Quantification of input and compositional variations of calciturbidites in a Middle Triassic basinal succession (Seceda, Dolomites, Southern Alps)

Abstract

Triassic calciturbidites were studied in a 100-m long core and nearby outcrops of the basinal Buchenstein Formation to determine composition and thickness variations. The quantity of recognized turbidite sediment relative to background sediment changes from 15% (by volume) in the lower part to 60% in the upper part, reflecting the steady progradation of nearby platforms. The composition of the sand fraction of 214 turbidites was point-counted in thin sections. Micrite peloids (average 23%) and lithoclasts (16%) are by far the most dominant constituents. They are interpreted as two different varieties of in-situ precipitated micrite (automicrite), which probably formed under the influence of microbes and constitute the principal building material of the adjacent platforms. Platform-derived skeletal grains amount to only 0.5%. Variations in turbidite composition were quantified using Spearman's rank correlation and cluster analysis. The most significant compositional variations seem to be related to hydrodynamic sorting in the turbidity currents and to the gradual shift from distal to more proximal turbidites in the core as the platforms prograded basinward. Cluster analysis of the 214 samples shows a major subdivision into micrite and sparite dominated turbidites. Clusters associated with micritedominated turbidites are enriched in Radiolaria and thinshelled bivalves, whereas the clusters related to sparitedominated turbidites show an abundance of lithoclasts. This subdivision seems strongly related to sorting effects in a turbidity current. Point-counting of turbidites in nearby outcrops revealed a lateral variation in composition. Proximal turbidites are sparite-dominated and enriched in lithoclasts, distal portions are chiefly micrite with an open-ocean biota (thin-shelled bivalves, Radiolaria). This differentiation resembles the vertical change in composition of thick turbidite beds, and is attributed to different settling rates of the various grains in the turbidity current. There is no indication that turbidite composition fluctuated significantly under the influence of sea-level fluctuations. This is not surprising because the dominant automicrite facies of the platforms only migrates laterally, but does not change much during sealevel cycles.

Lateral variations in sediment composition and bedding in Middle Triassic interplatform basins (Buchenstein Formation, southern Alps, Italy)

“…To date, the occurrence of resedimented shallow‐water debris in the Buchenstein Formation has only been described from the topmost unit of the formation (‘Upper Bänderkalke’; Viel, 1979; Bosellini & Ferri, 1980; Brack & Rieber, 1993). A detailed analysis of the Seceda core revealed the presence of calciturbidites throughout the entire formation, with values of 8–15 vol% in the ‘Knollenkalke’ (Maurer et al ., 2002). In the nearby outcrops and in other locations in the basin, the occurrence of calciturbidites has now also been demonstrated (Figs 4 and 5).…”

“…The levels with the lowest amount of carbonate form the boundaries between individual beds. A comparison of laminated and bioturbated facies in the slabbed Seceda core shows that, in the ‘Knollenkalke’, all bedding structures smaller than ≈ 1 cm are erased by bioturbation (Maurer et al ., 2002).…”

“…A particularly lively debate has taken place about the possible orbital controls on Middle Triassic platform cycles, because the result of time‐series analyses of platform cycles are at variance with the time constraints provided by radiometric age dating of ash layers (Hinnov & Goldhammer, 1991; Brack et al ., 1996; Mundil et al ., 1996; Hardie & Hinnov, 1997; Preto et al ., 2001; Zühlke et al ., 2002). The deposits of the interplatform basins play a crucial role in this debate because they contain most of the volcanic ash layers in undisturbed, continuous successions, and record the platform history in numerous layers of shallow‐water debris (Bosellini & Ferri, 1980; Brack & Rieber, 1993; Maurer, 1999; Brack & Muttoni, 2000; Maurer et al ., 2002). The Seceda coring project is an attempt by an international working group to provide detailed documentation of the sedimentology, stratigraphy and depositional rhythms of a mid‐Triassic basinal succession (Buchenstein Formation) as a basis for discussion of orbital cycles and chronostratigraphy (Brack et al ., 2000).…”

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“…Normal grading and parallel lamination suggest deposition via turbidites. Sparite-dominated varieties (MF 9, 10) are proximal turbidites, whereas micrite-rich microfacies types (MF 2,(4)(5)(6)(7)8,11) are interpreted as distal turbidites (see Maurer et al, 2003). Mud-supported bioclastic-intraclastic floatstone (MF 12) and intraclastic rudstone (MF 13) might be debris-flow deposits (Mullins & Cook, 1986;Eberli, 1991).…”

„Oázis a sivatagban” — különösen gazdag ősmaradvány-együttes a fosszíliaszegény üllési Szegedi Dolomitból

“…Az együttesben a Hoyenella nagyszámú előfordulása, valamint a Trochammina cf. almtalensis jelenléte sekélytengeri, zátonyháttér-lagúna környezetet igazol (BÉRCZINÉ MAKK 1996, MAURER & RETTORI 2002. Ezt támasztja alá az algatelep-foszlányok előfordulása is (2. ábra, i-k).…”