Abstract

Seismic-scale outcrops of lower Pennsylvanian steep-margined carbonate platforms in northern Spain (Sierra de Cuera, Cantabrian Mountains) provide the opportunity to link depositional facies to platform-interior architecture. Lithofacies character, vertical stacking patterns, and spatial distribution of depositional facies and shallowing-upward cycles were investigated to provide a semiquantitative depositional model for inner-platform strata.

A 70-m-thick succession of eight shallowing-upward cycles (2.5-15 m thick) was studied across a 2-km-wide transect. Cycles consist of algal bioherms and skeletal packstone (lithofacies association B) deposited in an open marine and sub-wave-base environment, which are locally lateral to crinoid-dominated packstone (lithofacies association D). Lithofacies association B developed on a transgressive, one-meter-thick high-energy coated-grain grainstone (lithofacies association A) and is overlain by open marine to restricted lagoonal deposits (lithofacies association C) indicative of decreased paleo-water depth. Cycle boundaries are marine flooding surfaces occurring at the base of lithofacies A and the top of lithofacies C. Petrographic and outcrop evidence of subaerial exposure is conspicuously lacking.

Cycle and lithofacies thickness varies laterally, and all the lithofacies can be either continuous for the 2 km width of investigation or discontinuous, terminating within tens to hundreds of meters. In most of the cycles, the lithofacies succession changes laterally because of lateral facies transitions (between lithofacies B and D) and pinching out of lithofacies A and C. Meter-scale variations in depositional topography are related to the stacked lens-shaped bioherms assigned to lithofacies B and appear to control the thickness and lateral continuity of the overlying facies.

Sierra de Cuera strata do not resemble other Pennsylvanian cycles driven by high-frequency, high-amplitude glacio-eustasy, which are commonly characterized by subaerial exposure surfaces developed on subtidal deposits, except for the absence of peritidal facies and the presence of low-relief algal bioherms on the platform top. The necessary accommodation to enhance the growth of bioherms on the platform top was probably due to high-frequency (100-240 ky) moderate-amplitude (nearly 40 m) sea-level fluctuations combined with subsidence rates of 70-140 m/My. Tectonic subsidence might have been responsible for the lack of evidence of subaerial exposure surfaces in the examined Moscovian strata. The studied inner-platform lithofacies character and architecture were controlled by the interplay of high-frequency changes in accommodation and the presence of meter-scale depositional topography attributed to lateral variations of carbonate accumulation.

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