André Strasser

Sequence stratigraphy: methodology and nomenclature

The recurrence of the same types of sequence stratigraphic surface through geologic time defines cycles of change in accommodation or sediment supply, which correspond to sequences in the rock record. These cycles may be symmetrical or asymmetrical, and may or may not include all types of systems tracts that may be expected within a fully developed sequence. Depending on the scale of observation, sequences and their bounding surfaces may be ascribed to different hierarchical orders. Stratal stacking patterns combine to define trends in geometric character that include upstepping, forestepping, backstepping and downstepping, expressing three types of shoreline shift: forced regression (forestepping and downstepping at the shoreline), normal regression (forestepping and upstepping at the shoreline) and transgression (backstepping at the shoreline). Stacking patterns that are independent of shoreline trajectories may also be defined on the basis of changes in depositional style that can be correlated regionally. All stratal stacking patterns reflect the interplay of the same two fundamental variables, namely accommodation (the space available for potential sediment accumulation) and sediment supply. Deposits defined by specific stratal stacking patterns form the basic constituents of any sequence stratigraphic unit, from sequence to systems tract and parasequence. Changes in stratal stacking patterns define the position and timing of key sequence stratigraphic surfaces. Precisely which surfaces are selected as sequence boundaries varies as a function of which surfaces are best expressed within the context of the depositional setting and the preservation of facies relationships and stratal stacking patterns in that succession. The high degree of variability in the expression of sequence stratigraphic units and bounding surfaces in the rock record means ideally that the methodology used to analyze their depositional setting should be flexible from one sequence stratigraphic approach to another. Construction of this framework ensures the success of the method in terms of its objectives to provide a process-based understanding of the stratigraphic architecture. The purpose of this paper is to emphasize a standard but flexible methodology that remains objective.

DEPOSITIONAL SEQUENCES IN SHALLOW CARBONATE-DOMINATED SEDIMENTARY SYSTEMS : CONCEPTS FOR A HIGH-RESOLUTION ANALYSIS

Abstract Oxfordian and Berriasian sections representing shallow-water, carbonate-dominated sedimentary systems have been studied in the Swiss and French Jura, in Spain, and in Normandy. They all display a hierarchical stacking of depositional sequences. Facies evolution and stacking pattern allow to define elementary, small-scale, medium-scale, and large-scale sequences. Some depositional sequences display well-marked sequence boundaries, others are limited by transgressive or maximum-flooding surfaces. The hierarchical organisation of such sequence-stratigraphic elements implies that sea-level fluctuations were an important factor in their formation, and that these fluctuations had different frequencies. The superposition of high-frequency sea-level changes on a long-term sea-level trend leads to repetition of diagnostic surfaces, defining sequence-boundary and maximum-flooding zones wherein the corresponding high-frequency surfaces are well developed. Chronostratigraphic tie points permit us to estimate the duration of large-scale sequences. This time control and the observed hierarchical stacking suggest that the high-frequency sea-level changes were controlled by climatic cycles in the Milankovitch frequency band. The variability of stacking pattern and facies evolution between sections illustrates the complexity of the studied environments. Furthermore, because of the minimal accommodation space available in these shallow-water settings, much of the geologic time is not recorded. Nevertheless, detailed analysis of the depositional sequences allows the interpretation of the evolution of the sedimentary system with a high time resolution. Thus, there is a potential to monitor sedimentological, ecological, and diagenetic processes on a time scale of 20 to 100 ka.

Microbialites and micro-encrusters in shallow coral bioherms (middle to late Oxfordian, Swiss Jura Mountains)

SummaryBenthic microbial crusts (microbialites or microbolites) are an important component of Middle to Upper Oxfordian shallow-water coral bioherms in the Swiss Jura. They display stromatolitic (laminated), thrombolitic (clotted), and leiolitic (structureless) fabrics, which are distributed heterogeneously throughout the studied sections. The bioherms can be subdivided into coral-microbialite facies, microbialite-dominated facies, and sediment matrix.Macroscopic and microscopic study reveals that microbialitic encrustations commonly occur in two layers. The first one is directly in contact with the substrate and composed of leiolite (locally stromatolite) and a well-diversified micro-encruster fauna; the second one fills the remaining porosity partly or completely with thrombolite and low-diversity micro-encrusters. The growth of the first layer accompanies the growth of the coral reef and thus formed under the same environmental conditions. The second layer is the result of a moving encrustation front filling the remaining porosity (micro- and macrocavities) inside the reef, below the living surface. Both layers play an important role in early cementation. Phototrophic cyanobacteria probably intervene in the formation of the first encrustation zone, whereas heterotrophic bacteria associated to acidic, Ca2+-binding macromolecules in biofilms are thought to contribute to the thrombolite inside the reef body. When coral growth cannot take pace with microbialite development, the thrombolite from reaches the surface of the construction and finally covers the reef. The result is a thick interval of thrombolite, which can be interpreted as being related to an ecological crisis in coral-reef evolution.A semi-quantitative analysis of the relative abundance of microbialite types and associated micro-encrusters permits to better constrain the processes leading to a reef crisis. Four micro-encruster associations can be distinguished, and each follows an evolutionary trend in the studied section:Terebella-Tubiphytes dominated,Serpula-Berenicea dominated,Litho-codium dominated, andBacinella dominated. These trends are interpreted to reflect changes in environmental conditions. Bioerosion generally is at its maximum before and after abundant growth of microbialite.According to microbialite-bioerosion relationships and shifts in micro-encruster associations, we propose that the evolution towards a coral-reef crisis involves four main phases: (1) An oligotrophic to low mesotrophic phase when low water turbidity and good oxygenation allow phototrophic metabolisms. This leads to a maximum of coral diversity and development of light-dependent micro-encrusters. (2) A low-mesotrophic phase when increased turbidity and slack water circulation reduce the photic zone and favor heterotrophic micro- and macrofauna. Bioerosion through bivalves increases. (3) A high-mesotrophic phase when environmental conditions are so bad that only microbiatite can be produced. (4) A eutrophic phase when carbonate production is inhibited by high nutrient input and clay flocculation as a result of increased terrestrial run-off.The observed evolutionary trends are not directly linked to changes in bathymetry, but sea-level fluctuations played an important role in opening and closing the depositional environments on the shallow platform. Climatic changes contributed in modulating the influx of siliciclastics and nutrients, and the alkalinity of the water. Demise of coral reefs generally coincides with low sea level and humid climate. Sea-level and climatic fluctuations and, consequently, the crises in reef growth are linked to orbital cycles in the Milandkovitch frequency band.

Nutritional Modes in Coral—Microbialite Reefs (Jurassic, Oxfordian, Switzerland): Evolution of Trophic Structure as a Response to Environmental Change

Abstract Detailed study of Oxfordian coral-microbialite reefs in the Swiss Jura Mountains has identified major paleoecological variations in space and time, which are attributed to environmental changes. Micro- and macroscale semi-quantitative analyses of microbialite types, micro-encrusters, bioerosion, corals, and other macrofauna composing the reefal facies were performed. Three main trophic structures (dominant nutritional modes) were recognized: phototrophic-dominated, balanced photo-heterotrophic, and heterotrophic-dominated. A phototrophic (light-dependant) fauna dominated reefs growing in pure carbonate and nutrient- poor environments, where sedimentation rate was the main factor controlling reef growth. In mixed siliciclastic-carbonate platform environments, a balanced photo-heterotrophic fauna with periodical shifts to heterotrophic-dominated associations was induced by freshwater and sediment run-off into closed, shallow lagoons. In this case, the main factors controlling reef growth were the distribution and accumulation of terrigenous sediment on the platform and/or associated nutrient availability. The balanced photo-heterotrophic structure found in mixed carbonate-siliciclastic settings produced the most diversified reefs, suggesting that these Oxfordian reefs preferentially thrived in water moderately charged with nutrients (mesotrophic environment). In the case of strong siliciclastic accumulation and/or strong increase in nutrient availability, coral reef diversity dropped drastically and heterotrophs dominated the trophic structure. A model of the evolution of trophic structure in these reefs as a function of the governing environmental factors is proposed. Focusing on the dominant nutritional mode at each step in reef evolution allows a detailed characterization of reefal structure and a better understanding of the processes leading to coral reef settlement, development, and demise.

Formation and Taphonomy of Human Footprints in Microbial Mats of Present-Day Tidal-flat Environments: Implications for the Study of Fossil Footprints

This study concerns the formation, taphonomy, and preservation of human footprints in microbial mats of present-day tidal-flat environments. Due to differences in water content and nature of the microbial mats and the underlying sediment, a wide range of footprint morphologies was produced by the same trackmaker. Most true tracks are subjected to modification due to taphonomic processes, leading to modified true tracks. In addition to formation of biolaminites, microbial mats play a major role in the preservation of footprints on tidal flats. A footprint may be consolidated by desiccation or lithification of the mat, or by ongoing growth of the mat. The latter process may lead to the formation of overtracks. Among consolidated or (partially) lithified footprints found on present-day tidal flats, poorly defined true tracks, modified true tracks, and overtracks were most frequently encountered while unmodified and well-defined true tracks are rather rare. We suggest that modified true tracks and overtracks make up an important percentage of fossil footprints and that they may be as common as undertracks. However, making unambiguous distinctions between poorly defined true tracks, modified true tracks, undertracks, and overtracks in the fossil record will remain a difficult task, which necessitates systematic excavation of footprints combined with careful analysis of the encasing sediment.

Black-Pebble Occurrence and Genesis in Holocene Carbonate Sediments (Florida Keys, Bahamas, and Tunisia)

ABSTRACT Black carbonate lithoclasts, along with blackened peloids, ooids, skeletal fragments, and entire limestone beds are found in the shallow subtidal, intertidal, and supratidal zones of modern and ancient carbonate environments. They are often associated with pedogenic features such as root-traces or caliche. The blackening is due to impregnation of the sediment by dissolved, colloidal, or finely particulate organic substances in an anoxic and alkaline environment or microenvironment. The organic matter is derived from decayed algae and/or decayed or burnt higher terrestrial plants. Iron sulfides contribute to the blackening, especially in samples containing algal matter. A complex interplay of adsorption of organic matter on carbonate-crystal surfaces, neomorphism, and microcrystalline cementation in the vadose or freshwater phreatic zones is thought to fix the organic matter and make the black coloration relatively resistant to oxidation. Black pebbles form through reworking of the preferentially cemented and blackened sediment by coastal erosion. They are mostly relics because the less-consolidated host sediment is washed away. Black pebbles may thus be valuable indicators of ancient coastal and terrestrial environments.

Tyrrhenian coastal deposits from Sardinia (Italy): a petrographic record of high sea levels and shifting climate belts during the last interglacial (isotopic substage 5e)

Abstract Detailed petrographic and sedimentological analysis of Tyrrhenian coastal deposits from four sites in Sardinia provides new data about climate and sea-level changes during the early part of the last interglacial (isotopic substage 5e) and shows that facies analysis is not only useful for the identification of ancient depositional environments, but also represents a powerful tool for evaluating past climatic conditions and sea-level forcing mechanisms. Two shallowing-upward sequences (sensu Ward, W.C., Brady, M.J., 1979. Strandline sedimentation of carbonate grainstones, Upper Pleistocene, Yucatan Peninsula, Mexico. Am. Assoc. Pet. Geol. Bull. 63, 362–369), commonly separated by an erosion surface and displaying complex geometric relationships, occur above modern sea level at several locations along the shorelines of Sardinia. These sequences include upper shoreface to backshore deposits and were formed during separate sea-level highstands attributed to isotopic substage 5e. Our recent revision of these deposits indicates that each sequence presents distinctive petrographic characteristics. The older succession essentially contains mixed calcarenites, rich in quartz grains and lithic fragments, whereas the younger sediments are dominated by carbonate bioclasts such as red algae, coral and mollusk fragments. Both units apparently accumulated in a similar depositional setting, but during different climatic regimes. The petrography of the lower unit reflects a temperate-humid period characterized by increased weathering and continental erosion, whereas the composition of the upper one suggests a dryer, and possibly warmer, climate favoring active carbonate production. The disparity in petrographic composition between sequences probably records the migration of atmospheric circulation cells, i.e. a shift of climate belts, before and after the ∼128-ka insolation maximum. Our climatic data are coherent with the calculated insolation curves for isotopic substage 5e and thus support the Milankovitch hypothesis for the origin of Pleistocene climates. In contrast, our sedimentological data clearly show the occurrence of two distinctive highstands of sea level during this time period, which does not agree with the hypothesis. It follows that the relation between orbitally induced insolation variations, climate and sea level is not necessarily straightforward.

Cyclostratigraphy - concepts, definitions, and applications

Cyclostratigraphy is the subdiscipline of stratigraphy that deals with the identification, characterization, correlation, and interpretation of cyclic variations in the stratigraphic record and, in particular, with their application in geochronology by improving the accuracy and resolution of time-stratigraphic frameworks. As such it uses astronomical cycles of known periodicities to date and interpret the sedimentary record. The most important of these cycles are the Earth’s orbital cycles of precession, obliquity, and eccentricity (Milankovitch cycles), which result from perturbations of the Earth’s orbit and its rotational axis. They have periods ranging from 20 to 400 kyr, and even up to millions of years. These cycles translate (via orbital-induced changes in insolation) into climatic, oceanographic, sedimentary, and biological changes that are potentially recorded in the sedimentary archives through geologic time. Many case studies have demonstrated that detailed analysis of the sedimentary record (stacking patterns of beds, disconformities, facies changes, fluctuations in biological composition, and/or changes in geochemical composition) enables identification of these cycles with high confidence. Once the relationship between the sedimentary record and the orbital forcing is established, an unprecedented high time resolution becomes available, providing a precise and accurate framework for the timing of Earth system processes. For the younger part of the geologic past, astronomical time scales have been constructed by tuning cyclic palaeoclimatic records to orbital and insolation target curves; these time scales are directly tied to the Present. In addition, the astronomical tuning has been used to calibrate the 40Ar/39Ar dating method. In the older geologic past, “floating” astronomical time scales provide a high time resolution for stratigraphic intervals, even if their radiometric age is subject to the error margins of the dating techniques. Because the term “sedimentary cycle” is used in many different ways by the geologic community and does not always imply time significance, we propose using “astrocycle” once the cycle periodicity has been demonstrated by a thorough cyclostratigraphic analysis. Authors’ addresses: André Strasser, Department of Geosciences, University of Fribourg, CH-1700 Fribourg, Switzerland, e-mail: andreas.strasser@unifr.ch; Frederik J. Hilgen, Faculty of Earth Sciences, University of Utrecht, 3584 CD Utrecht, The Netherlands, e-mail: fhilgen@geo.uu.nl; Philip H. Heckel, Department of Geoscience, University of Iowa, Iowa City, Iowa 52242, U.S.A., e-mail: philip-heckel@uiowa.edu DOI: 10.1127/0078-0421/2006/0042-0075 0078-00421/06/0042-0075

Sequential evolution and diagenesis of Pleistocene coral reefs (South Sinai, Egypt)

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Formation of Holocene Limestone Sequences by Progradation, Cementation, and Erosion: Two Examples from the Bahamas

Abstract The uplifted Pleistocene coral reefs in South Sinai represent two major depositional sequences. They generally form two morphological terraces, which locally are tilted and offset by still active faulting. The major reef sequences are composed of several small-scale sequences. Corals grew rapidly when accommodation space was created by eustatic sea-level rise or fault-block subsidence. As the accommodation decreased, coral rubble and siliciclastic sands prograded over reefal and lagoonal sediments, thus creating a shallowing-upward sequence. Terrigenous input was further stimulated by a more humid climate during interglacial times. 230 Th/ 234 U dating places the older reef cycle between 350,000 and 270,000 years B.P., which corresponds to the interglacial period of isotope stage 9. The younger reef cycle has been dated between 140.000 and 60,000 years B.P. (isotope stage 5). An intermediate, relic sequence found in only one outcrop can be attributed to isotope stage 7. Correlation of the small-scale sequences is difficult: age-dating is not precise enough, and local tectonic activity in many cases overruled the smaller eustatic fluctuations. The younger reef sequence commonly exhibits marine cements, whereas the older sequence was exposed to freshwater dissolution and cementation. An important feature is the locally pervasive dolomitization especially of the older reef. Oxygen and carbon isotope values suggest that the dolomite formed in a seawater-dominated mixing zone. Multiple phases of dissolution, cementation, and dolomitization point to a very complex diagenetic history.