organic carbon during lowered sea levels near the Permo-Triassic boundary to bring about a lowering of oceanic del ¹³C levels. However, a decrease in productivity and burial of organic carbon at or near the boundary could bring about a similar carbon isotopic shift. Berner (1989b) attributed this decreased organic carbon burial to the increase in continentality and aridity toward the end of the Permian. A decrease in marine organic productivity is likely to be involved in this process as well, however, as has been shown for a comparable isotopic excursion at the Cretaceous-Tertiary boundary (Zachos and Arthur 1986; Zachos et al. 1989). In any case, the establishment of detailed and reliable secular variation curves will greatly facilitate accurate modeling of Permo-Triassic boundary events.

OXYGEN ISOTOPE EXCHANGE DURING NEAR-SURFACE DIAGENESIS IN A MISSISSIPPIAN CARBONATE-EVAPORITE SYSTEM [abs.]:

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

American Association of Petroleum Geologists 1995 Annual Convention Program, v. 4, p. 99A, 1995.

Abstract: The Mississippian (Tournaisian-Visean) Arroyo Peñasco Group of northern New Mexico consists of thin interval of carbonate and siliciclastic strata which have undergone complex diagenesis. A thick breccia, the Macho member of the Tererro Formation, caps intensely recrystallized carbonates and evaporites of the Espiritu Santo Formation throughout most of the study area, and is interpreted to result from collapse of carbonates in association with intrastratal dissolution of bedded sulfates during a mid-Visean subaerial exposure. The Macho member clearly predated the deposition of overlying late Visean units.

Oxygen isotopic data for both the calcitized evaporites and "dedolomites" are much more depleted than one would expect for the low paleolatitude marginal-marine setting and near surface diagenetic reactions; the data suggest a higher temperature, burial diagenetic origin. To reconcile the petrographic and field data with the oxygen isotopic information, water/rock interactions were modeled. From the calculations, it was determined that calcites with depleted oxygen isotopic compositions could be generated from dissolution of interbedded dolomite and gypsum units with the sulfate undergoing microbial reduction. These waters could then precipitate calcites with depleted isotopic values, even at low surficial temperatures.

Integration of observational data with the results of water/rock modeling indicates that near-surface evaporite dissolution not only controlled carbonate diagenetic reactions, but also strongly affected their preserved isotopic record. Calcites with depleted oxygen isotopic values can be generated not only at elevated temperatures during burial, but may also occur under surficial conditions in association with evaporite dissolution and sulfate reduction.

WIDESPREAD VENTING OF METHANE-RICH FLUIDS LATE CRETACEOUS IN (CAMPANIAN) SUBMARINE SPRINGS (TEPEE-BUTTES), WESTERN INTERIOR SEAWAY, USA

Kauffman, E. G., Arthur, M. A., Howe, B. and Scholle, P. A.

Geology, v. 24, p. 799-802, 1996.

Abstract: From New Mexico to Montana, Campanian shales contain abundant submarine spring and seep deposits aligned along early Laramide basement faults. From spring conduit to sea floor, characteristic lithofacies are (a) a vuggy pelletoid micrite core to 30 m height, 5 m width; (b) lucinoid bivalve coquina in pelletoid micrite or grainstone; (c) carbonate slump breccias in shale; (d) concretion-bearing shale; and (e) organic-rich shale, Stable isotope analyses of early, zoned, marine core rock cements suggest (a) an oxidized methane source for the carbonate carbon, (b) venting of methane-charged fluids along active fracture zones, and (c) methanogenesis in older and coeval organic carbon-rich muds. Six consistently zoned foraminifer and macrofaunal communities suggest a strong environmental stress gradient over a few metres from spring vents to the adjacent sea floor. These methane springs and their biotas were dynamic and episodically active over a 1.25 m.y. time span.

REGIONAL SETTING AND ROLE OF METEORIC WATER IN DOLOMITE FORMATION AND DIAGENESIS IN AN EVAPORITE BASIN : STUDIES IN THE ZECHSTEIN (PERMIAN) DEPOSITS OF POLAND

Peryt, T. M. and P. A. Scholle

Sedimentology, v. 43, p. 1005-1023, 1996.

Abstract: The Zechstein Basin of Poland was an area of widespread cyclical deposition of carbonates and evaporites during Late Permian time. The Zechstein shelves, along both the northern and the southern margins of the basin, were sites of shallow-water sedimentation during the formation of the Main Dolomite and Platy Dolomite, two widespread carbonate units. These units consist of oolitic, peloidal, skeletal, micritic and evaporitic carbonates formed in depositional settings ranging from open marine to coastal (lagoonal, sabkha and salina). Although originally deposited as limestones, the Main Dolomite and Platy Dolomite are inferred to have been completely replaced by dolomite through very early stage (essentially penecontemporaneous) reflux of hypersaline brines.

The dolomites of the two basin margins, however, have very different petrographic and isotopic characteristics. Many northern shelf dolomites show early stage calcitization (dedolomitization) and even, in some cases, evidence of a subsequent redolomitization event. These northern shelf samples also have a broad range of carbon and oxygen isotopic ratios (up to 12 per mil for oxygen). Samples from the southern shelf, on the other hand, are petrographically much simpler; they do not show complex calcitization and redolomitization patterns. Likewise, their isotopic values are much more tightly clustered, with only about a 5 per mil range of oxygen isotopic ratios.

The differences between dolomites of the same age from the northern and southern margins are best explained by regional variations in river water influx during episodic exposure events associated with regional or global sea-level fluctuations. The distribution of elastic terrigenous materials and palaeokarstic features indicate that areas of the northern shelf had extensive river input, an influx largely lacking on the southern shelf. Early formed dolomites appear to have been calcitized during sea-level lowstands through the infiltration of meteoric fluids into the evaporitic dolomites created during the previous highstand. In some cases, redolomitization occurred when meteoric fluids were again replaced by hypersaline brines during subsequent sea-level highstands.

Although repeated sea-level fluctuations are clearly evident in these strata, it is likely that associated climatic changes (rainfall variations) also played a role in forming these complex diagenetic patterns. Age-equivalent strata from Texas and New Mexico (from sites at much lower palaeolatitudes) show no such alteration patterns; samples from Greenland (slightly higher palaeolatitudes) show even more intense diagenetic alteration during depositional cycles. Thus, the examination of patterns of diagenesis may be useful in interpreting ancient, palaeolatitudinally sensitive climate patterns.

INTERACTIVE CD ROMS IN THE GEOSCIENCES: AN INSTRUCTIONAL TOOL [abs.]

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

American Association of Petroleum Geologists 1997 Annual Convention Program, v. 6, p. A118, 1997.

Abstract: In the geological sciences, interactive CD ROMs are becoming a common way to provide self-paced training in either an academic or corporate setting. Unlike more traditional modes of teaching, interactive CD ROMs permit the individual to easily integrate a variety of medias--still pictures (petrography, photography, diagrams, etc.), video and sound. This allows the incorporation of information from megascopic to microscopic scales and pacing that maximizes student learning rates.

Kodak Photo CDs provide the most basic form of interactivity and are easily brought into the classroom using a Photo CD player and a television or are playable on Macintosh-, PC- and Sun-based computer systems.

More complex interactive CD ROMs require a computer, but they provide more advanced capabilities. A single module can be developed for variety of skill levels (or languages) by using different sound or text tracks with a common set of visual data. Glossaries and bibliographies can easily be integrated into modules to provide additional levels of information. Also, tests can provide feedback on the individuals comprehension and progress as they work their way through the material.

While interactive CD ROMs may never completely take the place of being on field trips or taking classes, they are an ideal way to supplement an individual's knowledge by providing them with a non-threatening environment to first get acquainted with the material. In addition, interactive CD ROMs can now be inexpensively created using a variety of desktop computer systems and programs.

FORMATION AND DIAGENESIS OF BEDDING CYCLES IN UPPERMOST CRETACEOUS CHALKS OF THE DAN FIELD, DANISH NORTH-SEA

Scholle, P. A., Albrechtsen, T. and Tirsgaard, H.

Sedimentology, v. 45, p. 223-243, 1998.

Abstract: Metre-scale lithologic cycles, visible in core and on logs from Maastrichtian chalks of the Dan Field, were examined to determine their mechanisms of deposition and relation to hydrocarbon production. The lower parts of cycles consist of porous, cream-coloured, largely non-stylolitic, commonly laminated chalk with limited bioturbation (mainly escape burrows). Cycles are capped by thinner intervals of white to grey, hard, stylolitic chalk with concentrations of bioclastic material, intense burrowing and few preserved primary sedimentary structures. The cycle caps contain nearly twice as much Mg as compared to the more porous parts of cycles and also have slightly larger del ¹⁸O values (-4.1 per mil for the caps; -4.4 per mil for porous zones). There is a significant reduction of average cycle thickness, as well as total thickness of the Maastrichtian chalk section, from SW to NE across the Dan Field. The cycle thinning largely results from a reduced thickness of porous chalks from the lower parts of cycles and thus is reflected in lower average porosity and permeability on the NE side of the field. These data indicate that episodic winnowing removed fine-grained constituents from highstanding northeastern areas. Porous cycle bases were deposited at relatively high rates that precluded complete bioturbation; preserved laminae, coupled with escape burrows, reflect episodic sediment influx in areas that flank the seafloor highs. Cycle tops apparently accumulated more slowly (throughout the region, but especially on seafloor highs), perhaps because of reduced productivity of planktic organisms. Slower sedimentation allowed more complete bioturbation and destruction of sedimentary structures, and also led to incipient high-magnesium calcite seafloor cementation (sufficient to yield firmer sediment and enhanced burrow preservation, but not to form true hardgrounds). Thus, the elevated magnesium contents and reduced porosity of the cycle caps reflect very early diagenetic processes that were only partially modified by burial diagenesis.

Rates of chalk deposition, as inferred from physical and geochemical evidence, appear to be a significant control on reservoir characteristics in North Sea chalks. The highest average porosities and permeabilities are found in areas with the highest sediment accumulation rates where seafloor diagenesis is minimized. Topographic depressions at the time of sedimentation can thus be expected to have the best production characteristics, and synsedimentary topographic highs should have the thinnest sections and the poorest petrophysical properties.

DIAGENESIS OF THE CAPITAN FORMATION FOREREEF FACIES (PERMIAN, WEST TEXAS AND NEW MEXICO)

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

in Saller, A.H., Harris, P.M., Kirkland, B.L., and Mazzullo, S.J., eds., Geologic Framework of the Capitan Reef: Tulsa, OK, SEPM Special Publication No. 65, p. 193-210, 1999.

Abstract: The diagenesis of the Capitan forereef can be divided into four overlapping episodes: 1) early marine diagenesis; 2) early burial diagenesis involving normal marine to hypersaline fluids; 3) late burial diagenesis; and 4) uplift related diagenesis. Early meteoric diagenesis was not identified in the Capitan forereef. Early marine diagenesis is limited to minor isopachous cement in upper forereef grainstones. The greatest amount of diagenesis in the Capitan forereef occurred during early burial diagenesis in marine to hypersaline pore fluids. The lower forereef facies was largely altered by marine pore fluids; fabrics include moldic porosity, fine blocky spar and overgrowth cementation, silicification, and rare aragonite neomorphism. The upper forereef facies, however, was nearly completely dolomitized by mesosaline fluids mainly derived by seepage reflux from the near-backreef carbonate lagoon facies. The middle forereef facies contains both styles of alteration. During and/or subsequent to dolomitization, hypersaline fluids completely cemented the reef and forereef with evaporites (gypsum and/or anhydrite). Deeper burial processes include stylolites and partial recrystallization of the early fabric-preserving dolomite to a fabric-destructive dolomite. During uplift, erosion of overlying Ochoan evaporites allowed the influx of meteoric water into the Capitan Formation. This led first to hydration of anhydrite to gypsum and kaolinitization of feldspars and later to complete dissolution of evaporites and precipitation of coarse blocky spars II and III in the resulting porosity.

Most diagenetic models focus on timing of alteration and the fluid composition. In the case of the Capitan forereef facies, however, the sedimentology was a major factor controlling the distribution of diagenetic fabrics. For example, debris-flow deposits (unsorted packstones to rudstones) are partially to completely dolomitized while interbedded turbidity-current deposits (graded packstones to wackestones) are limestone with fine blocky spar and aragonite alteration. The changing patterns of diagenesis from the upper forereef to the lower forereef are at least partially controlled by the change from a predominance of debris-flow deposits in the upper forereef to turbidity-current deposits in the lower forereef.

FORMATION AND DIAGENESIS OF A COOL-WATER HARDGROUND IN THE OLIGOCENE NILE GROUP, WESTLAND, NEW ZEALAND [abs.]

Scholle, P. A., and Lewis, D. W.

American Association of Petroleum Geologists, 1999 Annual Convention Program, v. 8, p. A125.

Abstract: A hiatal surface in the Nile Group provides an example of hardground formation on a narrow and steep, tectonically active, shelf-slope area in a cool-water setting. The sediment contains bivalves, bryozoans and brachiopods in a micritic matrix. Seafloor cementation, followed by extensive boring of the hardened sediment, provided pathways for shallow sub-seafloor dissolution of originally aragonitic bivalves without collapse of surrounding matrix. Bivalve molds were then filled with infiltrated or authigenic sediment--detrital terrigenous quartz and feldspar grains and peloidal glauconite (materials that are scarce in surrounding matrix), plus micritic and peloidal carbonate sediment. Carbonate mold-fills were later replaced by equant, medium-crystalline dolomite. Most of that dolomite, in turn, was altered to a "rusty" dedolomite (ferroan calcite after dolomite). This combination of noted features leads to the following conclusions: 1) initial diagenesis had to be essentially penecontemporaneous; 2) a slow-down or cessation of carbonate production led to early lithification, presumably by Mg-calcite cement, and allowed time for formation of glauconite and accumulation of terrigenous sediment; 3) dolomitization of mold-filling sediment apparently also occurred in a near-seafloor setting, probably as a result of prolonged contact with seawater, as determined from stable isotope geochemistry; 4) subsequent burial and uplift of these strata led to formation of additional calcite cements and near-surface alteration of dolomite to calcite.

The Nile Group hardgrounds differ in many respects from those found in other, broader shelf settings of similar age in the Tertiary of New Zealand--they have far less phosphate and far more dolomite and are much less regionally extensive. Not enough is known about these sediments yet to fully explain such differences.

A SECOND LOOK AT THE DEPOSITION AND DIAGENESIS OF NORTH SEA CHALKS

Scholle, P.A

International Association of Sedimentologists, European Regional Meeting Abstracts volume, Copenhagen, August 1999, p 222-224.

Extended Abstract: Discovery in the 1970's of giant oil fields in Cretaceous-Paleocene chalks of the North Sea, along with oil strikes in similar reservoirs in the United States and the Middle East, spurred extensive investigations about the general principles of deposition and diagenesis of chalks. The depositional studies determined that most, but not all, chalks were deposited as pelagic sediments. Pelagic chalks are characterized by distinctive rhythmic bedding (chalk-marl or equivalent cycles), typically controlled by Milankovitch-scale astronomical variations over 20,000 to 100,000-year intervals. Most such pelagic deposits are extensively bioturbated and may contain bands of chert nodules. Other chalk deposits, including some of the most oil productive ones, however, reflect mass-flow (debris flow or slump) redeposition of earlier pelagic sediments.

Diagenetic studies in the 1970-85 period demonstrated that, under normal circumstances, physical compaction (dewatering plus grain reorientation and/or breakage) and pressure-related chemical dissolution and reprecipitation rapidly destroy chalk porosity during burial. After as little as 3-3.5 km of burial, chalks will have lost more than 90 percent of their porosity and retain virtually no permeability (Scholle, 1977). The fact that some of the world's largest and most prolific chalk oil fields exist at such depths, however, indicates that the diagenetic alteration of chalks must be strongly influenced by other, at least locally important, processes. The two most commonly identified factors are overpressuring and early oil or gas entry into the chalk. Both factors are thought to slow or terminate pressure-induced dissolution and/or cement reprecipitation, leading to substantial preservation of whatever porosity remains at the time that these factors becomes effective. The porosity-preserving effects of anomalous pore pressures have been clearly demonstrated in many North Sea reservoirs; the effects of hydrocarbons are complex, however, as they probably are dependent on the levels of hydrocarbon saturation as well as the timing of entry and entrapment.

This "first-look" at chalks provided a satisfactory model for hydrocarbon exploration during the pre-1990's era of predominantly vertical drilling. In that period, successful hydrocarbon exploration, especially in offshore areas, depended on finding thick, moderately permeable, hydrocarbon-productive chalk sections with relatively uniform properties that provided for good reservoir continuity. In this setting, it was mainly an understanding of "average" chalk properties was needed and small-scale variability was rarely investigated. The development of sophisticated horizontal drilling and completion technologies in the 1990's, however, made it possible to drill in, and produce oil and gas from, relatively thin beds of exceptional porosity and permeability. This has forced a second look at the bedding-scale variability of chalks — the very thing that was previously ignored in order to produce general models of deposition and diagenesis.

This second-generation work has shown that more subtle sedimentologic and diagenetic factors control smaller-scale porosity variations. Those variations occur on vertical scales of individual beds to packages of beds that are tens of meters or more in thickness. They may also be reflected in geographic inhomogeneities (on kilometer or larger scales) from crest to flanks of oil fields or on one side of a field versus another. There appear to be two major controls on such variations: grain size and short-term sedimentation rate. Grain size appears to reflect the evolutionary changes in the average sizes of constituent coccoliths (reaching a maximum in the late Maastrichtian) as well as the degree of breakage or disaggregation of coccoliths and sorting or winnowing by bottom currents. Grain size variations affect not only porosity, but also relative permeability (which increases with average grain size) and capillary pressure relationships. Grain size effects should thus be both petrophysically and petrographically measurable and thus relatively easily identifiable.

Variations in short-term sedimentation rates are less easily definable but may be of even greater significance. The simplest cases involve mass flow resedimentation by debris flows, slumps, or other mechanisms. Debris flow reworking of chalks from the flanks of the North Sea Central Graben has long been identified as a major factor in producing especially high porosity chalk reservoirs in the Norwegian sector (e.g., Kennedy, 1987). Recently, however, more subtle forms of "persistent" (as opposed to episodic) current reworking of chalks have also been shown to produce high porosity chalks in a number of settings (Hovorka and Nance, 1994; Scholle et al., 1998). Such "persistent" reworking is widespread in shelf chalks and can lead to substantial local variations in accumulation rates. The reasons why sediment reworking and high-rate accumulation lead to high porosity preservation in chalks remain somewhat enigmatic. The nature of chalk cycles may provide some insights, however. Each cycle is composed of a basal chalk interval and a slightly to substantially more marly cap. The base is commonly laminated, contains few burrows, is porous, has few stylolites or solution seams, and is thought to reflect a relatively high rate of sediment accumulation. The capping portion of each cycle typically is more completely bioturbated, contains more pressure-solution features, has 4 to 8 percent less porosity than the adjacent cycle base, shows indications of more substantial sea-floor cementation, and is therefore thought to reflect lower rates of sedimentation. Persistent reworking produces thicker depositional cycles in proximal flank areas (coupled with thinning of cycles atop paleo-"highs"), but almost all the thickening takes place in the cycle bases, not in the caps. Thus, there is an association between higher sediment accumulation rates and the features found in cycle bases: lower average clay content, fewer stylolites and solution seams, more lamination, less bioturbation, less syndepositional marine cementation, and higher porosity. In addition, some of the persistently resedimented beds, at least in proximal areas, also are richer in whole or only slightly fragmented coccoliths which have proportionally smaller surface area to volume ratios for overgrowth cementation.

All these factors lead to retardation of cementation (both early and late) in rapidly sedimented chalks, resulting in preferential preservation of porosity. The minimization of pressure solution features in rapidly sedimented chalks is especially important in this context because there is little evidence for intergranular dissolution in most chalks. Thus, thin zones of solution seams and stylolites probably provide much of the burial-stage cement that is responsible for the vast majority of porosity loss in chalks. Differential porosity may be further accentuated by selective entry of hydrocarbons into more porous and permeable chalks, resulting in inhibition of cement precipitation in those zones. Studies of chalk heterogeneity thus have reaffirmed the conclusions of early chalk studies, but have also added another layer of detail to our understanding of chalk deposition and diagenesis. Such studies are also expanding the areas considered prospective for hydrocarbon production from chalk reservoirs. Far more work is needed to further refine these new ideas, to integrate them with modern 3-D seismic data, and to implement them in hydrocarbon exploitation.

TIMING OF POROSITY GENERATION IN LOWER CRETACEOUS RESERVOIRS (SHUAIBA AND KHARAIB FORMATIONS), BLOCK 5, OFFSHORE QATAR

Scholle, P.A., Ulmer-Scholle, D., Jeppesen, M.W., and Simonsen, L.

AAPG Bulletin, v. 83, p. 2040, 1999.

Abstract: The fine-grained Shuaiba and Kharaib carbonate reservoirs of Block 5 offshore Qatar are characterised by extensive secondary porosity. This porosity includes biomoulds, leached intergranular calcite cements (resurrecting primary porosity), solution-enlarged fractures, and centimeter-sized vugs. Extensive secondary porosity, ascribed to early meteoric diagenesis, have been described from the UAE and Oman. Carbon and oxygen isotope geochemistry, core observations, and petrography, however, indicate that the Block 5 reservoirs were deposited in deeper-shelf settings that were not exposed to early meteoric diagenesis. Early diagenesis was dominated by marine cementation, burial diagenetic aragonite dissolution or neomorphism, extensive calcite cementation, localized multi-stage dolomitization, and substantial small scale fracturing. Early calcite cements contain single-phase water-filled inclusions, indicating formation at temperatures of max. 35°C. The main phase of porosity development postdates early diagenesis and appears to be related to large-scale throughput of highly corrosive pore fluids. Extensive corrosion of earlier cements, development of solution-enlarged fractures and vugs, and the presence of two-phase fluid inclusions in cements in secondary pores support a late origin of this porosity. Primary fluid inclusions were formed at 55-60°C (slightly higher than present-day ambient temperatures); secondary inclusions reveal the short-lived presence of much hotter (120-140°C) waters. Abundant hydrocarbon-filled inclusions in some cements indicate leaching by hot brines that may have just preceded or accompanied hydrocarbon migration. A likely mechanism for the influx of such fluids is episodic expulsion from nearby overthrust belts (Zagros or Oman Mountains).