Outcrop Cycle Stratigraphy of Shallow Ramp Deposits<subtitle>The Late Cretaceous Series on the Castilian Ramp (Northern Spain)</subtitle>

Abstract

Latest Albian to late Maastrichtian sedimentary deposits on the Castilian ramp (northern Spain) contain a great diversity of facies punctuated by numerous biosedimentary discontinuities that reveal many paleoenvironmental changes and provide a precise record of stratigraphic cycles. These deposits represent a transgressive-regressive depositional megacycle (i.e., a very long-term cycle, 34 my). This late Cretaceous megacycle is subdivided into four long-term (4-20 my) transgressive-regressive depositional cycles: (1) latest Albian to middle Cenomanian cycle, (2) late Cenomanian to earliest Coniacian cycle, (3) early/middle Coniacian to late Santonian cycle and (4) latest Santonian to late Maastrichtian cycle. These long-term cycles are made up of 13 well-characterized short-term transgressive-regressive depositional cycles, plus other cycles of shorter duration. Most of the cycles recognized correlate with those defined from the series of the steeper, distal part of the ramp and from the outer series of the deep basque Basin. The megacycle and the long- and short-term cycles were largely the result of the tectonic behavior of the Basque-Cantabrian passive margin and were closely controlled by the evolution of the bay of Biscay in conjunction with the relative movements of the Iberian and European plates. AIM OF THE STUDY AND METHODOLOGY The best way to pinpoint the factors that govern the cyclic pattern of sedimentation is to conduct an exhaustive analysis of regional cycle stratigraphy. The late Cretaceous deposits of the Castilian sedimentary basin in northern Spain are highly ame- nable to this type of analysis because: 1. the series crop out widely, so the three-dimensional arrange- ment o f t he l ithostratigraph ic u nits c an b e e asily reconstructed; 2. the series can be studied across the full extent of the sedi- mentary basin, from shoreline to continental slope and from the time the basin formed; and 3. the sedimentary basin was a shallow epicontinental ramp on which almost all the environmental changes, especially those related to relative sea-level fluctuations, were very well r ecorded b y b oth c arbonate a nd s iliciclastic sedimentation; 4. the biostratigraphic data are good enough to ensure precise