Sequence stratigraphy and bedding rhythms of an outer ramp limestone succession (Late Kimmeridgian, Northeast Spain)

Abstract

Facies, stratal and spectral analyses of an outer ramp lime mudstone succession (Aguilón, north Iberian Ranges, Spain) are presented in this work. The studied succession is Late Kimmeridgian ( eudoxus and beckeri zones) in age and comprises the transgressive and highstand deposits of a third-order depositional sequence. A number of higher-order sequences (bundles and sets of bundles) have been identified based on the comparative analysis of the bedding planes. The bundles and sets of bundles show a well-defined stratal pattern. Spectral analysis has provided further independent confirmation of the cyclical nature of the bundles and sets of bundles defined from field analysis. The bundles have variable thickness (from 1 to 2 m) and are formed by up to 10 micritic beds. They have been related to sea-level changes controlled by the orbital precession cycle, affecting the shallow productivity area. A significant amount of the lime mudstones accumulated in outer ramp settings were derived from resedimentation of the shallow carbonate production areas. Many of the bundles show a lower interval with a thinning and fining-up trend, indicating a progressive decrease of the carbonate production (and carbonate export) during periods of high-frequency sea-level rise. The late transgressive and highstand deposits show sets of bundles (groups of five bundles, from 5 to 8 m) probably related to sea-level changes controlled by the short eccentricity cycle. The overall thickness and the stacking pattern observed in the sets of bundles are controlled by the long-term sea-level variation. The sets of bundles located in the late transgressive deposits show thinner micritic beds in their lower or middle part. The sets of bundles found in the highstand deposits are thinner and show a thickening-up and thinning-up trend. On the studied carbonate ramp, during periods of long-term sea-level rise, the overall carbonate production (and carbonate export) is high, although the superposition of the high-frequency sea-level rises may result in episodic flooding and drowning of the shallow ramp areas. During periods of long-term, early highstand of sea level, the overall carbonate production (and carbonate export) is more reduced, but it has maximum peaks during the transgressive (and early highstand) intervals of the high-frequency sea-level cycles. Sedimentation during the long-term sea-level fall (late highstand) was scarce and discontinuous in the outer ramp area, and resulted in the overall thickness reduction of the sets of bundles.