Western Dolomites Research Articles

Growth mode of Middle Triassic carbonate platforms in the Western Dolomites (Southern Alps, Italy)

Abstract The correlation of carbonate platform and slope sediments with biostratigraphically and radiometrically dated basinal successions in the Western Dolomites allows the reconstruction of the growth of a Middle Triassic carbonate platform (Schlern/Rosengarten) over a time interval of approximately five ammonoid zones (Reitzi, Secedensis, Curionii, Gredleri, Archelaus zones). The whole platform evolution had a total duration of

Morphological features of Triasina Hantkeni Majzon, 1954 (Foraminiferida, Aulotortidae) and remarks on the test wall structure

The Late Triassic foraminifer Triasina hantkeni Majzon, 1954 (Involutinacea, Aulotortidae) is herein studied from isolated material of the Sella Group (Piz Boé, Western Dolomites, Italy). Morphological features of the test and the internal pillars are figured and described on the basis of isolated specimens. An interpretation of the lamellar test-wall structure is tentatively proposed by the comparison with the lamellar model of the genus Aulotortus Weynschenk. The lectotype of Triasina hantkeni is herein designated.

High resolution UPb dating of Middle Triassic volcaniclastics: Time-scale calibration and verification of tuning parameters for carbonate sedimentation

We report high-resolution single-zircon UPb age data for Middle Triassic volcaniclastic intercalations in biostratigraphically calibrated pelagic successions of the Southern Alps. The results require a redefinition of the chronometric scale for the Middle Triassic. Moreover, they do not support current models relating cyclic sedimentation in platform carbonates of the Dolomites to orbital tuning. Tight concordant age clusters were obtained for five volcaniclastic layers in three ammonoid biozones of late Anisian to early Ladinian age. Two layers in the (Nevadites) Secedensis Zone yielded identical mean 206Pb 238U ages of 241.2 + 0.8/−0.8 Ma and 241.2 + 0.8/−0.6 Ma (errors given at the 95% confidence level). A layer in the Gredleri Zone is dated at 238.8 + 0.5/−0.2 Ma, and two horizons in the Archelaus Zone yield similar ages of 237.9 + 1.0/−0.7 Ma and 238.0 + 0.4/−0.7 Ma. These results are significantly older than the age values of 233–235 Ma assigned to the Anisian/Ladinian boundary by several current time scales [1–4]. Moreover, our estimate of 240.7–241.3 Ma (depending on biostratigraphic collocation) for the Anisian/Ladinian boundary casts doubts on the reliability of age values of 245–250 Ma proposed by most time scales for the Permian/Triassic boundary. The occurrence of pelagic fossils in basinal sediments as well as in age-equivalent shallow marine Middle Triassic platform carbonates in the Dolomites allows the sedimentary sequences of both environments to be correlated. The 800 m thick Latemar platform (western Dolomites) is characterized by cyclic stacking patterns, which have been interpreted as results of Milankovitch-type high-frequency/low-amplitude sea-level fluctuations. The 12 m.y. interval of platform growth postulated from the assignment of orbital periodicities to the platform carbonate cycles [5,6] is in conflict with a maximum time span of 4.7 m.y. allowed by the present zircon data.

Saxifraga depressa and Saxifragetum depressae: an endemic species and association of the western Dolomites (Italy)

Saxifraga depressa grows only on the summits of eleven siliceous massives in the western Dolomites. It is restricted to the habitat of steep N-faced screes lying above 2000 m. Syntaxonomically, Saxifraga depressa characterizes the association Saxifragetum depressae which is here described for the first time with two subassociations. With respect to the closely allied Oxyrietum digynae, the Saxifragetum depressae colonizes debris richer in fine material. The soil pH in the habitats of the Saxifragetum depressae varies from acid to slightly basic, mainly as a result of lithological differences in the parent rocks. The habitats having a subneutral or basic soil are colonized by a well-defined subassociation of the Saxifragetum depressae, which is differentiated by some basiphilous species.

Microfacies of carbonate slope boulders: Indicator of the source area (Middle triassic: Mahlknecht cliff, western Dolomites)

Paleontological and microfacies criteria of limestone boulders occurring within megablocks deposited on clinogonal slopes provide indications of the source area of Middle Triassic allochthonous carbonates exposed in the Mahlknecht cliff, Seiser Alm, Dolomites. Microfacies, biotic composition, a high percentage of low-growing binding and baffling communities (sponges, algae, microproblematica) as well as the remarkable coincidence with distributional patterns observed on other Ladinian paleoslopes indicate a source area characterized by common bioconstructions, which is situated on a subtidal upper and middle foreslope and not at the platform/slope-margin or on the carbonate platforms.

Tectonics of the Dolomites (southern alps, northern Italy)

In post-Variscan times the Dolomites underwent a number of tectonic events, which may be summarized as follows: Permian and Triassic rifting phases broke the area into NS trending basins with different degrees of subsidence. A Middle Triassic transpressive event then deformed the region along a N70°E axis, generating flower structures within the basement. Volcano-tectonic domal uplift and subsequent caldera formation occurred at the same time as the Late Ladinian magmatism. Early Jurassic rifting also controlled the subsidence which increased eastward. This long period of deformation was followed by a pre-Neogene (Late Cretaceous-Palaeogene ?) EW (ENE-WSW) compression which generated a W-vergent belt, possibly equivalent to the folded foreland of the Dinaric chain. A 70 km EW section of the Dolomites indicates shortening of at least 10 km. During the Neogene the Dolomites, as far north as the Insubric Lineament, were the innermost part of a S-vergent thrust belt: the basement of the Dolomites was thrust southwards along the Valsugana Line onto the sedimentary cover of the Venetian Prealps for at least 10 km. This caused a regional uplift of 3–5 km. The Valsugana Line and its backthrusts on the northern side of the central Dolomites generated a 60 km wide pop-up in the form of a synclinorium within which the sedimentary cover adapted itself mainly by flexural-slip often forming triangle zones. The shortening linked to this folding is about 5 km with Neogene thrusts faulting and folding pre-existing thrust-planes. On the north-eastern side of the Dolomites, Neogene deformation is apparently more strictly controlled by the transpressive effects of the Insubric Lineament and shortening of the sedimentary cover may be greater than in the central Dolomites. Minor deformation linked to the Giudicarie belt is present in the western Dolomites. The present structure of the Dolomites is thus the result of a number of tectonic events of different significance and different strike. Only a 3-dimensional restoration can unravel the true structure of the Dolomites.

Aspects of middle and late triassic palynology. 6. Palynological investigations in the Ladinian and lower Karnian of the Western Dolomites, Italy

A palynostratigraphical investigation in the Buchenstein Group, the Wengen Group, the San Cassiano Formation and the Raibl Group of the western Dolomites has resulted in a palynological characterization of the Ladinian Stage and the Cordevolian Substage of the Karnian in the classic type-region for these units of Triassic chronostratigraphical subdivision. Seven characteristic phases in the compositional development of successive palynological assemblages can be recognized: the thiergartii—vicentinense phase (Upper Anisian, Illyrian Substage), the plurianulatus—novimundanus phase and the plurianulatus—secatus phase (Lower Ladinian, approximately Fassanian Substage), the secatus—dimorphus phase and the secatus—vigens phase (Upper Ladinian, approximately Longobardian Substage), the vigens—densus phase (Lower Karnian, Cordevolian Substage), and the densus maljawkinae phase (Middle Karnian, Julian Substage). A palynological concept of the Ladinian and Cordevolian is in reasonable agreement with the classic concepts of the units. Boundaries, however, do not coincide with lithostratigraphical boundaries. It remains difficult to correlate the palynological information with alternative concepts based on ammonoid zonation outside the type-region of the Ladinian. Palynostratigraphically it is unrealistic to consider the Cordevolian to be a synonym of the Julian. The results of quantitative analyses suggest arid climatic conditions. Local humid environments progressively increased during the Ladinian but gradually disappeared during the Cordevolian. These humid environments may explain the occurrence of southern (Gondwana) elements in the assemblages. The following new taxa and new combinations are formally introduced: Bocciaspora nov. gen., Sellaspora nov. gen., Gordonispora nov. gen., Lycopodiacidites kokenii nov. sp., Sellaspora rugoverrucata nov. sp., Sellaspora foveorugulata nov. sp., Krytomisporis ervii nov. sp., Bocciaspora blackstonensis (De Jersey) nov. comb., Gordonispora fossulata (Balme) nov. comb., Gordonispora lubrica (Orlowska-Zwolińska) nov. comb., Weylandites magmus (Bose et Kar) nov. comb., Partitisporites tenebrosus (Scheuring) nov. comb., Partitisporites maljawkinae (Klaus) nov. comb., and Partitisporites quadruplicis (Scheuring) nov. comb. The morphon concept is applied informally to unite morphologically closely related species within the group of the Circumpolles.

Recent Acquisitions at the Natural History Museum

AMONG recent acquisitions in the Department of Zoology are the mounted head of a Mexican bighorn sheep presented by Mr. John Lawson, the head of a Newfoundland caribou presented by Mr. W. Lawson, and the head of a woodland caribou presented by Captain D. A. Lawson. The study collection has been enriched by an Argali sheep skin from Samarkand, the gift of Mr. Douglas Carruthers, and a tiger skin and skull from Perak presented by the Zoological Society. Miss Emma Hutchinson of Grantfield, Leominster, Herefordshire, has presented to the Museum the collection, contained in four cabinets, of British Lepidoptera made at Leominster by her mother and other members of the family, mostly between 1860 and 1900, though Miss Hutchinson has added to it odd specimens and notes up to 1936. The specimens number some 10,000, and included with them are Miss Hutchinson's note-books in which all the records are kept. Among donations to the Geological Department is one from Mr. C. T. A. Gaster, who collected the material during many years of intensive study of the chalk of the South Downs. This collection includes 10,000 Polyzoa, 3,200 Echinoderms, 700 Sponges, 360 Annelids, 85 Mollusca, and 450 Brachiopods, and forms a valuable source of information on the succession of faunas in the Chalk. The Mineral Department has acquired by gift from Dame Maria Ogilvie Gordon, a carefully labelled series of rocks and minerals which were collected by her in the Monzoni district, Val di Fassa, Italy. She first visited this region in 1891 when on Baron von Richthofen's geological excursion. Since then she has carefully studied the structure of the western Dolomites and has made a geological map of the whole area. The present gift is a first selection of all the original material studied. Prof. S. J. Shand, of Stellenbosch University, has given a fine series of igneous rocks from localities in South Africa, South-West Africa, and Kenya Colony.