Stratigraphy and sedimentology of the Miocene distal-alluvial and lacustrine deposits of Plana Negra-Sancho Abarca, Ebro Basin

The stratigraphic and sedimentologic study of Miocene deposits in the Plana Negra-Sancho Abarca area fills a gap in the Ebro Basin knowledge. Five main lithofacies were characterized and mapped. The boundary between regional tectosedimentary units T5 and T6 is recognized as a conformity at a major facies change. Unit T5, up to 130m thick, consists of mudstones with interbedded sandstones and limestones. Unit T6, with 30 to 62m of preserved thickness, is made of limestones, marls and minor mudstones. Correlation of the studied outcrops with nearby sections that already had detailed magnetoestratigraphic and biostratigraphic data allows dating the studied succession as Burdigalian (early Miocene)-Langhian (middle Miocene), from C5Dr to C5Cn, and the boundary between units T5 and T6 to be located at 16.062-16.099 Ma.

Seventeen sedimentary facies associated in seven vertical facies sequences document deposition in fluvial-distal alluvial, palustrine and lacustrine environments. A shallow (2-4m deep) lake body was locus for charophytes, ostracods, bivalves and gastropods to thrive. Occasional bottom currents reworked and transported allochems and fine-siliciclastic sediment offshore, forming laminated limestones. Bioturbation and desiccation features are abundant, indicating wide palustrine areas and episodic exposure. Low-sinuosity channels associated to the Pyrenean-derived Luna fluvial system run across the alluvial-plain.

Four evolutionary stages of alluvial progradation-retrogradation and retraction-expansion of the palustrine-lacustrine system, likely related to Pyrenean tectonic activity, are distinguished. Maximum lacustrine expansion took place at the beginning of unit T6 deposition. This is consistent with initial conditions of increasing humidity of the Middle Miocene Climatic Optimum (MMCO).