Chemostratigraphic correlation of upper Pliensbachian-lower Toarcian sections reveals significant gaps in the Alpine-Carpathian hemipelagic/pelagic successions

The Earth’s biogeochemical systems have experienced profound disruptions in the late Early Jurassic (latest Pliensbachian and early Toarcian, ~184-181.5 Ma). Two events are recognised over these ages, the earlier Pliensbachian-Toarcian boundary event (Pl-To) and the later and more severe Jenkyns Event. Accompanied carbon cycle perturbations left behind characteristic carbon isotope excursions (CIE) in the sedimentary record, which are registered in marine biogenic and abiotic carbonates as well as in terrestrial and marine organic matter. A short negative CIE by 1-4 ‰ commonly appears in the geological records of the Pl-To. A characteristic positive CIE expands through the entire lower Toarcian, which is interrupted by a large magnitude negative CIE by 4-6 ‰, marking the main phase of the Jenkyns Event in the late Tenuicostatum and early Serpentinum biochrons. The negative CIEs are widely linked to the concomitant activity of the Karoo & Ferrar LIP and/or methane release from terrestrial and marine reservoirs. On the other hand, the positive CIE is generally regarded as a sign of enhanced organic matter burial. The characteristic trends of these CIEs offer remarkable potential for chemostratigraphic correlation of various sections and lithologies.
Hemipelagic/pelagic upper Pliensbachian to lower Toarcian successions are common in the Alpine-Carpathian-Pannonian Mega-unit. In the late Early Jurassic, the area was part of the wider Adriatic plate, located west of the Neo-Tethys, between the open ocean and the rifting zone of the future Penninic Ocean. Due to its distal location and tectonic isolation, coarse siliciclastic input was subordinate in the region and open marine sedimentation of marls, black shale and limestones was predominant. Changes in sea level, seawater chemistry to more acidic and possibly increased bottom current activity profoundly affected sedimentation during the Pl-To and the Jenkyns Event, resulting in variable thicknesses, condensations and gaps.
Several sections with available high-resolution carbon isotope records were selected for chemostratigraphic correlation: Bächental and Sachrang (Eastern Alps, Austria and Germany); Skladana Skala (Central Western Carpathians, Slovakia); Úrkút, Tölgyhát, Kisgerecse and Bánya Hill (Transdanubian Range, Hungary). Correlating with existing chemostratigraphic schemes revealed a pattern of gaps and condensations that could be a systematic feature for the region. In condensed pelagic succession (Kisgerecse and Bánya Hill), the entire lower Toarcian is commonly missing and only the late phase of the Jenkyns Event is preserved. Other hemipelagic records exhibit significant condensations coinciding with the negative CIE marking the main phase of the Jenkyns Event (Skladana Skala and Tölgyhát). Extensive successions (Úrkút, Bächental and Sachrang) are characterised by thick black shales deposited during the Jenkyns Event and with significant manganese enrichment, which mainly corresponds to the lowermost Toarcian and Pl-To.