Page 26 - Geologic Investigations in the Lake Valley Area, Sierra County, New Mexico

Rubio Peak. In some places, tuff of the Sugarlump Tuff lies directly on jasperoid. At no place is the volcanic rock altered, indicating that the contacts with jasperoid are of neither intru­ sive nor hydrothermal replacement origin. These relationships date the jasperoid as pre-Rubio Peak.
The Sugarlump Tuff unconformably overlies Rubio Peak Formation on a surface of low relief throughout the map area. In the upper reaches of Jaralosa Creek, the outcrop of Sugar- lump crosses the contact between flows and tuffaceous rocks of the Rubio Peak. Evidently, the Rubio Peak Formation was tilted slightly and eroded before deposition of the Sugarlump. Epiclastic rocks are not abundant at the base of the Sugarlump. On the west side of Berrenda Mountain, the Sugarlump Tuff rests directly on about 9 m of interbedded tuff and breccia. These deposits appear to be largely or entirely volcaniclastic in origin and do not indicate extensive erosion of the underlying volcanic rocks.
A landslide megabreccia composed largely of fragments of Kneeling Nun Tuff and Sugarlump Tuff unconformably overlies
these formations and the Rubio Peak Formation at McClede Mountain (fig. 5). Fragments ranging up to tens of meters in size evidently slid off a west-facing escarpment in the Sugar- lump and Kneeling Nun Tuffs. Scarps are exposed north and south of McClede Mountain, where megabreccia and the overly­ ing basal tuff of the Mimbres Peak Formation abut Kneeling Nun and Sugarlump Tuffs along a steep, discordant surface. Foliation in some of the largest blocks of tuff is nearly horizon­ tal, indicating little rotation during sliding. The megabreccia has been described in detail by Elston (1989), who interpreted it as a landslide deposit formed outside the Emory cauldron during collapse and erosion of the caldera wall. The landslide and asso­ ciated scarp are part of the erosional surface on which the over- lying Mimbres Peak Formation was deposited.
Note: A combined list of references for all chapters of this Professional Paper is located at the end of this volume, beginning on page 76.
Geology of the Lake Valley Area 17