In Contrast To Earlier Hypotheses
The marine Upper Permian and Marine parts the Permian-Triassic boundary of the Southern Alps between Val Adige (Southern Tyrol, northern Italy) and the Karawanken Mountains (northern Yugoslavia) had been studied with respect to microfacies, paleogeography and biostratigraphy. The Upper Permian consists of the Bellerophon Formation (20-360 m thick). The decrease part of the conformably overlying Tesero Horizon (4-6 m thick). The Bellerophon Formation was deposited in a marginal basin of the Tethys on a homoclinal ramp dipping slightly eastward. The ramp developed on a passive continental margin upon the clastic sediments of the Val Gardena Sandstone Formation. Slow subsidence caused by extension of the crust was liable for the accumulation of thick shallow-marine sequences exhibiting a transgressive growth from the base to the top.
The depositional space exhibits four parts as distinguished by lithology. These are from W to E:
1. 1. A near-coastal surroundings with strong clastic influx in an irregular alternation of dolomite and gypsum;
2. 2. An inside shelf space with a restricted setting and the next sedimentary cycles: regressive gypsum-dolomite cycles (sabkha cycles) in the lower part, followed by dolomites deposited in a restricted atmosphere and limestone-dolomite cycles with a transgressive tendency within the higher half;
3. 3. An inside shelf space with a much less restricted setting; isoclinally folded subaqueous laminated gypsum in the lower part is followed by limestone-dolomite cycles and rauhwacke of restricted atmosphere in the middle half and an alternation of limestones and marls at the top, producing a cyclic low/high energy sedimentary sample;
4. 4. An off-shore a part of the inner shelf area with thin evaporitic sequences (rauhwacke) in the decrease half and utterly dolomitized carbonates within the center and higher half, deposited underneath low-energy conditions.
The depositional space of the Bellerophon Formation is subdivided into two basins produced by a fault-bound structural high (Comelico High) which is answerable for the extra restricted setting within the western part.
The microfacies analysis of the evaporitic sequences led to a refinement of the sabkha mannequin of BOSELLINI & HARDIE (1973). In the event you loved this informative article and you would like to receive more details concerning marine parts, stackoverflow.com, boat hardware generously visit our own web site. The limestones are subdivided according to water motion (micrites-sparites) and the diversity of organisms (dominance of molluscs and ostracods, and of algae and foraminifera). The transgressive megacycle culminates within the deposition of an open marine grapestone facies. Freshwater influence affected all elements of the Bellerophon Formation throughout early diagenesis.
The distribution of the microfacies varieties enables of the topography of the sea flooring to be reconstructed as well as a distinction to be made between a restricted lagoon, an open lagoon and an open shallow-marine area. The open marine influence shifts towards the coast in course of time, filling the slight relief brought on by little shoals on the sea flooring. The highest of the Bellerophon Formation represents a really shallow open marine area.
The grapestone facies repeatedly grades up into the oolites of the basal Tesero Horizon, followed by a regression indicated by algal mats. In the decrease half these mats are reworked to intraclasts; in the upper half they characterize a muddy tidal flat.
Synsedimentary faults cause a various development of the basal Tesero Horizon in several areas: While the oolites of the western basin transgrede, an emersion horizon on the highest of the Bellerophon Formation is found in a small area on the japanese margin of the Comelico High; it’s, in flip, followed by peritidal deposits. The skinny oolites deposited in the japanese basin, which grade constantly from the underlying grapestone facies, represent a peritidal member in the regressive sequence leading to the formation of algal mats. At the top of the Tesero Horizon, a new transgression is manifested by the subtidal biomicrites of the Mazzin Member.
Algae and foraminifera are strongly influenced by the atmosphere: Euryhaline organisms likeGymnocodium, Atractyliopsis, agglutinated foraminifera,Earlandia andHemigordius occur in the strongly hypersalinar atmosphere of the basal evaporitic sequences and in the peritidal deposits of the Tesero Horizon as well as in biosparites of the open marine, whereas the stenohaline Fusulinids, Biseriamminids and numerous Nodosariids are restricted to the open marine setting.
A biostratigraphic subdivision of the lower and center parts of the Bellerophon Formation into zones with algae and foraminifera shouldn’t be doable due to this restricted setting. In the limestones of the higher half two levels (Dzhulfian. Dorashamian) might be distinguished.
In contrast to earlier hypotheses, the Permian-Triassic boundary lies throughout the Tesero Horizon at the bottom of sediments representing deposition on a muddy tidal flat. Here the Permian microfauna and microflora grew to become extinct, a mixed fauna with Permian-kind brachiopods and Triassic-type bivalves appeared. In South China this blended fauna is related withOtoceras woodwardi, the index ammonite of the Lowermost Triassic.