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Evidences of salt tectonics around the Guadalquivir bank, Gulf of Cádiz (SW Iberia)

Abstract

The geology of offshore southwestern Iberia can be summarized into:

  1. 1.Pre-Cambrian / Paleozoic basement,
  2. 2.Triassic - Early Jurassic rifting,
  3. 3.Mesozoic platform passive margin,
  4. 4.Paleogene fold-thrust belt (Olistostrome chaotic unit) and
  5. 5.Neogene sedimentary cover.

During the Quaternary, an important number of submarine mounds have been generated around Gulf of Cádiz by halokinesis (Late Triassic – Early Jurassic evaporites) and argilokinesis (over-pressured Miocene shales). The Guadalquivir bank located in the Algarve basin covers an area of approximately 40 km2 (250 – 300 meters water depth) and it has been traditionally interpreted as a basement - Mesozoic outcrop in agreement with dredged samples containing lithological associations ranging from Paleozoic to Upper Miocene ages. This bank lies above the northeast - southwest Guadalquivir – Albufeira basement structural high running for more than 100 kms along the westernmost side of Gulf of Cádiz.

Preliminary analysis of similar submarine mounds around the area suggested that they correspond to halokinetic features associated with salt-diapirs triggered from Late Triassic - Early Jurassic evaporites remobilized during the Late Tertiary – Quaternary.

A possible halokinetic origin of the Guadalquivir bank is proposed here using 2D / 3D seismic lines interpretation. The resulting geological model has been calibrated with gravimetric and magnetic data suggesting that basement – Mesozoic units traditionally interpreted as in situ outcrops, would not reach the sea-bottom. The discrepancy between the in situ origin for the basement - Mesozoic outcrops along the Guadalquivir bank and our model could be explained considering at least, two hypotheses:

  • – Samples belong to Tertiary conglomeratic outcrops, or
  • – Samples belong to exotic blocks extruded by halokinetic processes.

Maximum horizontal stress (NNW-SSE) could be responsible of faults reactivation and basement uplift underneath tertiary units (“thick-skin” tectonics). Active halokinesis and argilokinesis processes are probably triggered by Quaternary stress field.

Acknowledgments: We acknowledge Partex Oil and Gas geosciences staff for strong interaction and fruitful discussions.