The geological and geoarchaeological significance of Cerro Pedernal, Rio Arriba County, New Mexico

—Cerro Pedernal is an isolated basalt-capped peak of substantial geologic and geoarchaeological significance. The peak is located on the Colorado Plateau near the boundary of the Rio Grande rift. Oligocene-Miocene stratigraphic units on Cerro Pedernal correlate to thicker sections within the rift and document initiation of rift-basin subsidence before 25 Ma. Cessation or near cessation of sedimentation on the nascent rift margin led to hypothesized extensive weathering and formation of pedogenic calcrete horizons that were later buried by volcaniclastic deposits that overlapped the rift margin. Diagenesis of vitric volcaniclastic detritus likely led to silica replacement of the calcareous soils to form the Pedernal chert, which was commonly used for lithic-tool manufacture for more than 13 millennia of human occupation in northern New Mexico. Heavily utilized chert quarries in the Cerro Pedernal-San Pedro Parks region were important lithic-material sources, but so were redeposited cobbles of chert that are ubiquitous in alluvial deposits on the Colorado Plateau and in the Rio Grande rift. The combination of primary and secondary chert sources has confounded efforts to determine the exact source locations of artifact raw materials. Fire, along with simple prying tools and hammerstones were likely used to dislodge large chert pieces for tool manufacture. Preliminary working of cores and bifaces produced large volumes of irregular flakes and rejected pieces that were mistaken by some early workers as finished products of more ancient tool-making cultures. FIGURE 1. View of Cerro Pedernal from the north. Miocene basalt caps a pinnacle of poorly exposed lower and middle Tertiary strata that rise above a base of Mesozoic rocks. Photo courtesy of Lisa W.Huckell. 426 SMITH AND HUCKELL Basalt flows and the intrusion of contemporary dikes along faults; they proposed that principal rift-basin subsidence in the Abiquiu embayment initiated at about 10 Ma. Smith et al. (2002) point out, however, that the basalt flows preserve a northward paleoslope from the Jemez Mountains that was eroded on southwest-dipping rift-basin fill (Fig. 2). The angular unconformity between the basalt and underlying strata implies that most basin subsidence occurred before basalt extrusion. Figure 3 illustrates the correlation of Cerro Pedernal stratigraphy to the stratigraphic section in the Cañones area in the western Rio Grande rift and only 5 km east of the peak. The correlation of rift-basin stratigraphy mapped by Manley (1982) and Moore (2000) to the undeformed section at Cerro Pedernal highlights evidence for the pre-basalt rifting history. Middle Miocene strata of the Tesuque Formation thicken dramatically from about 65 m at Cerro Pedernal to 330 m southeast of Cañones, indicating substantial movement on the Gonzales and Cañones faults (Figs. 2 and 3) before eruption of the Lobato Basalt. The underlying Abiquiu Formation consists of two thick members, a lower member of Precambrian-clast gravel and conglomerate, and an upper member of volcaniclastic sandstone and conglomerate (Smith, 1938; Vazzana, 1980; Moore, 2000). The lower member thickens only slightly between Cerro Pedernal and Cañones, whereas the upper member is roughly 25% thicker at Cañones (Fig. 3). Recognition of stratigraphic subdivisions within the upper member defined by changing volcanic provenance (Smith, 1995), allowed Moore (2000) and Smith et al. (2002) to further determine that the thinner section at Cerro Pedernal lacks the lowermost parts of the upper member that are present in outcrop near Cañones and farther east near Abiquiu. Clasts of the 25 Ma Amalia Tuff and sandstones with abundant quartz and alkali feldspar eroded from the tuff and coeval volcanic rocks of the Latir volcanic field (north of Taos, NM) occur at the base of the upper member at Cerro Pedernal (Smith et al., 2002; Fig. 3). This same marker horizon is about 15 m above the base of the upper member near Cañones and is more than 80 m above the base near Abiquiu (Smith et al., 2002). These observations imply that basin subsidence and rift-margin faulting were underway during Abiquiu Formation deposition, which commenced in this area before eruption of the 25 Ma Amalia Tuff (Smith et al., 2002). Deposition of approximately 400 m of Abiquiu Formation outside of the rift at Cerro Pedernal also implies aggradation mechanisms other than rift-basin subsidence. Moore (2000) and Smith et al. (2002) provide the results of simple flexural-loading and sediment-transport models that can partially account for deposition of the Abiquiu Formation by two mechanisms. First, the loading of the crust by construction of the coeval San Juan and Latir volcanic fields (in southern Colorado and north-central New Mexico, respectively) likely produced a flexural moat of sufficient dimensions to produce sediment accommodation space FIGURE 2. Generalized geologic map of the Abiquiu region showing the location of Cerro Pedernal near the boundary between the Colorado Plateau and the Abiquiu embayment of the Rio Grande rift (after Smith et al., 2002). FIGURE 3. Correlation of Oligocene and Miocene rocks eastward from Cerro Pedernal shows abrupt thickening of strata across rift-margin faults, except for the Pedernal member, which is restricted to the footwall of the Cañones fault. Thickening across faults implies syndepositional movement on rift-margin faults. The restricted distribution of the Pedernal member, along with condensation of the lowest strata of the upper member implied by the lowest appearance of Amalia Tuff clasts, is consistent with an initial pedogenic origin for the Pedernal member. 427 GEOLOGICAL AND GEOARCHAEOLOGICAL SIGNIFICANCE OF CERRO PEDERNAL in the Cerro Pedernal area. Second, the inundation of rivers by large volumes of volcanically produced pyroclastic sediment would likely lead to regrading of river profiles that would cause modest aggradation in this region. This volcanism-driven aggradation allowed sedimentation to extend beyond the nascent rift margin, whereas stratigraphic thickening into the rift, relative to the section at Cerro Pedernal, demonstrates the contemporaneity of deposition and rift-basin subsidence (Smith et al., 2002). Nature and Origin of the “Pedernal Chert” In Abiquiu Formation outcrops west of the Cañones fault, the Pedernal member is present between the lower and upper members (Fig. 3; Bryan, 1938, 1939; Smith, 1938; Church and Hack, 1939). The Pedernal member is a 4to 8-m thick interval of gravel that resembles the lower member but is notable for the presence of one to four, discontinuous, irregular bodies of chalcedonic and cherty silica with varying proportions of associated calcite. Figure 4 presents the approximate extent of the Pedernal chert member as mapped by Church and Hack and others (Banks, 1990; LeTourneau, 2000). Most of the chert is massive with scattered pebbles but in places also consists of nodular cement within gravel. The siliceous layers compose the informally known “Pedernal chert.” The siliceous horizons are best exposed as outcrop ledges more than 2 m thick near the tapered base of Cerro Pedernal (at an elevation of ~1900 m), but continue southwestward around the western margin of the Jemez Mountains and form much of the summit plateau of the Sierra Nacimiento at the San Pedro Parks (at an elevation of 3100-3200 m; Fig. 4; Church and Hack, 1939). Although sedimentary deep-sea chert and chert replacement of limestone are common rocks, the Pedernal chert is very unusual as a nonmarine occurrence of stratiform silica. The siliceous horizons are very heterogeneous in thickness, color, and texture. Individual layers range from 20 cm to more than 2 m thick. Upper and lower contacts are undulatory and are sharp in some places but gradational through a nodular-chert zone in other places. As Bryan (1939, p. 17) described it: “The chert is commonly white to pearly gray in color though in places the color may vary considerably. Near the base and sometimes near the top the chert shows bands 1/4” to 1/2” thick and is black in color. In places near the top, weathering has changed the color to pink, red, or yellow. The red color also occurs as flecks or spots in the white to pearly gray mass. Generally the yellow color is associated with clear, very translucent phases of the chert. This translucent type usually occurrs in small masses near the top of the bed.” Most exposures consist of vitreous chert (equant, microcrystalline quartz) and chalcedony (fibrous quartz) dominated by translucent gray and yellowish gray colors with patches and swirls of more opaque red, maroon, and black coloration. Discontinuous micritic calcite, in horizons as much as 50 centimeters thick, is present in the lower parts of, or beneath, most siliceous layers. Petrographic observations by Vazzana (1980) and Moore (2000) show evidence of replacement of micritic calcite by both chalcedony and chert, with subsequent infilling of vugs by chalcedony, drusy quartz, or sparry calcite. Although cropping out only in a narrow stratigraphic interval over an area of only 300 km2, chert pebbles and cobbles from this unit are notable in rift-basin and younger valley-fill alluvial deposits southward into the Albuquerque basin, westward into the San Juan basin, and eastward into the Española basin (Fig. 4). The persistence of the siliceous clasts attests to the resistance to weathering and the hardness of Pedernal chert. No researchers have systematically studied the siliceous horizons of the Pedernal member, although Vazzana (1980) and Moore (2000) provided some important observations and data as part of broad studies of the Abiquiu Formation. Vazzana (1980) described mineralogical and whole-rock-geochemical variations within sandy gravel below a Pedernal chert horizon that he ascribed to a buried weathering profile. Along with the widespread replacement of micritic calcite by silica, Vazzana (1980) and Moore (2