Limestone–marl alternations as environmental archives and the role of early diagenesis: a critical review

Limestone–marl alternations and other micritic calcareous rhythmites have long appealed to sedimentologists, as they appeared to directly reflect high-frequency environmental change. In particular, when orbital forcing gained popularity amongst sedimentologists and paleoclimatologists in 1980s, such rhythmites seemed to offer an ideal tool for high-resolution chronostratigraphy and environmental reconstruction. However, in spite of the fact that orbital forcing has become a routine interpretation of calcareous rhythmites, and that the processes of formation of calcareous rhythmites are considered well understood, research in the past 10 years again has questioned their primary origin and their direct interpretability. Detailed petrographic, paleontological, and geochemical data from numerous successions through geological time provided the basis for testing whether or not the regular alternation of limestone beds and marl or shale interlayers represents bimodally fluctuating environmental conditions in a direct way. In particular, these data, supplemented by box model simulations, imply that post-depositional alteration (diagenesis) has the potential to not only seriously distort primary environmental signals, but also to mimic primary signals. This questions the use of micritic calcareous rhythmites for high-resolution chronostratigraphy and for environmental interpretations where independent data of diagenetically inert parameters are not available. Diagenetic changes appear to have a yet widely underestimated influence on the appearance of limestone–marl alternations and other calcareous rhythmites. The aim of the present review is to summarize new approaches and give an overview of our research results in this field of the past decade. This review also aims at pointing to still enigmatic aspects that need to be addressed before the interpretation of micritic calcareous rhythmites can be considered a reliable tool for high-resolution chronostratigraphy and paleoenvironmental interpretation. “This difference [between shale and limestone] does not find definite expression in the chemical composition but appeals to the eye.” Gilbert (1895)     

Sub-wavebase cross-bedded grainstones on a distally steepened carbonate ramp, Upper Miocene, Menorca, Spain

Cross‐bedded grainstones on carbonate ramps and shelves are commonly related to the locus of major wave energy absorption such as shorelines, shoals or shelf breaks. In contrast, on the Early Tortonian carbonate platform of Menorca (Balearic Islands), coarse‐grained, cross‐bedded grainstones are found at a distance from the palaeoshoreline where they were deposited below the wavebase. Excellent exposures along continuous outcrops on the sea cliffs of Menorca reveal the depositional profile and three‐dimensional distribution of the different facies belts of the Tortonian ramp depositional system. Basinward from the palaeoshoreline, fan deltas and beach deposits pass into 5‐km‐wide gently dipping bioturbated dolopackstone (inner and middle ramp), then into 12–20°‐dipping dolograinstone/rudstone clinobeds (ramp slope) and, finally, into subhorizontal fine‐grained basinal dolowackestone to dolopackstone (outer ramp). In this Miocene example, coarse‐grained grainstones exist in five different settings other than beach deposits: (1) on the middle ramp, where cross‐bedded grainstones were deposited by currents roughly parallel to the shoreline at 40–70 m estimated water depth and are interbedded with gently dipping bioturbated dolomitized packstones; (2) on the upper slope, where clinobeds are composed mostly of in situ rhodoliths and red‐algae fragments; (3) on the lower slope, as small‐scale bedforms (small three‐dimensional subaqueous dunes) migrating parallel to the slope; (4) at the transition between the lower slope and the outer ramp, where mollusc‐rich and rhodolithic rudstones and grainstones, interbedded in dolomitized laminated wackestones containing abundant planktonic foraminifera, infill slide/slump scars as upslope‐backstepping bodies (backsets); (5) at the toe of the slope, where coarse skeletal grainstones indicate bedform migration parallel to the platform margin, induced by currents at more than 150 m estimated water depth. This Late Miocene example also illustrates how changes in intrabasinal environmental conditions (nutrients and/or temperature) may produce changes in stratal patterns and facies architecture if they affect the biological system. Two depositional sequences compose the Miocene platform on Menorca, where a reef‐rimmed platform prograded onto an earlier distally steepened ramp. The transition from the ramp to the reef‐rimmed platform was effected by an increase in accommodation space caused by ecological changes, promoting a shift from a grain‐ to a framework‐producing biota.     

NMR Measurements in Carbonate Rocks: Problems and an Approach to a Solution

Carbonate rocks are well known for their complex petrophysical behavior where, in contrast to siliciclastic rocks, different parameters, including porosity and permeability, usually are not directly related. This behavior is the result of thorough reorganization of porosity during diagenesis, and it turns prediction of reservoir quality of carbonate rocks into a challenge. The study presented here deals with the problem of utilizing NMR techniques in prediction of petrophysical properties in carbonates.We employ a visual porosity classification as a priori knowledge for better interpreting NMR data for prediction purposes. This allows for choice of suitable T₂ cutoff values to differentiate movable from bound fluids adapted for the specific carbonate rock, thus resulting in better interpretation of NMR data. The approach of using a genetic pore type classification for adapting the conventional method for T₂ cutoff determination, which originally was developed for siliciclastic rocks, is promising. Similarly, for permeability determination on the basis of NMR measurements, the classification of carbonate rocks based on porosity types also shows potential. The approach implemented here has the promise to provide a basis of standardized interpretation of NMR data from carbonate rocks.Acknowledgment We are grateful to Baker Hughes INTEQ, Celle, for permission to publish the results of this work. This study was part of I.S.s Masters Thesis at Hannover University that was carried out in collaboration with Baker Hughes INTEQ, Celle, Germany. We appreciate comments of an anonymous referee and PAGEOPH editor Brian J. Mitchell.     

Untersuchungen über die Petrographie kulmischer Kieselschiefer

Zusammenfassung Profile von kulmischen Kieselschiefern des hessischen Hinterlandes wurden optisch, röntgenographisch, chemisch, spektralanalytisch und differential-thermoanalytisch untersucht. Die nicht-lyditischen Bestandteile wurden besonders berlicksichtigt.Als Vergleichsmaterial dienten kulmische Kieselschiefer aus dem Harz, sowie tertiäre (Monterey), silurische und algonkische Kieselschiefer. Es wurde festgestellt, daß in der Kulmstufe II–III (dunkle und bunte Lydite) im hessischen Hinterland und im Harz zahlreiche, gering mächtige Tuffhorizonte auftreten; ihre Häufigkeit nimmt nach obenhin zu. Die bunten Lydite führen tuffitische Bänke, deren Grundmasse ursprünglich kalkig war. Die bisher nur aus dem Harz bekannten, kieseligen Tuffe des Kulms (Typ Lerbacher Adinole) Bind auch im hessischen Hinterland verbreitet.Met Röntgen- und DTA-Aufnahmen wurde zu zeigen versucht, daß die Kieselsäure seit der Entstehung des Sediments folgende Phasenumwandlungen erfahren hat:Opal Cristobalit mikrokristalliner Quarz (Chalzedon, Quarzin), und daß in paläozoischen Kieselsedimenten noch amorphe oder stark fehlgeordnete Kieselsäure vorhanden sein kann.Die Entstehung der kulmischen Kieselschiefer wird diskutiert.     

Sediment partitioning and winnowing in a mixed eolian-marine system (Mauritanian shelf)

Continental shelf systems are highly dynamic sedimentary environments, where sediments from biogenic production as well as from terrigenous sources are redistributed in the shelf depositional system, and partly exported off the shelf to the slope and the deep sea. The Golfe d’Arguin (Mauritania, NW Africa) is dominated by such redistribution processes, involving clastic silt imported as dust from the Sahara desert and biogenic carbonates of marine origin. Indeed, surface-sediment grain size and mineralogy show a clear north–south partitioning of sediment type. Fine material is winnowed from the northern part of the gulf, and transported toward the southern part off the Banc d’Arguin, where coarse silt settles on the outer shelf and upper slope, at least down to 600 m water depth. Particles of the fine silt fraction, estimated in terms of eolian material collected aboard the research vessel, are thought to be exported further offshore as they correspond to grain sizes previously reported from adjacent deep-sea sediments. These findings suggest that the interpretation of dust records from the continental slope and rise off NW Africa must consider reworking and partitioning processes active on the Mauritanian shelf.     

Sealevel history recorded in the Pleistocene carbonate sequence in IODP Hole 310‐M0005D,off Tahiti

Material cored during the Integrated Ocean Drilling Program (IODP) Expedition 310 ‘Tahiti Sea Level’ revealed that the fossil reef systems around Tahiti are composed of two major stratigraphic sequences: (i) a last deglacial sequence; and (ii) an older Pleistocene sequence. The older Pleistocene carbonate sequence is composed of reef deposits associated with volcaniclastic sediments and was preserved in Hole 310‐M0005D drilled off Maraa. Within an approximately 70‐m‐thick older Pleistocene sequence (33.22–101.93 m below seafloor; 92.85–161.56 m below present sealevel) in this hole, 11 depositional units are defined by lithological changes, sedimentological features, and paleontological characteristics and are numbered sequentially from the top of the hole downward (Subunits P1–P11). Paleowater depths inferred from nongeniculate coralline algae, combined with those determined by using corals and larger foraminifers, suggest two major sealevel rises during the deposition of the older Pleistocene sequence. Of these, the second sealevel rise is associated with an intervening sealevel drop. It is likely that the second sealevel rise corresponds to that during Termination II (TII, the penultimate deglaciation, from Marine Isotope Stages 6 to 5e). Therefore, the intervening sealevel drop can be correlated with that known as the ‘sealevel reversal’ during TII. Because there are limited data on the Pleistocene reef systems in the tropical South Pacific Ocean, this study provides important information about Pleistocene sealevel history, the evolution of coral reef ecosystems, and the responses of coral reefs to Quaternary climate changes.     

Heterozoan carbonates from the equatorial rocky reefs of the Galápagos Archipelago

Strongly influenced by seasonal and interannual (i.e. El Niño‐Southern Oscillation) upwelling, the equatorial setting of the Galápagos Archipelago is divided into well‐defined temperature, nutrient and calcium carbonate saturation (Ω_aragonite) regions. To understand the relationship between oceanographic properties and sediment grain associations, grain size, carbonate content and components from sea floor surface samples were analysed, representing the main geographical regions of the Galápagos Archipelago. The shallow‐water rocky reefs of the Galápagos Archipelago are characterized by mixed carbonate–siliciclastic slightly gravelly sands. Despite minor differences in carbonate content, major differences exist in the distribution and composition of key carbonate producing biota. Halimeda is absent and benthic foraminifera occur in extremely low abundance. The western side of the Galápagos Archipelago is strongly influenced by nutrient‐rich, low‐Ω_aragonite, subtropical water, which generates a heterozoan carbonate biofacies in a tropical realm resembling cold‐water counterparts (i.e. serpulid, echinoderm, gastropod, barnacle and bryozoan‐rich facies). The Central East region is composed of a transitional‐heterozoan biofacies. Biofacies observed in the northern region have an increased occurrence of tropical corals, albeit with a minor overall contribution to the carbonate components. Although the temperature gradient would allow for a broader distribution of photozoan biofacies, the increased nutrient concentration and related reduced light penetration from the upwelled waters favour heterozoan carbonate factories, mimicking cool‐water, deeper or higher latitude environments. The recent sedimentary record of the Galápagos Archipelago presents a range of tropical heterozoan carbonate communities, responding to more than simply latitude or temperature but a much more complex mixture of physical, evolutionary and geological processes.     

Multiproxy approach to understanding the origin of Cretaceous pelagic limestone–marl alternations (DSDP site 391, Blake-Bahama Basin)

Limestone–marl alternations are usually directly interpreted to reflect cyclic palaeoenvironmental signals. However, uncertainty in such interpretations stems from the differential diagenesis that most limestone–marl alternations have undergone. Differential diagenesis results in markedly different alterations between limestones and marls and in the loss of comparability of many measurable parameters. For an unequivocal interpretation of the origin of rhythmic alternations, diagenetically robust parameters or parameters that clearly indicate the degree of diagenetic bias are required. The present study uses a multiproxy approach (independent biotic, sedimentary and geochemical parameters) in order to unravel the palaeoenvironmental signal recorded in Valanginian (Early Cretaceous) limestone–marl alternations from the Blake‐Bahama Basin (DSDP site 391). Using this approach, terrestrial and marine influences can be distinguished, changes in nutrient levels estimated and prediagenetic differences in the non‐carbonate fraction constrained. Surprisingly, no systematic variations in any of these parameters were observed between limestone and marl layers, implying that none of these was directly responsible for the formation of the rhythmic alternation. Hence, none of the current models of sedimentary formation of limestone–marl rhythmites is applicable here. Calcareous nannofossils are equally well preserved in limestone and marl layers, ruling out their dissolution in marl layers as a source of the calcite cement in the limestone beds. Sr values of 700–900 p.p.m. indicate that aragonite may have been present in the original, pelagic sediment. The assumption of fine‐grained sedimentary aragonite imported from nearby carbonate platforms as the source of the cement would explain a number of otherwise enigmatic features in these rhythmites, including the source of the calcite cement observed in the limestones, the equally good preservation of calcareous nannofossils in limestones and marls and the higher concentration of calcareous nannofossils in marl layers. The study demonstrates that examination of diagenetically inert parameters or parameters in which diagenetic effects can be filtered can yield unexpected results. Clearly, careful analysis of such parameters needs to be undertaken in order to make reliable palaeoenvironmental interpretations from rhythmite successions.     

Response of large benthic foraminifera to climate and local changes: Implications for future carbonate production

Large benthic foraminifera are major carbonate components in tropical carbonate platforms, important carbonate producers, stratigraphic tools and powerful bioindicators (proxies) of environmental change. The application of large benthic foraminifera in tropical coral reef environments has gained considerable momentum in recent years. These modern ecological assessments are often carried out by micropalaeontologists or ecologists with expertise in the identification of foraminifera. However, large benthic foraminifera have been under-represented in favour of macro reef-builders, for example, corals and calcareous algae. Large benthic foraminifera contribute about 5% to modern reef-scale carbonate sediment production. Their substantial size and abundance are reflected by their symbiotic association with the living algae inside their tests. When the foraminiferal holobiont (the combination between the large benthic foraminifera host and the microalgal photosymbiont) dies, the remaining calcareous test renourishes sediment supply, which maintains and stabilizes shorelines and low-lying islands. Geological records reveal episodes (i.e. late Palaeocene and early Eocene epochs) of prolific carbonate production in warmer oceans than today, and in the absence of corals. This begs for deeper consideration of how large benthic foraminifera will respond under future climatic scenarios of higher atmospheric carbon dioxide (pCO₂) and to warmer oceans. In addition, studies highlighting the complex evolutionary associations between large benthic foraminifera hosts and their algal photosymbionts, as well as to associated habitats, suggest the potential for increased tolerance to a wide range of conditions. However, the full range of environments where large benthic foraminifera currently dwell is not well-understood in terms of present and future carbonate production, and impact of stressors. The evidence for acclimatization, at least by a few species of well-studied large benthic foraminifera, under intensifying climate change and within degrading reef ecosystems, is a prelude to future host–symbiont resilience under different climatic regimes and habitats than today. This review also highlights knowledge gaps in current understanding of large benthic foraminifera as prolific calcium carbonate producers across shallow carbonate shelf and slope environments under changing ocean conditions.