Timing and duration of the early Hettangian marine inundation in the Polish Basin: organic carbon isotopes and astronomical calibration of the Triassic-Jurassic transition of the Niekłań PIG core (Holy Cross Mountains, SE Poland)

Stratigraphy of non-marine sections and dynamics of marine transgressions in the deep geological past are important, yet challenging issues. Here we discuss results obtained from the Triassic–Jurassic (T–J) boundary interval of the Niekłań PIG drillcore section (Holy Cross Mountains or HCM, SE Poland). The core represents an expanded record of continental to marginal-marine facies, deposited within the axial part of the Polish Basin. The section is dated based on an integrated approach, utilizing palynologic proxies, stable carbon isotope correlations, and astronomical tuning of the section. Palaeoenvironment and its evolution is established by means of high-resolution sedimentology, clay mineral assemblages, and geochemical weathering indices (chemical index of alteration, CIA). The early to mid-Rhaetian was seasonal and progressively warmer and more humid, culminating with a hot and humid climate in the late Rhaetian; the earliest Jurassic was slightly cooler, yet still warm and humid, with year-round rainfall. Characteristic for the T/J boundary is the occurrence of two fern spikes (uppermost Rhaetian and lowermost Hettangian), which are associated with vegetation crises; this interval is also marked by wildfire indicators. Recognition of short eccentricity (~100 kyr) cycles enables calculation of sedimentation rates as well as temporal interpretation of the early Rhaetian marine inundation onto coastal plains of the Polish Basin. To begin with, over a period of ~200 kyr, a base-level rise led in the HCM region to a shift from fluvial to lacustrine depositional systems, while the final flooding marked by the onset of marine facies took another ~350 kyr. Additionally, observations allow inference of the magnitude of the early Hettangian relative sea-level rise, estimated herein to have been about 55 m.