Thrust-Generated, Back-Fill Stacking of Alluvial Fan Sequences, South-Central Pyrenees, Spain (La Pobla De Segur Conglomerates)

  1. L. E. Frostick3 and
  2. R. J. Steel4
  1. D. Mellere

Published Online: 16 APR 2009

DOI: 10.1002/9781444304053.ch14

Tectonic Controls and Signatures in Sedimentary Successions

Tectonic Controls and Signatures in Sedimentary Successions

How to Cite

Mellere, D. (1994) Thrust-Generated, Back-Fill Stacking of Alluvial Fan Sequences, South-Central Pyrenees, Spain (La Pobla De Segur Conglomerates), in Tectonic Controls and Signatures in Sedimentary Successions (eds L. E. Frostick and R. J. Steel), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304053.ch14

Editor Information

  1. 3

    Reading, UK

  2. 4

    Bergen, Norway

Author Information

  1. Dipartimento di Geologia, Geofisica e Paleontologia, Via Giotto 1, Universita' di Padova, Italy

  1. Geologisk Institut, Allegaten 41, Universitet i Bergen, 5007 Bergen, Norway

Publication History

  1. Published Online: 16 APR 2009
  2. Published Print: 28 FEB 1994

ISBN Information

Print ISBN: 9780632037452

Online ISBN: 9781444304053



  • thrust generated back-fill stacking of alluvial fan sequences;
  • La Pobla de Segur conglomerates;
  • the Pessonada allogroup;
  • the Ermita allogroup;
  • the Senterada allogroup;
  • structural disposition of clastic groups


La Pobla de Segur Conglomerates (Collegats Formation) represent a Late Eocene and Oligocene alluvial fan complex developed within an intermontane basin, in the south-central part of the Pyrenean Chain, to the south of the Axial Zone antiformal stack (Nogueres). The conglomerates onlap a previously deformed substratum of Mesozoic and Palaeozoic rocks. They are organized into a series of stacked wedge-shaped bodies reaching a cumulative thickness of about 3500 m. In a vertical succession, five main allogroups (Pessonada, Ermita, Pallaresa, Senterada and Antist), comprising more than 20 alluvial fan lobes have been recognized. The allogroups are distinguished on the basis of underlying major unconformity surfaces, clast composition and palaeocurrent patterns. The structural relationship between successive alluvial fan lobes and their substratum has allowed the establishment of a chronology of deformation in this thrust-controlled basin. Each of the five allogroups can be tied to a specific stage of structuring.

The basal Pessonada allogroup is composed of small, local alluvial fans deposited at the southern active margin of the basin. Relationships between tectonic activity and the contemporaneous sedimentation have demonstrated a break-back thrusting sequence. The Ermita allogroup consists of units representing small fan-deltas which prograded into a shallow lake. The widespread fine-grained materials (floodbasin and lacustrine) suggest a period of initial subsidence linked to the onset of deep thrusting along the northern margin of the basin. The basinwards progradation of the polymict alluvial fan conglomerates of the Pallaresa allogroup define the third stage. Tectonic activity had shifted towards the northern margin of the basin and was characterized by the emergence of backthrusts related to the paroxysmal growth of the adjacent Nogueres structure. The imbricated system of backthrusts led to the development of a minor basin to the north, whose infill (Senterada allogroup) defines the fourth stage: alluvial units, bounded by tectonic structures, are deformed in progressive unconformities demonstrating their contemporaneous deposition with emplacement of the backthrusts. Only the youngest alluvial deposits of the Antist allogroup are not deformed and record the cessation of activity in the Nogueres Zone (fifth stage). The general north–south geometry of the Collegats Formation shows a stepwise, backfilling pattern of deposition for the allogroups. There was a northward migration of the sedimentary depocentres as a response to deep seated thrust emplacement along the northern margin. The Collegats Formation, rather than being merely post-tectonic infill, has been shown to be Late Eocene–Oligocene syntectonic deposits.