RECONNAISANCE PETROLOGY OF THE LATE CRETACEOUS MCDERMOTT FORMATION, SW COLORADO

The McDermott Formation is a Late K/Early Tertiary volcaniclastic sedimentary unit exposed along the northwestern margin of the San Juan Basin. It overlies the Kirtland Formation unconformably and underlies the Animas Formation, with which it intergrades. The unit has a maximum thickness of 250 feet, and consists of conglomerate, sandstone, and mudstone, including abundant debris avalanche deposits. Conglomerate units are mostly composed of grey, porphyritic-aphanitic trachyandesite. Phenocrysts include sericitized plagioclase, clinopyroxene, altered hornblende, opaques and minor biotite and apatite. We studied six samples selected for variability in appearance from three sections: five trachyandesites are benmoreites and one a latite, according to the TAS classification. Only two samples show normative quartz when allotted 15% of iron as Fe203; one sample is nepheline normative.

A sample/primitive mantle spider diagram for McDermott samples shows an orogenic signature as evinced by positive Ba and negative Nb anomalies. K/Ba vs. Ba suggests mixing of a high Ba source to lower K/Ba. One McDermott sample contains over twice the amount of Ba as any other sample. The very high Ba content may be the result of assimilation of felsic material, as indicated by the presence of xenocrystic K-feldspar and quartz, and normative quartz in its norm.

Comparison of our data with published analyses from the Allard Stock in the La Plata Mountains is consistent with derivation of the McDermott material from an eroded volcanic center related to the La Plata intrusions, but additional research will be necessary to develop this hypothesis.