--> ABSTRACT: Evolution of a Lahar-Dominated Sequence: Cochiti Formation, Jemez Mountains, New Mexico, by Bruce W. Nocita; #91022 (1989)

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Evolution of a Lahar-Dominated Sequence: Cochiti Formation, Jemez Mountains, New Mexico

Bruce W. Nocita

The northern (proximal) facies of the Cochiti Formation consists mainly of very coarse-grained conglomerate deposits that crop out on the southern and southeastern flank of the Jemez Mountains, New Mexico. The Cochiti Formation in this area is interbedded with volcanic rocks of the Keres Group and ranges in age from approximately 13-6.5 Ma.

Sedimentary rocks in the study area are dominated by debris-flow deposits (lahars) and hyperconcentrated flood-flow deposits. Stream deposits comprise a minor fraction of this proximal facies association. Interbedded with the sediments are at least three volcanic flows and numerous pyroclastic units. Detailed stratigraphic analysis in the eastern portion of the study area, where outcrops are thickest, shows development of cycles of sedimentation following the emplacement of a volcanic flow unit.

The exposed stratigraphic thickness varies considerably from west to east. The westernmost exposures, in Paliza Canyon, have a maximum thickness of about 30 m. The eastern part of the study area near St. Peters Dome and Boundary Peak has exposures up to 110 m in thickness. The nature of the deposits also varies from west to east. In the western portion of the study area, about 41% of the sedimentary accumulations are stream deposits, which is greater than in the east. Volcanic flows are commonly overlain by stream deposits. Hyperconcentrated flood-flow deposits comprise 23% of the western exposures, and 36% are matrix-supported debris-flow deposits. The easternmost outcrops, on the other hand, are debris-flow dominated, with single deposits as thick as 20 m.

Previous workers have attributed westward thickening to faulting along the western margin of the Rio Grande rift; however, facies associations and paleocurrents do not support this interpretation. The proximal Cochiti Formation may represent not one but two main depositional sites.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.