Interplay of orbital forcing and tectonic pulses in the Cambrian Iberian platform, NE Spain

The stratigraphy of carbonate/shale couplets, cycles and cycle-stacking pat terns in a Cambrian shallow water platform (Iberian Chains, NE Spain) are r elated to sea-level changes driven by orbital forcing and by tectonic pulse s. The interplay of both effects can be discriminated in the Iberian fault- controlled platform, in which the tectonic activity can be analysed by accu rate and detailed biostratigraphic correlations based on trilobite zonation . The stratigraphic hierarchy of rhythmically interbedded limestones and sh ales, in two coeval but structurally separated geodynamic settings, yields cycle ratios of 1.44:1. This ratio is supported by time thickness and spect ral analysis, which is based on a graphic method of analysis: the Map of Gr ey Lines. The cycle ratio seems to be evidence for orbital forcing by obliq uity and precession cycles predicted for early Paleozoic time. Carbonate/sh ale couplets, the smallest rhythmic units recognisable in the field, repres ent short-term, periodic fluctuations in supply of terrigenous sediments an d carbonate productivity of uncertain origin, which could be associated wit h one of several harmonics of the former orbital cycles. The pulsating tect onic activity was approximated by using a quantitative analysis of tectonic ally induced subsidence (Shaw method). Recurrence frequencies of tectonic p ulses were estimated and dated by biostratigraphy. As a result, tectonic di sturbances in the Cambrian Iberian platform show an episodic periodicity co mparable to that of orbital eccentricity cycles, which could mask their rec ognition.