Deciphering the history of hydrologic and climatic changes on carbonate lowstand surfaces: calcrete and organic-matter/evaporite facies association on a palimpsest Middle Jurassic landscape from Portugal

• Complex palimpsest exposure record preserves paleoclimate and hydrological changes.
• Calcrete/organic-matter/evaporite facies association exhibiting high variability
• Comparisons with modern and ancient coast marginal carbonate lowstand deposystems

The unusual occurrence of calcretes and prominent organic matter in the Middle Jurassic (Lower Bathonian, Serra de Aire Formation) of the Lusitanian Basin of western Portugal (Western Iberian Margin) revealed a complex palimpsest exposure record, here interpreted as reflecting hydrological changes caused by phases of emergence and immersion. It serves as a potential model for understanding stratigraphic development at lowstand surfaces in carbonate successions. The exposure-dominated facies association grades upwards into peritidal and lagoonal limestones, and the interval is assigned to the regressive peak of a Transgressive-Regressive Facies Cycle (2nd order) of the thick Middle Jurassic carbonate ramp succession.The Galinha Quarry, Fátima region, NE of Lisbon, a type section for this lowstand assemblage, exhibits varied calcretes, with black-clasts, interbedded with, and grading into: organic-rich marly/clayey seams and lenses, locally with carbonate nodules; carbonates with evaporite traces; microbial laminites; black-clast and fenestral limestones; some lithofacies are dolomitized. The palynofacies contains phytoclasts associated with less refractory, more prone to degradation components, which suggests natural combustion/pyrolysis (wild fires). The lowstand surface represents a low relief landscape with small depressions/ponds bordering a more distal marginal-littoral setting; the partly subaerial and partly subaqueous settings were subjected to lengthy exposure and to fluctuating, very shallow water bodies and water table. Coeval climatic regime was a seasonally dry/wet one, with dry/semi-arid phases dominating over the sub-humid, as shown by the combined record of intense calcrete development, rhizogenic structures, microbial mats, brecciation, desiccation, evaporites and wild fire evidence. However, sea level rise caused changes to shallow, sea-water influenced restricted lagoonal-peritidal settings.Comparisons and differences with modern and ancient coast marginal carbonates are made to provide a guide to the variability in such lowstand deposystems in the stratigraphic record.