Page 25 - Geologic Investigations in the Lake Valley Area, Sierra County, New Mexico

Jasperoid
Figure 9. Hills of jasper­ oid in Fusselman Dolo­ mite. Unaltered Trp (Rubio Peak Formation) and Ts (Sugarlump Tuff ) overlie exhumed jasper­ oid hill. View looking east across State Highway 27 near Jaralosa Creek.
Ts
Trp
faults were mapped mainly in Paleozoic rocks, they cut outliers of Rubio Peak Formation on Apache Hill. Most of the faults could not be traced east or west of Apache Hill, but the two long­ est faults were traced across Quartzite Ridge. A large east-trend­ ing fault north of Apache Hill extends west across Quartzite Ridge and displaces a splay of the Berrenda fault zone and the Rubio Peak Formation. Slickenlines measured on Quartzite Ridge indicate both dip-slip and strike-slip movement (fig. 8C). A second east-trending fault south of Apache Hill crosses Quartzite Ridge and merges with or abuts the main strand of the Lake Valley fault system.
The origin of the east-tending faults is problematic. Their trend, as well as that of the adjacent southern segment of the Lake Valley fault system, is subparallel to the trend of Laramide faults postulated nearby by Seager and others (1986), suggesting the possibility that the east-trending faults may have first origi­ nated during Laramide thrusting. However, the observed offset of the Rubio Peak Formation and of the Berrenda fault zone reveals major movement on east-trending faults during Oli­ gocene or later extension. The easterly trend may result from stress accommodation where the Berrenda fault zone ends, or it may represent development of en echelon fractures along the Lake Valley fault system, or it may have yet another explanation, unidentified by us. Other east-trending faults cut the northern segment of the Lake Valley fault system; their origin is also not understood by us.
Eocene and Oligocene Unconformities and Paleotopographic Features
Unconformities underlie and separate volcanic and sedimentary rocks of Eocene and Oligocene age. In ascending order, unconformities occur at the base of the Rubio Peak
Formation (and, where present, the Love Ranch Formation), the base of the Sugarlump Tuff, and the base of the Mimbres Peak Formation and the base of the interlayered volcanic rocks of McClede Spring. Other, local unconformities may also occur in the complexly intertongued volcanic rocks of McClede Spring and Thurman Formation.
Unconformities at the base of the Love Ranch and Rubio Peak Formations may be of various ages, but they are discussed together because they cannot be distinguished in the map area. As reported by Seager and others (1986), the Love Ranch For­ mation overlies the Lake Valley Limestone north of Lake Valley townsite and west of the north end of Quartzite Ridge; it is in turn overlain by Rubio Peak Formation at these localities. At most localities, however, the Rubio Peak directly overlies rocks of Paleozoic age.
North of O Bar O Spring, in the western part of the map area, laharic breccia and flow rocks of the Rubio Peak Forma­ tion rest directly on limestone of the Pennsylvanian Magdalena Group. However, evidence of residual weathering prior to dep­ osition of the Rubio Peak was observed locally. As seen in a gully cut by an ephemeral stream, surfaces on the Magdalena limestone beneath the Rubio Peak are coated with thick argilla­ ceous deposits; cracks in limestone are likewise filled with the deposits, and unstratified breccia immediately overlying the limestone contains red matrix and abundant fragments of lime- stone as well as andesitic rock. Love Ranch Formation mapped there by Seager and others (1982) was found to consist of poorly exposed remnants of loosely consolidated gravel resting on Rubio Peak Formation.
Northeast of the junction of Jaralosa Creek and State Highway 27, paleohills of jasperoid in the Silurian Fusselman Dolomite, locally overlain by Rubio Peak and Sugarlump For­ mations, have been exhumed by present-day erosion (fig. 9). Jasperoid hills stand as much as 30 m above the base of the
16 Geologic Investigations in the Lake Valley Area, Sierra County, New Mexico