Abstract: Intra- and Extrabasinal Controls on Mid-Tertiary Sandstones and Conglomerates, Jaca Basin, Northern Spain

Larry T. Middleton, Janae Wallace

The Jaca Basin in northern Spain contains thick sequences of non-marine siliciclastics and carbonates. Three units of Oligocene to Miocene age; the Petilla Member of the Campodarbe Formation, the Bernues Conglomerate and the Uncastillo Formation record major changes in alluvial architecture and the result of intrabasinal thrusting, and geomorphic partitioning of numerous subbasins.

The Petilla is up to 2000 m thick and exhibits sheet-like geometries. It consists of hybrid sandstones deposited in broad ephemeral channels and in proximal channel areas on levees and crevasse splays. The overlying Bernues Conglomerate is up to 2000 m thick and consist of well-rounded clasts up to 50 cm. These clast-supported conglomerates were deposited in a rapidly subsiding basin associated with movements on numerous thrusts. Syndepositional deformation is common. The Bernues conglomerate accumulated on coalescing alluvial fans and adjacent braid plains. The overlying Uncastillo Formation is up to 700 m thick and is comprised of ribbon-like hybrid sandstones encased in mudstones. Deposition occurred in high sinuosity channel complexes. The ribbon-like geometries of the Uncastillo re a result of the increased input of fine-grained sediments coupled with higher subsidence rates relative to the Petilla and adjustments to syndepositional thrusting.

Paleocurrent patterns, petrofacies distributions and locations of thrust within the Jaca Basin indicate deposition in a series of geomorphically partitioned subbasins. Tectonic partitioning of this thrust-top basin is evidenced by differential rates of subsidence for the three units and syndepositional thrusting.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California