ABSTRACT

Near the town of Zumaya in northern Spain, upper Maastrichtian strata consist of limestone/marl rhythms with thin, interbedded sandstones. The marls and limestones are primarily pelagic and hemipelagic deposits, while the sandstones are the product of turbidity currents. Stratigraphic variations in the volume of marl and limestone form three hierarchical or composite cycles. The first-order cycles consist of 20-30-m alternations between stratal sequences consisting of interbedded limestone and marl, and sequences composed almost exclusively of marl. Within the interbedded limestone/marl sequences, second-order cycles occur that consist of 5-10-m alternations between limestone or marl domination. The third-order cycles consist of the bed-by-bed alternation of limestone and marl. These hree composite cycles are the product of shifts in the clay-carbonate ratio on various rock- and time-stratigraphic scales.

In order to determine if the cyclicity was the result of changes in the supply of siliciclastic material or carbonate material, detailed stratigraphic measurements were made of volume percent, average bed thickness, and stratigraphic frequency for each lithology. Product-moment coefficients indicate that a negative correlation exists between percentage marl and sandstone frequency and a positive correlation exists between limestone percent and sandstone frequency. Assuming that turbidite deposition is not genetically related to limestone or marl deposition, the correlation coefficients suggest that there were increased sedimentation rates during marl accumulation. Correlation coefficients also showed that increases in amount of marl in a section occur through bed thickening rather tha the increased number of marl beds. In contrast, limestone-dominated intervals are achieved by reduction of marl-bed thickness and increases in marl and limestone bed frequency. This confirms the suggestion that marl-dominated episodes were associated with increases in the sedimentation rate. From this analysis, we propose that changes in the influx of siliciclastic material, and not changes in productivity or dissolution, led to the production of limestone/marl rhythms as well as the first- and second-order cycles seen in the Zumaya section.