Abstract:
The middle Oxfordian warming climate and sea-level rise initiated the development of vast carbonate platforms
in some western European basins. At the same time, however, siliciclastics and siliceous sponges dominated
certain marginal areas of the Iberian ramp. There, siliciclastic input was particularly prominent during the latest
Oxfordian and may have been related to a global sea-level fall, synsedimentary tectonic activity, or humid climatic
conditions in the hinterland. Field analyses and computer modelling have been previously used to determine
the factors that controlled sedimentation. However, it is still unclear if the specific conditions that prevailed
during the latest Oxfordian were due to eustasy, tectonics or climate, and when precisely they occurred. Here,
we document major changes in sedimentological, micropalaeontological, and mineralogical records on the Iberian
ramp during this interval. Detailed sedimentary facies and palynofacies analyses combined with sequencestratigraphic
and cyclostratigraphic analyses of the Ricla Barranco section enable the establishment of a highresolution
time frame. Based on the quartz and mica percentage fluctuations, one large- and seven small-scale
sequences are defined. The large-scale sequence boundaries correlate with third-order sequence boundaries
Ox 8 and Kim 1 defined by Hardenbol et al. (1998). The large-scale maximum-flooding surface corresponds to
the base of the most calcareous interval and to the maximum abundance of marine phytoplankton and opaque,
equidimensional phytoclasts. The small-scale sequences correspond to the 100-kyr orbital eccentricity cycle.
Calcareous nannofossils and clay minerals were used as palaeoclimatic proxies. Nannofossil abundances and
fluxes are lower in the upper part than in the lower part of the interval studied, suggesting a decrease in seasurface
trophic conditions, also shown by an increase in the relative abundance of oligotrophic taxa. This upper
part is also characterised by an increase in smectite, which coincides with the base of the large-scale highstand
deposit, and is interpreted as reflecting the establishment of dry conditions. A first increase in smectite occurs
in the lower part of the succession, and coincides with high percentages of quartz and mica. This latter mineralogical
assemblage is interpreted as recording the onset of the Late Jurassic to Early Cretaceous rifting stage,which
occurred just before the Planula–Galar ammonite subzone transition. The present study points out a return
toward optimumconditions for carbonate sedimentation only 300 kyr after the prominent increase in siliciclastic
input due to tectonic activity. The recovery of carbonate production was accompanied by a global sea-level rise
and by decreasing rainfall on nearby land.
