Early Jurassic Flood Basalt Volcanism on the Franz Josef Land Archipelago: Geological and Palynostratigraphical Data

Abstract

Biostratigraphic evidence of the presence of Early Jurassic flood basalts on the Franz Josef Land archipelago is presented. The flood basalts form layered section with two units, which is not discovered for Early Cretaceous basalts. The lower unit is composed by large-columnar basalts (colonnade), and the upper unit by small-columnar (entablature) chaotic-fan basalts. On the Hooker Island, the basalt flow is exposed on the Sedov Plateau, on the Lunacharsky Rock Cape and, possibly, on Al’banov Cape. On the southern slope of the Sedov Plateau, the basalt flow overlaps sands and sandstones, which contain palynoassemblage of the lower Toarcian. In the Lunacharsky Rock Cape outcrop, the underlying basalt sands are of the Pliensbachian to Early Toarcian chronostratigraphic interval. Apart from the Hooker Island, we observed basalts with the “colonnade/entablature” on three other islands: Scott Keltie, May and Leigh-Smith. The most complete section was found in the western part of the Leigh-Smith Island, where basalts are underlain and overlapped by sand units. The underlying sands in contact with basalts have a quenching zone. There is no quenching zone at the contact with the overlapping sands. A palynocomplex from the lower sand unit is early Toarcian in age. The palynocomplex found in the upper sand unit indicates its accumulation in the interval from the lower part of the late Toarcian to the early Aalenian. A palynological study of the underlying and overlying deposits of the basalt flow has shown that the flow is underlain by continental and coastal-marine sediments of the Pliensbachian to the upper part of the early Toarcian age interval. Basalt flow is overlain by the earliest late Toarcian–early Aalenian marine sediments. According to the modern chronostratigraphic scale, the age of the basalt flow can be estimated as approximately 180 million years, which is quite consistent with the earlier obtained 40Ar/39Ar data of 189.1 ± 11.4 million years. These data indicate that the basalt flow was formed during a narrow stratigraphic interval of the uppermost lower–earliest upper Toarcian.