Detailed field mapping of a Middle Triassic carbonate buildup, the Latemar in the western Dolomites, northern Italy, has been carried out. The Latemar is an isolated carbonate buildup that nucleates on a fault-bounded structural high (horst) cut into the underlying late Anisian carbonate bank of the Contrin Fm. This study demonstrates that extensional synsedimentary tectonics is the main factor controlling its geometry and provides an age for this tectonic phase. In an early phase, slopes were mostly composed of well bedded, clinostratified grainstones and rudstones. In a later stage, the deposition of grainstones was accompanied by the emplacement of clinostratified megabreccias. The upper portion of slopes is a microbial boundstone with abundant Tubiphytes and patches or lenses of grainstone. Boundstones may occasionally expand into the platform interior and downward to the base of the slope. The depositional profile was that of a mounded platform. The buildup is dissected by a dense framework of high angle fractures and faults, and by magmatic and sedimentary dikes, exhibiting two principal directions trending NNW–SSE and ENE–WSW. Faults trending WNW–ESE were also observed. Magmatic dikes are related to the emplacement of the nearby Predazzo intrusion and are thus upper Ladinian. Kinematic indicators of strike-slip activity were observed on fault planes trending NNE–SSW and NNW–SSE, that can be attributed to Cenozoic Alpine tectonics. Faults, magmatic dikes and sedimentary dikes show systematic cross-cutting relationships, with strike-slip faults cutting magmatic dikes, and magmatic dikes cutting sedimentary (neptunian) dikes. ENE–WSW and WNW–ESE faults are cut by all other structures, and record the oldest tectonic activity in the region. Structural analysis attributes this tectonic phase to an extensional stress field, with a direction of maximum extension oriented ca. N–S. Several lines of evidence, including sealed faults and growth wedge geometries allow us to recognize this old tectonic phase as syndepositional, and to date it to the latest Anisian (avisianum and crassus ammonoid biochronozones). The horse-shoe shape of the platform and its margins, rectilinear for long tracts, are explained by the existence of a network of extensional faults, whose activity lasted for part of the growth of the platform. The Latemar is thus a polygonal, fault-block platform controlled by synsedimentary tectonics. The resulting sedimentary architecture is that of a growth wedge, deposited probably on the uplifted side of a submerged half graben. Continuous reactivation of faults at platform margins locally determined anomalous facies transitions, with well layered platform interior directly in contact with the clinostratified slopes.
The Latemar: A Middle Triassic polygonal fault-block platform controlled by synsedimentary tectonics
FRANCESCHI M;
2011-01-01
Abstract
Detailed field mapping of a Middle Triassic carbonate buildup, the Latemar in the western Dolomites, northern Italy, has been carried out. The Latemar is an isolated carbonate buildup that nucleates on a fault-bounded structural high (horst) cut into the underlying late Anisian carbonate bank of the Contrin Fm. This study demonstrates that extensional synsedimentary tectonics is the main factor controlling its geometry and provides an age for this tectonic phase. In an early phase, slopes were mostly composed of well bedded, clinostratified grainstones and rudstones. In a later stage, the deposition of grainstones was accompanied by the emplacement of clinostratified megabreccias. The upper portion of slopes is a microbial boundstone with abundant Tubiphytes and patches or lenses of grainstone. Boundstones may occasionally expand into the platform interior and downward to the base of the slope. The depositional profile was that of a mounded platform. The buildup is dissected by a dense framework of high angle fractures and faults, and by magmatic and sedimentary dikes, exhibiting two principal directions trending NNW–SSE and ENE–WSW. Faults trending WNW–ESE were also observed. Magmatic dikes are related to the emplacement of the nearby Predazzo intrusion and are thus upper Ladinian. Kinematic indicators of strike-slip activity were observed on fault planes trending NNE–SSW and NNW–SSE, that can be attributed to Cenozoic Alpine tectonics. Faults, magmatic dikes and sedimentary dikes show systematic cross-cutting relationships, with strike-slip faults cutting magmatic dikes, and magmatic dikes cutting sedimentary (neptunian) dikes. ENE–WSW and WNW–ESE faults are cut by all other structures, and record the oldest tectonic activity in the region. Structural analysis attributes this tectonic phase to an extensional stress field, with a direction of maximum extension oriented ca. N–S. Several lines of evidence, including sealed faults and growth wedge geometries allow us to recognize this old tectonic phase as syndepositional, and to date it to the latest Anisian (avisianum and crassus ammonoid biochronozones). The horse-shoe shape of the platform and its margins, rectilinear for long tracts, are explained by the existence of a network of extensional faults, whose activity lasted for part of the growth of the platform. The Latemar is thus a polygonal, fault-block platform controlled by synsedimentary tectonics. The resulting sedimentary architecture is that of a growth wedge, deposited probably on the uplifted side of a submerged half graben. Continuous reactivation of faults at platform margins locally determined anomalous facies transitions, with well layered platform interior directly in contact with the clinostratified slopes.Pubblicazioni consigliate
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