Feature and duration of metre-scale sequences in a storm-dominated carbonate ramp setting (Kimmeridgian, northeastern Spain)

Abstract

Metre-scale sequences may result from the combined effects of allocyclic and autocyclic processes which are closely inter-related. The carbonate ramp that developed northwest of the Iberian Basin during the late Kimmeridgian was affected by northwestward migrating cyclones. Marl–limestone alternations that settled in mid-ramp environments contain abundant mm to cm thick coarse-grained accumulations that have been related to these events. The aim of this paper is to determine the impact of storm-induced processes on the metre-scale sequence features. Four sections (R3, R4, R6, and R7), which are 5 to 7 m in thickness, were studied bed-by-bed along a 4 km-long outcrop, which shows the transition between the shallow and the relatively deep realms of the middle ramp. Metre-scale sequences were defined and correlated along this outcrop according to the detailed microfacies analysis of host, fine-grained deposits, palynofacies and sequence-stratigraphic analyses, and carbon- and oxygen-isotope measurements. The evolution through time of sedimentary features such as the size of quartz grains and the relative abundance of grains other than quartz (i.e., muscovite, bivalve, ooid, and intraclast) does not correlate from one section to the other, suggesting that the finest as well as the coarsest sediment was reworked in these storm-dominated environments. Small- and medium-scale sequences are defined according to changes in alternation, marly interbed, and limestone bed thickness, and correlated from one section to the other. Because of the effects of storms on sediment distribution and preservation, sequence boundaries coincide with thin alternations and marly interbeds in the most proximal sections (i.e., R3, R4), while they correspond to thin alternations and limestone beds in the most distal sections (i.e., R6, R7). Field observations and palynofacies analyses confirm this sequence-stratigraphic analysis. The excursions in carbon- and oxygen-isotope values are consistent with the lithological correlations, but in themselves are not conclusive. Marl–limestone alternations, and small-, and medium-scale sequences are hierarchically stacked, suggesting an orbital control on sedimentation with alternations lasting 20 kyr, small-scale sequences, 100 kyr, and medium-scale sequences, 400 kyr. As biostratigraphic analyses and spectral analysis are not the most appropriate tools to validate this time calibration in such a short interval and highly dynamic system, an alternative approach is developed, which is based on the quantification of the rates of sediment accumulation, preservation, and sea-level rise.