Sequence stratigraphy and carbonate platform organization of the Devonian Santa Lucia Formation, Cantabrian Mountains, NW-Spain

In this paper, the sedimentology and the stratigraphic architecture of the Devonian Santa Lucia Formation in the Cantabrian Mountains of NW-Spain are described. The Santa Lucia Formation consists of 11 different facies that can be attributed to peritidal/lagoonal, intertidal and subtidal facies associations. These facies associations are arranged in small-scale sedimentary cycles. Three different settings of small-scale sedimentary cycles are recognized: intertidal/supratidal, shallow subtidal/intertidal and subtidal cycles. These cycles reflect spatial differences in the reaction of the depositional system to small-scale relative sea-level changes. Small-scale stratigraphic cycles are stacked into seven medium-scale cycles that in turn are integral parts of three larger-scale cycles. Most of the Santa Lucia Formation (sequences 2–6) forms one major large-scale cycle, whereas sequences 1 and 7 are part of an underlying and an overlying cycle, respectively. Eustatic sea-level changes exerted major control on the formation of these large-scale sequences, whereas the medium-scale cycles seem to be co-controlled by regional tectonism and eustasy. Small-scale cycles seem to be the product of high frequency, eustatic sea-level changes. During the deposition of the Santa Lucia Formation, the morphology of the carbonate platform changed from a gently south-dipping ramp to a rimmed shelf and back to a gently dipping ramp.     

A facies model for an Early Aptian carbonate platform (Zamaia, Spain)

The Cretaceous (Early Aptian, uppermost Bedoulian, Dufrenoyia furcata Zone) Zamaia Formation is a carbonate unit, up to 224 m thick and 1.5 km wide, which formed on a regional coastal sea bordering the continental Iberian craton. A high-resolution, facies-based, stratigraphic framework is presented using facies mapping and vertical-log characterization. The depositional succession consists of a shallow estuarine facies of the Ereza Fm overlain by shallow-water rudist limestones (Zamaia Fm) building relief over positive tectonic blocks and separated by an intraplatform depression. The margins of these shallow-water rudist buildups record low-angle transitional slopes toward the adjacent surrounding basins. Syn-depositional faulting is responsible for differential subsidence and creation of highs and lows, and local emplacement of limestone olistoliths and slope breccias. Two main carbonate phases are separated by an intervening siliciclastic-carbonate estuarine episode. The platform carbonates are composed of repetitive swallowing-upward cycles, commonly ending with a paleokarstic surface. Depositional systems tracts within sequences are recognized on the basis of facies patterns and are interpreted in terms of variations of relative sea level. Both Zamaia carbonate platform phases were terminated by a relative sea-level fall and karstification, immediately followed by a relative sea-level rise. This study refines our understanding of the paleogeography and sea-level history in the Early Cretaceous Aptian of the Basque-Cantabrian Basin. The detailed information on biostratigraphy and lithostratigraphy provides a foundation for regional to global correlations.     

A sequence stratigraphical model for the Late Ludfordian (Silurian) of Gotland,Sweden: implications for timing between changes in sea level,palaeoecology, and the global carbon cycle

This paper investigates a time interval within the Late Ludfordian (Late Silurian), involving changes in faunal composition (the Lau Event), a major positive carbon isotope excursion (CIE), and contemporaneous sea-level changes in remote palaeo-basins. Based on the Silurian strata of Gotland (Sweden), we integrate sequence stratigraphy, carbon isotope stratigraphy, and platform-scale palaeoecological changes associated with this turbulent time period in Earth history. Three depositional sequences (sequences Nos. 1–3), including two separate periods of forced regression (falling stage systems tracts, FSSTs) are identified from outcrop and drillcore studies. The sequence stratigraphical framework is interpreted to reflect glacio-eustatic sea-level changes. The CIE starts at the onset of the initial FSST (sequence No. 1), just below the last appearance datum of the conodont Polygnathoides siluricus. The values increase through the ensuing lowstand and transgressive systems tracts (LST and TST) of sequence No. 2 and peak in the following highstand systems tract (HST). A second forced regression (FSST of sequence No. 2) took place in the lower Ozarkodina snajdri Zone. δ¹³C data are scarce from these siliciclastic strata, but inferably remain high. The δ¹³C values increase within the LST and earliest TST of sequence No. 3, before a decreasing trend starts within the early TST. δ¹³C values return to pre-excursion levels within the ensuing HST. The CIE is closely associated with an increase in stromatolites (mats and oncoids) across a wide range of depths and sedimentary environments, and correlations to other basins indicate a global increase in cyanobacterial activity. A drastic decline in level-bottom benthic faunas during the FSST of sequence No. 2 is, however, interpreted as a local response to the progradation of a delta complex (the Burgsvik Sandstone). Biological carbonate production replenishes rapidly within the TST of sequence No. 3, succeeding a thin LST dominated by reworked siliciclastics and chemically precipitated carbonates (ooids). The detailed relationship between the CIE and sea-level change presented herein is not fully consistent with previous reports on the CIEs associated with the lower Silurian Ireviken and Mulde events, respectively. Based on our facies analysis and sequence stratigraphical interpretation, two main mechanisms are suggested as responsible for the Late Ludfordian CIE: (1) a change in the riverine C-weathering flux towards the ¹³C end member following glacio-eustatically induced subaerial exposure of carbonate platforms throughout the tropics, and, (2) increased photosynthetic activity by benthic cyanobacteria exaggerating the δ¹³C values of precipitated carbonates.     

Patch-reef morphology as a proxy for Holocene sea-level variability,Northern Florida Keys,USA

A portion of the northern Florida Keys reef tract was mapped with the NASA Experimental Advanced Airborne Research Lidar (EAARL) and the morphology of patch reefs was related to variations in Holocene sea level. Following creation of a lidar digital elevation model (DEM), geospatial analyses delineated morphologic attributes of 1,034 patch reefs (reef depth, basal area, height, volume, and topographic complexity). Morphometric analysis revealed two morphologically different populations of patch reefs associated with two distinct depth intervals above and below a water depth of 7.7 m. Compared to shallow reefs, the deep reefs were smaller in area and volume and showed no trend in topographic complexity relative to water depth. Shallow reefs were more variable in area and volume and became flatter and less topographically complex with decreasing water depth. The knoll-like morphology of deep reefs was interpreted as consistent with steady and relatively rapidly rising early Holocene sea level that restricted the lateral growth of reefs. The morphology of shallow “pancake-shaped” reefs at the highest platform elevations was interpreted as consistent with fluctuating sea level during the late Holocene. Although the ultimate cause for the morphometric depth trends remains open to interpretation, these interpretations are compatible with a recent eustatic sea-level curve that hindcasts fluctuating late Holocene sea level. Thus it is suggested that the morphologic differences represent two stages of reef accretion that occurred during different sea-level conditions.     

Microbial boundstone dominated carbonate slope (Upper Carboniferous,N Spain): Microfacies,lithofacies distribution and stratal geometry

Summary The Carboniferous, particularly during the Serpukhovian and Bashkirian time, was a period of scarce shallow-water calcimicrobial-microbialite reef growth. Organic frameworks developed on high-rising platforms are, however, recorded in the Precaspian Basin subsurface, Kazakhstan, Russia, Japan and Spain and represent uncommon occurrences within the general trend of low accumulation rates and scarcity of shallow-water reefs. Sierra del Cuera (Cantabrian Mountains, N Spain) is a well-exposed high-rising carbonate platform of Late Carboniferous (Bashkirian-Moscovian) age with a microbial boundstone-dominated slope dipping from 20° up to 45°. Kilometer-scale continuous exposures allow the detailed documentation of slope geometry and lithofacies spatial distribution. This study aims to develop a depositional model of steep-margined Late Paleozoic platforms built by microbial carbonates and to contribute to the understanding of the controlling factors on lithofacies characteristics, stacking patterns, accumulation rates and evolution of the depositional architecture of systems, which differ from light-dependent coralgal platform margins. From the platform break to depths of nearly 300 m, the slope is dominated by massive cement-rich boundstone, which accumulated through the biologically induced precipitation of micrite. Boundstone facies (type A) with peloidal carbonate mud, fenestellid and fistuliporid bryozoans, sponge-like molds and primary cavities filled by radiaxial fibrous cement occurs all over the slope but dominates the deeper settings. Type B boundstone consists of globose centimeter-scale laminated accretionary structures, which commonly host botryoidal cement in growth cavities. The laminae nucleate around fenestellid bryozoans, sponges, Renalcis and Girvanella-like filaments. Type B boundstone typically occurs at depths between 20–150 m to locally more than 300 m and forms the bulk of the Bashkirian prograding slope. The uppermost slope boundstone (type C; between 0 and 20–100 m depth) includes peloidal micrite, radiaxial fibrous cement, bryozoans, sponge molds, Donezella, Renalcis, Girvanella, Ortonella, calcareous algae and calcitornellid foraminifers. From depths of 80–200 m to 450 m, 1–30 m thick lenses of crinoidal packstone, spiculitic wackestone, and bryozoan biocementstone with red-stained micrite matrix are episodically intercalated with boundstone and breccias. These layers increase in number from the uppermost Bashkirian to the Moscovian in parallel with the change from a rapidly prograding to an aggrading architecture. The red-stained strata share comparable features with Lower Carboniferous deeper-water mud-mound facies and were deposited during relative rises of sea level and pauses in boundstone production. Rapid relative sea-level rises might have been associated with changes in oceanographic conditions not favourable for thecalcimicrobial boundstone growth, such as upwelling of colder, nutrient-rich waters lifting the thermocline to depths of 80–200 m. Downslope of 150–300 m, boundstones interfinger with layers of matrix-free breccias, lenses of matrix-rich breccias, platform- and slope-derived grainstone and crinoidal packstone. Clast-supported breccias bound by radiaxial cement are produced by rock falls and avalanches coeval to boundstone growth. Matrix-rich breccias are debris flow deposits triggered by the accumulation of red-stained layers. Debris flows develop following the relative sea-level rises, which favour the deposition of micrite-rich lithofacies on the slope rather than being related to relative sea-level falls and subaerial exposures. The steep slope angles are the result of in situ growth and rapid stabilization by marine cement in the uppermost part, passing into a detrital talus, which rests at the angle of repose of noncohesive material. In the Moscovian, the aggradational architecture and steeper clinoforms are the result of increased accommodation space due to tectonic subsidence and due to a reduction of slope accumulation rates (from 240±45−605±35 m/My to 130±5 m/My). The increasing number of red-stained layers and the decrease of boundstone productivity are attributed to environmental changes in the adjacent basin, in particular during relative rises of sea level and to possible cooling due to icehouse conditions. The geometry of the depositional system appears to be controlled by boundstone growth rates. During the Bashkirian, the boundstone growth potential is at least 10 times greater than average values for ancient carbonate systems. The slope progradation rates (nearly 400–1000 m/My) are similar to the highest values deduced for the Holocene Bahamian prograding platform margin. The fundamental differences with modern systems are that progradation of the microbial-boundstone dominated steep slope is primarily controlled by boundstone growth rates rather than by highstand shedding from the platform top and that boundstone growth is largely independent from light and controlled by the physicochemical characteristics of seawater.     

Cycle stacking pattern, diagenesis and reservoir geology of peritidal dolostones, Trigonodus-Dolomite, Upper Muschelkalk (Middle Triassic, SW-Germany)

Summary Peritidal dolostones (Trigonodus-Dolomite) characterize the back-bank environment of the Upper Muschelkalk (Middle Triassic) carbonate ramp of SW-Germany. These deposits represent the Late Highstand Systems Tract (HST) of the ‘Third-Order’ Middle to Upper Muschelkalk depositional sequence. The HST forms an overall shallowing-upward trend and is build by a progradational stack of 1–2 m thick shallowing-upward cycles. The latter vary from subtidal-to-intertidal cycles at the base of the investigated section to intertidal-to-supratidal cycles at the top of the section. Six major facies types can be recognized: subtidal associations are characterized by oolithic grainstones, lagoonal oncolithic wackestones and peloidal mudstones. Intertidal associations are characterized by ostracod wackestones and laminated mudstones, supratidal facies consist of laminated mudstones with tepee horizons and flat pebble conglomerates as well as paleosol horizons. Thin section petrography, cathodoluminescence-microscopy and stable isotope geochemistry reveal a complex dolomitization history (evaporative dolomitization; burial dolomitization). The strong negative oxygen isotope signatures(−3.28 to−5.85‰) point out burial dolomitization as the dominant stage. The Trigonodus-Dolomite shows intercrystalline porosity and some vuggy porosity. Subtidal dolo-grainstones with idiotopic texture at the base of the investigated section have fair permeabilities (5–30 mD) and high porosities (14–32%). Inter-to supratidal dolo-wackestones and dolo-mudstones with xenotopic texture at the top of the section have very low permeabilities (0.3–1.0 mD) and lower porosities (11–16%). The reservoir characteristics with lateral continuity of porous and permeable zones at the base of the section and less porous and impermeable zones at the top again reflect the stacking pattern of shallowing-upward cycles within the overall shallowing-upward trend of the HST. Primary facies and dolomitization processes thus control the distribution of porosity and permeability.     

Carboniferous-Permian long-term sea-level change inferred from Panthalassan oceanic atoll stratigraphy

This paper traces late Palaeozoic second-order sea-level change based on the stratigraphy of the Akiyoshi Limestone, an accreted Carboniferous-Permian Panthalassan atoll carbonate succession in SW Japan. The estimated subsidence of a volcanic edifice and the variable rate of carbonate accumulation reveal the long-term sea-level history of the late Palaeozoic. During Bashkirian time, just after the lowstand stage at the mid-Carboniferous boundary, a slow progressive long-term sea-level rise began. This sea-level rise then increased greatly during Moscovian time. The following Kasimovian to Asselian interval represents a stable highstand stage. Beginning in the Sakmarian, sea level fell slowly and became stable in the Yakhtashian (= Artinskian). This stable sea level was maintained into the Midian (= Capitanian), although a small-scale sea-level rise of approximately 55–70 m is recognized in the Murgabian (= Wordian). The most noteworthy aspect of this change of sea level is the rapid sea-level rise during the Moscovian that created a very large accommodation space and resulted in accumulation of a thick carbonate succession. The eustatic rise from the earliest Bashkirian lowstand to the latest Moscovian highstand had an amplitude of approximately 230–240 m. Such a large-scale eustatic rise in the long-term sea-level change would most likely be caused by greater uplift of the ocean floor along the mid-oceanic ridges, as a result of an increase in the production of oceanic crust during that time.     

Mid-oceanic carbonate platforms as oceanic dipsticks: examples from the Pacific

Framebuilders of Cenozoic coral reefs are limited by their photic requirements to the contemporaneous sea-level, and therefore shallow water reef facies are reliable paleo sea-level indicators. Sea-level lowstands leave no record on coral reefs in areas subject to tectonic uplift, such as the Huon Peninsula, New Guinea, but are recorded by coral reefs in areas subject to tectonic subsidence. A eustatic sea-level fall which exceeds the rate of subsidence subaeriallyexposes the upper section of the reef complex, creating a meteoric ground water system whose diagenetic imprint on the reef carbonates offers a good indicator of a sea-level stillstand. Cenozoic reef platforms thus may contain records of sea-level fluctuations, whether eustatic and global, or tectonic and local. Those reef platforms which developed on seamounts formed in mid-oceanic plate settings are particularly useful for the study of eustatic sea-level changes because their subsidence history is relatively simple, and the tectonic factor can be accounted for when estimating the eustatic sea-level component. Conventional petrographic and biostratigraphic methods used to delineate erosional unconformities in Cenozoic carbonate sections are often deficient. We demonstrate here that stable oxygen and carbon isotopes of the carbonates can reveal the location of both the exposure surface and the paleo water table with greater confidence on account of the specific imprint of meteoric diagenesis. In addition, the⁸⁷Sr/⁸⁶Sr isotope technique offers a promising dating tool of disconformities linked to sea-level lowstands with a resolution superior to the conventional biostratigraphic techniques. Although oxygen, carbon, and strontium isotopes monitor different aspects of global sea-level changes, when used in conjunction they provide deeper insights into the past than either one could achieve alone. Examples from previous and ongoing studies of Pacific mid-oceanic carbonate platforms illustrate the potential of the isotope techniques to unravel sea-level changes. At Midway Atoll, stable carbon and oxygen isotopes along with lithologic and biostratigraphic data suggest a sharp eustatic sealevel fall during the Early Miocene and a series of rapid, brief eustatic fluctuations during the Pliocene-Quaternary. The frequency and timing of the latter is supported by sea-level data from Enewetak Atoll obtained on the basis of detailed strontium isotopes and lithology. The Enewetak data also indicate a series of rapid, brief eustatic fluctuations around the Early-Middle Miocene boundary. At Niue, a carbonate platform about 500 km south of Samoa, oxygen, carbon, and strontium isotope records cover the critical interval of the Miocene-Pliocene boundary and show two distinct disconformities. The mid-oceanic carbonate platforms offer a testing ground of Vail-Haq type eustatic sea-level curves derived primarily from sections along passive continental margins and continental interiors. We show that Neogene sea-level data obtained from Midway, Enewetak, and Niue differ from Vail and Hardenbol's contemporaneous sea-level curve and support Haq et al.'s version.     

Age and composition of carbonate shoreface sediments, Kailua Bay, Oahu, Hawaii

The origin, age, and dynamics of carbonate sediments in Kailua Bay on Oahu, Hawaii, are described. The shoreface (from shoreline to 4 km offshore) consists of a broad (5 km²) fringing coral reef ecosystem bisected by a sinuous, shore-normal, sand-filled paleostream channel 200–300 m wide. The median grain diameter of surface sands is finest on the beach face (<0.3 mm) and increases offshore along the channel axis. Kailua sands are >90% biogenic carbonate, dominated by skeletal fragments of coralline algae (e.g. Porolithon, up to 50%) followed by the calcareous green alga Halimeda (up to 32%), coral fragments (1–24%), mollusc fragments (6–21%), and benthic foraminifera (1–10%). Sand composition and age across the shoreface are correlated to carbonate production. Corals and coralline algae, principal builders of the reef framework, are younger and more abundant in sands along the channel axis and in offshore reefal areas, while Halimeda, molluscs, and foraminifera are younger and more dominant in nearshore waters shoreward of the main region of framework building. Shoreface sediments are relatively old. Of 20 calibrated radiocarbon dates on skeletal constituents of sand, only three are younger than 500 years b.p.; six are 500–1000 years b.p.; six are 1000–2000 years b.p.; and five are 2000–5000 years b.p. Dated fine sands are older than medium to coarse sands and hence may constitute a reservoir of fossil carbonate that is distributed over the entire shoreface. Dominance of fossiliferous sand indicates long storage times for carbonate grains, which tend to decrease in size with age, such that the entire period of relative sea-level inundation (∼5000 years) is represented in the sediment. Despite an apparently healthy modern coral ecosystem, the surficial sand pool of Kailua Bay is dominated by sand reflecting an antecedent system, possibly one that existed under a +1–2 m sea-level high stand during the mid- to late Holocene. Accepted: 20 December 1999     

Depositional sequences in the Kimmeridgian of the Vocontian Basin (France) controlled by carbonate export from shallow-water platforms

Detailed sedimentological analyses and sequential and cyclostratigraphic interpretations in the Kimmeridgian of the Swiss Jura and the Vocontian Basin lead to a high-resolution correlation from the platform to the basin where the biostratigraphy is well established. Several orders of depositional sequences are defined. Their duration is estimated from the time frame given in the sequence-chronostratigraphic chart of Hardenbol et al. (1998). It is suggested that an elementary sequence formed in tune with the 20 ky precession cycle. Small-scale and medium-scale sequences correspond to the 100 and 400 ky eccentricity cycles, respectively. The platform-to-basin correlation shows that the composition of the hemipelagic and pelagic deposits depends to a large part on cyclical variations of carbonate production in shallow-marine environments and subsequent export to the basin. The distribution of thick versus thin marl-limestone alternations and carbonate-dominated versus marl-dominated intervals observed in the basinal sections is explained by the superposition of high- and low-frequency sea-level changes that controlled the carbonate productivity on the platform and the export potential of carbonate mud to the basin.     

Facies models of a shallow-water carbonate ramp based on distribution of non-skeletal grains (Kimmeridgian,Spain)

The internal facies and sequence architecture of a Late Jurassic (Late Kimmeridgian) shallow carbonate ramp was reconstructed after the analysis and correlation of 17 logs located south of Teruel (northeast Spain). The studied rocks are arranged in five high-frequency sequences A–E (5–26 m thick) bounded by discontinuities traceable across the entire study area (20 × 25 km). Facies analysis across these sequences resulted in the reconstruction of three sedimentary models showing the transition from interior ramp environments (i.e., lagoon, backshoal, and shoal) to the progressively deeper foreshoal and offshore areas. Coral-microbial reefs (meter-sized patch and pinnacle reefs) have a variable development throughout the sequences, mostly in the foreshoal and offshore-proximal environments. The preferential occurrence and down-dip gradation of non-skeletal carbonate grains has been evaluated across the three models: low-energy peloidal-dominated, intermittent high-energy oolitic-dominated and high-energy oolitic–oncolitic dominated. The predominance of these non-skeletal grains in the shoal facies was mainly controlled by the hydrodynamic conditions and spatial heterogeneity of terrigenous input. The models illustrate particular cases of down-dip size-decrease of the resedimented grains (ooids, peloids, oncoids) due to storm-induced density flows. Offshore coarsening of certain particles (intraclasts, oncoids) is locally observed in the mid-ramp areas favorable for microbial activity, involving coral-microbial reef and oncoid development. The observed facies variations can be applicable to carbonate platforms including similar non-skeletal components, where outcrop conditions make the recognition of their three-dimensional distribution difficult.     

Increased sea level promotes coral cover on shallow reef flats in the Andaman Sea, eastern Indian Ocean

Sea level in the Indian Ocean is subject to considerable temporal and spatial variabilities. During the period 1960–2009 at Phuket, Thailand, in the NE Indian Ocean, mean sea level increased by 2.7 mm y⁻¹. Regular monitoring of coral cover on fringing reef flats at Phuket since 1979 revealed a sensitive response of this habitat to both transient sea-level depressions and sea-level elevation. Since 1987 when more frequent sampling began, coral cover was positively correlated with the mean sea level experienced over the preceding months. Changing mean sea level explained a high proportion of the observed variation in cover, with overall increasing sea levels and a lack of negative sea-level anomalies promoting cover on the outer reef flats. Concomitantly, there have been no changes in reef community structure or any apparent shifts in zonation patterns across the reef. While future benefits of continued increases in mean sea level on reef flats in the region will be constrained by the frequency and intensity of sea-level depressions associated with the Indian Ocean Dipole, and bleaching events, the overall picture for these shallow reefs is a positive one as they respond to increasing sea level and show rapid recovery from environmental disturbances.     

Mammalian Faunal Dynamics During the Last 1.8 Million Years of the Cretaceous in Garfield County, Montana

This study provides an analysis of biotic change in successive mammalian communities during the last 1.8 million years of the Cretaceous (67.3–65.58 Ma) from the Hell Creek Formation in Garfield County, Montana. Results show changes in relative abundances of species, mean individual body size, and to some extent taxonomic composition through the Hell Creek Formation. These results are interpreted as “normal” mammalian responses to fluctuating temperatures during the latest Cretaceous. By contrast, the extinction of 22–27 mammalian species at or near the Cretaceous-Tertiary (K-T) boundary cannot be explained by the coincident cooling interval alone. At the scale of temporal resolution available, these fossil data are inconsistent with an extended gradual pattern of extinction (linear-response) and are most consistent with either a non-linear response pattern for the K-T extinction, resulting from the accumulated stress of multiple long- and short-term environmental perturbations (e.g., climate change, sea-level regression, volcanism, an extraterrestrial impact), or a single, short-term cause (an extraterrestrial impact).     

Joint composite-rotation adiabatic-sweep isotope filtration

Joint composite-rotation adiabatic-sweep isotope filters are derived by combining the composite-rotation [Stuart AC et al. (1999) J Am Chem Soc 121: 5346–5347] and adiabatic-sweep [Zwahlen C et al. (1997) J Am Chem Soc 119:6711–6721; Kupče E, Freeman R (1997) J Magn Reson 127:36–48] approaches. The joint isotope filters have improved broadband filtration performance, even for extreme values of the one-bond ¹H–¹³C scalar coupling constants in proteins and RNA molecules. An average Hamiltonian analysis is used to describe evolution of the heteronuclear scalar coupling interaction during the adiabatic sweeps within the isotope filter sequences. The new isotope filter elements permit improved selective detection of NMR resonance signals originating from ¹H spins attached to an unlabeled natural abundance component of a complex in which the other components are labeled with ¹³C and ¹⁵N isotopes.     

Facies architecture,depositional environments,and sequence stratigraphy of the Middle Cambrian Fasham and Deh-Sufiyan Formations in the central Alborz,Iran

Based on their lithologic characteristics and stratal geometries, the Middle Cambrian Fasham and Deh-Sufiyan Formations of the lower Mila Group in the Central Alborz, northern Iran, exhibit 39 lithofacies representing several supratidal to deep subtidal facies belts. The siliciclastic successions of the Fasham Formation are divided into two facies associations, suggesting deposition in a tide-dominated, open-mouthed estuarine setting. The mixed, predominantly carbonate successions of the Deh-Sufiyan Formation are grouped into ten facies associations. Four depositional zones are recognized on the Deh-Sufiyan ramp: basinal, outer ramp (deep subtidal associations), mid ramp (shallow subtidal to lower intertidal associations), and inner ramp (shoal and upper intertidal to supratidal associations). These facies associations are arranged in small-scale sedimentary cycles, i.e., peritidal, shallow subtidal, and deep subtidal cycles. These cycles reflect spatial differences in the reaction of the depositional system to small-scale relative sea-level changes. Small-scale cycles are stacked into medium-scale cycles that in turn are building blocks of large-scale cycles. Systematic changes in stacking pattern (cycle thickness, cycle type, and facies proportion) allow to reconstruct long-term changes in sea-level. Six large-scale cycles (S1–S6) have been identified and are interpreted as depositional sequences showing retrogradational (transgressive systems tract) and progradational (highstand systems tract) packages of facies associations. The six depositional sequences provide the basis for inter-regional sequence stratigraphic correlations and have been controlled by eustatic sea-level changes.     

Facies analysis of a large-scale Jurassic shelf-lagoon: the Kamar-e-Mehdi Formation of east-central Iran

The Callovian–Lower Kimmeridgian Kamar-e-Mehdi Formation of the Tabas Block (east-central Iran) is an up to 1,350-m-thick, fine-grained, marly-calcareous unit containing a basal Echellon Limestone Member (up to 180 m thick) and a terminal Nar Limestone Member (up to 100 m thick). The formation was deposited in a relatively deep shelf-lagoon that was part of the large-scale carbonate system of the Esfandiar Subgroup, extending N–S for about 500 km along the strike with a width of up to 100 km. The lagoonal Kamar-e-Mehdi Formation shows sedimentation rates of 150 m/myr, twice as high as those of the shelf-edge carbonate barrier (Esfandiar Platform). The repetitive lithologies and uniform depositional environment suggest equilibrium conditions between sedimentation and subsidence, related to constant slow rotation of the Tabas fault-block around a horizontal axis, the platform sitting on the crest, and the lagoon occupying the dip-slope. Lagoonal sedimentation was dominated by suspended carbonate mud and peloids from the eastern Esfandiar Platform whereas the subordinate siliciclastic material was derived from the west (Yazd Block). The diverse macrobenthos (mainly bivalves) suggests fully marine conditions for the major part of the Kamar-e-Mehdi Formation. However, towards the upper part, biotic impoverishment and the deposition of skeletal-poor, evaporitic sediments indicate increasing restriction. The overlying Magu Gypsum Formation marks the end of an arid basin-fill cycle and possibly forms an effective seal for hydrocarbon reservoirs in that area. The Esfandiar Subgroup was a Neotethys-facing carbonate margin, forming part of a belt of carbonate systems tracking the margins of the Iran Plate during Callovian to Late Jurassic times.     

Palaeoceanographic differences of early Late Aptian black shale events in the Vocontian Basin (SE France)

A high-resolution study focussing on the distribution of calcareous nannofossils and carbon isotopes was carried out to improve the understanding of mid-Cretaceous black shale formation. The studied interval of the early Late Aptian is characterized by two major black shale couplets, the Niveau Noire 4 (NN4) and Niveau Noire Calcaire 2 (NNC2), of the Serre Chaitieu section in the Vocontian Basin (SE France; Bréhéret, 1997). This interval occurs within a long-term negative carbon isotope excursion of > 1.5‰ following the Early Aptian Oceanic Anoxic Event 1a (OAE1a). In contrast to the local NN4 black shales, the black shales of NNC2 are of regional significance and occur at the end of the major negative carbon isotope excursion of the early Late Aptian. Time equivalent black shales are suggested to be coeval with black shales found in the Western Tethys and Atlantic Ocean (Herrle et al., 2004). Calcareous nannofossil analyses and carbon isotopes indicate higher surface water productivity (mesotrophic), warmer surface water, and higher sea-level during the formation of the NN4 black shales. In contrast, the formation of the NNC2 black shales took place during a cooler phase, lower surface water productivity, and lower sea-level. A sea-level fall may cause a restriction of water mass exchange between the open-marine Western Tethys and the Vocontian Basin. This resulted in a longer residence time of the bottom water, decreased ventilation and less mixing of surface waters and thus enhanced preservation of organic matter at the sea floor. Our results indicate that the black shale formation of NN4 and NNC2 was caused by different processes such as increased surface water productivity and enhanced preservation of organic matter at the sea floor. Thus, we emphasize the role of different forcing factors which control the formation of local and regional black shales. The most important factors are sea-level fluctuations, increasing productivity, and changes in precipitation and evaporation rates.     

Development of facies and C/O-isotopes in transects through the Ludlow of Gotland: Evidence for global and local influences on a shallow-marine environment

Summary The Silurian of Gotland is characterized by repeated changes in depositional facies development. The deposition of uniform sequences of micritic limestones and marls was interrupted four times by the growth of reef complexes and the formation of expanded carbonate platforms. Coinciding with these, often abrupt, facies changes extinction events occurred which predominantly affected nektonic and planktonic organisms. Ratios of carbon- and oxygen-isotopes covary with the facies development. Periods in which the deposition of limestonemarl alternations prevailed are characterized by relatively low C- and O-isotope values. During periods of enhanced reef growth isotope values are high. For these changes,Bickert et al. (1997) assume climatic changes between humid “H-periods”, with estuarine circulation systems and cutrophic surface waters with decreased salinity in marginal seas, and arid “A-periods”, with an antiestuarine circulation and oligotrophic, stronger saline surface waters. In order to separate local and regional influences on the isotopic development from the global trend, the interactions between facies formation and isotope record have to be clarified. For this purpose, the patterns of isotope values in the upper part of the Silurian sequence on Gotland (upper Wenlock —upper Ludlow) has been determined and stratigraphically correlated along four transects through different facies areas. Facies formation during this time interval was investigated by differentiation and mapping of twelve facies complexes in the southern part of Gotland. These include shelf areas, reef complexes with patch reefs and biostromes, backreef facies, and marginal-marine deposits. The good correspondence between the carbon-isotope records of the four transects suggests that local environmental conditions in the different facies areas did not influence the δ¹³C values. Therefore, a supra-regional or even global mechanism for the C-isotope variations is likely. In contrast to carbon istopes, the oxygen-isotope values of the four transects generally show parallel trends, but higher variabilities and in parts distinctly deviating developments with a trend to more negative values. These are interpreted as an effect of local warming in small shallow-water areas which developed during arid periods in reef complexes and backreef areas. The boundaries between A-periods and H-periods, as defined by δ¹³C values, which are interpreted as isochrones, can be mapped. From the upper Homerian to the Pridolian six parastratigraphic isotope zones are defined which only partly match the stratigraphic division ofHede (1942, 1960). The isotope stratigraphy results in an improved correlation between the shallow and marginal-marine areas in the eastern part of Gotland and the uniform shelf areas at the west coast of the island. Furthermore, a detailed relationship between the development of carbon and oxygen isotope ratios, the carbonate facies formation, and the succession of palaeontological events could be observed. At the transition from H-periods to A-periods, major extinction events occurred prior to the first increase of δ¹³C and δ¹⁸O values. Extinction events affected conodonts, graptolites, acritarchs, chitinozoans, and vertebrates and resulted in impoverished nektonic and planktonic communities. The reef-building benthos was less affected. Parallel to a first slight increase of isotope values, facies began to change, and reefs developed in suitable locations. The subsequent rapid increase of C- and O-isotope values occurred contemporarily with strong facies changes and a short-term drop of sea-level. Oligotrophic conditions in the later stages of A-periods led to strong reef growth and to an expansion of carbonate platforms. The transitions from A-periods to H-periods were more gradual. The δ¹³C values decreased slowly, but reef growth continued. Later the reefs retreated and were covered by the prograding depositional facies of shelf areas. The diversity of planktonic and nektonic communities increased again. The close relationship between facies formation, palaeontological events and isotope records in the Silurian suggests common steering mechanisms but gives no indication of the causes for the repeated extincion events related to H-period/A-period transitions. Especially the observation, that strong extinctions occurred prior to changes of isotope values and facies, points to causes that left no signals in the geological record. Hypothetical causes like collapse of trophical nets, anoxias, or cooling events are not evident in the sediment record or do not fit into the regular succession of period transitions.     

Sequence stratigraphy of Eocene incised valley clastics and associated sediments,Island of Rab,northern Adriatic Sea,Croatia

Clastic sediments of Middle–Late Eocene age were studied on the Island of Rab (northern Adriatic Sea, Croatia) in order to reconstruct their depositional history, depositional environments, and geometry of sandstone bodies. Detailed outcrop logging and mapping revealed the response of depositional systems to frequent relative sea-level changes, which initiated significant basinward and landward shifts of facies, respectively. Tidal sandstones are commonly underlain by shoreface sandstones, and overlain by offshore sandy marls, whereas the latter are again overlain by shoreface sandstones. Major relative sea-level falls initiated basinward shift of depositional systems and the incision of incised valleys or estuaries, and consequently truncated the underlying shelf sediments. In some cases, the accelerated sea-level fall caused rapid shoreface progradation which is interpreted as a forced regression. Relative sea-level rise caused flooding of the incised relief, and deposition of tidal sandstone bodies which overlie type-I sequence boundaries. The coarse lag sediment of these sequence boundaries locally disappears laterally, and the boundary is granulometrically less prominent. All of the major bounding surfaces have been recognized in the sections studied, although the maximum flooding surface is recognized as a thin “zone” instead of a single surface. Altogether, 28 complete sequences, and 15 parasequences are recognized in the informal unit of the Lopar sandstones, documenting the depositional response to high-frequency relative sea-level oscillations. They have so far not been recognized in the Eocene of the eastern Adriatic region.     

MIS 21 and the Mid-Pleistocene climate transition: Climate and sea-level variation from a sediment core in Osaka Bay,Japan

We report climate and sea-level variation for the marine oxygen isotope stage (MIS) 21, encompassing the end of the Mid-Pleistocene climate transition (MPT), based on pollen, diatom, and sulfur records from a 50-m thick sequence in a core from Osaka Bay. An extremely warm climate coincided with the sea level highstand of substage 21.5, when the warm-temperate element Quercus (Cyclobalanopsis) exceeds 40% of total arboreal pollen. This was followed by a warm-temperate to temperate and humid climate that continued until the end of MIS 21. A linear age model shows that climate was dominated by precessional cyclicity, with an inverse correlation between temperature and precipitation. The postglacial sea-level rise reached its highest peak in substage 21.5, when paleo-Osaka Bay reached its maximum extent including Kyoto and Nara Basins. At this time pelagic diatoms were dominant in the central part of the bay. Sea level dropped below the Osaka Bay sill (about ? 60 m at present) during substage 21.4, followed by a rise above the sill in substage 21.3, and a drop at 21.2. Sea level remained below the sill during substage 21.1. The thermal maximum and sea level peak occurred just after the rapid postglacial sea level rise, after which there was a gradual decline in temperature and sea level accompanied by precession-related oscillations; these features are typical of the post-MPT interglacials dominated by 100-ka cyclicity. These features may be a sign of termination of the MPT.