GeoResources - Sequence Stratigraphy and Basin Development, Anisian, Central Southern Alps, Details 2


	High-Resolution Sequence Stratigraphy and Basin Development of the Central Southern Alpine Basin (Anisian, Triassic, Dolomites, N-Italy) Details 2

	Sequence stratigraphy (continued) Sequence An 3 covers the late Bithynian to late Pelsonian (late Anisian). Thicknesses vary between 0-320 m, the absolute time interval is approximately 2.0 My. In the W- and S-Dolomites the basal sequence boundary An 2/An 3 represents a type 1-boundary. Alluvial breccias (LST) and shoreface sandstones (early TST, Voltago Fm.) cover the deep erosional relief at the sequence boundary. In basin margin to lower ramp areas clastic and carbonate-clastic lowstand wedges developed. Subsequently, carbonate-clastic successions were deposited in shallow subtidal to subtidal shelf areas (late TST). In the NE-Dolomites, no subaerial erosion occurred at the sequence boundary An 2/An 3. Here, the sequence boundary is represented by a correlative conformity near the top of the Low. Sarl Fm. Incipient drowning of the carbonate ramp took place in the An 3-TST. In the early HST these deep sub-tidal successions are overlain by the carbonate ramp of the Up. Sarl Fm. Ramps were distally and proximally steepened and prograded over wide parts of the N-, E- and S-Dolomites. They comprised the first reefs after the Permian/Triassic faunal crisis. Marine inlets with oxic to anoxic conditions cut the late Pelsonian ramps of the Up. Sarl Fm. A wide basin only developed in the E-Dolomites (Ampezzo Basin).

	Isopach map of sequence An 3 (Central Southern Alps, Dolomites)

	Map of depositional settings during sequence An 3 HST (Central Southern Alps, Dolomites)

	Sequence An 4 covers the early to middle Illyrian (late Anisian). Thicknesses vary between 30-170 m, the absolute time interval is approximately 2.0 My. The regional structural high in the W-Dolomites was inundated. During the An 4- HST proximally to distally steepened ramps (Contrin Fm.) developed again. Oxic to anoxic marine inlets and basins (Bivera, Ambata, Moena Fms.) separated the individual ramps of the Contrin Fm.

	Isopach map of sequence An 4 (Central Southern Alps, Dolomites)

	Map of depositional settings during sequence An 1 HST (Central Southern Alps, Dolomites)

	Basin architecture During the sequences An 1 and An 2 depositional centers remained stationary in the NE- and E-Dolomites. The significant changes in basin architecture took place in the An 2- HST to An 3-TST, that is in a time interval of less than 1.5 My. The basin is characterized by strong compartmentalization. Different subbasins with independently evolving depositional systems developed. During sequence An 3, the main depositional center shifted 35-40 km towards the west, to the eastern margin of the structural high in the W-Dolomites. This intermittent depositional center already became inactive during sequence An 4. In contrast to the strong differential accommodation during sequence An 3, relatively homogeneous accomodation rates with little regional variations established in sequence An 4.

	Basin architecture during sequence An 1 to An 4, W-E trending transects across Central Southern Alps

	Comparison of sequence stratigraphic data (early to late Triassic) Basin and shelf development is primarily controlled by changes in accommodation space, which results from absolute sea-level changes and changes in subsidence. In order to quantify the controlling factors of shelf development, (semi)quantitative data about absolute sea-level changes have to be incorporated in the model. Data must be derived from various sedimentary basins in different plate-tectonic settings. This approach is essential for analyzing regional and larger subsidence trends and for isolating the effects of absolute sea-level changes (Rüffer & Zühlke, 1995; in: Haq (ed.) Sequence stratigraphy and depositional response, 161-207, Kluwer). Sequence stratigraphic data from the Dolomites have been compared to sequence stratigraphic data from other sedimentary basins in the W-Tethys (Northern Calcareous Alps, Lombardy), from epicontinental basins (German, Silesian Basins; SE-France), from the SW-Tethys, the E-Tethys (China) and from the Arctic Basin (Barents Sea, Sverdrup Basin). 3rd order sea-level changes in these basins are correlatable during the late Scythian (Spathian) to late Anisian (Illyrian) and the late early Ladinian (late Fassanian) to early Carnian (Julian). Correlations indicate, that in spite of different subsidence trends and other regional/local controls in these invidual basins, 3rd order sea-level changes exerted the primary control on shelf development.

	Subsidence modeling (Dolomites) Laterally homogeneous subsidence rates characterized the late Permian (Tatarian) to early Triassic (late Scythian) basin development in the central Southern Alps. Tectonic and total subsidence rates increased gradually in the middle to late Scythian. Marked differential subsidence/uplift occurred between the late Scythian and the early Carnian. In the NE-Dolomites, total subsidence rates increased from (+150) mm/ky in the late Scythian to (+250)-(+270) mm/ky in the early Pelsonian. Small uplift rates prevailed during the middle and late Pelsonian. Total subsidence returned in the Illyrian with rates of (+50)-(+110) mm/ky. In the NW-Dolomites total subsidence rates varied between (+80)-(+150) mm/ ky in the late Scythian to Aegean. Late Aegean and Bithynian show total uplift rates of up to (-50) mm/ky. Small uplift/ subsidence rates between (-10) mm/ky and (+20) mm/ ka occurred in the Pelsonian. In the middle and late Illyrian total subsidence rates increased significantly up to (+250) mm/ky. The eastward shift of the depositional center in the late Anisian was controlled by the initiation of growth faults at the eastern margin of the structural high in the W-Dolomites. Laterally homogeneous total subsidence rates reappeared only in the late Carnian and persisted until the early Jurassic.

	Geohistory and genetic subsidence compents of the NE-Dolomites Parameters: stratigraphic basin fill (age, thickness, lithology, initial porosity, specific weight, diagenesis coefficient, paleobathymetry, mantle density)

	Geohistory and genetic subsidence compents of the NE-Dolomites Parameters: stratigraphic basin fill (age, thickness, lithology, initial porosity, specific weight, diagenesis coefficient, paleobathymetry, mantle density)

	Controls on shelf development In the Triassic constant subsidence trends existed over time intervals between 1-4 My. Abrupt changes in tectonic subsidence occured over time intervals as little as 0.2-0.4 My. In the Ansisian of the Dolomites, sequence stratigraphic surfaces (transgressive, maximum flooding surfaces, sequence boundaries) in time intervals or areas with total subsidence rates of (±0)-(+20) mm/ky and <(+200) mm/ky are correlatable with sequence stratigraphic surfaces in other sedimentary basins in- and outside the Tethys. Sequence stratigraphic surfaces in time intervals or areas with total uplift/subsidence rates of <(±0)-(-20) mm//ky and >(+200) mm/ka are not correlatable with sequence stratigraphic surfaces in other sedimentary basins in- and outside the Tethys.This leads to a general model of controls on shelf development in the Anisian of the central Southern Alps. Eustatic sea-level changes represent the main control on shelf development in time intervals or areas with total subsidence rates of (±0)-(+20) mm/ky and <(+200) mm/ky (subsidence window). In time intervals or areas with total subsidence rates of <(±0)-(+20) mm/ky and >(+200) mm/ ky differential total subsidence primarily controlled the shelf development. Changes in total subsidence rates within the window of (±0/+20)-(+200) mm/ky did not control the basic evolution of individual 3rd order sequences. However, they influenced the internal architecture of sequences (internal geometries, thicknesses), facies (shallowing/deepeningupward trends, stacking patterns) and the types of sequence boundaries (or correlative conformities). Internal controls and external controls on the shelf development were closely related.

	Presentations [available soon]

	Publications Zühlke, R., 2000, Fazies, hochauflösende Sequenzstratigraphie und Beckenentwicklung im Anis (mittlere Trias) der Dolomiten (Südalpin, N-Italien): Gaea Heidelbergensis, v. 6, 368 p. Zühlke, R. and Bechstädt, T., 1998, Sequence stratigraphy, Controls on Deposition and basin development in the Middle Triassic of the Central Southern Alps (N-Italy). Abstract Volume, International Association of Sedimentologists, 15th International Sedimentological Congress, April 12-17, Alicante, 849-851. Rüffer, T., and Zühlke, R., 1995, Sequence stratigraphy and sea-level changes in the Early to Middle Triassic of the Alps: a global comparison, in Haq, B.U., ed., Sequence stratigraphy and depositional response to eustatic, tectonic and climatic forcing: Amsterdam, Kluwer, p. 161-207.

	Links [available soon]

	Funding Organisations German Research Fund (DFG)


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GeoResources - Sequence Stratigraphy and Basin Development, Anisian, Central Southern Alps, Details 2

High-Resolution Sequence Stratigraphy and Basin Development of the Central Southern Alpine Basin (Anisian, Triassic, Dolomites, N-Italy)