Palustrine sedimentation and associated features—grainification and pseudo-microkarst—in the Middle Miocene (intermediate unit) of the Madrid Basin, Spain

Abstract

A rather thick sequence (50–75 m) of lacustrine limestones forms the uppermost part of the Intermediate Unit (Middle Miocene) in the NE part of the Madrid Basin, central Spain. A gradual change from distal alluvial to open lake facies is recorded in this area. Red mudstones passing vertically to strongly mottled limestones are typical of the transition from distal alluvial to marginal lake deposits. A wide variety of facies occur in the marginal lacustrine facies association, all of them showing pedogenic features (e.g. root traces, alveolar septal structures, pseudo-microkarst). Micrites and biomicrites are interpreted as open lacustrine deposits. Although not as strongly pedogenically modified as the marginal lacustrine facies, they still show evidence of subaerial exposure. The carbonate sequence was deposited in a low-gradient, shallow-lake system in which slight fluctuations in the water level caused extensive emergence. Pedogenic processes affected most of the lake deposits, but modification was more intense in the marginal facies. Biogenic activity and mechanical reworking at lake margins led to the formation of pedogenic-diagenetic pseudo-microkarst and peloidal and/or intraclastic limestones.