Relationship of Clay Minerals to Depositional Environment in the Non-Marine Eocene Pontils Group, Se Ebro Basin (Spain)

Abstract

ABSTRACT The Eocene sequence along the central part of the SE margin of the Tertiary Ebro Basin (Igualada area) consists of alternating marine and non-marine deposits up to 2000 m thick. Non-marine conditions persisted through the Cuisian--early Bartonian (early-middle Eocene) and resulted in the deposition of the Pontils Group (up to 900 m thick). During the late Bartonian--early Priabonian (late Eocene), marine environments prevailed in this area, leading to the deposition of the Santa Maria Group. Paleoenvironmental interpretations of the Pontils Group were obtained from sedimentology data of an earlier study. The nonmarine environments recorded in this group are: alluvial fans, flood plains, mudflats, playa-lakes, and shallow freshwater lakes. X-ray diffraction, TEM, SEM, EDX, and chemical analyses of mudstones and marls from the Pontils Group indicate that the diverse environments also can be distinguished from one another on the basis of the clay-mineral content. Illite is present in all samples of the Pontils Group. Palygorskite, with minor amounts of illite and sometimes chlorite, is the dominant clay mineral in flood plain and mud flat sediments. Palygorskite is also predominant in sabkha, playa-lake and carbonate-lagoonal sediments. This clay mineral is absent in sediments from fresh, organic-rich lakes. Kaolinite is relatively abundant in mudstones from the organic-rich lakes. Smectite is relatively abundant in the sediments from subaqueous environments but is almost always absent from the sediments which were subjected to subaerial exposure and water-table oscillations such as flood plain and mudflat deposits. According to these clay-mineral distribution trends, the organic-rich lacustrine, lagoonal, and marine sediments preserve the detrital clay-minerals. The original detrital assemblages were mostly formed by illite, associated with chlorite, kaolinite and subordinate smectite. In the environments which underwent repeated flooding and desiccation, chlorite and smectite were eventually eliminated and palygorskite was formed. Microscopic observation suggests that palygorskite was formed in mudflats and flood-plains mainly by the transformation of smectite and/or illite in a Mg-rich environment. In the sabkha facies, the abundance of palygorskite, frequently associated with smectite, dolomite, and chert, suggests palygorskite neoformation in a Si- and Mg-rich environment.