Oriented Concretions, Ionian Coast, Italy: Evidence of Groundwater Flow Direction

Abstract

ABSTRACT Highly elongate, aligned, calcite-cemented concretions are present in exposed shallow-marine Pleistocene sands and gravels along the Basilicata coast of the Ionian Sea, southern Italy. Concretions are shaped like rods (pencils and cigars, 0.5-3 cm in diameter) and thin blades (031 cm thick and 1-5 cm wide). Most have aspect ratios greater than 5 and some reach 30. Most concretions are in parallel-laminated beds, but a few are in cross-stratified and ripple-cross-laminated beds. Concretions cut across foresets and inclined laminae. The standard deviations of long axes at each of six localities with more than eight measurements are less than 7° the standard deviation of the orientation of concretions from 30 localities over a 400 km2 area is less than 17°. Previous interpretations for elongate carbonate concretions in sandstones include growth in the direction of groundwater flow, growth parallel with current-oriented grains and/or paleocurrents, and selective cementation in the troughs of wave ripples. The Basilicata concretions are oriented perpendicular to the local coastline and parallel with the direction of present-day groundwater flow. The groundwater flow direction was probably the same during the Pleistocene. The evidence indicates that the concretions grew parallel with groundwater flow and were not influenced by sedimentary structures or grain fabric. The isotopic values (13C and 18O) range from 10 to -1 [PDB] and from -5 to -3 [PDB], respectively, and textures of calcite cement in the concretions indicate that the calcite was precipitated from meteoric groundwater. Nannobacteria may have mediated precipitation of calcite.