Early diagenetic alteration of shallow-marine mixed sandstones: An example from the lower eocene Roda sandstone member, Tremp-Graus basin, Spain

Abstract

A Lower Eocene shallow-marine sandstone complex, composed of calcareous quartzose arkoses, was modified partly by diagenesis relatively shortly after deposition. The sandstone complex forms the lower member of the Roda Formation, which represents part of the Palaeogene fill of the Tremp-Graus Basin in the southern Pyrenees, Spain. Early diagenesis comprised the introduction of matrix into the sandstone framework through mechanical infiltration and bioturbation, and the precipitation of a, mainly aragonitic, rim-cement. This early diagenetic modification occurred in the uppermost parts of sandstone bodies, beneath abandonment surfaces. The early cement stabilized the sandstone framework, and counteracted mechanical compaction. In sandstones lacking this early cement, on the other hand, mechanical compaction severely reduced the primary porosity during a first burial phase, causing the development of a thightly compacted fabric. Mechanical compaction was highly effective because of a high content of ductile grains. Afterwards, two major erosional phases truncated the sandstone complex in the northern part of the area studied. Probably through the introduction of meteoric water, unstable carbonate phases, such as aragonite and high-Mg calcite, were replaced by low-Mg calcite, whereas a contemporaneous second calcite cement generation was precipitated. After the erosional phases, the sandstones were buried once again until recent uplift and denudation brought them to the surface.