Geometry and structure associated to gas-charged sediments and recent growth faults in the Ebro Delta (Spain)

Abstract

In the Ebro Delta, NE Spain, small- to large-scale synsedimentary deformation has been observed by means of high- and low-resolution seismics correlated with a network of wells. Lithoseismic units involved are middle–upper Pleistocene clays and gravels and Holocene deltaic sequence. Gassy sediments and large river-floor pockmarks are identified from seismic profiles along the Ebro Delta. Gas-charged sediments are identified by zones of acoustic turbidity and other acoustic anomalies on seismic profiles. Most of the gas is believed to be biogenic in origin, resulting from the decay of organic matter contained within rapidly developing shelf margin deltas. A group of large symmetric river-floor pockmarks also occurs along the inner delta plain, both on the river-floor surface and buried in the sediment column. Listric fault geometry is a typical scope-shaped plane with a steep upper surface (40–60°) that passes downward into a horizontal shear plane. The shear plane appears seismically as chaotic and convoluted high-amplitude reflectors and is correlated with well data showing layers that include gravel, sand and clays. Associated structural features include reverse drag of reflectors, antithetic faults, and fluid escapes (methane/water). The growth faulting along the Ebro Delta includes a break-away and thin, near-surface layer created by overstepping of a sandy Holocene delta slope. The over-steeping lies down-dip from growth faults along the shelf-break. The overall growth faults province appears to represent a deeper-seated general movement of the delta mass involving differential loading, overpressure of fluids, diapiric movements, and extensional tectonics that cause listric faults and associated structures at different scales. The recent growth faults are regularly spaced, with the distance between successive faults controlled by the depth to the detachment layer. Three main fault systems have been identified: A first or primary system spaced 5.2–10.5 km above a detachment layer at 55–95 m deep; a second system spaced 3.5–6 km; and a third system spaced 1.5–2.5 km above a detachment layer at 25–30 m. The slope angle calculated by empirical equations ranges between 0.5° and 1.0° for the three systems.