GeoScienceWorld
Volume

Advances in Carbonate Sequence Stratigraphy:

Application to Reservoirs, Outcrops and Models

Edited by Paul M. (Mitch) Harris and J.A. (Toni) Simo

Abstract

Advances in Carbonate Sequence Stratigraphy: Application to Reservoirs, Outcrops, and Models - The SEPM Research Symposium for the 1997 AAPG-SEPM Annual Meeting in Dallas, Texas, was organized around the theme ?Advances in Carbonate Sequence Stratigraphy ? Application to Reservoirs, Outcrops and Models.? The symposium proved to be thought-provoking and forwardlooking and showed how sequence stratigraphy is influencing many aspects in our understanding of carbonates. This SEPM Special Publication contains studies that represent advances in our understanding of stratigraphy and the distribution of carbonate facies and diagenesis within a sequence stratigraphic framework. The case studies in this volume reinforce the observation that the most important factor controlling stratigraphic relationships in accommodation change, which is generally a product of variations in eustatic sea level and tectonic subsidence. The sequence stratigraphic studies included in this publication develop a better framework for interpreting depostional and diagenetic processes in carbonate strata. The subject matter falls generally into five main categories: (1) platform to basin stratigraphic correlation and facies distribution, (2) high-resolution sequence framework bridging the gap between seismic-scale and reservoir-scale stratigraphy, (3) cycle-stacking patterns and their relationship to longer-term sequences, (4) sequence stratigraphy and facies of slope and basin carbonates, and (5) paleoceanographic factors that cause variation in carbonate deposition.

  1. Page 1
    Abstract

    The SEPM Research Symposium for the 1997 AAPG–SEPM Annual Meeting in Dallas, Texas, was organized around the theme "Advances in Carbonate Sequence Stratigraphy—Application to Reservoirs, Outcrops, and Models." The symposium consisted of both oral and poster presentations. The sessions proved to be thought-provoking and forward-looking and showed how sequence stratigraphy is influencing many aspects in our understanding of carbonates. The very favorable response to the symposium encouraged many of the authors to publish their material and led to this SEPM Special Publication.

    1. Page 11
      Abstract
      Present address: Exxon Production Research Company, P.O. Box 2189, Houston, TX 77252-2189

      A distinctive second-order (5–20 m.y.) stratigraphic signature is present within upper Paleozoic, evaporite-related, carbonate-platform successions of the Paradox and Permian basins in the southwestern United States. This supersequence scale of cyclicity occurs under icehouse and transitional conditions and in synorogenic and postorogenic phases of basin development and appears to control the depositional and stratigraphic-trapping components of petroleum systems in these basins. Primary controls on the development of second-order architecture include basin geometry and subsidence patterns, sediment supply, and long-term sea-level changes. Climate and paleoceanography are important controls on facies development within sequences. Supersequences are defined as long-term transgressive-regressive cycles that may be symmetric or asymmetric and that are composed of third-order sequences.

      The most normal, open-marine depositional conditions occur during the trasgressive phase of second-order cycles. Organic-rich, source-rock-quality (source-prone) lime mudstones are deposited across shelves and in the basin. Carbonate reservoirs are best developed during second-order transgression and turnaround to highstand. Open-marine conditions and diverse skeletal-grain bank deposition occur during third-order highstands of second-order transgressions. Early marine cement is relatively minor in occurrence, and reservoir quality is excellent. During the second-order turnaround and early regression, shelf areas become more restricted, and third-order highstands are dominated by nonskeletal-grain (ooid and peloid) bank-margin deposition. During the later stages of second-order regression, if platform progradation accesses a deeper basin or encounters elevated salinities, or if siliciclastic input increases, the biota become more specialized, marine cementation increases, and reservoir quality is reduced.

      The Lower Pennsylvanian (Morrowan-lower Missourian) supersequence, in the Paradox basin of the Four Corners region, United States, is a symmetrical cycle of approximately 20–22 m.y. duration and is composed of third-order composite depositional sequences, six of which are present in the Paradox basin. Third-order transgressive systems tracts contain organic-rich, black, laminated lime mudstones. Third-order highstand systems tracts of the second-order transgression are characterized by thick mound facies containing a diverse biota and skeletal grainstone banks. Third-order highstands of the second-order turnaround are composed of thick aggradational to progradational stacks of cyclically deposited, relatively thin, phylloid-algal buildups and areally extensive, porous nonskeletal grainstone banks. Third-order highstands of the second-order late-regressive phase are characterized by abundant siliciclastics and nonporous crinoid-rich, skeletal grainstone banks.

      The Upper Permian (Guadalupian) supersequence in the Permian basin, west Texas, United States, is an asymmetrical cycle, consisting of 11 third-order depositional sequences; it was deposited over a period of about 10 to 12 m.y. Normal-marine conditions existed over a wide area during second-order transgression, and skeletal-grain bank margins exhibit the most diverse biota. Deep-water lime mudstones are source prone. The third-order highstands of the initial second-order regressive phase are characterized by prograding ooid and peloid ramp margins with excellent reservoir quality. Cemented, upright margins of organic-cement boundstone with steep (35°–40°) foreslopes and poorer reservoir quality developed during the late stages of second-order regression.

      Postdepositional compactional drape of turnaround third-order highstand bank margins over underlying structural blocks, combined with shelfward facies changes to nonporous inner-shelf strata, provides a combined structural and stratigraphic trapping configuration for large petroleum accumulations. Stratigraphic traps form (1) during second-order transgressions where isolated platforms and buildups develop and are draped by late transgressive mudstones and (2) in shelf settings during the transgression and turnaround at facies changes from porous carbonate sandstones to nonporous lagoonal dolomites and evaporites.

    2. Page 35
      Abstract

      Sequence boundaries were originally defined as bounding surfaces of conformably stratified units. Within this descriptive definition, two types of boundaries were specificaljy recognized and widely used—the boundary between highstand and lowstand systems tracts and that between highstand systems tract and shelf-margin wedge. A third type of boundary, the flooding surface between a highstand tract and an overlying transgressive tract, is increasingly used as a sequence boundary, particularly in carbonates. It should be formally recognized as a specific type of sequence boundary because it is prone to developing long submarine hiatuses, occurs commonly in the geologic record, particularly on drowned carbonate platforms, tends to be a pronounced reflector and unconformity in seismic images, and represents a logical third combination in the interplay of subsidence and eustasy—one in which the rate of subsidence exceeds even the most rapid fall of sea level during a particular eustatic cycle. Including type 3 unconformities as sequence boundaries has two advantages: (1) the general definition of sequence boundary remains descriptive and thus workable even where the boundary must be drawn solely on geometric grounds without supporting evidence for sea-level fall and exposure, and (2) the designation of boundary types 1 and 2 can be restricted to surfaces with reliable evidence for relative fall in sea level.

      On drowned reefs and carbonate platforms, the hiatuses associated with type 3 unconformities frequently exceed 10 m.y., occasionally even 100 m.y. Relicts of pelagic cover in hardgrounds, along with marine diagenesis, attest to the submarine, erosional nature of these hiatuses. On rapidly flooded siliciclastic shelves, the effects are similar but less pronounced. The mechanism behind this persistent and powerful marine erosion is amplification of sluggish oceanic tides by the interaction with sharp topography. This process continues and expands the effects of marine shoreface erosion at greater water depth.

    3. Page 47
      Abstract
      Present address: Earthworks LLC, and Theoretical & Applied Geology, 50 Day Street, Norwalk, CT 06854

      Mapping Upper Devonian reef complexes along 80 km of the southern Napier Range in the Canning basin, Western Australia, has documented three generations (A, B, C) of Frasnian reef-rimmed carbonate-siliciclastic platforms (B and C are backstepped). The tops of these platforms are sequence boundaries. The seaward margins of the platforms are typically upright escarpments developed by both reef growth and collapse. Backstepping of platform B over platform A is most consistent with backstepping during a sea-level rise following a sea-level fall. The internal stratigraphy within platform A predicts this fall and suggests that the fall was predominantly eustatic. This finding is also consistent with the record of the sea-level rise recorded by the lower strata of platform B.

      Platform B, which lacks subaerial exposure features, marks a major episode of margin reconfiguration by collapse along nephmian fractures (oblique, parallel, and normal to platform trends). Overlying strata record a prolonged postcollapse submergence prior to and during the development of backstepped platform C. During submergence, the near-vertical escarpments were coated by calcareous cyanobacteria. The stratigraphic sequence shows no evidence of eustatic control on this backstepping. The record of the period between the termination of platform B deposition and backstepping to form platform C is most consistent with rift-related tectonic movement that submerged the platform, produced an extensive submarine erosional escarpment, and forced backstepping to occur. Tectonic movement effecting a change in relative sea level and backstepping is consistent with (1) the regional extent of a mixed carbonate-siliciclastic conglomerate that indicates erosion of the interior of several platforms, (2) the structural control on neptunian-fracture and platform-margin orientations, (3) siliciclastic distribution, and (4) large (meter-scale) Precambrian boulders. The above interpretation implies that basinal wedges with "lowstand geometries" in the subsurface actually record technically induced sedimentation during a sea-level rise.

    4. Page 75
      Abstract

      This study evaluates the role of compaction-induced differential subsidence as a control on carbonate-platform architecture, sequence development, and early diagenesis in Permian strata of the Delaware and Midland basins, west Texas and southeast New Mexico. The geometry and stratal patterns of compaction-modified sequences identified from outcrops in the Sierra Diablo, Guadalupe, and Brokeoff Mountains are comparable to those observed in seismic lines from the Northwest and Eastern shelf areas. The modification of platform geometries by differential compaction varies spatially and temporally in the studied area and is most important where Leonardian and Guadalupian highstand strata (sequences and sequence sets) have prograded over antecedent aggradational and/or erosional platform margins and wedge-shaped packages of compactible basin facies.

      A "compaction hinge" delineates two mechanically distinct parts of a carbonate platform with contrasting stratal patterns, facies, and accommodation histories. The hinge normally develops in highstand strata deformed by compaction-induced differential subsidence over a preexisting platform margin. Updip of the compaction hinge, platform topsets are generally parallel and of constant thickness, whereas downdip, the same topsets thicken, dip, and diverge basinward toward a downward-deflected depositional platform margin. The alteration of platform architecture by differential compaction during a sea-level lowstand forms a compaction-modified sequence boundary. During a lowstand, flexural deformation driven by compaction-induced differential subsidence (at an outcrop-constrained average rate of 0.19 m/k.y.) modifies platform-top physiography and can generate relief and fractures to control facies patterns, stacking patterns, and the distribution of penetrative paleokarst. The resulting compaction-modified platform architectures and sequence boundaries show a characteristic suite of features that allows their identification using the criteria presented herein. The progressive further steepening of the dips formed across compaction-modified sequence boundaries during burial aids their recognition in outcrops and especially on regional seismic lines.

      The control of compaction-induced differential subsidence on carbonate-platform development has previously been underestimated. Differential compaction can significantly modify the development and dynamics of carbonate platforms and holds important implications for the interpretation of stacking patterns, platform architecture, paleoecology, diagenesis, and accommodation history.

    1. Page 107
      Abstract

      The Upper Mississippian (Chesterian) Ste. Genevieve and Paoli Formations of the Illinois basin are composed of five carbonate and mixed carbonate-siliciclastic fourth-order (100–500 k.y.) sequences. Interpretive cross sections based on detailed measured sections of 57 closely spaced outcrops and cores show that sequence-bounding carbonate and siliciclastic paleosol horizons and distinctive marker beds can be correlated basin-wide, within the available biostratigraphic framework. Each sequence is composed of as many as nine regionally developed parasequences. Carbonate parasequences have patchy updip carbonate eolianites at the base, overlain by laterally discontinuous ooid tidal ridge and skeletal bank facies and laterally extensive muddy carbonate caps. Mixed carbonate-siliciclastic parasequences are composed of skeletal limestone capped by fos-siliferous shale. Carbonate tidal-flat facies are rare in the succession.

      The sequences were produced by moderate—amplitude, fourth-order (100–500 k.y.) glacio-euslatic sea-level changes, whereas the parasequences were likely produced by fifth-order (10–100 k.y.) glacioeustatic sea-level changes during a time of transition from the greenhouse conditions of the Early Mississippian to the icehouse conditions of the late Paleozoic. This transition is reflected in upward change from carbonate-dominated, disconformity-bounded sequences with little lowstand erosion at the base to mixed carbonate-clastic sequences with significant lowstand erosion at the top of the study interval and in overlying sequences. These changes were likely caused by an increase in the magnitude of sea-level fluctuations coupled with increasingly humid wet-dry seasonal climate. The sequences may be stacked into one or two third-order (0.5–5 m.y.) composite sequences that are bounded by relatively well developed unconformities. However, the magnitude of the fourth-order signal makes third-order bundling less clear than in strata deposited during greenhouse times of lower-amplitude fourth- and fifth-order sea-level fluctuations. The dominance of fourth-order disconformity-bounded sequences and the lack of regional tidal-flat facies reflect moderate-amplitude eustasy associated with the transition from global greenhouse climates of the Early Mississippian to times of abundant global ice in the late Paleozoic.

    2. Page 127
      Abstract
      Present address: Sonai Exploration Company, P.O. Box 4792, Houston, Texas 77210–4792

      The strata] architecture of the Holder Formation (Pennsylvanian, Virgilian) in the Sacramento Mountains, New Mexico, reveals the complexities of mixed carbonate-siliciclastic depositional systems. Thirty-three measured sections, correlated by physical tracing, and three research cores and well logs provide an extensive database for examination of two- and three-dimensional stratigraphic architecture. In the Holder Formation, carbonate and siliciclastic facies are arranged into 22 shallowing-upward high-frequency sequences, each bounded by either a surface of subaerial exposure developed on subtidal strata (an abnormal subaerial-exposure surface) or its correlative conformity. Vertical facies successions within individual high-frequency sequences vary significantly across the narrow shelf, as shown in the late 1960s by James Lee Wilson. High-frequency sequence boundaries cross lateral facies transitions and extend down paleoslope; such boundaries are the highest-resolution, correlatable chrono-straligraphic surfaces in this succession.

      Subaerial exposure of subtidal strata, fluvial incision through marine strata, and major downward and basinward facies shifts indicate diat relative changes in sea-level significantly influenced stratal architecture. Measured minimum amplitudes of relative falls in sea level are as great as 32 m and exceed measured thicknesses of high-frequency sequences, suggesting that accommodation space was not completely filled. Stratigraphic response to relative changes in sea level was controlled by spatial and temporal changes in sediment supply and production and shelf position. Relative lows in sea level and the early parts of relative rises in sea level were characterized by siliciclastic sediment bypass of the emergent shelf and deposition and onlap in more downdip positions. Flooding of the shelf is indicated by a vertical facies transition from alluvial siliciclastics to marine carbonates and shales. During relative highs in sea level, shelf-wide carbonate sedimentation dominated, although localized deltaic siliciclastics were deposited on the shelf during the early part of the relative high. Responses to relative changes in sea level were mediated by shelf position, and strata at different shelf positions reflect distinct parts of the sea-level curve.

      Syndepositional structural deformation affected stratigraphic architecture at several scales through its influence on siliciclastic sediment supply. subsidence, and uplift; such deformation accounts for some of the considerable vertical and lateral complexity in the system as first described by J. L. Wilson. A relatively steep depositional gradient and depositional topography during deposition of the lower part of the Holder Formation (partly controlled by La Luz anticline) led to distinct lateral changes in facies and laterally compressed facies belts; the less pronounced topography and gradient that existed during deposition of the upper Holder Formation, because of in-filled topography and the waning influence of La Luz anticline, resulted in more laterally continuous sheetlike deposits.

    3. Page 151
      Abstract

      Cyclic marine carbonate and marine to terrestrial siliciclastic strata in the upper Wolfcampian Chase Group and basal Wellington Formation were deposited on a low-relief, south-sloping ramp overlying the buried Nemaha ridge. The section comprises eight depositional sequences, each of which consists of two higher-frequency cyclothems of transgressive-regressive character. Each cyclothem includes a transgressive systems tract and ensuing highstand systems tract, and the maximum flooding surface of each sequence consistently occurs in the basal cyclothem of that sequence. Cyclothems are separated by relative lowstand systems tracts of dominantly marginal-marine facies, whereas sequence boundaries are either regionally extensive unconformities or lowstand systems tracts of paleosols. Thickness and facies-stacking patterns of the sequences are arranged so as to define three third-order cycles; component sequences and cyclothems are regarded as fourth- and fifth-order cycles, respectively. The regional occurrence of these cycles is interpreted to suggest glacio-eustatic forcing.

      The complex interna] architecture of the sequences reflects interplay among forcing parameters and periodic syndepositional tectonism along the Nemaha ridge. Although there is coincidence between maxima of tectonic activity and degree of architectural complexity of sequences within each third-order cycle, the episodic nature of tectonism does not appear to have been consistent enough to have forced cyclicity in this section. Rather, syndepositional tectonism merely overprinted thickness variations of systems tracts within sequences on the fundamentally glacio-eustatically forced sequence stratigraphic architecture of this section.

    4. Page 169
      Abstract

      Dip-oriented profiles of the Yates-Capitan shelf margin were independently constructed from exceptional outcrops in McKittrick and Slaughter Canyons in the Guadalupe Mountains of west Texas and southern New Mexico. Comparison of the two profiles reveals significant similarities in the position and character of high-frequency sequence boundaries, the internal architecture of facies tracts and cycle-stacking patterns, and offlap angles of both the shelf and reef. The evolution of the Yates-Capitan shelf margin is recorded by systematic long-term trends in key depositional variables measured on individual high-frequency sequences in each canyon. This comparison of the two profiles, located along strike 25 km apart, provides a three-dimensional model of the extent and variability of genetic components of the Late Permian margin of the Northwest shelf of the Delaware basin.

      Comparison of the sequence stratigraphic models for the Yates Formation in McKittrick and Slaughter Canyons indicates that four complete high-frequency sequences can be confidently correlated along strike. Fundamental architectural characteristics of the Yates-Capitan shelf margin are evident in the comparative profiles from each canyon. (1) The Yates Formation in both canyons exhibits initial aggradational geometries followed by strong progradational patterns. The volume of the shelf-crest pisolite complex in the Yates progressively expands through time in concert with a reciprocal contraction of the outer-shelf facies tract. The time-equivalent Capitan reef in both canyons exhibits remarkably similar patterns of stepwise alternations of aggradational and progradational growth that relate to changes in high-frequency sequence architecture through time. (2) The thickness of the Yates varies considerably both across the dip of the shelf margin and along strike between the two canyons. The 240%—600% increases in downdip thickness primarily reflect the inherited depositional topography. The thickness variation along the 25-km strike distance may indicate significant lateral variability in subsidence and/or accumulation rate along the Yates-Capitan shelf margin. (3) Several individual siliciclastic beds can be correlated between each canyon, and the distribution of siliciclastics within individual high-frequency sequences exhibits similar patterns of retrogradation, aggradation, and progradation. Correlative facies-stacking patterns integrated with long-term variations in progradation:aggradation ratio and derived offlap angle act as sensitive indicators of relative changes in base level.

      Comparison of our integrated field observations with subsurface data from the northern Northwest shelf (65–90 km away), Central Basin platform (approximately 150 km away), and Eastern shelf (>300 km away) reveals a consistent, basin-wide pattern of the internal architecture of the high-frequency sequences that compose the Yates Formation. These widespread similarities provide compelling evidence for regional sea-level control on sequence development around the Permian basin.

    1. Page 187
      Abstract

      Stacking-patterns analysis is defined as the identification, interpretation, and correlation of chronostratigraphic units from vertical patterns in facies, diagenetic attributes, and the thickness of high-resolution depositional cycles (parasequences). Such analysis has been widely used in outcrop and subsurface studies of shallow-marine carbonate strata to establish a stratigraphic hierarchy and erect a sequence stratigraphic framework. Recent debate about the levels of order and randomness preserved in the record of cyclic platform-carbonate strata, however, points to the likelihood that a broad spectrum of stratigraphic behavior exists, ranging from nearly random (stochastic) to highly ordered (deterministic). To evaluate the levels of order versus randomness inherent in different depositional systems and to detect secular variations in stratigraphic style, we have constructed a digital database of high-quality stratigraphic data from diverse settings and ages ranging across the Phanerozoic; we have analyzed these data (1) by employing statistical tests (Markov chain analysis, runs test, Durbin-Walson test, entropy analysis) and (2) through visual and analytical comparison of normalized stratigraphic plots.

      The results of our analysis indicate that six empirical classes of stratigraphic style are recognizable. The classes are grouped by age and consist of (1) Proterozoic through Early Ordovician, (2) Silurian-Devonian, (3) Pennsylvanian, Early Permian, Late Triassic, and Neogene, (4) middle to Late Permian, (5) Early to Middle Triassic, and (6) Jurassic to Cretaceous. Differences in stratigraphic style between classes are interpreted to have resulted from differences in the amplitude of eustatic sea-level signatures between icehouse climates (with high-amplitude glacio-eustasy) and greenhouse climates (with low-amplitude eustasy) as well as differences in levels of autocyclicity dictated by tectonic setting and platform configuration. Secular variations in amplitudes of sea-level fluctuation have resulted in differences in the strength of allocyclic forcing and are reflected in the stratigraphic record by the degree to which sediment fill accurately recorded accommodation space.

      Icehouse platforms (Pennsylvanian, Early Permian, and Neogene) contain stacks of fifth- or fourth-order high-frequency sequences capped with subaerial exposure surfaces, relatively chaotic vertical thickness distributions, and high lateral continuity. High lateral continuity of individual fifth- or fourth-order sequences is interpreted to result from the strong allocyclic forcing potential during icehouse periods. Third-order composite sequences are best identified from regional retrogradational to progradational patterns of platform-margin facies. Chaotic vertical thickness distributions and long-term stationarity (i.e., lacking in long-term directional change) in facies patterns of high-frequency sequences, resulting from incomplete recording of accommodation space, however, hamper the identification of the composite sequences on the basis of individual vertical sections in the platform interior.

      Greenhouse platforms contain broad transgressive-regressive (or landward-stepping to seaward-stepping) sequences characterized by (1) long-term systematic change in parasequence thickness and facies proportions, (2) gross shallowing and deepening shifts in facies tracts defining maximum regressive and transgressive intervals, and (3) in some cases, a capping surface with extensive subaerial diagenetic modification (e.g., karst). Proterozoic through Ordovician sequences record long-term systematic changes in parasequence thickness and facies proportions. Jurassic and Cretaceous sequences tend to be represented as gross shallowing and deepening shifts in facies tracts with high levels of randomness at the parasequence scale due to greater levels of autocyclicity and the development of facies mosaics. Potential for greater discontinuity at the parasequence scale results from weaker allocyclic forcing by diminished fourth- and fifth-order amplitudes. The superposition of low-amplitude, high-frequency sea-level fluctuations on higher-amplitude fluctuations of lower frequencies, however, sets up the potential for the accurate recording of long-term accommodation and a full expression of the stratigraphic hierarchy (i.e., third-, fourth-, and fifth-order stratigraphic cycles).

    2. Page 227
      Abstract
      *Present address: Teikoku Oil de Venezuela, Caracas, Venezuela

      Upper Paleozoic limestones were studied in the subsurface on the eastern side of the Central Basin platform, west Texas, to determine the effect of long-term change in accommodation on short-term cycles. Approximately 87 cycles are present in five stratigraphic intervals that are (from bottom to top) the Strawn, Canyon, Cisco, Wolfcamp detritai interval, and Wolfcamp "reef interval. The lower three intervals are Middle to Upper Pennsylvanian, and the two Wolfcamp intervals are Lower Permian. The average cycle thickness is 3.7 m. Most cycles are bounded by subaerial-exposure surfaces, and each cycle apparently represents a glacio-eustatic fluctuation of sea level at an average frequency of approximately 163,000 yr per cycle. Characteristics of cycles change upward from the base of the Strawn (well depth of 2950 m [9700 ft]) to the top of the Cisco (depth of 2730 m [8950 ft]), and those changes include a general shallowing of depositional facies (more grainstones), a decrease in average cycle thickness (an average of 5.2 decreasing to 1.9 m), an increase in amount of shale, and a decrease in δ13C and δ13O values of bulk-rock samples. These patterns are interpreted to be a response to a long-term decrease in rate of accommodation. The changes upward through the Wolfcampian (between depths of 2620 and 2730 m [8600 and 8950 ft]), however, include a general thickening of the depositional cycles (an average of 2 increasing to 4.5 m), an increase in δ13C and δ18O values, and more deep-water facies (more wackestones and mudstones), apparently in response to a long-term increase in accommodation. A major drowning and backstepping of the carbonate-platform margin occurred at the top of the Wolfcamp reef interval.

      The time represented by each cycle includes (1) time when the sediment surface was below sea level accumulating carbonate sediment and (2) time when the sediment surface was subjected to meteoric diagenesis above sea level and no carbonate sediment accumulated. Meteoric diagenesis apparently caused the stable carbon and oxygen isotope compositions of entire limestone cycles to become lighter with more prolonged subaerial exposure. Low δ13C and δ18O values in thin cycles suggest that the cycles were subjected to progressively longer subaerial exposures as the rate of accommodation decreased; finally, a major unconformity was created at the top of the Pennsylvanian. This scenario supports the idea that, compared to thin cycles, thick cycles spent more time below sea level accumulating sediment and less time above sea level being subjected to meteoric diagenesis. The most widespread porosity occurs in cycles of intermediate or greater thickness (>2 m) that were subaerially exposed for brief to moderately long periods (estimated at 5,000–50,000 yr) of freshwater diagenesis (upper part of the Strawn, lower and middle parts of the Canyon, and Wolfcamp reef intervals). Thin cycles (<2 m thick; upper part of the Cisco and lower part of the Wolfcamp detrital interval) have little porosity because (I) longer subaerial exposure resulted in more cements filling most matrix pores and (2) vugs, fractures, fissures, and caverns that formed during prolonged subaerial exposure were filled by shale.

    1. Page 247
      Abstract
      Present address: RSMAS, University of Miami, 4600 Rickenbacher Causeway, Miami, Florida 33149-1098

      Periplatform carbonates of flat-topped, steep-sided carbonate platforms are well known to record the varying environmental conditions that prevail on the platform top. The depositional record of slope sediments on carbonate ramps sensu lato is less understood. This study describes and interprets the textural and compositional signatures of Pliocene periplatform sediments from the core CLINO, located on the leeward paleoslope of the Great Bahama Bank (i.e., the present-day margin). During the Pliocene, the morphology of the Great Bahama Bank evolved from a distally steepened ramp into a steep-sided, flat-topped platform. Hence, the Pliocene deposits offer an important opportunity to compare slope sediments of the different morphology types.

      Compositional analyses reveal that the upper Pliocene periplatform sediments, deposited on the slope of a steep-sided platform, clearly record the repetitive submergence and subaerial exposure of the platform top. Highstand deposits are characterized by a predominance of fine-grained sedimentary matrix exported from the platform interior (mainly aragonite needles). The lowstand deposits, in contrast, typically consist of coarse-grained skeletal material and cortoids, with fine-grained material from the platform interior being rare. The interpretation of sea-level highstand and lowstand deposits is substantiated by palynomorph assemblages that offer a direct proof of the conditions that prevailed on the platform top. Sea-level lowstand assemblages contain high proportions of terrestrial pollen, whereas the highstand assemblages are dominated by dinoflagellate cysts. The selected upper Pliocene section thus represents a straightforward image of highstand shedding.

      The selected lower Pliocene section was deposited when the platform exhibited a distally steepened ramp morphology and shows a more complex picture. Composition of the sediments is rather uniform and does not reveal the clear cyclic nature seen in the upper Pliocene strata. This difference is partly attributed to the ramp morphology's being less sensitive to sea-level fluctuations, because facies belts are able to migrate upslope and downslope, following the changing sea level. Additionally, however, sea-level fluctuations in the early Pliocene were overlain by an overall sea-level rise that could have obscured the signatures of short-term sea-level drops.

      Diagenesis of the periplatform sediments was found to be strongly influenced by sea-level-induced compositional changes in the upper Pliocene section. As seen in scanning electron microscopic examinations, the coarse-grained lowstand deposits are diagenetically more mature than the fine-gained highstand intervals. This pattern is thought to reflect the high primary permeability of the coarse-grained lowstand layers. Fine-grained highstand deposits from the upper Pliocene s20ection and the overall fine-grained lower Pliocene sediments, in contrast, appear much less mature. The low initial permeability seems to have restricted fluid flow that could have driven diagenetic alterations.

    2. Page 275
      Abstract
      *Present address: Korea National Oil Corp., 1588-14, Kwanyan-dong ,Anyang City, Kyungki-do, Korea 431-711

      The Middle to Upper Ordovician Decorah and Galena Formations in Wisconsin are composed of stratigraphically condensed sequences(net accumulation rate : 0.35--0.9 cm/k .y., characterized by extreme lateral cont inuity of facie s and dominated by fine-grained carbonate with numer ousomission surfaces. Depositional sequences are defined by relatively conformable successions bounded by identified or interpreted exposure surfaces across which siliciclastics and skeletal grains increase abruptly. Depositional sequences are subdivided in a nested hierarchy into cycle sets,cycles, and omission-surface-bounded packages.

      The lower sequence, sequence D, corresponds to the Decorah Formation and has a siliciclastic-rich fining-upward succession followed by a coarsening-upward carbonate succession. The phosphate-enriched condensed section above the fining-upward succession represents maximum transgression .Cycles in seq uence D are represented by the upward increases in ske letal-grain and tempestite abundance. The middle sequence, sequence G 1, corresponds to the Galena Formation (lower to middle Dunleith Member) and shows an overall transition from a shale- and skeletal-grain-rich base to a carbonate-mud-dominated top. Both shale content and the abundance of skel etal-grain-supported rocks increase northward in eastern Wisconsin and westward in southern Wisconsin toward the inferred paleoshore line. The upper sequence, sequence G2, corresponds to the Galena Formation (upper part of Dunleith, Wise Lake, and Dubuque Members) and consists of a skeletal-grain-rich base with minor shale content grading upward into mudstones and into a skeletal-grain-rich top.

      Cycle sets and cycles in the sequences G I and G2 are characterized by relatively sharp boundaries across which there is an abrupt increase in shale content and skeletal-grain-rich facies grading upward to a carbonate-mud-dominated facies. The asymmetry of cycles-represented by the abrupt increase followed by gradual decrease in shale content and skeletal grains-is incon sistent with progradation of high-energy shale-rich facies over low-energy mudstone. This facies stacking pattern is better explained by reciprocal sedimentation in which skeletal-grain-rich shale deposition and carbonate-mud deposition are disassociated from each other both in time and space. The omission-surface-bounded pack ages form the smallest recognized sedimentary units within the sequences Gland G2 and represent alternations of periods of sediment deposition and starvation.

      The Decorah and Galena Formations are dominated by mudstone-rich lithofacies and abundant omission surfaces, and their contained fauna consists predominantly of heterotrophs. These characteristics are in contrast with tho se of the Lower Ordovician and Silurian "Bahama-like" carbonate strata. Apparently, in addition to tectonics, antecedent topography, and changes in eu stasy, siliciclastic influx, and storm transport, the paleoclimatic and paleoceanographic conditions in the Late Ordovician epeiric sea had important controls on depositional models, sequence development, and facies stacking patterns.

    3. Page 291
      Abstract
      *Present addresses: (van Konijnenburg) SIEP-RTS, Volmerlaan 8, P.O. Box 60, NL-2280 AB Rijswijk, The Netherlands;*Department of Earth Sciences, University of Soulliern California, Los Angeles, California 90089–0740

      Carbonate slopes are key areas for correlating shelf and basinal sediments. They provide important clues to the problem of correlation of sequential units on platforms with unconformity-bounded and conformable units on the slope and in the deep basin. This study aims to define packages of genetically related sedimentary units in a base-of-slope succession and to compare them to second-order sequences defined on a nearby carbonate-platform margin.

      The Lower Cretaceous to Oligocene pelagic and redeposited carbonate sediments, now lithified and exposed in the Gran Sasso d'Italia area, were deposited along the base of slope separating the Lazio-Abruzzi carbonate platform to the southwest and the Maiella platform margin of the Apulia carbonate platform to the southeast from the Umbria-Marche basin to the north. Three major facies types can be distinguished: (1) breccias and megabreccias, containing a large variety of platform- and slope-derived lithoclasts, originating from the collapse of platform-margin areas, and minor skeletal fragments; (2) rurbiditic packstone and grainstone beds, with mainly skeletal fragments and minor amounts of smaller lithoclasts shed from the nearby platforms; and (3) pelagic foraminiferal lime mudstones and wackestones. The interplay of the different sedimentary processes caused changes in facies associations, not only downslope but also along depositional strike. Although slope deposits were reworked, most of the redeposited material was derived from the adjacent platforms and, therefore, provides additional evidence for platform evolution.

      The sedimentary succession of the Gran Sasso area is subdivided into six formations, separated from each other by unconformities or rapid vertical changes in facies associations. Each of these formations is characterized by a distinct depositional system, which changed drastically across formational boundaries. These changes in depositional system were largely controlled by changes in sedimentary regime that took place on the adjacent platforms and that were related to second-order (10–20 m.y.) platform cycles. The platform cycles were in turn controlled by relative changes in sea level, which in the case of the middle Cretaceous unconformity were related to tectonic events. A comparison of geohistory (i.e., depth vs. time) curves of the Maiella platform-margin, the Gran Sasso base of slope, and the Umbria-Marche basin deposits indicates that platform evolution also appears to have controlled the amount of sediment that accumulated on the base of slope. Along the base of slope, higher sedimentation rates during times of massive redeposition were compensated by periods of submarine erosion and nondeposition, resulting in bulk sedimentation rates comparable to those in the Umbria-Marche basin where pelagic sedimentation was essentially continuous.

    4. Page 317
      Abstract
      *Present address Elf Exploration UK PLC, I Claymore Drive, Bridge of Don, Aberdeen AB23 8GB United Kingdom

      Facies of Campanian chalk in the Paris basin are shown to change as a function of accommodation cycles. Fourteen facies—identified in the field at the Chaux-de-Boran and Précy-sur-Oise quarries at Précy-sur-Oise near the Oise River, north of Paris, and studied by thin-section and SEM (scanning electron microscope) analysis—make up four well-defined facies associations. Three nannofacies compose the background chalk sedimentation. Cherts are more abundant in the progradational phases of 5–10-m-thick, higher-frequency cycles. Depositional environments were generally below fair-weather wave base and built up from below storm wave base. The tops of some shallowing-upward cycles were reworked by current activity, perhaps near to fair-weather wave base.

      Patterns of chert distribution and abundance, early-cemented ftrmgrounds, and evidence of storm-current reworking allow interpretation in terms of progradation, aggradation, and sediment bypass. Two sedimentological models illustrate the contrasting conditions under increasing and decreasing accommodation. Sedimentation took place over a low-relief profile far from any shoreline, with low-angle clinoforms prograding from storm-winnowed platform areas that lay above storm wave base into slightly deeper lows.

      Symmetrical to asymmetrical cycles of facies associations, corresponding to progradational-aggradational cycles, define genetic stratigraphic units. These stratigraphic units resulted from cycles of increasing and decreasing accommodation of at least two frequencies. Major hardgrounds record sediment bypass at the turnarounds from longer-term decreasing to increasing accommodation.

      A simple seismic model shows that the abundance and distribution of chert may explain amplitude changes observed on regional seismic data. Progradational intervals with higher concentrations of chert and capped by hardgrounds should create continuous reflections on a seismic line. These reflections should be of variable amplitude, depending upon the spacing between individual chert layers.

    1. Page 335
      Abstract
      *Present address Mobil(22). PO. Box 139082, Dallas Texas 75313

      The Middle Jurassic Vajont Limestone is predominantly composed of oolite that was shed from the Friuli platform and resedimented by gravity flows into the Belluno basin. Detailed petrographic, geochemical, and biostratigraphic study of a 325-m-thick section reveals stratigraphic variations in nonskeletal-grain and skeletal-grain content and in δ13C, δ18O, and 87Sr/86Sr ratios measured in radial calcite ooids. New nannofossil and ammonite data provide tight age constraints on the stratigraphic section and suggest that deposition occurred during the interval between the late Aalenian/early Bajocian boundary and the late Bajocian—a period of approximately 4.5 m.y. Thus the grain contents and the isotope compositions of Vajont gravity-flow deposits are interpreted as a basinal record of the compositional, chemical, and sequence stratigraphic evolution of the Friuli platform during part of the Middle Jurassic.

      Point-count analysis indicates that Vajont gravity-flow deposits are predominantly composed of nonskeletal grains (average = 89% by volume) with lesser amounts of skeletal grains (11%). Nonskeletal grain types of the Vajont Limestone include ooids (44%), peloids (30%), oncoids (7%), grain fragments (4%), and grapestone (3.75%). Overall, the Vajont exhibits an upward increase in skeletal, peloid, and grapestone contents and a concomitant decrease in ooid and oncoid content. Within the Vajont stratigraphic section, variations in the grain type forming the rock—either skeletal grains or nonskeletal grains—and the δ13C values of radial calcite ooids define four decameter-scale sedimentologie associations. Each of two nonskeletal-grain-dominated associations is paired with an overlying skeletal-grain-rich association, and each of the couplets so formed is interpreted to represent an individual cycle of transgressive and highstand sedimentation (i.e., to be a third-order sequence). The absence of a definitive lowstand compositional signature—i.e., skeletal-grain—dominated gravity-flow deposits—between the two third-order sequences is attributed to the sustained flooding of the Friuli platform. Sustained flooding was most likely achieved by a combination of rapid platform subsidence and a long-term (second-order) rise in global sea level during middle Aalenian-late Bajocian time.

      The 13C fluctuations that are displayed by Vajont radial ooids are in agreement with 13C profiles defined by study of time-equivalent pelagic marine calcite. Similarly, 87Sr/86Sr ratios measured in Vajont radial ooids and other calcitic components are consistent with a seawater Sr isotope curve defined by study of time-equivalent calcitic fossils. Positive excursions in δ13C values (i.e., 13C enrichments) correlate with increased skeletal-grain content in Vajont gravity-flow deposits and proposed sea-level highstands. Times of relative depletion in 13C correlate with increases in the nonskeletal-grain content in Vajont gravity-flow deposits and proposed transgressions. Upward through the stratigraphic section, 87Sr/86Sr ratios exhibit an overall decrease on which is superimposed small increases that potentially correlate with maximum skeletal-grain contents within skeletal-grain-rich sedimentologic associations. These data suggest that third-order sea-level changes affected sediment production on the Friuli platform as well as the δ13C values and perhaps the 87Sr/86Sr ratios of ambient seawater.

      On the basis of the gradual upward enrichment in skeletal-grain content of Vajont gravity-flow deposits, the exposure of the Friuli platform following Vajont deposition, and the abrupt transition to skeletal-grain—dominated gravity-flow deposits in the overlying Fonzaso Formation (Callovian-Oxfordian), we conclude that the Vajont Limestone is a basinal record of a second-order depositional sequence. Within this second-order sequence, the decameter-scale couplets of nonskeletal-gram-dominated and skeletal-grain-rich sedimentologie associations are defined by upward increases in skeletal-grain and 13C contents and are interpreted as the basinal record of two third-order depositional sequences. We use these results to support our proposal that third-order sequences in resedimented carbonates may be recognized through a combination of sedimentologie and chemostratigraphic studies

    2. Page 371
      Abstract

      In the central Mediterranean area, a major, second-order transgressive event spanning from the Burdigalian to the Serravallian is recorded by the widespread deposition of deeper-water facies on carbonate shelves and is matched to a large extent by changes in carbonate facies, including the sudden disappearance of corals and coral reefs and the corresponding rise in the abundance of temperate and cool-water carbonate facies. Two sections from the Maiella platform margin in the southern Apennines and the Hyblean Plateau in Sicily provide us with time series based on oxygen and carbon isotopes and detailed biostratigraphic and strontium isotope ages of unprecedented resolution to compare these facies changes to the global Neogene cooling trends and to the paleoceanographic conditions existing at the time in the Mediterranean area.

      An evaluation of the timing of facies change indicates that the major cooling interval in the Neogene, between 14 and 12 Ma, postdates the change from tropical to temperate carbonate facies in the Mediterranean, which began at approximately 20 Ma. Our oxygen isotope record from the Hyblean Plateau suggests relatively cool temperatures during the Serravallian (within biozones N10-N12) with progressive warming within biozone N11 (11.9–11.8 Ma). This signature is opposite to the one recorded in pelagic sections in other locations of the world and is interpreted to reflect regional conditions within the Mediterranean, as after 20 Ma, the connection to the Indian Ocean became closed or reduced to a very shallow si11, which prevented the outflow and inflow of intermediate and deeper waters.

      The second-order transgressive event in the central Mediterranean coincides with an increase in the δ13C values of skeletal carbonates. These isotope data, combined with biotic evidence, suggest increased productivity of surface waters during the transgressive event. The onset of this event predates by approximately 3 m.y. the major positive carbon isotope excursion recorded worldwide in pelagic sections (the Monterey event); its termination coincides with the end of the Monterey excursion.

      Our data indicate how variations in water temperatures, coupled with drastic changes in water circulation, severely affected rates of carbonate production during the deposition of a second-order sequence and had the effect of accentuating the transgressive trend within the sequences. Rates of carbonate production reached a minimum during the transgressive and early-highstand systems tracts. This low production was not linked to a decrease in the size of the carbonate-producing area along the depositional ramp but rather to changes in production rates due to environmental change. A better understanding of the response of carbonate facies to paleoceanographic changes needs to be developed, so that these changes can be used to predict sequence stratigraphic architecture in a given time slice and to better estimate the reliability of the sea-level signature preserved in carbonate sequences.

    3. Page 385
      Abstract

      The last glacial interval (oxygen isotope stages 4–2 or 75–12 ka) is considered overall as a sea-level lowstand relative to today (Holocene, i.e., since 12 ka) and the previous interglacial highstand stage 5 (130–75 ka). During this stage 4–2 interval, an earlier lagoon and barrier-reef system—established on the edge of the southern Belize shelf margin during the previous stage 5 interglacial highstand—was completely exposed. The exposed stage 5 lagoon area became a siliciclastic fluvial plain bounded on its eastern side by a ridge of karst topography created by the exposure of the stage 5 barrier reef.

      A high-resolution single-channel seismic survey in the northern part of the present Southern Shelf Lagoon off the coast of Belize was used to image two late Quaternary lowstand fluvial drainage systems consisting of a series of well-developed incised valleys, trending parallel to the strike of the exposed stage 5 barrier reef. The divide between both drainage systems, at the latitude of Dangriga (just south of 17°N), appears to be rooted on a topographic high composed of upper Tertiary deposits. South of Dangriga, the system of buried incised valleys drained the area toward the south and merged into Victoria Channel and other channels separating modern rhomboid reefs. North of Dangriga, the system of buried incised valleys drained into the single, partially filled, and deeply incised valley of the English Cay Channel. This valley cut through the exposed stage 5 barrier-reef and back-barrier-reef system and linked the fluvial drainage system north of Dangriga to the Turneffe Basin.

      The bathymetric contours at the mouth of the English Cay Channel protrude eastward across a thick, fanlike sedimentary body characterized by a series of stacked, prograding, and laterally shifting lobes. This sedimentary body is interpreted as a lowstand shelf-edge delta, similar in surface area to the modern highstand delta of the Belize River on which Belize City is located. Southerly directed currents partially reworked the lowstand shelfedge delta and created an elongated slope fan at the toe of slope of the exposed barrier reef.

      A model, tied to the established sea-level curve from 20 ka to the present, is developed to explain the lowstand geometry and morphology of the incised valleys and the sedimentary deposits in front of the modern Belize Barrier Reef. This model also describes, in a tentative time frame, the nature of the transgressive sedimentary fills in the valleys. Moreover, correlation of the single-channel high-resolution seismic grid along the west-em margin of the Turneffe Basin with a multichannel seismic dip line shows that the lowstand shelf-edge delta at the mouth of the English Cay Channel and the slope fan, developed on the toe of the barrier reef farther south, correspond to the latest lowstand prograding sedimentary bodies that are mostly observed underlying the modern barrier-reef and back-barrier-reef system. The establishment and growth of carbonates over siliciclastic deposits—expressed in the colonization of patch reefs over the lowstand shelf-edge delta, beach ridges, and flooded levees—appear to represent a possible mechanism by which the present barrier reef established itself. Results of our study suggest that carbonate deposition occurred during a very restricted time interval of the overall sea-level highstand and that the carbonates rest on a substratum of lowstand and transgressive siliciclastic deposits. However, the establishment of barrier reefs, stacked on top of one another during successive interglacial highstands, though exposed during intervening glacial lowstands, has influenced the orientation of the lowstand incised valleys on the exposed shelf. These valleys trend parallel to the strike of the exposed barrier reefs.

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