During Upper Jurassic to Lower Cretaceous rifting associated with the opening of the North Atlantic, the Cameros Basin formed on the northwest margin of the Iberian Trough. During rifting, reactivated Late Hercynian faults controlled sediment distribution and architecture. The basin fill consists of > 6,000 m of fluvial and lacustrine sediments, with occasional marine influence, comprising six unconformity-bounded depositional sequences. The siliciclastic Cuerda del Pozo Formation, 800-1200 m thick and corresponding to the fourth depositional sequence, is built of sandstone and mixed conglomerate-sandstone channel bodies of various kinds interbedded with floodplain mudstones. It was deposited in a complex fluvial system with multiple, generally low-sinuosity channels passing northeastward into fluvial and lacustrine environments. We analyze the fluvial architecture of the Cuerda del Pozo in the central part of Cameros Basin, where the fluvial system is transitional between proximal and distal facies. Three hectometer-scale sequences showing changes in depositional architecture are recognized. Changes in kind, density., and degree of interconnection of the channel bodies are controlled by subsidence rate in relation to frequency of channel avulsion. The observed coarsening-upward trend is interpreted as reflecting a progradational event.

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