End-Triassic crisis and 'unreefing' led to the demise of the Dachstein carbonate platform: A revised model and evidence from the Transdanubian Range, Hungary

摘要

The Dachstein platform was an extensive carbonate platform developed on the westernmost shelf of the Neotethys during the Late Triassic, now preserved in various tectonic units disrupted during the Alpine orogeny. Despite being the focus of a multitude of sedimentological, paleontological and other studies, the demise of this platform remains controversial, with contrasting views on the timing and causes of cessation of its growth, the duration of the gap above, which at many places includes the Triassic-Jurassic boundary (TJB), and the depositional environment of overlying strata. Here we present new carbonate sedimentological, stable isotope and cyclostratigraphic data from sections in the Transdanubian Range (Hungary) which capture the termination of uppermost Triassic Dachstein Limestone and the onset of Hettangian (Early Jurassic) sedimentation following a hiatus. Previously, the TJB in the Transdanubian Range was regarded as a textbook case of a tectonically-driven platform drowning event or, alternatively, cessation of carbonate production due to emergence caused by a significant sea level fall at the TJB. However, recognition of global biotic change and environmental perturbations at the TJB calls for an assessment of their possible role in the demise of the Dachstein platform.Oxygen and carbon isotopic composition of bulk carbonates were measured in sections at Ko?ris-hegy (Bakony Mts.), Tata (Tata Horst), and Vo & uml;ro & uml;sh & iacute;d (Gerecse Mts.) Paleogeographically, these three sections represent a proximal to distal platform transect. Other sections at Pisznice and To & uml;lgyh & acute;at (Gerecse Mts.) yielded additional sedimentological data. The sharp surface separating the Dachstein Limestone from the overlying Jurassic formations carries no or only minimal relief at outcrop scale. Thin section studies reveal small-scale irregularities, stylolites, microborings with ferruginous filling, or a thin clay-rich layer at the TJB, indicative of a submarine, or perhaps polygenetic, hardground. In the first meters of the lowermost Jurassic beds abundant ooids occur, and crinoids become common. In each of the studied sections, an abrupt negative carbon isotope shift is recorded at the TJB, and a gradual rebound to more positive values characterizes the lowermost Jurassic strata. Chemostratigraphy allows correlation with sections elsewhere. In the Transdanubian Range, the initial carbon isotope excursion and at least the first part of the purported main carbon isotope excursion are not preserved due to the gap at the TJB. Combined bio-and cyclostratigraphy of lowermost Jurassic strata permits an astrochronologic duration estimate of the early Hettangian hiatus that was not longer than a few hundreds of thousand years.Our results highlight the role of submarine erosion, perhaps partly related to acidification, and point to an abrupt change in carbonate production related to the end-Triassic extinction of several groups in the platform system. & ldquo;Unreefing & rdquo;, the ecological collapse of reefs, led to a regime shift, the transformation of the rimmed platform to a carbonate ramp, with a significant gap in production and preservation of carbonate sediment. This model is not uniformly applicable to Late Triassic platforms as several of them, unlike the Dachstein platform,survived with unrimmed architecture in the Early Jurassic elsewhere on the Neotethyan shelf. However, the model may explain features of the carbonate platform sedimentary record across other events with reef collapse.