Lofer Cycles Research Articles

Primer registro de reptiles marinos triásicos (notosaurios, sauropterigios) del Complejo Alpujárride (Zonas Internas de la Cordillera Bética, España)

Este trabajo documenta el primer registro de vertebrados fósiles, concretamente de un reptil marino (sau­ropterygia), en las Zonas Internas de la Cordillera Bética. El desarrollo de plataformas epicontinentales en el Tetis durante la fragmentación de Pangea a partir del Triásico Inferior tardío favoreció la radiación de los sau­ropterigios, grupo de reptiles diápsidos marinos que incluye a los clados Placodontiformes, Pachypleurosauria, Nothosauroidea y Pistosauroidea. Los materiales triásicos del Complejo Alpujárride de las Zonas Internas de la Cordillera Bética, SE de España, incluyen los depósitos carbonatados correspondientes a las plataformas epicon­tinentales desarrolladas durante el Triásico medio-superior en el margen norte del Macizo Messomediterraneo. El intervalo estratigráfico seleccionado se encuentra en la Sierra de Lújar (Granada) y es una sucesión de 17,5 m de potencia con una alternancia de margas y calizas fosilíferas con abundante bioturbación y restos de macroinver­tebrados. Se han identificado ciclos loferíticos con laminación a la base que cambia a un sedimento intensamente bioturbado con abundantes bivalvos hacia el techo del ciclo. El intervalo de estudio representa facies someras en un ambiente inter- o submareal como indican los ciclos loferíticos y la presencia de brechas carbonatadas en el techo de la secuencia, presumiblemente relacionadas con tormentas. Una costilla dorsal de 15 cm de longitud de un sauropterigio, probablemente un notosaurio, ha sido descubierta en esta serie en buen estado de conser­vación y sin evidencias de fragmentación, perforaciones o encostrantes. Este estudio muestra por primera vez la presencia de restos fósiles de reptiles marinos en el Complejo Alpujárride, reptiles que vivieron en aguas someras durante el Triásico al igual que se ha documentado en el Paleomargen Ibérico y en algunas cuencas triásicas de Europa central.

CORELLATION OF LATE TRIASSIC AND EARLY JURASSIC LOFER-TYPE CARBONATES FROM THE PELOPONNESUS PENINSULA, GREECE

Correlation of the Late Triassic carbonate successions, formed at the passive Pelagonian margin (SE of Dhidymi Mt.) with the Early-Middle Liassic successions of the Gavrovo-Tripolitza zone (SE of Leonidion), in eastern and central Peloponnesus respectively, is attempted. Detailed microfacies analysis revealed that the studied carbonate formations were deposited in analogous restricted inner-platform environments (lagoon-peritidal domain) and are composed of meter-scale, shallowing-upward, mostly incomplete peritidal cycles. The top of the supratidal and/ or shallow subtidal deposits are often affected by meteoric diagenesis tracing sea-level lowering and periodic emersions episodes. The well-developed pedogenic features observed in the Early Jurassic platform carbonates indicate long-lasting subaerial exposure intervals and semi-arid to arid climate. Instead, the Late Triassic strata preserve vadose diagenetic indices which point to shorter exposure events, weaker meteoric alteration and slightly wetter climatic conditions. The detected sedimentological features suggest the occurrence of wide lagoonal-peritidal depositional systems during Late Triassic and Early Jurassic, eastern and western of the Pindos basin, respectively. The basic facies pattern and the meter-scale cyclicity show many analogies with the Lofer cycles of the Alpine Triassic, supporting that in the Hellenides the Dachstein-type platform systems evolved till Early-Middle Liassic (Gavrovo-Tripolitza zone).

Microbial laminite versus rooted and burrowed caps on peritidal cycles: Salinity control on parasequence development, Early Cretaceous isolated carbonate platform, Croatia

Some parasequences in the interior of the Early Cretaceous isolated carbonate platform, Croatia, which formed under a greenhouse climate, lack the typical regressive tidal flat laminites. Instead, these bioturbated emergent parasequences have burrowed and rooted upper parts, capped with thin greenish paleo­ sols, and are reminiscent of Triassic “Lofer cycles.” We propose that the bioturbated cycle tops reflect coastal salinities that are normal marine, brackish, or metahaline, whereas the transgressive (rare) and abundant regressive tidal flat laminites formed under hypersaline coastal waters. Thus, these features may be used to track gross salinity changes of coastal waters on the platform through time. In addi­ tion, we suggest ways to evaluate whether the salinity changes relate to hypersaline embay­ ments on the platform or changes in the cli­ mate at the parasequence scale. We propose that the bioturbated emer­ gent parasequences could have formed under semiarid conditions, seaward of subtle , shallow embayments on the platform. Con­ temporaneously, in the bayheads, micro­ bial laminites developed—these would require time­equivalent updip and downdip laminate­capped and bioturbated emer­ gence–capped parasequences. However, the para sequences could also have been con­ trolled by climate changes. The laminite­ capped parasequences could have resulted from salinity increase with shallowing of the platform interior under semiarid condi­ tions (stable climate). In contrast, the bio­ turbated emergence–capped parasequences could have formed during times of more humid climate punctuating the overall semi­ arid climate. The humid phase would have favored brackish to normal marine salinities in the coastal zone, along with macrophytes, and intense animal and plant bioturbation, which would suppress development of lami­ nite caps. Thus, these parasequences provide a means of tracking gross salinity of coastal waters, and if climate induced, then they have implications for fluctuating climates in greenhouse worlds.

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.

Characteristics of the Lofer cyclicity in the type locality of the Dachstein Limestone (Dachstein Plateau, Austria)

In the present paper, the results of our studies in the type locality of the Dachstein Limestone are summarised in order to contribute to the correct interpretation of the Lofer cycles. In the sections studied on the Dachstein Plateau, the boundaries of the Lofer cycles are usually erosional disconformities showing karstification features. Penetration by karstic solution was not more than a few decimetres, since during the recurrent sea-level drops the platform only slightly emerged above sea level. The reddish or greenish argillaceous carbonate interlayers (facies A) cannot be interpreted as in situ palaeosol horizons. They are tidal flat deposits consisting predominantly of subtidal carbonate mud redeposited by storms that was mixed with reworked airborne fine carbonate particles and argillite and/or reworked lateritic soil, which were accumulated on the subaerially exposed platform. Rip-ups from consolidated sediment, blackened intraclasts and skeletons of tidal flat biota may have also contributed to the sediment of facies A. Erosional boundaries of most of the investigated cycles, and definite features of karstic solution beneath the disconformities, suggest periodical drops of sea level followed by a renewed transgression. This appears to confirm the allocyclic model for the explanation of the origin of the Lofer cycles.

Differential severity of Permian–Triassic environmental changes on Tethyan shallow-water carbonate platforms

Significantly different Guadalupian–Late Triassic patterns are observed in the evolution of attached and isolated carbonate platforms of the southwestern Tethys (Oman Mountains). Close to the rim of the attached Arabian platform, carbonates of the Saiq and Mahil Formations reveal an almost complete Permian–Triassic sedimentary record. Guadalupian–Changhsingian 3rd order sequences consist of fossiliferous transgressive systems tracts and monotonous highstand systems tracts with mud/wackestone and coral bafflestone. The youngest Changhsingian beds are bioturbated floatstone with crinoids, sponges and bryozoans. All sediments indicate a healthy, tropical carbonate production. Above, a unique facies change begins with a pyrite-encrusted omission surface. Greenish mudstone rich in authigenic pyrite infills the relief of the unconformity and is overlain by clastic sediment and by laminated, microbialite-bearing carbonate. Unfossiliferous sediments and seafloor cements indicate a change in carbonate production towards abiotic processes. Prevailing anoxic conditions were interrupted by seven oxic event beds, as indicated either by low-diversity and small-sized ichnotaxa or by shell beds with low-diversity bivalve and crinoid assemblages. By comparison with published data, the described sedimentary sequence can be assigned to the Changhsingian–earliest Griesbachian. Beginning probably with the Anisian, bioturbated Griesbachian–Dienerian recovery period and the unconformity below to the latest grain-supported sediment textures mark the return to biogenic tropical carbonate production under oxic conditions. The Middle–Late Triassic carbonate platform consists of stacked high-frequency shallowing upward cycles. By contrast, carbonate production of Neo-Tethyan isolated platforms was discontinuous and interrupted by a large gap. Guadalupian deposits of the Al Jil Formation consist of bioclastic limestone typical of a tropical carbonate production. The uppermost bed, an impoverished bioclastic packstone capped by an unconformity, marks the onset of platform drowning which resulted from the end-Guadalupian mass extinction. Above, a polymict breccia witnessed rift pulses of the Neo-Tethys. The overlying pelagic mud- and packstone contains radiolarians and rare foraminifera of Lopingian age, and overlying microbialites. In the Carnian, tropical shallow-water carbonate production restarted with a low-relief platform and culminated in a Norian–Rhaetian reef-rimmed platform. Stacked Lofer cycles dominated the inner platform of Jebel Kawr (Misfah Formation). We here propose a differential onset and severity of the Late Permian mass extinctions for carbonate platforms. On the Arabian Plate, tropical carbonate production collapsed after the end-Lopingian mass extinction and was replaced by microbialites and sea-floor cements during the earliest Triassic. After approximately six million years, tropical shallow-water carbonate production resumed in the Middle Triassic. Neo-Tethyan isolated platforms drowned shortly after the end-Guadalupian mass extinction and did not recover before the Late Triassic. Absence of shallow-water limestone suggests that carbonate production of isolated platforms ceased for about 30 million years, a period exceeding the recovery of most marine ecosystems.

Orbitally forced Lofer cycles in the Dachstein Limestone of the Julian Alps (northeastern Italy)

Lofer cyclothems of the Upper Triassic Dachstein Limestone of the Julian Alps (northeastern Italy) are the focus of a new sedimentological-cyclostratigraphic study. A 112-cycle-long field section reveals that the meter-scale basic cycle shallows upward, from subtidal megalodont-bearing subfacies to intertidal laminites, capped by a paleosol subfacies. Time-series analysis of a correlative grayscale scan of a high-resolution field photograph shows that a 166-m-thick succession of Lofer cycles has time-frequency characteristics consistent with Earth's precession index, suggesting that the Lofer cycles were generated by eustatic sea-level oscillations that were forced by Milankovitch cycles.

Lofer cyclothems revisited (Late triassic, Northern Alps, Austria)

Fischer's classic study (1964) of the Upper Triassic “Lofer” cyclothems in the Dachstein Limestone of the Northern Calcareous Alps was seminal to many studies of the Dachstein and to carbonate cycles globally.Fischer's idealized cycle is deepening upward, ABC in his terminology, where member A is a surface or interval of subaerial exposure, B is tidal deposits, and C is shallow subtidal. Studies of the Dachstein from the elsewhere in the northern Alps have substantiatedFischer's upward-deepening ABC cycle, butGoldhammer et al. (1990) andSatterley (1996a) reinterpreted the type Lofer cycles as shoaling upward.

Shallow Marine Record of Orbitally Forced Cyclicity in a Late Triassic Carbonate Platform, Hungary

ABSTRACT Well preserved examples of Milankovitch-driven cyclicity in the Triassic sedimentary record include Triassic-Jurassic lake sedimentary cycles, marine cyclic carbonates on small Middle Triassic platforms in the Dolomite Alps, and off-shelf facies equivalent to Late Triassic large passive-margin platforms. In contrast, the Late Triassic Hungarian shallow marine carbonate platform that formed on the southern margin of Tethys records a far from perfect Milankovitch eustatic signal. These carbonate cycles contain subtidal skeletal wackestone/packstone, tidal laminites, and paleosols. The cycles include not only transgressive laminites (the classic Lofer cycles): some cycles contain regressive laminites, whereas other cycles have both transgressive and regressive laminites. The stratigraphic successions do not show the clear bundling of five precessional cycles into larger-eccentricity cycles. The poor record of Milankovitch sea-level changes is interpreted to be due to many missed beats in the platform stratigraphy. Missed beats are evidenced by (1) caliches and paleosols and (2) thick amalgamated subtidal carbonates, and may result from precessional sea-level fluctuations either not flooding the platform or flooding it too deeply to allow shallowing up to sea le el in one precessional beat. Fourth-order bundling of the cycles is weak, with fewer than the typical 5 cycles/100 k.y. bundle and far less than 20 cycles for inferred 400 k.y. eccentricity bundles. Fischer plots of the Hungarian cycles show third-order bundling that matches Aigner's (1992) sea-level curve from the German Muschelkalk basin but is less similar to the Haq et al. (1987) global sea-level curve.

Ancient platform carbonates with well-developed Lofer cyclicity: new candidates for magnetostratigraphy and geodynamically oriented palaeomagnetism

SUMMARY Young platform carbonates seem to preserve continuous records of the magnetic field. In contrast, ancient ones often fail to yield a meaningful palaeomagnetic signal. The frequent failure of the palaeomagnetic method in the platform carbonates of the AlpineMediterranean zone resulted in an investigation as to what extent the original lithology is responsible for this situation. The material selected for the study represents Late Triassic platform facies with welldeveloped Lofer cycles (metre-scale peritidal-subtidal cycles) of the Transdanubian Central Range in Hungary, with evidence for minimal late diagenetic, tectonic or secondary alteration processes. By selecting an area where the expected declinations for the Late Triassic are rotated in a counterclockwise manner by a significant amount, relative to the present north, we hope to recognize ancient natural remanence on the basis of both consistency and departure from the Earth's present field direction. For the study, different members of Lofer cycles were drilled at two sections (13 km apart). 71 and 38 samples, respectively, were taken from the two sections and fully oriented in the field. Using standard palaeomagnetic processing techniques, ancient natural remanence was isolated in all 'A', 'B' and 'C' members of the Lofer cycle, although different members were characterized by different magnetic parameters ('C' members were diamagnetic with very low NRM intensity; 'A' members had positive susceptibility and relatively strong NRM, and 'B' members intermediate properties). The mean palaeomagnetic directions isolated for the two sections, based on 58 and 30 samples, respectively, are D = 306, I = 47, k = 43, ay5 = 3 and D = 300, I = 52, k = 26, ay5 = 5 before and D, = 31 3, I, = and D, = 307, I, = 52 after tectonic correction. Both normal and reversed polarities were observed, and there were several polarity reversals, even in short sections. These results suggest that platform carbonates with Lofer cycles are likely to preserve primary remanence, thus they are suitable for magnetostratigraphy studies, if free from karstic cavities and fissures. The presence of 'A' members is very favourable for the preservation of the original magnetization (since general water circulation is inhibited by these members), but these may represent condensed intervals, thus the sedimentary record in these members is not necessarily complete. Despite the above limitations and also the possibility of gaps at sequence boundaries, platform carbonates are good candidates for future definition of magnetostratigraphic scales, for example in the Late Triassic; they also promise palaeomagnetic results of considerable tectonic importance, since platform carbonates with Lofer cycles are widespread in the Alpine-Mediterranean zone.

Cyclic Carbonate Sedimentation in the Upper Triassic Dachstein Limestone, Austria: The Role of Patterns of Sediment Supply and Tectonics in a Platform-Reef-Basin System

ABSTRACT Factors that controlled the deposition of (i) peritidal Lofer cycles on a carbonate platform, and (ii) platform margin (reef-slope) deposits have been deduced at localities in the Austrian Alps (the Steinernes Meer and Hochkonig Massif). These locations are part of an entire Late Triassic platform-reef-basin system that is preserved with most original stratigraphic relationships intact. Platform Lofer cycles shallow upward from a subtidal grainstone through a variety of intertidal dolomitic mudstones to a supratidal weathering horizon (soil). Lofer cycles show random, non-hierarchical stacking patterns, limited lateral continuity, varied progradation directions, complete shoaling (98% of cycles), and very low stratigraphic completeness (only 1-20%). Exponential frequency distributions of cycle thickness suggest random, aperiodic cycle deposition, rather than regular deposition in response to regular eustatic sea-level oscillations. Sediments in the adjacent reef complex record storms and the lateral migration of sand shoals and stromatolite-capped banks, not sea-level fluctuations and intermittent subaerial exposure. On the basis of these observations, in contrast to many previous interpretations, Lofer cycles are interpreted to be mostly autocycles formed within a tidal-flat island system by lateral migration of wide, low, emergent banks separated by shallow subtidal areas. Preservation potential of individual cycles is thought to have been low; reworking was almost certainly very important in this system. Extensional regional tectonics in the Late Triassic exerted a long-term control over the development of the tidal-flat island system on the platform top, and appears to have left a overprint in Lofer cycle successions. Differential subsidence of individual platforms across the region is suggested by substantial regional thickness variations (1200-3000 m) in the Norian/Rhaetian platform carbonates of the Northern Calcareous Alps. Two important deepening events in the Steinernes Meer section almost caused platform drowning, and correlate with a lowermost Rhaetian transgression in the Western Tethys. Many other events may have gone unrecorded on the platform. Within a 716 m thick measured succession of Lofer cycles, intervals of enhanced paleokarst development nd stacked intertidal to supratidal beds are present with 20-75 m vertical spacing. These platform units are interpreted to represent prolonged periods in which greater areas of the platform were occupied by intertidal to supratidal sediments. These units correlate with thick units of debris flows on the reef slope. What may be tectonic highstands (the result of a vertical movement) are recorded as packstone to grainstone deposition on the reef slope. A holistic model driven by aperiodic fault-controlled downdropping, resulting in switching loci of sediment export patterns from a continuously operative subtidal carbonate factory (the reef complex) best explains vertical facies patterns in the platform and reef-slope successions. It is not a static sea-level model, although thi d-, fourth-, and fifth-order eustasy is not required. The model could explain the sporadic occurrence of shallowing-up cycles in the adjacent Kossen Basin. Late Triassic eustatic sea-level fluctuations were ineffective in controlling sedimentation as a result of the processes described above.

The genesis of Lofer cycles of the Dachstein Limestone, Northern Calcareous Alps, Austria

Peritidal carbonate platform Lofer cycles have been regarded as a record of sea-level oscillations driven by variations in the earth's orbit. Fresh analysis of a newly measured 700 m thick section from the Steinernes Meer is presented here. Estimation of stratigraphic completeness indicates that this sequence is only 0.75–9.5% complete, strongly suggesting that Milankovitch patterns are not preserved. Analysis of cycle thickness-frequency distributions reveals an exponential pattern, strongly suggestive of aperiodic, random deposition rather than periodic deposition in response to orbital forcing. Evidence for the importance of autocyclic processes (e.g. lateral migration of inter- to supratidal mudbanks and subtidal areas) and syn-sedimentary tectonic downfaulting events within this already active Late Triassic extensional zone is presented.

The genesis of Lofer cycles of the Dachstein Limestone, Northern Calcareous Alps, Austria

The genesis of Lofer cycles of the Dachstein Limestone, Northern Calcareous Alps, Austria

Mangrove-generated structures and depositional model of the pleistocene fort thompson formation (Florida plateau)

The carbonate Fort Thompson Formation (Pleistocene) of the South Florida platform consists of regular alternations of marine, brackish and terrestrial lithofacies 1) mollusk packstones and grainstones (estuaries, banks, patch-reefs, bioturbated nearshore zone, marsh flat); 2) mangrove peats; 3)Helisoma wackestones (saw grass prairie, freshwater pond, buttonwood hummock), and 4) freshwater swamp. The corresponding environments represent a low-energy coastline. Lithofacies are arranged into stacked sequences. Three modal asymmetric regressive parasequences (‘A’;‘B’;‘C’) reflect relative sea-level rises; the lower part of sequence ‘A’ is analogous to the Lofer cycles; sequence ‘B’ is similar to the Holocene cycle from Florida Bay; sequence ‘C’ is analogous to present-day swamp cycles forming in the Everglades. Correlations of transgressive surfaces point out a wedge with parasequences thinning-out and disappearing towards a hinge located to the west. Geometrical considerations suggest that rotational subsidence was initially the main control on sedimentation and led to the deposition of parasequences ‘A’. Subsequently, its attenuation highlighted eustatic changes which are more apparent higher up in the Formation; parasequences ‘B’ and ‘C’ formed under the main influence of eustatic factors.