Peritidal Cycles Research Articles

Ichnology as a tool in carbonate reservoir characterization: A case study from the Permian – Triassic Khuff Formation in the Middle East

ABSTRACTIn the Upper Permian to Lower Triassic Khuff Formation in the Arabian Gulf, a vast shallow-marine carbonate platform developed broad facies belts with little significant changes in the lithofacies. However, trace fossil assemblages and ichnofabrics, in combination with sedimentological observations, serve in subdividing this platform and in distinguishing sub-environments. From proximal to distal, these are sabkha and salina, tidal flat, restricted lagoon, open lagoon, platform margin, shoreface/inner ramp, slope/outer ramp and basin/deeper intra-shelf. In this way, changes in relative sea level can be better reconstructed and guide the sequence stratigraphic interpretation. Meter-scale shallowing-upward cycles dominate the succession and, in addition to conventional methods, bioturbation, trace fossil assemblages and tiering patterns aid in interpreting subtidal, lower and upper intertidal and supratidal portions of these peritidal cycles. Bioturbation (and cryptobioturbation) have an impact on the primary reservoir quality before diagenetic processes overprint the deposits. For instance, deposit-feeders (such as vermiform organisms) introduce a certain amount of mud and decrease porosity and permeability considerably, whereas others like the Zoophycos-producers fill their dwellings with ooid grains and turn a mudstone from a barrier to a flow unit. This novel study demonstrates the value of ichnological information in carbonate reservoir characterization and the significance of trace fossil analysis in facies interpretation, reservoir zonation and the impact of bioturbation on the reservoir quality.

How fracture systems affect permeability development in shallow-water carbonate rocks: An example from the Gargano Peninsula, Italy

Fracture networks control the permeability of many reservoirs. Since the fracture patterns of petroleum reservoirs in situ are difficult to study in detail, field analogues are very important for understanding their fracture-related permeability. Here we present the results of a study of the fracture system of carbonate rocks of Lower Cretaceous age in a quarry associated with the damage zone and fault core of a major fault zone on the Gargano Peninsula in South Italy. We measured the attitude of 1541 fractures and faults along several vertical and horizontal scan lines. There are two main fracture sets: one strikes between E–W and ESE–WNW, the other NNE–SSW. A total of 675 fracture-spacing measurements indicate log-normal spacing distributions, with an arithmetic mean fracture spacing of 0.29 m and a median of 0.15 m. The data, plotted on a log–log plot, suggest three main spacing subpopulations, each of which follows approximately a power law with different fractal dimensions. Subpopulation 1, where the spacing ranges from 1 to 10 cm and the straight-line slope D (“fractal dimension”) is 0.20, represents fractures confined to laminated carbonate mudstones (multilayers) that form the microbial mat deposits of a peritidal cycle. Subpopulation 2, where the spacing ranges from 11 to 55 cm and D is 0.77, represents fractures confined to thicker layers, forming a part of a peritidal cycle, the contacts of which are marked by stylolites. Subpopulation 3, where the spacing ranges from 56 to 243 cm and D is 2.81, represents fractures that dissect comparatively thick units of an entire peritidal cycle. For the spacing, the minimum coefficient of variation, C v, defined as standard deviation divided by the mean, is 1.00 (essentially randomly spaced fractures) while its maximum C v is 1.62, suggesting that some fractures form clusters, some clusters being denser than others. The clusters, composed of fractures with varying attitudes and therefore commonly intersecting, are likely to contribute significantly to the overall permeability of the carbonate rock. Fracture-aperture (opening) data ( N = 324) also show a log-normal size distribution, with a mean opening of 1.01 cm and median of 0.29 cm. Log–log plots indicate that a part of this data groups into two subpopulations, I and II, each of which follows approximately a power law. The straight-line slope D (“the fractal dimension”) of subpopulation I is 0.46, whereas that of subpopulation II is 1.49. We present boundary-element models showing that laminated carbonate mudstones and their contacts modify the local stress fields so as to encourage fracture offset and, commonly, arrest. Our results also show that when a fluid-driven subpopulation 2 fracture approaches subpopulation 1 fractures, the induced tensile stresses may result in the opening up of many of the subpopulation 1 fractures directly above the tip of the subpopulation 2 fractures. If, in addition, the contacts between the multilayers are weak, they also tend to open up, thus generating a large interconnected cluster of vertical fractures and horizontal contacts. The results suggest that the tensile stresses induced by a comparatively large fluid-driven subpopulation 2 fracture may contribute to the formation of an interconnected cluster of subpopulation 1 fractures and associated contacts, thereby significantly increasing the permeability of the carbonate rock.

THE TRIASSIC/JURASSIC BOUNDARY IN A PERITIDAL CARBONATE PLATFORM OF THE PELAGONIAN DOMAIN: THE MOUNT MESSAPION SECTION (CHALKIDA, GREECE)

In the Mount Messapion area (Chalkida, Greece) a continuous and expanded section of Triassic/Jurassic (T/J) limestone is exposed. This section consists of a 710 m thick pile of shallowing-upward peritidal cycles; the persistence of the facies and the lack of paleoenvironmental changes across the T/J boundary allowed studying the distribution of shallow water microfossils. The T/J boundary is placed in the upper part of the section defined by correspondence with (i) the last occurrence of Triasina hantkeni , (ii) the abrupt disappearance of megalodontid faunas and (iii) the presence of Early Jurassic microfossil assemblages. This paleontological reorganization happens suddenly, it is not controlled by any facies change, and surprisingly, it seems to produce no evident modification in the vertical stacking pattern of the cycles. A detailed facies analysis, performed along a 290 m thick stratigraphic interval (60 m above the T/J boundary and 230 m below), allowed the recognition of peritidal cycles: five different elementary cyclothemes are described and their distribution along the section is given. This integrated stratigraphic study attempts to highlight the relationship between the changes of carbonate producers and sea level fluctuations across the T/J boundary.

Facies analysis of Lofer cycles (Upper Triassic), in the Argolis Peninsula (Greece)

The Upper Triassic carbonate sediments of Argolis Peninsula are part of the Upper Triassic–Lower Jurassic extensive and thick neritic carbonate formations (Pantokrator facies) that formed at the passive Pelagonian margin and are considered as Dachstein-type platform carbonates. Facies analysis of the Upper Triassic “Lofer-type” lagoonal–peritidal cycles in the Dhidimi area, proved that cycles, although mostly incomplete, were regressive shallowing-upward. The ideal elementary cyclothems are meter-scale in thickness and begin with a subtidal bed (Member C), represented by a peloidal dolostone with megalodonts (wackestone or packstone), being followed by a stromatolitic intertidal dolomitic mudstone and/or fenestral intertidal dolomitic mudstone (Member B) that is overlain by dolocrete (terrestrial stromatolites or pisoidic dolomite) or a supratidal “soil conglomerate” in red micritic matrix (Member A). Lofer-cycle boundaries are defined at the erosional surfaces and accordingly the Lofer cyclothems are unconformity-bounded units. Due to common post-depositional truncation of the subtidal and intertidal facies, the supratidal members prevail, being developed, in places, directly upon subaerial exposure surfaces (erosionally reduced cyclothems). Peritidal layers are characterized by a well-expressed lamination, sheet cracks, tepee structures, fenestral pores and karst dissolution cavities. The studied lagoonal–peritidal cycles are considered to have been deposited in a tidal-flat setting (inner platform), repeatedly exposed under subaerial conditions, in the context of a broader tropical rimmed platform. Although the studied area was tectonically active due to rift-activity and the autocyclic processes should also be taken in consideration, the great lateral correlatability of cycles, the facies shifting and the widespread erosion that resulted in superposition of supratidal-pedogenic facies directly upon subtidal members (subaerial erosional unconformity), indicating a sea-level drop, reflect allocyclic control via high-frequency eustatic sea-level oscillation (orbital forcing). Sediment deposition occurred during low-stand system tract (LST), that probably continued also in the transgressive system tract (TST) and reflects an overall sea-level fall. Under these conditions dissolution and cement precipitation episodes, as well development of paleosols and karsts, were triggered, during a relatively less arid interval.

Microfacies and cycle stacking pattern in Liassic peritidal carbonate platform strata, Gavrovo-Tripolitza platform, Peloponnesus, Greece

Platform carbonate sediments of Liassic age cropping out in the area of the Pigadi-Fokianos Gulf (SE of Leonidion, Peloponnesus) have been investigated in order to determine their depositional environment. Facies analysis allowed the recognition of several microfacies types and their cyclic stacking pattern. The carbonates were deposited in a restricted inner platform environment (lagoon-peritidal domain) and are arranged into small-scale shallowing-upward cycles. Palaeosol horizons containing typical pedogenic features are developed on the top of the peritidal facies or are directly superimposed on subtidal deposits, forming diagenetic caps. This implies repeated sea-level fluctuations and periodic emersion episodes. The presence of orbitally forced cyclicity though is mostly probable, cannot be clearly documented by the available data. The studied carbonates are comparable with other coeval analogous peritidal cycles of the same age along the southern margin of the Tethys.

A Proposed Integrated Multi-Signature Model for Peritidal Cycles in Carbonates

Abstract A new integrated multi-signature model for meter-scale peritidal carbonate cycles presented here is relevant to the interpretation of these cycles in the stratigraphic record. Meter-scale shallowing-upward (micro)facies cycles are the most common type of peritidal parasequence. Peritidal parasequences are a fundamental component of carbonate sequence stratigraphic models. However, the existence of meter-scale cycles in the geological record has been questioned owing to the failure to prove statistically the existence of these cycles using data on (micro)facies and stacking pattern taken from previous studies. Nevertheless, the shallowing-upwards (micro)facies interpretive template remains widely used to identify these cycles and parasequences. We propose that the sedimentary signatures of meter-scale peritidal cycles vary systematically during the various phases of an underlying 3rd-order relative sea-level cycle. Although during specific phases of a 3rd-order relative sea-level cycle meter-scale peritidal cycles may be characterized by shallowing-upward units (e.g., early transgressive systems tract), during other periods the stratigraphic signal of cyclicity may be more cryptic and complex, causing them to be overlooked (e.g., late transgressive systems tract and highstand systems tract). We also propose that there is a half-cycle phase shift in periods of deposition and the formation of drowning surfaces, relative to 4th- to 5th-order relative sea-level cycles, over the course of a 3rd-order relative sea-level cycle. These points have ramifications for defining parasequences and using stacking patterns to determine peritidal cycle periodicity. Specifically, parasequences defined using flooding surfaces identify sedimentary units that inconsistently partition 4th- to 5th-order relative sea-level cycles at different stages of a 3rd-order relative sea-level cycle. We propose testable, new physical stratigraphic signatures for identifying meter-scale cycles that have consistent genetic significance, each cycle recording a relative sea-level cycle characterized by a 4th- to 5th-order relative sea-level rise and fall. Meter-scale peritidal cycles are recorded by an ordered pattern of systematically varying (micro)facies signatures related to their position within a 3rd-order relative sea-level cycle. The integrated multi-signature model of meter-scale peritidal carbonate cycles we propose here may assist in explaining the apparently contradictory evidence both for and against their existence in the geological record. As a result of the detailed consideration in this paper of the nature of parasequences and their variations with 3rd-order relative sea-level changes, a redefinition of parasequence is required and one is presented here.

Peritidal sedimentary depositional facies and carbon isotope variation across K/T boundary carbonates from NW Adriatic platform

The K/T boundary in the Karst region (Italy and Slovenia) is well documented in correspondence to shallow water limestones. Its evidence is usually related to palaeontological data (disappearance of Cretaceous taxa and subsequent appearance of Tertiary ones), palaeomagnetism (Ch 29r) iridium anomaly and negative shift of δ 13C. In Italy, the Padriciano section highlights the K/T boundary included in the basal breccia of a peritidal cycle. Recent investigations brought to light a short well exposed new stratigraphic sequence containing the K/T boundary, 100 m east of Padriciano, as demonstrated by lithological, palaeontological and new isotopic data. The recording of a new peritidal setting denotes stressed environmental conditions. The findings of the biota mostly correspond to the lagoonal limestones of the peritidal cycle. It consists of small opportunistic thin shelled foraminifers, ostracods and gastropods. At Padriciano, the environmental stability occurred after episodes of strong fresh water influence. The intraclasts, oncolites and stromatolites have been observed in the Padriciano section. The broken shell fragments, algae and foraminifera have been found associated with fenestral fabric. These features indicate deposition in a shallow marine (intertidal to supratidal) environment. Algal boundstones and microbial (stromatolitic) laminae, crenulated domal laminae and crenulated conical laminae have been studied in petrographic sections for microstructures. The K/T boundary section has been analysed for the carbon and oxygen isotopic ratios. The δ 13C values range from − 3.62 to − 10.01‰ (V-PDB) and the δ 18O values range from − 3.85 to − 5.47‰ (V-PDB). This extreme depletion in the δ 13C record has already been reported from other K/T boundary sections both in the Padriciano area (Italy) and in Slovenia (Dolenja Vas sections) and may be related to global climate change but also to local environmental changes occurring in the NW Adriatic Platform.

Neoproterozoic Molar-tooth Structure and Constraint of Depositional Facies and Environment in the North China Platform in Jiangsu, Anhui and Liaoning, Eastern China

Molar tooth structure (MTS) represented by complex ptygmatical shapes is widely distributed in the Proterozoic of the world. MTS filled by fine, equant sparry calcite (or dolomite) displays an abrupt contact with hosting rocks, which are mainly composed of carbonaceous micrites and fine-grained carbonates with local silts and storm-dominated deposits with graded, cross or wave beddings, numerous erosional surfaces and truncated and fills or guttered bases. Occurrence of MTS suggests a result of the constraint of sedimentary facies, and the storm-base in ramp settings is the maximum depth for the formation of MTS. Vertical succession of MTS-bearing carbonates shows a deposition stacked by high-frequency shallow subtidal and peritidal cycles. An individual cyclic MTS-bearing sequence is characterized by thinning, shallowing and dynamic decreasing-upward, and peritidal caps of purple red iron and organic carbonaceous sediments with more complicated shapes of MTS are common on the top of individual MTS-bearing sequences.

Role of mechanical stratigraphy on fracture development in carbonate reservoirs: Insights from outcropping shallow water carbonates in the Umbria–Marche Apennines, Italy

Studies on mechanical stratigraphy show that a link exists among facies, sedimentary cycles, diagenesis and fracturing. Understanding this link is fundamental for characterising fluid flow in natural reservoirs, especially carbonate ones. This work investigates the field of evidence through a case study in the Umbria–Marche Apennines: the Lower Jurassic shallow water carbonates of the Calcare Massiccio unit, a potential carbonate reservoir. A multidisciplinary approach is used, involving stratigraphy, sedimentology and structural geology. The studied succession crops out at the core of an ENE-verging anticline, located at Campolarzo, within the Umbria–Marche Apennines. The Calcare Massiccio here consists of high-frequency, metre-scale, shallowing upward, peritidal cycles. Three different cycles (thickness ranging from 0.3 to 3 m) have been recognised: type A (incomplete asymmetric cycle), type B (complete asymmetric cycle with sheet cracks) and type C (complete asymmetric cycle with tepees). Sedimentary textures vary according to the depositional environment. The subtidal facies is almost entirely mud-supported, with the exception of type A cycle, which locally can be grain-supported. The intertidal facies is from mud- to grain-supported and commonly laminated. The supratidal facies may be either absent (type A) or represented by vadose pisoid caliches, sheet cracks (type B) and tepees (type C). The overprinting of diagenetic facies (early diagenesis) is particularly evident in the supratidal intervals of type C cycle (tepee structures) and in the inter-supratidal intervals of type B cycle (sheet cracks). The fracture pattern within the Calcare Massiccio mainly consists of systematic joints organized in two prominent mutually perpendicular sets (orthogonal joints) and two subordinate non-perpendicular sets (diagonal joints). The orthogonal sets are interpreted as tensile joints developed sub-parallel (longitudinal joints) and sub-perpendicular (cross joints) to the NNW–SSE axial trend of the host anticline. The diagonal joints are interpreted as shear joints. In the study area, the fracture density is strongly controlled by heterogeneities of rock properties between and within facies, which in turn are determined by sedimentary textures and, dominantly, by the combination of sedimentary and diagenetic facies, e.g., by the petrofacies. The control played by the petrofacies distribution across the stratigraphic succession may justify strong variations in the fracture density (up to ca. 80% or more) within the same sedimentary cycle, as well as fracture terminations, independently from tectonic causes. A systematic decrease in fracture density going from subtidal to intertidal to inter-supratidal and tepee facies is evident. Minimum values are observed on tepee structures; fractures often terminate against supratidal intervals with tepee. The lowest fracture density characterises petrofacies where early diagenetic processes are more pronounced (i.e., the intertidal facies and the inter-supratidal facies with tepee, strongly affected by early dissolution cavities filled by early cements). No obvious correlation is observed between the fracture density and the thickness of the petrofacies layer.

Lithofacies, diagenesis and cyclicity of the ‘Lower Member’ of the Khuff Formation (Late Permian), Al Qasim Province, Saudi Arabia

The ‘Lower Member’ of the Khuff Formation, exposed along the western margin of the Arabian Basin (Al Qasim Province, Saudi Arabia) comprises seven carbonate and two siliciclastic lithofacies. The carbonate lithofacies include laminated lime mudstone, ostracode wackestone, foraminiferal wackestone, intraclastic wackestone, burrowed dolostone, lithoclastic grainstone and caliche. The clastic lithofacies are represented by claystone and quartzarenite. These lithofacies are arranged vertically into shallowing-upward or emergence cycles, the boundaries between which were delineated by field observations. Peritidal as well as subtidal cycles built-up the Lower Member of the Khuff Formation. The peritidal cycles consist of laminated lime mudstone or subtidal claystone, overlain by intertidal burrowed dolostone, which is capped in most cases by supratidal lithoclastic grainstone, and locally by subaerial caliche deposits. These cycles were probably formed by allocyclic mechanism due to the effect of sea level changes and intrabasinal tectonics. The subtidal cycles comprise laminated lime mudstone, claystone, foraminiferal wackestone, and ostracode wackestone, capped by intertidal burrowed dolostone or intraclastic wackestone. These cycles may be formed due to internal factors, e.g. carbonate production, changes in water circulation, salinity, depth, chemistry, energy, temperature and nutrient content. Diagenetic studies on the depositional cycles show that the basal part of cycle (subtidal zone) includes micrite cementation, micritization and silicification. The middle part (intertidal zone) was affected by early dolomitization, while the upper part (supratidal zone) was subjected to dedolomitization, aggrading neomorphism, sparry calcite cementation and evaporite-carbonate replacement.

Application of ground-penetrating radar, digital optical borehole images, and cores for characterization of porosity hydraulic conductivity and paleokarst in the Biscayne aquifer, southeastern Florida, USA

This paper presents examples of ground-penetrating radar (GPR) data from two study sites in southeastern Florida where karstic Pleistocene platform carbonates that comprise the unconfined Biscayne aquifer were imaged. Important features shown on resultant GPR profiles include: (1) upward and lateral qualitative interpretative distribution of porosity and hydraulic conductivity; (2) paleotopographic relief on karstic subaerial exposure surfaces; and (3) vertical stacking of chronostratigraphic high-frequency cycles (HFCs). These characteristics were verified by comparison to rock properties observed and measured in core samples, and identified in digital optical borehole images. Results demonstrate that an empirical relation exists between measured whole-core porosity and hydraulic conductivity, observed porosity on digital optical borehole images, formation conductivity, and GPR reflection amplitudes—as porosity and hydraulic conductivity determined from core and borehole images increases, formation conductivity increases, and GPR reflection amplitude decreases. This relation allows for qualitative interpretation of the vertical and lateral distribution of porosity and hydraulic conductivity within HFCs. Two subtidal HFCs in the uppermost Biscayne aquifer have significantly unique populations of whole-core porosity values and vertical hydraulic conductivity values. Porosity measurements from one cycle has a median value about two to three times greater than the values from the other HFC, and median values of vertical hydraulic-conductivity about three orders of magnitude higher than the other HFC. The HFC with the higher porosity and hydraulic conductivity values is shown as a discrete package of relatively low-amplitude reflections, whereas the HFC characterized by lower porosity and hydraulic-conductivity measurements is expressed by higher amplitude reflections. Porosity and hydraulic-conductivity values measured from whole-core samples, and vuggy porosity identified on digital borehole images from shallowing-upward, peritidal HFCs show that the highest porosity occurs at the base of the cycles, moderate porosity at the middle of the cycles, and lowest porosity occurs at the top of cycles. Hydraulic conductivity is also highest at the base of the peritidal cycles and lowest in the middle to upper parts of cycles. This change in porosity and hydraulic conductivity from bottom to top is visible as an upward variation in reflection amplitude on GPR profiles—lowest amplitudes at the base and highest at the cycle tops. This study demonstrates that GPR can be used to show the qualitative distribution of porosity and hydraulic conductivity within a cycle-stratigraphic framework composed of carbonate HFCs. The distribution of porosity and hydraulic conductivity within HFCs is related to depositional textures. The upward and lateral patterns of the rock facies within the HFCs can be translated to geophysical-log properties and radar facies configurations that could aid in interpretation and prediction of ground-water flow through a carbonate aquifer.

Neoproterozoic Chuar Group (∼800–742Ma), Grand Canyon: a record of cyclic marine deposition during global cooling and supercontinent rifting

Abstract Chuar Group sediments were deposited in a marine cratonic basin synchronously with marine deposition in other basins of western North America and Australia. The top of the Chuar Group is 742 Ma; thus, this succession offers a potentially important record of global climatic and tectonic changes during the mid-Neoproterozoic — a critical time leading to possible global glaciation during supercontinent rifting. Chuar Group cycles and sequences may record glacioeustatic fluctuations during the climatic transition into the Sturtian Ice Age (∼750–700 Ma), as well as extension related to the dispersal of Rodinia. The mid-Neoproterozoic Chuar Group (1600 m thick) is predominantly composed of mudrock with subordinate meter-scale beds of dolomite and sandstone. Facies analysis leads to the interpretation of a wave- and tidal-influenced marine depositional system. Diagnostic marine features include: (1) marine fossils and high local pyrite content in the mudrock facies; (2) mudcracked mud-draped symmetric ripples and reverse flow indicators in the sandstone facies; (3) facies associations between all facies; and (4) no unequivocal terrestrial deposits. Chuar facies stack into ∼320 dolomite- and sandstone-capped meter-scale cycles (1–20 m thick) and non-cyclic intervals of uniform mudrock facies (20–150 m thick). Nearly all cycles have mudrock bases indicating subtidal water depths. Dolomite-capped cycles shallow to peritidal environments (peritidal cycles), some indicating subaerial exposure of varying degrees (exposure cycles). Sandstone-capped cycles shallow to peritidal environments with no evidence of prolonged exposure (peritidal cycles). Non-cyclic intervals represent the deepest water depths (∼10s of meters). Peritidal cycles and non-cyclic intervals dominate the lower and middle Chuar Group. In the upper Chuar Group, the percentage of exposure cycles increases, as does the thickness of individual cycles and non-cyclic intervals, and the degree of subaerial exposure in the cycle caps. Correlation of cycles across 10 km shows that many cycles are laterally continuous and some vary in thickness. Some thickness changes are coincident with local and regional extensional structures. These cycle trends reflect a resolvable mixture of tectonic and climatic controls. Cycles are interpreted to be high frequency and glacioeustatically-controlled based on comparison of cycle character and thickness to Phanerozoic examples, and lateral continuity of many cycles, respectively. Cycles in the lower and middle Chuar Group show characteristics suggestive of low-amplitude sea-level changes similar to meter-scale cycles from global greenhouse climates in the Phanerozoic. Upper Chuar Group cycles show characteristics suggestive of moderate-amplitude sea-level changes similar to Phanerozoic greenhouse–icehouse transition climates, indicating an increase in continental ice volume. Variability in cycle thickness and lateral pinching of some cycle caps is controlled by variable tectonic subsidence, evidenced by thickening trends coincident with active rift-related structures, and also by variability in sediment supply. The presence of dolomite caps, in this otherwise siliciclastic succession, is interpreted to be due largely to rapid changes in climate related to short-term glacioeustatically-controlled sea-level changes. Four crude lithostratigraphic sequences (150–775 m thick) are defined based on dolomite-poor to dolomite-rich stratigraphic intervals. Long-term changes in cycle cap-lithology may be driven by long-term changes in climate, similar to short-term glacioeustatic controls on meter-scale cycle cap lithology. Large-scale thickness variability in these sequences is due to local differential tectonic subsidence.

Anatomy of a cyclically packaged Mesoproterozoic carbonate ramp in northern Canada

Carbonates in the upper member of the Mesoproterozoic Victor Bay Formation are dominated by lime mud and packaged in cycles of 20–50 m. These thicknesses exceed those of classic shallowing-upward cycles by almost a factor of 10. Stratigraphic and sedimentological evidence suggests high-amplitude, high-frequency glacio-eustatic cyclicity, and thus a cool global climate ca. 1.2 Ga. The Victor Bay ramp is one of several late Proterozoic carbonate platforms where the proportions of lime mud, carbonate grains, and microbialites are more typical of younger Phanerozoic successions which followed the global waning of stromatolites. Facies distribution in the study area is compatible with deposition on a low-energy, microtidal, distally steepened ramp. Outer-ramp facies are hemipelagic lime mudstone, shale, carbonaceous rhythmite, and debrites. Mid-ramp facies are molar-tooth limestone tempestite with microspar-intraclast lags. In a marine environment where stromatolitic and oolitic facies were otherwise rare, large stromatolitic reefs developed at the mid-ramp, coeval with inner-ramp facies of microspar grainstone, intertidal dolomitic microbial laminite, and supratidal evaporitic red shale. Deep-subtidal, outer-ramp cycles occur in the southwestern part of the study area. Black dolomitic shale at the base is overlain by ribbon, nodular, and carbonaceous carbonate facies, all of which exhibit signs of synsedimentary disruption. Cycles in the northeast are shallow-subtidal and peritidal in character. Shallow-subtidal cycles consist of basal deep-water facies, and an upper layer of subtidal molar-tooth limestone tempestite interbedded with microspar calcarenite facies. Peritidal cycles are identical to shallow-subtidal cycles except that they contain a cap of dolomitic tidal-flat microbial laminite, and rarely of red shale sabkha facies or of sandy polymictic conglomerate. A transect along the wall of a valley extending 8.5 km perpendicular to depositional strike reveals progradation of inner-ramp tidal flats over outer- and mid-ramp facies during shoaling. The maximum basinward progradation of peritidal facies coincides with a zone of slope failure that may have promoted the development of the stromatolitic reefs. The sea-level history of the Victor Bay Formation is represented by three hectometre-scale sequences. An initial flooding event resulted in deposition of the lower Victor Bay shale member. Upper-member carbonate cycles were then deposited during highstand. Mid-ramp slumping was followed by late-highstand reef development. The second sequence began with development of an inner-ramp lowstand unconformity and a thick mid-ramp lowstand wedge. A second transgression promoted a more modest phase of reef development at the mid-ramp and shallow-water deposition continued inboard. A third and final transgressive episode eventually led to flooding of the backstepping ramp. Overall consistent cycle thickness and absence of truncated cycles, as well as the high rate and amount of creation of accommodation space, suggest that the periodicity and amplitude of sea-level fluctuation were relatively uniform, and point to a eustatic rather than tectonic mechanism of relative sea-level change. High-amplitude, high-frequency eustatic sea-level change is characteristic of icehouse worlds in which short-term, large-scale sea-level fluctuations accompany rapidly changing ice volumes affected by Milankovitch orbital forcing. Packaging of cyclic Upper Victor Bay carbonates therefore supports the hypothesis of a late Mesoproterozoic glacial period, as proposed by previous workers.

Long‐distance correlation between tectonic‐controlled, isolated carbonate platforms by cyclostratigraphy and sequence stratigraphy in the Devonian of South China

During the early Middle Devonian in South China, an extensive carbonate platform was broken up through extension to create a complex pattern of platforms, and interplatform basins. In Givetian and Frasnian carbonate successions, five depositional facies, including peritidal, restricted shallow subtidal, semi‐restricted subtidal, intermediate subtidal and deep subtidal facies, and 18 lithofacies units are recognized from measured sections on three isolated platforms. These deposits are arranged into metre‐scale, upward‐shallowing peritidal and subtidal cycles. Nine third‐order sequences are identified from changes in cycle stacking patterns, vertical facies changes and the stratigraphic distribution of subaerial exposure indicators. These sequences mostly consist of a lower transgressive part and an upper regressive part. Transgressive packages are dominated by thicker‐than‐average subtidal cycles, and regressive packages by thinner‐than‐average peritidal cycles. Sequence boundaries are transitional zones composed of stacked, high‐frequency, thinner‐than‐average cycles with upward‐increasing intensity of subaerial exposure, rather than individual, laterally traceable surfaces. These sequences can be further grouped into catch‐up and keep‐up sequence sets from the long‐term (second‐order) changes in accommodation and vertical facies changes. Catch‐up sequences are characterized by relatively thick cycle packages with a high percentage of intermediate to shallow subtidal facies, and even deep subtidal facies locally within some individual sequences, recording long‐term accommodation gain. Keep‐up sequences are characterized by relatively thin cycle packages with a high percentage of peritidal facies within sequences, recording long‐term accommodation loss. Correlation of long‐term accommodation changes expressed by Fischer plots reveals that during the late Givetian to early Frasnian increased accommodation loss on platforms coincided with increased accommodation gain in interplatform basins. This suggests that movement on faults resulted in the relative uplift of platforms and subsidence of interplatform basins. In the early Frasnian, extensive siliceous deposits in most interplatform basins and megabreccias at basin margins correspond to exposure disconformities on platforms.

Cyclic sedimentation across a Middle Ordovician carbonate ramp (Duwibong Formation), Korea

The Middle Ordovician Duwibong Formation (about 100 m thick), Korea, comprises various lithotypes deposited across a carbonate ramp. Their stacking patterns constitute several kinds of meter-scale, shallowing-upward carbonate cycles. Lithofacies associations are grouped into four depositional facies: deep- to mid-ramp, shoal-complex, lagoonal, and tidal-flat facies. These facies are composed of distinctive depositional cycles: deep subtidal, shallow subtidal, restricted marine, and peritidal cycles, respectively. The subtidal cycles are capped by subtidal lithofacies and indicate incomplete shallowing to the peritidal zone. The restricted marine and peritidal cycles are capped by tidal flat lithofacies and show evidence of subaerial exposure. These cycles were formed by higher frequency sea-level fluctuations with durations of 120 ky (fifth order), which were superimposed on the longer term sea-level events, and by sediment redistribution by storm-induced currents and waves. The stratigraphic succession of the Duwibong Formation represents a general regressive trend. The vertical facies change records the transition from a deep- to mid-ramp to shoal, to lagoon, into a peritidal zone. The depositional system of the Duwibong Formation was influenced by frequent storms, especially on the deep ramp to mid-ramp seaward of ooid shoals. The storm deposits comprise about 20% of the Duwibong sequence.

Sequence Stratigraphy of Lower Cretaceous (Barremian-Albian) Carbonate Platforms of Northeastern Mexico: Regional and Global Correlations

ABSTRACT The Lower Cretaceous Cupido and Coahuila platforms of northeastern Mexico form part of the extensive carbonate platform system that surrounded the ancestral Gulf of Mexico. A sequence-stratigraphic model for these Barremian to Albian platforms was constructed from regional correlations of vertical cycle stacking patterns constrained by newly acquired biostratigraphic data and C and Sr isotope stratigraphy. The Cupido shelf lagoon (Barremian-Aptian) is composed of up to 150 peritidal cycles, which stack into high-frequency sequences that are correlated across the shelf. Cupido high-frequency sequences build into a lower partial composite sequence (Cu1), an overlying full composite sequence (Cu2), and the transgressive base of a third composite sequence (Cu-Co3) related to backstepping of the Cupido platform. The highstand part of Cu-Co3 is recorded in evaporite and carbonate facies of the Coahuila platform (Aptian-Albian). More than 80 meter-scale evaporite-to-carbonate cycles characterize the restricted interior of the Coahuila platform and stack into two full composite sequences (Co4, Co5). The Coahuila platform was drowned in the latest Albian or earliest Cenomanian, terminating shallow-water carbonate sedimentation in the region. Regional correlations of the depositional sequences in northeast Mexico with coeval platforms in Texas and the northern Gulf coast indicate that the lower two composite sequences of the Cupido shelf correlate with the lower part of the Sligo-Hosston platform in Texas. The transgressive systems tract of the third composite sequence of the Cupido-La Pena is recorded in Texas by the upper Sligo and Pearsall formations. The highstand systems tract and the disconformable top of the third composite sequence (Coahuila platform) is likely coeval with the upper Glen Rose platform in Texas. The fourth and fifth composite sequences are correlative with the Fredericksburg platform and the Washita-Devils River platforms of Texas, respectively. We conservatively correlated our sequence-stratigraphic framework with composite global sequences and eustatic curves derived from several other basins. Given the available time resolution, results are variable, but sequence boundaries at the top of Cu-Co3 and Co5 appear to have clear interbasinal equivalents.

Low‐ and high‐frequency sea‐level changes control peritidal carbonate cycles, facies and dolomitization in the Rock of Gibraltar (Early Jurassic, Iberian Peninsula)

The Rock of Gibraltar comprises two tectonically separated limbs of an isolated klippe of Liassic Gibraltar Limestone Formation. Both limbs have similar, c. 400 m thick sequences of inner carbonate platform facies arranged in high‐frequency, metre‐scale, shallowing‐upward, peritidal cycles with emergent, caliche caps. Four cycle types are recognized on the basis of vertically repeated successions of different sedimentary structures, lithologies, facies and biota. When compared with other Liassic cycles from fault‐bound platforms of the western Mediterranean region all are found to be of similar scale, facies and cycle type. Likely common origins are through Milankovitch band allocyclicity, or autocyclic tidal flat progradation superimposed on regional subsidence. Within the Gibraltar Limestone high‐frequency cycles are superimposed on a low‐frequency (third order?) cyclicity that is revealed, through the use of Fischer plots, to control the occurrrence of facies, biota, high‐frequency cycle types and dolomitization. Falling sea‐level and lowstand phases, with reduced accommodation space, are typified by restricted, inner platform facies and cycles and by early reflux dolomitization. Transgressive and highstand phases, with more accommodation space, are characterized by the absence of early dolomites, the incoming of inner platform microfossils (i.e. foraminifera and calcareous algae) and by less restricted marine facies (i.e. oncoids, shelly rudstones, packstones and grainstones). Fischer plots have demonstrable value in the correlation and analysis of tectonically separated and geographically isolated cyclic sequences that lack prominent marker beds or stratigraphically useful biotas.

High-resolution sequence stratigraphic correlation in the Upper Jurassic (Kimmeridgian)-Upper Cretaceous (Cenomanian) peritidal carbonate deposits (Western Taurides, Turkey)

Upper Jurassic (Kimmeridgian)±Upper Cretaceous (Cenomanian) inner platform carbonates in the Western Taurides are composed of metre-scale upward-shallowing cyclic deposits (parasequences) and important karstic surfaces capping some of the cycles. Peritidal cycles (shallow subtidal facies capped by tidal-¯at laminites or fenestrate limestones) are regressive- and transgressive-prone (upward-deepening followed by upward-shallowing facies trends). Subtidal cycles are of two types and indicate incomplete shallowing. Submerged subtidal cycles are composed of deeper subtidal facies overlain by shallow subtidal facies. Exposed subtidal cycles consist of deeper subtidal facies overlain by shallow subtidal facies that are capped by features indicative of prolonged subaerial exposure. Subtidal facies occur characteristically in the Jurassic, while peritidal cycles are typical for the Lower Cretaceous of the region. Within the foraminiferal and dasyclad algal biostratigraphic framework, four karst breccia levels are recognized as the boundaries of major second-order cycles, introduced for the ®rst time in this study. These levels correspond to the Kimmeridgian±Portlandian boundary, mid-Early Valanginian, mid-Early Aptian and mid-Cenomanian and represent important sea level falls which affected the distribution of foraminiferal fauna and dasyclad ¯ora of the Taurus carbonate platform. Within the Kimmeridgian±Cenomanian interval 26 third-order sequences (types 1 and 2) are recognized. These sequences are the records of eustatic sea level ¯uctuations rather than the records of local tectonic events because the boundaries of the sequences representing 1±4 Ma intervals are correlative with global sea level falls. Third-order sequences and metre-scale cyclic deposits are the major units used for long-distance, high-resolution sequence stratigraphic correlation in the Western Taurides. Metre-scale cyclic deposits (parasequences) in the Cretaceous show genetical stacking patterns within third-order sequences and correspond to fourth-order sequences representing 100±200 ka. These cycles are possibly the E2 signal (126 ka) of the orbital eccentricity cycles of the Milankovitch band. The slight deviation of values, calculated for parasequences, from the mean value of eccentricity cycles can be explained by the currently imprecise geochronology established in the Cretaceous and missed sea level oscillations when the platform lay above fluctuating sea level.

Controls on cyclostratigraphy of Lower Cretaceous carbonates and evaporites, Cupido and Coahuila platforms, northeastern Mexico

The Lower Cretaceous Cupido (Barremian-Aptian) and Coahuila (Albian) carbonate platforms of northeastern Mexico exhibit thick successions of meter-scale cycles deposited in three unique paleoenvironmental settings. (1) The Cupido shelf lagoon is composed of peritidal carbonate cycles deposited in the protected lee of a reef-rimmed to barrier-shoal margin. (2) The restricted Coahuila ramp interior consists of alternations of subtidal evaporites and peritidal carbonates. (3) The deep-water parts of both the Cupido and Coahuila platforms are composed of foraminiferal wackestones and lime mudstones interspersed with firmgrounds and hardgrounds in a cyclic arrangement. Vertical successions of meter-scale evaporitic cycles and peritidal cycles exhibit systematic stacking patterns that build into intermediate-scale high-frequency sequences (tens to hundreds of meters thick), and large-scale composite sequences (hundreds of meters thick) that can be correlated across the Cupido and Coahuila platforms. These large-scale stacking patterns are interpreted to reflect longterm accommodation events and, when combined with the scale-independent architecture of all genetic units, permit the inference that all three meter-scale cycle types on the Cupido and Coahuila platforms are also governed by relative sea-level change. The composition, thickness, and number of meter-scale cycles within individual high-frequency sequences can be highly variable across the Cupido and Coahuila platforms, however, even though the overall upward-shallowing patterns are evident. The lateral complexity of cycle architecture and distribution is interpreted to be a natural response to fluctuations in regional climate interacting with autogenic processes such as variations in carbonate production and dispersal, intensity and frequency of tropical storms and monsoons, thermohaline circulation patterns, and ambient ocean chemistry and temperature. These interacting processes created laterally variable physiographic and oceanographic conditions across the Cupido and Coahuila platforms, complicating the sedimentary record generated by the composite sea-level signal. Well-documented evidence from Barremian-Cenomanian pelagic cycles throughout the Tethyan seaway strongly indicates that Milankovitch-driven climatic changes operated during the Early Cretaceous. Contemporaneous shallow-marine cyclicity in several locations suggests that these climatic changes may have had globally widespread effects. In an effort to link the shallow-water and deep-water realms, we propose a model whereby Milankovitch-driven global climatic changes generated low-amplitude, high-frequency eustatic fluctuations through some combination of thermal expansion and contraction of ocean water, waxing and waning of small ice caps and alpine glaciers, and changes in the storage capacity of aquifers and lakes to produce meter-scale cycles across Lower Cretaceous shallow-marine platforms.

Origin and modification of early dolomites in cyclic shallow platform carbonates, Yeongheung Formation (middle Ordovician), Korea

The middle Ordovician Yeongheung Formation, Korea, consists of numerous metre-scale, shallowing-upward cycles, which were deposited on a shallow-marine carbonate platform. Depositional sequences developed on the inner platform were completely dolomitized regardless of constituting lithology, whereas those developed on the outer platform were partially dolomitized. The amount of replacement dolomite within a metre-scale, shallowing-upward peritidal cycle decreases downward from the peritidal cycle cap in the partially dolomitized depositional sequences. The cross-platform distribution of dolomites and their stratigraphic position in the sequences suggest that the Yeongheung dolomite formed syndepositionally. The common existence of pseudomorphs of evaporite minerals and nodules in the Yeongheung Formation indicates that dolomitization of the Yeongheung Formation occurred in a sabkha-type tidal-flat environment. It also occurred during each cycle of deposition influenced by high-frequency sea-level oscillations. The present-day Yeongheung replacement dolomites are characterized by near-stoichiometric to stoichiometric composition (49.9 to 52.1 CaCO 3 mole%), and low strontium concentration (52 to 133 ppm). The δ 18O value of the replacement dolomites ranges from −7.7 to −5.2‰ (PDB); this is much depleted relative to that of contemporary sabkha-type dolomites. These geochemical characteristics of Yeongheung dolomite indicate significant post-depositional modification. The common existence of oomolds, leached skeletal grains and dissolved evaporites in the Yeongheung Formation illustrates invasion of meteoric water into the sediments. Recrystallization of Yeongheung dolomites in the infiltrated meteoric water at slightly elevated temperatures during shallow burial resulted in lighter δ 18O values and lower strontium concentration.