The aftermath of the Carnian carbonate platform demise: a basinal perspective (Dolomites, Southern Alps)

Abstract

In the Dolomites of northernmost Italy the carbonate-platform growth came to a standstill late in the Early Carnian (Late Triassic). The response to this shutdown of shallow-water carbonate production in the interplatform basins is largely unknown because erosion has removed most of the soft basinal sediments, giving rise to today's scenic landscape of the Dolomites. Mapping in the central part of the Dolomites and newly available core material has recently revealed a wellpreserved succession of basinal rocks within the Heiligkreuz Hospiz Basin (ital. Ospizio di Santa Croce Basin). In this paper, the regional depositional nature of arrested carbonate platform production is reconstructed by tracing its sedimentological record across the slope and into the basin. The uppermost St. Cassian Formation, the time-equivalent basinal rocks to the prograding carbonate platforms, is overlain by the Heiligkreuz Formation, whose basal succession was deposited in a restricted and oxygen-depleted environment immediately post-dating the platform demise. The succession consists mainly of mudrocks, marlstones, and peloidal packstones, with abundant low-diversity ostracod and pelecypod fauna and early diagenetic dolomite. C and O isotope values of the basal Heiligkreuz Formation, post-dating platform demise, average + 2AE4 and ) 2AE4&, respectively, and largely overlap the isotopic composition of St. Cassian carbonates. A shift toward slightly lower d 13 C values in the Heiligkreuz Formation may reflect incorporation of isotopically depleted C released during bacterial sulphate reduction in the Heiligkreuz sediments. Sedimentological, palaeobiological and geochemical indices suggest that nearnormal marine conditions persisted long after the shutdown of shallow water carbonate-platform growth, although there are clear indications of severely reduced oxygen levels in the restricted Heiligkreuz Hospiz interplatform basin. The Early Carnian platform demise induced a distinct switch in the locus of carbonate production from the shallow-water platform and slope to the basin floor and a decrease in the availability of dissolved oxygen in the basinal waters. It is inferred that anoxia extended at least temporarily to the top of the carbonate slope, as indicated by the onlap of normal-marine mounds by dark marlstones of the basal Heiligkreuz Formation.

A Tethys-wide mid-Carnian (Upper Triassic) carbonate productivity crisis: Evidence for the Alpine Reingraben Event from Spiti (Indian Himalaya)?

“…Jerz, 1966;Schuler, 1968;Grottenthaler, 1978;Brandner, 1984;Riedel, 1991;Mandl, 2000;Gawlick, 2000; and can be observed even in the Southern Alps (e.g. RüVer and Zamparelli, 1997;Flügel and Senowbary-Daryan, 2000;Stefani et al, 2004;Keim et al, 2006) and NE Iran (Donofrio, 1991;Ruttner et al, 1991;Ruttner, 1993). As similar developments are recorded from Indonesia (SE Tethys; Martini et al, 2000), China (NE Tethys and Panthalassa Ocean; Lehrmann et al, 2005) and the Tethyan Himalaya, one might assume a Tethyan-wide event.…”

Diversity and palaeoecology of a highly diverse Late Triassic marine biota from the Cassian Formation of north Italy

“…The strata exposed at these locations may in fact be lateral equivalents of the Heilig Kreuz Formation, which overlies the Cassian Formation in the Gadertal region, where it consists of basin sediments (see Keim et al . ).…”

“…; Keim et al . ). Sea surface temperatures were tropically warm, but with possible strong seasonal fluctuations (Nützel et al .…”

A new ammonoid fauna from the Carnian (Upper Triassic) Kasimlar Formation of the Taurus Mountains (Anatolia, Turkey)

“…Although there are some records of Julian as well as of Tuvalian ammonoids, for example, at the Heiligenkreuz Formation (Dolomites, SA; Keim et al . ; Breda et al . ), the majority of Lower–Upper Carnian sections are dated by conodonts (e.g.…”