Abstract The Middle Triassic carbonate buildups of the Dolomites show facies similarities with mud mounds but display apparent architectural elements of flat‐topped carbonate platforms. In order to test whether the facies similarities to mounds are also reflected in the internal buildup architecture, a three‐dimensional modelling study of the Middle Triassic Monte Cernera buildup has been carried out. The Cernera buildup exhibits apparent geometries suggesting a mounded platform in the lower and uppermost part of the buildup, separated by an interval with apparent platform geometry and a retrogradational platform interior, which is difficult to explain with a flat top platform model. For this purpose, a number of three‐dimensional models were constructed using the three‐dimensional modelling programme petrel TM . Key geological horizons were constructed based on outcrop measurements, intermediate horizons were calculated in the modelling program, and the intersections of the modelled layers with a digital topography surface were displayed and compared with outcrop photographs. The models were refined stepwise until a best fit with the actual bedding architecture was achieved. The best fit model shows that the mounded geometries in the lower and uppermost part of the buildup are real architectural elements. The intermediate platform stage, about 1·5 km across, had probably retained a mounded top with a relief of up to 50 m, which is difficult to distinguish from an absolutely flat top, but necessary to explain the retrogradational platform interior. The present study shows that Monte Cernera was dominated by mounded geometries at all stages of platform development. The mounded geometry plus facies data suggest that the platform is a deep‐water accumulation, below the zone of intense wave energy, but within the photic zone. The Cernera represents a tropical buildup type, which did not have the capacity to grow into continuously wave‐swept environments because of the small size and the absence of a wave‐resistant energy barrier. Such buildup types are probably common after major crises in earth history, when reef organisms were virtually absent.
Abstract The Alpine Triassic units of Switzerland, Northern Italy and Western Austria offer an extensive geological archive, in which the enigmatic process of dolomite formation can be studied in a palaeoenvironmental context. Recent studies clearly demonstrate that large amounts of the Alpine Triassic dolomites are late diagenetic or hydrothermal. Nevertheless, as part of multiple generations of diagenetic overprint, some generations of fine‐crystalline, Ca‐rich dolomite appear strictly confined to their depositional facies and show signs of very early formation at surface temperatures in specific ancient depositional environments. In this review, three cases of Alpine Triassic dolomites are discussed, where dolomite rocks may have formed during or soon after sedimentation. The sedimentary facies indicate contrasting palaeoenvironmental conditions and, hence, document three different possible processes of dolomite formation: (i) In the Dolomite Mountains (Northern Italy), dolomite beds of the partly isolated Middle Triassic (Anisian/Ladinian) Latemar Platform are confined to the very top of shallowing‐upward lagoonal facies cycles. (ii) Dolomite beds of the San Giorgio Basin (Southern Switzerland), an intra‐platform basin that opened during the Anisian/Ladinian transition, are associated with organic carbon‐rich shales, which were deposited in a deeper water environment under anoxic conditions. (iii) In the entirely dolomitized platform facies of the Dolomia Principale (Hauptdolomit Formation), a very early generation of fine‐crystalline dolomite occurs in the shallowest part of evaporative peritidal cycles. This platform extended over thousands of square kilometres along the Tethys margin during the Late Triassic (Carnian and Norian) and large amounts of carbonate were deposited under hypersaline sabkha‐like conditions. Representing three distinct depositional environments, these three different Triassic systems show features in common with several dolomitization models developed from the study of modern dolomite‐forming environments; for example, the sabkha model, the evaporative lagoon/lake model, the organogenic model and the microbial model. Although these actualistic models may be applicable to reconstruct the palaeoenvironmental conditions during dolomite formation, dolomite‐forming processes during the Triassic were apparently quite different from the modern world in terms of distribution and scale. Recent developments in stable‐isotope geochemistry and high‐resolution geochemical probing offer the possibility to make better reconstructions of Triassic palaeoceanographic conditions and suggest a non‐actualistic approach to better understand dolomite formation during the Triassic.
Magnetostratigrapic and biostratigraphic data across the Anisian/Ladinian (Middle Triassic) boundary were obtained from the Frötschbach/Seceda section from the Dolomites region of northern Italy, and the Vlichos section from the Greek island of Hydra, where the Aghia Triada published section was also resampled. The Frötschbach/Seceda section includes two radiometrically dated (UPb) tuff levels and covers two of the three chief candidates for the position of the base of the Ladinian, namely at the base of the Secedensis Zone or the subsequent Curionii Zone. The Aghia Triada section yields biochronological evidence for the base of the Secedensis Zone, whose significance is, however, critically discussed in the light of the magnetostratigraphic correlation with Frötschbach/Seceda. The Vlichos section can be correlated with Aghia Triada and Frötschbach/Seceda by means of magnetic polarity stratigraphy and sparse fossil occurrences. The satisfactory correlation of the magnetozones allows us to construct a composite geomagnetic polarity sequence tied to Tethyan ammonoid and conodont biostratigraphy for about a 2.4 Myr interval across the Anisian/Ladinian boundary.
The Global boundary Stratotype Section and Point (GSSP) for the base of the Ladinian Stage (Middle Triassic) is defined in the Caffaro river bed (45°49'09.5''N,10°28'15.5''E),south of the village of Bagolino (Province of Brescia, northern Italy), at the base of a 15-20-cm-thick limestone bed overlying a distinct groove ("Chiesense groove") of limestone nodules in a shaly matrix, located about 5 m above the base of the Buchenstein Formation.The lower surface of the thick limestone bed has the lowest occurrence of the ammonoid Eoprotrachyceras curionii (base of the E. curionii Zone; onset of the Trachyceratidae ammonoid family).Secondary global markers in the uppermost Anisian include the lowest occurrence of conodont Neogondolella praehungarica and a brief normal-polarity magnetic zone recognized in closely correlated sections including the principal auxiliary section at Seceda in the Dolomites.The GSSP-level is bracketed by U-Pb single zircon age data from volcaniclastic horizons, indicating a boundary age of ca 241 Ma. Historical context of the Ladinian StageThe first formal recognition of a stratigraphic interval comprising what is now called Ladinian orginates from the subdivisions of the Triassic System proposed by E.v.Mojsisovics.Ammonoids served as the main biostratigraphic tool for these divisions.By 1874 and with later modifications (e.g., 1882), Mojsisovics used the name "Norian" for a stratigraphic interval including, at its base, the South Alpine Buchenstein Beds and siliceous limestones of Bakony (Hungary).Because Mojsisovics erroneously equated this interval with parts of the ammonoidrich Hallstatt-limestones, he used the term "Norian" as the stage name, which refers to the Norian Alps around Hallstatt near Salzburg (Austria).Later, these Hallstatt-ammonoids were found to be much younger.Bittner (1892) therefore proposed the term "ladinisch" (Ladinian, after the "Ladini"-people of the Dolomites area) as a new label for the stratigraphic interval comprising the South Alpine Buchenstein and Wengen Beds.Although not adopted by Mojsisovics et al. ( 1895), the Ladinian subsequently became the generally accepted stage name (e.g., Arthaber, 1906; for additional information and discussions of the history of the Ladinian see Brack &Rieber, 1994 andKozur, 1995).Only little progress was made in the following decades on the (bio)stratigraphy of the Buchenstein and equivalent intervals.In Mojsisovics' time, knowledge of ammonoid successions in the Anisian/Ladinian boundary interval was fragmentary and stratigraphic correlations rather speculative.For instance, the South Alpine Buchenstein Beds and siliceous limestones in Bakony were considered age equivalents and both attributed to the "T.reitzi" Zone.However, unambiguous evidence from recent biostratigraphic research on ammonoids, conodonts and radiolaria shows that the main ammonoid bearing interval of Mojsisovics' "yellow siliceous limestones of Bakony" (now called the "Vàszoly Fm.") partly predates and overlaps in age with only the lowermost parts of the South Alpine Buchenstein Formation.Starting in the 1960s numerous classical localities for Middle Triassic fossils of the Western Tethys were restudied and new schemes of ammonoid zones were sketched, with the Anisian/Ladinian boundary being placed at different stratigraphic positions.North American geologists meanwhile preferred to define their own stage boundary at yet another stratigraphic level (see e.g., schemes in Zapfe, 1983;Tozer, 1984).For a modern definition of the base of the Ladinian Stage, the perplexities associated with original concepts and historical usage of the Anisian/Ladinian boundary provide no useful basis for the positioning of the GSSP.However, the lower part of the South Alpine Buchenstein Formation is an adequate interval for the Ladinian GSSP because Bittner (1892) explicitly designated this stratigraphic unit as the oldest one of the stage.
