A detailed stratigraphic study associated with microfacies and strontium analysis has been performed on middle Miocene shallowwater carbonates developed in a wedge-top basin in the northern Apennines. More than 140 samples were collected in the representative outcrop of Torriana (Marecchia valley, Romagna Apennines). It covers the entire carbonate succession from the basal unconformity on Ligurian units to the terrigenous-rich sediments that mark the demise of the shelf. The modal analysis allowed to quantify the main framework components: an intrabasinal carbonate fraction (biogenic), a terrigenous fraction (largely siliciclastic) and an authigenic intrabasinal component mainly constituted by glauconitic grains. The carbonate succession starts with massive carbonates consisting of rudstones and grainstones dominated by bryozoans and echinoids. The basal few meters are rich in rhodolithes. The succession passes to planar-bedded grainstones to packstones with the same fossiliferous composition. The upper part is characterized by low angle cross-bedded grainstones to packstones with high detrital content, that gradually passes to mixed siliciclastic-carbonate sediments and finegrained hybrid arenites. An overall increase of the terrigenous input to the basin and the progressive deepening from a shallow shelf to outer shelf-slope environment caused the drowning of the shelf, marked by relatively high concentration of glauconitic grains. Our data indicate an almost exclusive intrabasinal supply during deposition of the basal portion. A vertical evolution is documented by a progressive increase in the terrigenous supply that replaces the bioclastic fraction. Preliminary 87Sr/86Sr data obtained from 7 carbonate samples allowed us to obtain absolute ages for the lower massive portion of the succession. For each sample we analyzed mollusk shells, bryozoans and bulk rock. Results indicate ages between 16,52-15,91 Ma, corresponding to the Burdigalian-lower Langhian interval. Comparing the values obtained from mollusk shells, bryozoans and bulk rock from samples at the same stratigraphic level, we observe similar results. This, according to petrographic analysis, permitted us to suppose a low diagenetic imprint of the rock.
Miocene carbonate shelf in the northern Apennines (S. Marino Fm): insights from microfacies, petrography and strontium isotopes / Argentino, Claudio; Salocchi, AURA CECILIA; Fontana, Daniela; Reghizzi, M; Grillenzoni, Claudia; Conti, Stefano. - ELETTRONICO. - (2015), pp. 1-91. (Intervento presentato al convegno XII GeoSed CONGRESS Italian Association for Sedimentary Geology tenutosi a Cagliari nel September 21-27, 2015).
Miocene carbonate shelf in the northern Apennines (S. Marino Fm): insights from microfacies, petrography and strontium isotopes
ARGENTINO, CLAUDIO;SALOCCHI, AURA CECILIA;FONTANA, Daniela;GRILLENZONI, CLAUDIA;CONTI, Stefano
2015
Abstract
A detailed stratigraphic study associated with microfacies and strontium analysis has been performed on middle Miocene shallowwater carbonates developed in a wedge-top basin in the northern Apennines. More than 140 samples were collected in the representative outcrop of Torriana (Marecchia valley, Romagna Apennines). It covers the entire carbonate succession from the basal unconformity on Ligurian units to the terrigenous-rich sediments that mark the demise of the shelf. The modal analysis allowed to quantify the main framework components: an intrabasinal carbonate fraction (biogenic), a terrigenous fraction (largely siliciclastic) and an authigenic intrabasinal component mainly constituted by glauconitic grains. The carbonate succession starts with massive carbonates consisting of rudstones and grainstones dominated by bryozoans and echinoids. The basal few meters are rich in rhodolithes. The succession passes to planar-bedded grainstones to packstones with the same fossiliferous composition. The upper part is characterized by low angle cross-bedded grainstones to packstones with high detrital content, that gradually passes to mixed siliciclastic-carbonate sediments and finegrained hybrid arenites. An overall increase of the terrigenous input to the basin and the progressive deepening from a shallow shelf to outer shelf-slope environment caused the drowning of the shelf, marked by relatively high concentration of glauconitic grains. Our data indicate an almost exclusive intrabasinal supply during deposition of the basal portion. A vertical evolution is documented by a progressive increase in the terrigenous supply that replaces the bioclastic fraction. Preliminary 87Sr/86Sr data obtained from 7 carbonate samples allowed us to obtain absolute ages for the lower massive portion of the succession. For each sample we analyzed mollusk shells, bryozoans and bulk rock. Results indicate ages between 16,52-15,91 Ma, corresponding to the Burdigalian-lower Langhian interval. Comparing the values obtained from mollusk shells, bryozoans and bulk rock from samples at the same stratigraphic level, we observe similar results. This, according to petrographic analysis, permitted us to suppose a low diagenetic imprint of the rock.
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