Abstract

In the southeastern Ebro basin of Spain, analysis of sequential fold history in combination with stacking patterns of conglomerate deposited in a growing syncline demonstrates the impact of local uplift attendant upon fold growth on nonmarine depositional systems in a foreland basin. The Penyagalera syncline is part of a belt of asymmetric folds developed in proximal Paleogene foreland-basin deposits by foreland-directed propagation of a detachment beneath the basin. Foreland-basin strata form a progradational succession of interbedded siltstone, sandstone, and evaporite overlain by conglomerate. Stratal stacking patterns and fold history indicate that conglomerate of the syncline marks the transition within the foreland-basin succession from foredeep to wedge-top depositional zones. The conglomerate is divided into three informal members separated by unconformities. The members are divided in turn into lithosomes separated by prominent bounding surfaces. Lithosomes represent depositional elements deposited by episodic progradation of alluvial-fan lobes. Older conglomerate members are more tightly folded than younger members, demonstrating that folding accompanied conglomerate deposition.

The uplift of opposing limbs of the Penyagalera syncline was diachronous. The asymmetric syncline comprises an overturned interior (southeastern) limb and a gently dipping exterior (northwestern) limb. The interior limb rotated to a steep to slightly overturned attitude before significant rotation and uplift of the exterior limb took place. Rotation of the interior fold limb was episodic and punctuated by periods of erosion of older conglomerate. Uplift rates during rotation events along the interior limb generally were more rapid than sediment-accumulation rates and caused sediment bypass to the foreland basin; however, sediment-accumulation rates overtook uplift rates during late limb rotation, causing alluvial-fan lithosomes to onlap previously formed unconformities. Offlap, signifying a slight excess of uplift over sediment-accumulation rates, is not observed in the syncline. On the exterior limb, sedimentation rates exceeded early limb-uplift rates, but the relationship of later folding and deposition is not preserved. Most synclinal closure postdated the youngest preserved lithosomes in the fold. Young lithosomes occupy a paleocanyon incised deeply into the interior limb of the syncline and Mesozoic strata of the thrust belt. The paleocanyon was backfilled during late aggradation of the Ebro basin, an event independent of local folding.

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