First-day road log: From Socorro to Magdalena, Datil, western Crosby Mountains, Sawtooth Mountains, Pie Town, Qemado, Quemado Lake
— Richard M. Chamberlin, Steven M. Cather, William C. McIntosh, Orin J. Anderson, and James C. Ratte

The First Day tour emphasizes the Cenozoic stratigraphy and structure of the Mogollon slope in northeastern Catron County. Gently south-dipping Eocene and Oligocene orogenic to volcanogenic strata as much as 1.3 mi thick mark the downwarped (loaded) southeastern margin of the stable Colorado Plateau microplate, referred to here as the Mogollon slope . US-60 west from Socorro generally follows strongly to moderately extended topographically lower terrain of the San Agustin arm of the Rio Grande rift until it crosses onto the Mogollon slope at the Red Lake fault zone west of Datil. The San Agustin arm marks a complex transition zone from rift to plateau; thus US-60 crosses several north-trending extensional basins and distended horsts between Socorro and Datil.

Stop 1 west of Datil provides an overlook of the Red Lake fault zone, a narrow NNE- trending belt of Neogene extension that locally cuts across the Mogollon slope. This mildly distended belt was probably guided by weak lithosphere below a zone of earlier Laramide transpression. The massive south-sloping core of the Datil Mountains visible to the west of Stop I locally defines an edge of the gently depressed Mogollon slope.

Stop 2 permits a hands-on look at upper Eocene ignimbrites of the Datil Group and intercalated volcaniclastic sedimentary apron deposits of the middle Spears Group that are well exposed on the west flank of the Crosby Mountains. Both epiclastic braided-stream deposits and syneruptive tuffaceous sandstones deposited by an overloaded stream system are locally well exposed here at Saulsberry Ranch.

Today's third and lunch stop is near Monument Rock in the eastern Sawtooth Mountains. Bold cliffs to the northeast of Monument Rock provide spectacular views of intense soft sediment deformation within the upper Eocene Dog Springs Formation. Intensely folded andesitic sandstones and upturned andesitic debris-flow deposits visible in these cliffs may reflect a seismically triggered regional liquefaction event of late Eocene age.

Stop 4 is along the late Oligocene Pie Town dike, where it forms a large wall-like outcrop at Bright Lake south of Pie Town. Country rocks of Eocene andesitic sandstone and the chilled margin of this basaltic-andesite dike are well exposed here. This great dike, approximately 45 mi long, indicates regional extension in late Oligocene time was to the WSW. From Pie Town the route follows US-60 west to Quemado and then south along NM-32 to the Apache National Forest and the Quemado Lake Recreation Area. Following a catered barbecue we will camp out below the tall ponderosa pines at the old Quemado Lake campground in the valley southeast of the reservoir.

The first 43.8 mi of this road log is modified and updated after the road log of Chapin et al. (1978), which emphasized then-newly-discovered calderas and their relationship to wellknown mining districts in the Mogollon-Datil volcanic field. This log and following road logs adopt the Tertiary stratigraphic nomenclature proposed by Cather et al. in this guidebook. Road logs also use 33 .4 Ma as the Eocene-Oligocene boundary following the lead of McIntosh et al. (1992); this boundary was previously placed at 36.6 Ma (Palmer, 1983). Ages of Tertiary ignimbrites and other volcanogenic units are from McIntosh et al. (1991, 1992) and papers in this guidebook unless otherwise specified.