The distinctive character of lumpy limestones (Early Jurassic, Lusitanian Basin, W Portugal)

Abstract

Ephemeral carbonate facies presenting restricted geographical and temporal occurrence hold the key to complete the knowledge of past environmental conditions. Far from representing environments widespread throughout the geological history, ephemeral lithofacies embody equally relevant but more unusual scenarios. This is the case of the lumpy limestones, a puzzling facies encased on a worldwide‐relevant stratigraphic section of the Portuguese Jurassic archive but still lacking a deeper understanding of the processes leading to their development. An extensive revision and update of the singularities of the lumpy limestones is presented, based on a systematic description of the lithic elements occurring in three sections. These represent different positions along the Pliensbachian (Early Jurassic) paleoplatform of the Lusitanian Basin (Western Portugal). The lumps were analysed regarding their abundance, distribution, appearance, and degree of sorting, based on field observation, macro‐ and microfacies inspection. Apart from shared stratigraphic trends, which highlight prevailing paleoenvironmental conditions during the Early Jurassic times, the main differences in spatial distribution across the Lusitanian Basin are also discussed, taking into account the specific paleogeographic setting of each studied locality. The presence of lumps but also absence is considered relevant, as for each case a specific paleoenvironmental context and/or syn‐ to post‐depositional processes can be assigned. A new lump classification scheme is brought forward, accounting for the high variety identified in lump architecture and aspect. A depositional model offering alternative paleoenvironmental scenarios is presented for the development of the variety of lump types identified, combining physical (mechanical) and biological processes involved in the occurrence and distribution of this facies. Depending on the presence/absence of lumps, lump type, abundance, and distribution, these scenarios include considerable fluctuations in oxygen availability and bottom current intensity, which affect deposition/accumulation, bioturbation, and related syn‐sedimentary conditions. Physical and mechanically dominated processes driven by sedimentary instability and/or heterogeneity are also introduced. A dynamic depositional framework is presented, clarifying the depositional to late‐diagenetic evolution accounting for the singularity of the lumpy limestone facies in the context of the Lower Jurassic in the western Iberia margin.