The Description and Origin of Intraformational Folds Near Tres Ritos and Holman Hill, New Mexico
D'Lugosz, Joseph J
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Intraformational folds in Pennsylvanian rocks (Sandia and Madera Formations) near Tres Ritos and Holman Hill, New Mexico, are found to be of different origins. Folds near Tres Ritos are characterized by relative uniformity of axial directions (N 15 E), parallelism of fold axes and the trend of a high-angle reverse fault, eastward vergenz (away from the fault), and their presence only on the east side of the fault. These relationships are interpreted as indicating that the folds are formed under lateral stress related to reverse faulting. Folds near Holman Hill are characterized by uniformity of axial directions (N 12 '.A , two directions of vergenz (SW-NE), detachment from enclosing nonfolded beds along shear planes, parallelism of enclosing nonfolded beds, and, after rotation of fold axes and enclosing nonfolded beds, parallelism of fold axes and the strike of the enclosing nonfolded beds. These relationships are interpreted as suggesting that the folds were formed by movement, under gravity, down a southwest paleoslope of approximately 8 or less. If the fault near Tres Ritos is of Laramide age, as suggested by Killer, Montgomery, and Sutherland (1963), then the folds there are also of Laramide age. The time of folding near Holman Hill could not be ascertained because the age of the proposed southwest paleoslope is unknown. A comparison of folds in the two areas shows characteristics reflecting differences in fold mechanisms, in lithology, and in bed thickness. Two directions of fold vergenz at Holman Hill, compared -co one direction at Tres Ritos, is the most conspicuous difference in fold styles of the two areas. The same sequence of Pennsylvanian sedimentation occurred in each area. After consolidation of these sediments folding occurred first at Holman Hill and then later at Tres Ritos. Both areas were then affected by northwest regional tilting of approximately 4, which probably took place in late- to post-Laramide time.