Abstract Sedimentology and stratigraphy are neighbors yet distinctly separate entities within the earth sciences. Sedimentology searches for the common traits of sedimentary rocks regardless of age as it reconstructs environments and processes of deposition and erosion from the sediment record. Stratigraphy, by contrast, concentrates on changes with time, on measuring time and correlating coeval events. Sequence stratigraphy straddles the boundary between the two fields. This book, dedicated to carbonate rocks, approaches sequence stratigraphy from its sedimentologic background. This book attempts to communicate by combining different specialities and different lines of reasoning, and by searching for principles underlying the bewildering diversity of carbonate rocks. It provides enough general background, in introductory chapters and appendices, to be easily digestible for sedimentologists and stratigraphers as well as earth scientists at large.

Abstract Unraveling Earth history is a core business of geology. However, understanding cause and effect of past events requires input from other disciplines that can study processes directly and do not have to reconstruct them from incom plete historic records. This introductory chapter summarizes a very limited number of concepts from neighboring disciplines that are relevant in this respect. The list is woefully incomplete. I did not try to simply cover the most im portant concepts; rather, I selected those that are highly rele vant for the topic of this book yet not sufficiently important for geology at large to be routinely covered by introductory texts or courses in geology.

Abstract Three rules capture the peculiar nature of carbonate depositional systems – carbonate sediments are largely of organic origin, they can form wave-resistant structures and they are easily altered by diagenesis because the original minerals are metastable. The implications of these rules are pervasive. We will encounter them throughout the chapters of this book, starting with the present review of principles that govern the production of sediments and the growth of reefs.

Abstract Depositional geometry of recent accumulations is an important tool for predicting the anatomy of sedimentary rocks in the subsurface. One advantage of sediment analysis by depositional geometry is that it can be performed on remote images, such as seismic or radar profiles and photos of distant or inaccessible outcrops. Geometry contains significant information on the internal structure and the depositional history of a formation. This chapter deals first with basic controls on the geometry of carbonate accumulations and the suitable terminology for their description. Subsequently, we turn to characteristic patterns associated with the three carbonate factories or specific depositional environments.

Abstract Carbonate rocks often overwhelm the untrained eye by a bewildering variety of textures, structures and grain types. Patchy diagenesis adds to the impression of almost chaotic diversity and irregularity. Upon closer inspection, the situation is not nearly as bad. If carbonate sediments are characterized by sedimentary structure, texture and grain kind, a recurring succession of facies belts can be recognized in shore-to-basin transects. These facies appear throughout the Phanerozoic and with only slight modification also in the late Precambrian. This surprising persistence indicates that the evolution of organisms in this time interval had only a modifying effect on the basic carbonate facies. The standard carbonate facies seem to capture trends dictated by other parameters such as the carbonate growth function, i.e. the distribution of growth rates as a function of depth and distance from shore, the degree of protection from waves and tidal currents, and the degree of restriction in the water exchange with the open sea. On the slopes, the declivity and the the balance between sedimentation and erosion are crucial controls. These principles are discussed in the next section, followed by a presentation of facies on ramps and rimmed platforms.

Abstract The standard facies model depicts depositional systems where sedimentation is in approximate equilibrium with intrinsic feedbacks and extrinsic controls. The assumption of equilibrium conditions is often unjustified and this chapter examines important causes of change in carbonate sedimentation with time. Chapter 2 presented evidence that sedimentation is inherently episodic or pulsating and that the record is riddled with hiatuses in a wide range of scales. The scaling of sedimentation rates and the intense lamination of unbioturbated sediments are two major arguments in favor of a non-steady model of sedimentation. This “Cantor model” of sedimentation ( Plotnick, 1986 ) does not invalidate facies models but limits their use. Facies models should be viewed as idealized equilibrium states that depositional systems strive to but do not always reach before being disturbed by extrinsic factors. The episodic nature of sedimentation implies that stratigraphic documentations solely in terms of standard facies belts, is inadequate and cannot do justice to the complexity of vertical successions. For carbonates, in particular, we need to consider changes through time imposed by system-internal feed back (“autocycles”), orbital cycles of the ocean-atmosphere system, organic evolution and long oscillations induced by plate tectonics and cosmic processes.

Abstract Careful definitions and consistent terminology have been a hallmark of sequence stratigraphy from the outset. By now, however, the field suffers from an inflation of terms. I will introduce and use only the most essential ones. Others are explained elsewhere and can be recovered via the index. The discussion in this chapter assumes that the following principles hold. Sedimentary bodies and thus all physical stratigraphic units are of finite extent - they are lenses in the most general sense of the term. Surfaces of erosion or non-deposition are also of limited extent. On the flat basin floors, the ultimate limits are set by the principle that siliciclastic sediment mass is conserved. Basins continually receive sediment and the water column cannot hold this material for any geologically significant time. As dissolution of siliciclastic sediment is negligible, the bulk must come rest on the basin floor. Thus, erosion in part of the basin must be compensated by increased sedimentation in others. For carbonate material, the conservation of mass does not hold for the abyssal sea floors but it remains a reason-able assumption for neritic and bathyal settings. The sediment record is dissected by hiatuses at all geologically relevant scales. The assumption of a “complete section” is an (often useful) idealization. The sediment record at any given locality reflects the history of relative sea-level movements. The history of eustasy requires either evidence of a world-wide phenomenon (such as orbital signals), or a global stack of records plus constraints from geodynamics. Geodynamic input is necessary because crust and mantle deform under the changing water load and therefore the same eustatic signal is recorded differently in different regions ( Watts, 2001 ).

Abstract Depositional systems resemble newspapers that all report on the events of the day but each with a different editorial bias. It behooves the reader to learn about the editorial bias of his paper. Similarly, the geologist ought to know about the bias of depositional systems in recording sea level and other environmental factors. The three carbonate factories each have their own bias in recording sequence-stratigraphic events and all three differ to varying degrees from the siliciclastic standard model. This is not to say, however, that sequence stratigraphy does not apply to these systems. On the contrary, comparative sedimentology of depositional systems clearly shows that the basic features of the standard model are shared by all depositional systems, showing once again the power of sequence stratigraphy as a unifying concept. In this chapter, we base our discussion of carbonate sequence stratigraphy primarily on the deposits of the T factory. They are volumetrically dominant in the geologic record and their sequence stratigraphy is best known. The sequence stratigraphy of the C and M factories is developed in chapter 8 by comparison with the T factory and the standard model.

Abstract The discussion of sequence stratigraphy started in chapter 6 by examinig the standard model and its observational support based largely on siliciclastics. Chaper 7 introduced carbonate sequence stratigraphy with a detailed look at the T factory - the best known carbonate system and the most productive one. The present chapter deals with the sequence stratigraphy of the C and M factories. It does so by high-lighting differences to the T factory. This is not to say, however, that the principles of marine carbonate production out-lined in chapters 2 and 7 are no longer valid simply because they are not discussed at length here. In the C and M factories, just as in the T factory, sediment is largely produced by organic activity within the depositional setting. Environmental factors therefore strongly influence the sediment type and the rate of production. Consequently, environmental change is a major competitor of sea-level change in shaping the sequence record.

Abstract Zircon dates and orbital interpretation of bedding rhythms have yielded very different estimates on the duration of Middle Triassi stages. Recently, a core was drilled in Middle Triassic basin sediments at Seceda (western Dolomites) to directly compare cyclostratigraphy with geochronologic data. Detailed study of facies, sediment sources, and transport mechanisms formed the basis of the statistical analysis of bedding rhythms that are based on a grayscale scan and a gamma-ray well log. Amplitude spectrograms reveal strong frequency components at f = 0.025 cycles/cm in the main nodular limestone interval (92–64 m core depth), corresponding to the dominant 40 cm bedding thickness. Significant spectral differences were found between the grayscale and gamma-ray bedding proxies, placing doubt on the appropriateness of the use of the latter as an effective tool in cyclostratigraphy. In the uppermost part of the succession (59–45 m core depth) calciturbidites constitute more than 50% of the rock volume. If turbidites and tuffs are removed from the rock column, the spectrogram in this interval becomes much smoother and significant peaks appear at higher frequencies. The signals of this pelagic background sedimentation were extracted by bandpass filtering and show strong similarities to Milankovitch cycles in the Quaternary. According to this cyclostrati-graphic interpretation, the dominant 40 cm bedding rhythm was produced by eccentricity, and the average sedimentation rate results in ~3.6 mm/ky. This estimate is in contrast to zircon data from volcaniclastic layers that bracket this core interval and suggest a sedimentation rate of 13.5 mm/ky. As it currently stands, neither of the two interpretations is yet fully satisfactory. Although the presence of orbital variations in the Triassic analogous to those predicted for the last 20 My remains questionable owing the presumed chaotic behavior of the planets, the zircon age data have uncertainties related to their origin that remain unaccounted for and require further investigation.

Abstract The origin of seismic reflections in slope deposits of a Miocene carbonate platform, offshore Sarawak, was studied using cores, well-log data, and two-dimensional seismic. This isolated carbonate platform has slope angles ranging from 2 to 25°. Our interpretation of the seismic data is that the asymmetric and high-rising platform (250–300 m relief) has different stratigraphic character for the southern and northern flanks. The southern slope was characterized by bypass or erosion throughout the aggrading phase of platform development. It was subsequently buried by shale with downbending, onlapping beds that indicate terrigenous sediment transport from the south. An alternative is folding during tectonic deformation. On the northern flank, the shale already started to pile up during platform aggradation. Phases of erosional or bypass conditions were short and alternated with two phases formed when platform debris interfingered with surrounding shale. Shale intercalations can be recognized seismically by negative reflections that quickly lose amplitude away from the platform. Although the overall shape of the platform is probably related to an older structural pattern of the Luconia Province, the asymmetry of the platform architecture and the distribution of sediments are most likely the results of paleowinds.

Abstract Ground penetrating radar (GPR) is a suitable technique for imaging sedimentary structures in the vadose zone because small texture-related capillary-pressure variations lead to changes in water content and electromagnetic properties. To study exactly how GPR reflections are generated by sedimentary structures, GPR profiles of an aeolian sedimentary succession are combined with measurements of textural, electromagnetic and water-retention characteristics from a trench. Time domain reflectometry indicates that small variations in texture in the high-angle dune sediment are associated with changes in water content. Synthetic modelling shows that these changes cause clear GPR reflections. In an experimental approach to estimate the radar response of structures below the wave resolution, i.e. features smaller than λ/4, variations in grain-size distribution and porosity in a thin section were used to reconstruct water-retention curves and impedance models of the thinly layered sediment. Synthetic radar records calculated from the impedance models show that reflections from the studied subcentimetre-scale structures are composites of interfering signals. Although these low-amplitude interfering signals will commonly be overprinted by more prominent reflections, they may cause reflection patterns that change with frequency and do not represent primary bedding.