Architecture of the tectonically influenced Sobrarbe deltaic complex in the Ainsa Basin, northern Spain

Abstract

The syn-tectonic Sobrarbe deltaic complex (Eocene of the Ainsa Basin, Spanish Pyrenees) is confined by lateral thrust ramps and influenced by intra-basinal growth anticlines. Six facies associations have been distinguished within the deltaic complex: (i) slope marls and turbidite sandstones; (ii) distal delta front silty and bioturbated sandstones; (iii) proximal delta front and delta plain deposits (tidal inlet, tidal flat, mouth-bar, bay, distributary channel, and floodplain environments); (iv) biogenic deposits at flooding surfaces; (v) collapse zone sediments; and (vi) Nummulite-dominated shallow-marine carbonates. The deltaic complex comprises four composite sequences, which each contain a number of minor sequences of the genetic (maximum flooding surface bounded) type. Regressive unconformities and their correlative surfaces separate the composite sequences (CS), and each CS can be divided into lowstand, transgressive and highstand components. At the base, the Comaron CS is a WNW-prograding deltaic complex containing six minor sequences characterized by basinward-stepping sandstone wedges separated by transgressive mudstones. Double regressive unconformities are often associated with these sandstone wedges proximally in the basin. Growth of the intra-basinal Arcusa anticline caused gentle folding of the Comaron CS, and syn-growth strata at the base of the overlying Las Gorgas CS contain a prominent forced regressive sandstone wedge (the Las Gorgas sandbody) and display thinning of bedsets onto the crest of the anticline. Moreover, it can be demonstrated that relative sea-level rise took place in the growth synclines at the same time as a fall in relative sea level occurred on the growth anticlines. The tectonic deformation caused a shift in the direction of deltaic progradation towards the north-northwest, as demonstrated by the well-developed clinoforms in the highstand sequence set of the Las Gorgas CS. A second phase of tectonic activity, this time triggering large-scale collapse of the delta front, occurred at the transition between the Las Gorgas and Barranco el Solano CS. Tilting of the basin floor led to the development of coeval transgressive and regressive strata separated by a fulcrum (line of zero net-vertical movement of the shoreline). In the post-tilt phase, reduced sediment supply and accelerated subsidence led to widespread transgression and a change from clastic- to carbonate-dominated sedimentation. This phase was terminated by a gradual increase in sediment supply causing progradation of a mixed clastic–carbonate system dominated by large Nummulite banks and bioturbated shoreface marls and sandstones. Finally, a pronounced fall in relative sea level associated with the transition to the Buil CS triggered slope instabilities and eventually established fluvial conditions all over the study area. With the exception of the anticline crests, the relatively high rate of tectonic subsidence in the study area suppressed relative sea-level falls, resulting in development of discontinuous lowstand strata and bounding surfaces. The segmented nature of many regressive unconformities may be related to incremental growth of thrust-related structures, and each segment is separated from neighbouring segments by gaps of limited vertical and lateral displacement. Precursors of progressive unconformities are seen in the upper proximal part of the Comaron CS by downdip `fanning' and updip convergence of minor sequences. These differentially tilted stratal units provide reliable indicators of syn-sedimentary tectonism.