The Jurassic – Cretaceous transition in the West Cameros Basin (Tera Group, Burgos, Spain): Sedimentological and palynostratigraphical insights

Abstract

The Jurassic–Cretaceous boundary transition is widely distributed within the West Cameros Basin. This horizon is constituted by the Tera Group, consisting of two depositional sequences (DS1 and DS2) comprising four geological formations spanning the Tithonian to the Berriasian. Here, a combined sedimentological and palynostratigraphic study was made covering the Tera Group. The sedimentological results for the DS 1 differentiate three facies associations (FA 1-1, 1-2 and 1-3) corresponding to floodplains and braided channels of a distributive fluvial system shifting to shallow lacustrine facies in a semi-arid climate. For the DS 2, three FA were also differentiated (FA 2-1, 2-2, and 2-3), corresponding to tidal-influenced meandering channels and floodplains in a distal fluvial to a delta-plain system that shifts to shallow lacustrine facies. The palynological results were only productive in samples from the Jaramillo Formation. The palynological record shows an assemblage dominated by spores including Concavissimisporites, Leptolepidites and Taurocusporites, as well as Classopollis pollen. According to the stratigraphic ranges of some selected taxa, the suggested chronostratigraphic assignment is lower Berriasian for the Jaramillo Fm. A comparison of the microfloras from the Tithonian–Berriasian recorded in the Iberian Peninsula was carried out. The following observations can be concluded: 1) the change in the plant communities, already observed since the Late Jurassic onwards, became more pronounced between the Tithonian–Berriasian boundary and the late Berriasian; and 2) a noteworthy occurrence of some biostratigraphically important taxa, specifically for the Tithonian–Berriasian interval.