Sedimentary basins formed and carried piggyback on active thrust sheets

Small sedimentary basins that formed on the Apennine (southern) margin of the Po basin complex, northern Italy, have been studied using borehole and seismic-reflection information, and their evolution can be dated using the largely marine biostratigraphy of the Miocene to Quaternary sediments. These studies show that the basins formed and were filled while being carried on moving thrust sheets, and they are therefore named piggyback basins. Each of these basins corresponds to the active phase of a different thrust-front ramp. On the Pyrenean (northern) margin of the Ebro basin complex, northern Spain, similar piggyback basins formed between Eocene and mid-Miocene time. They have since been uplifted and eroded and may therefore be studied in outcrop. In northern Spain, much of the basin-fill sediment was deposited in nonmarine environments.