Structural analysis and stratigraphic problems of Precambrian rocks of the Picuris Range, New Mexico

Abstract

Precambrian metasedimentary and amphibolite rocks of the Picuris Range have formerly been divided into the Ortega Group and the overlying Vadito Group. We present the results of a structural analysis of a small area straddling the contact between these two groups. Younging criteria and structural facing relationships show that the Vadito Group, as formerly defined, together with the Piedra Lumbre Formation of the Ortega Group, is an overturned sequence in fault contact with the remainder of the Ortega Group. The relative ages of the two groups are unknown, and their determination depends on interpretation of regional structure and structural history. We define four possible structural models, each of which would favor, but not require, reverse slip on the boundary fault, suggesting that the Vadito Group is probably in part or entirely older than the Ortega Group. Possible models for development of the observed geometry are: (1) rotational fault in major second generation folds; (2) a folded and faulted angular unconformity between the two groups; (3) major refolding of an earlier nappe-like structure; and (4) minor refolding and faulting of major first generation folds.