Long-term Callovian-Oxfordian sea-level changes and sedimentation in the Iberian carbonate platform (Jurassic, Spain): possible eustatic implications

摘要

Facies analysis across the carbonate platform developed during the Callovian-Oxfordian in the northern Iberian basin (Jurassic, Northeast Spain) is used to characterize successive stages of sedimentary evolution, including palaeoenvironmental reconstructions showing the distribution of a wide spectrum of facies, from ferruginous oolitic, peloidal, spongiolithic to intraclastic. The studied successions consist of two long-term transgressive-regressive cycles bounded by a major unconformity with a major gap, comprising at least the upper Lamberti (Callovian) and Mariae (Oxfordian) Zones. Major transgressive peaks of these two cycles occurred at the end of the Early Callovian (late Gracilis Zone) and at the end of the Middle Oxfordian. The Callovian and Oxfordian successions were further divided into three and seven higher frequency cycles, respectively. The modelling of two sections (i.e. Ricla and Tosos) located 40 km apart in the more subsident open platform areas, allows the reconstruction of two curves showing a similar evolution of long-term sea-level changes that are in theory eustatic, though subject to uncertainties derived form the assumptions required for their construction. The changes affecting the northern Iberian basin seem to reflect nearly homogeneous subsidence (rates around 2 cm kyr(-1)) combined with possible eustatic changes including an Early Callovian rise, a fall at the middle Callovian-earliest Oxfordian (i.e. the Anceps-Mariae Zones), with average long-term rates around 2 cm kyr(-1) (total fall of 40-60 m), a period of lowstand at the Early-Middle Oxfordian transition and a long-term rise at the Middle-Late Oxfordian transition (Transversarium and Bifurcatus Zones). Facies distribution across the Iberian platform indicates a progressive Middle-Late Callovian relative sea-level fall rather than a rapid relative sea-level fall at the end of the Callovian. After this falling episode, the progressive onlap over the swell areas during the Early Oxfordian and at the beginning of the Middle Oxfordian indicates a period of accommodation gain, which is explained by the combined effects of continuous subsidence across the platform and reduced sedimentation rates in spite of the possible eustatic lowstand. Eustatic lowstand, combined with other factors (ocean water circulation, volcanism) could help to explain the loss of carbonate production during the latest Callovian-Early Oxfordian, previous to the widespread eustatic rise and warning recorded at the onset of the Transversarium Zone (Middle Oxfordian).