Lower Visean sea-level changes in the northern Precaspian Basin

Abstract

This study presents the sequence stratigraphic interpretation of subsurface data in a Lower Carboniferous lagoon setting in the northern boundary zone of the Precaspian Basin. The proposed stratigraphic and facies distribution model of the lagoon fill enables the reconstruction of the relative sea-level history and refines the understanding of the geological evolution and eustatic changes in the Caspi region during the Lower Carboniferous. Semi-circular carbonate mounds were built in the Devonian period and Tournaisian stage enclosing a lagoon with an opening towards the Caspi Sea in a southeast direction. A sea-level fall at the end of the Tournaisian stage terminated the growth of the carbonate buildups; during the Lower Visean (Lower Carboniferous) time, the lagoon was filled with mixed siliciclastic and carbonate sediments. The lagoon fill is interpreted as a third-order composite sequence composed of three fourth-order sequences representing sea-level changes with an increased magnitude and frequency relative to the quiet conditions that prevailed during the previous carbonate buildup in Famennian-Tournaisian. The highstand systems tracts of the fourth-order sequences are likely Waulsortian-type carbonate banks, while the falling-stage and lowstand systems tracts are mostly clastic progradational wedges. The transgressive systems tracts include retrograding shale and grain shoal facies. A prominent falling-stage systems tract was detected in the third fourth-order sequence in the lagoon fill representing a significant sea-level fall in lower Visean time. The falling-stage systems tract is composed of three downstepping-prograding lobes indicating a continuously falling sea level punctuated by minor pauses. Seismic imaging revealed a series of distinct prograding wedges inside the falling stage lobes that are either accretionary bedset or high-frequency nested sequences. The observed fall in sea level during the Lower Visean may be associated with, and it may provide stratigraphic evidence for the beginning of Paleozoic glaciation on Gondwana.