LateWisconsin climatic inlerences from rock glaciers in south-centraland west-central New Mexico andeast-central Arizona byJohn W. Blagbrough, P0 Box8063, Albuquerque, NewMexico 87198 Abstract Inactive rock glaciersof late Wisconsin age occur at seven sites in south-central and west-central New Mexico and in east-centralArizona. They are at the base of steep talus in the heads of canyons and ravines and have surfacefeatures indicating they are ice-cemented (permafrost) forms that moved by the flow of interstitial ice. The rock glaciersindicate zones of alpine permafrost with lower levels that rise from approximately 2,400m in the east region to 2,950 m in the west. Within the zones the mean annual temperaturewas below freezing, and the climatewas marked by much diurnal freezing and thawing resulting in the production of large volumes of talus in favorableterrain. The snow cover was thin and of short duration, which fa- vored ground freezing and cryofraction. The rock glaciers in the east region occur near the late Wisconsin 0'C air isotherm and implv that the mean annual temperature was depressedapproximately 7 to 8'C during a periglacial episodein the late Wisconsin.A dry continental climate with a seasonaldistribution of precipitation similar to that of the present probably prevailed, and timberline former timberlines. may have been depresseda minimum of 1,240m. The rise in elevation of the rock glaciersfrom east to west acrossthe region is attributed to greater snowfall in west-centralNew Mexico and east-centralArizona, which reducedthe inten- sity and depth of ground freezing near the late Wisconsin 0"C air isotherm. Introduction Alsoin thisissue Rock glaciers are periglacial mass-movement deposits that have been described in many alpine areas throughout the world. Wahr- Oil andgas discovery wells of 1993 p.72 haftig (1987) proposed that "rock glacier" be restricted to those formations of blocky debris, extending outward and downslope NMGS1995 spring meeting p.77 moraines of from talus cones or from glaciers or the terminal (1930-1994) p.78 glaciers, that have moved in large part through the deformation RussellE. Clemons of interstitial ice (ice-cemented rock glaciers) or clear ice (ice-cored Statetaxes on naturalresource production p.79 rock glaciers) within them. He also emphasized that masses of blocky debris shown to have accumulated as landslides should NMGS1994 abstracts p. 79 be called a variety of landslide even though they have some mor- Upcominggeologic meetings p. 85 phologic characteristics of rock glaciers. Ice-cemented rock glaciers are cemented by interstitial ice or Service/News p. 86 contain large bodies of solid ice derived either from avalanche p.87 snow or refreezing of melt or spring water (Wahrhaftig and Cox, lndexto Volume16 1959), whereas ice-cored rock glaciers contain cores of massive Stafl notes p. 88 glacial ice (Potter, 1972, p.3027). Ice-cored rock glaciers are char- acterized by (1) saucer- or spoon-shaped depressions between NMGsubscription information p. 88 the base of the cirque headwalls and the rock glacier heads, (2) longitudinal furrows along both sides, (3) central meandering Gomingsoon furrows, and (4) conical or coalescing pits (White, t976). lce-ce- mented forms lack depressions at their heads and have contin- Bosquedel Apachehydrogeothermal study and avalanche slopes feeding onto their heads (Luckman uous talus Triassicstratigraphy and chronology and Crocket, 1978).They also are characterized by well-developed Iongitudinal and transverse ridges (White, 1976). central Andes (latitude 33'S) is 1,300 m below the equilibrium marily on t8O variations in ostracodshells. The late Wisconsin line of glaciers on the continental side in Argentina whereas on full-glacial climate between 35,000and 28,600years B.P. was a the maritime side in Chile, they are 700 m below the equilibrium dry and relativelycold period. An abruptwarming eventoccurred line. He ascribes this altitudinal variation to the difference in at28,6ffi yearsB.P., and warm temperaturescontinued until 21,800 climate between the two regions. Greater snow precipitation on years B.P.interrupted by cofd pulsesof short duration. The late the west side of the Andes reduces the intensity and depth of Wisconsinthermal minimum is definedby a cool and wet period freezing and raises the elevation of the lower limit of permafrost. between 21,800and 20,600years B.P. A rapid warming trend at Cort6 (1982b) suggested several regional.factors that promote 20,600 years B.P. marks the close of the late Wisconsin thermal the formation of rock glaciers. Among these are (1) diurnal freez- minimum and was followed by a period of climatic instability ing and thawing, which result in the generation of large volumes characterizedby rapid fluctuations in both temperature and pre- of debris, (2) a mean annual temperature below 0"C, (3) enough cipitation. moisture in the ground to promote cryofraction and gelifluction, (4) moderate to low amounts of precipitation in the form of snow Geographic, geologic, and modern climatic setting for the production of debris and some snow avalanches, and (5) Rock glaciersoccur at seven alpine sites between 33"15'and reduced snow cover that favors ground freezing and cryofraction. 34"15'North latitude in south-centraland west-centralNew Mex- ico and east-centralArizona (Fig. 1). The mountainshave summit Late Wisconsin climate elevationsbetween 2,638 and 3,316m and rise 450 to 1,370m The nature of the late Wisconsin climate in the southwest United above adjacentlowlands. Their slopesare cut by many steep- States has been controversial mainly because of the difference walled canyons with maximum depths of approximately 300 m. between interpretations of the physical evidence and that of the Numerous small canyons and ravines are tributary to the trunk fossil plant record. Models have ranged from climates with lower canyons. winter temperatures to climates with mild winters and cool sum- The rock glaciersin the eastregion are in the Sacramentosection mers, and from climates of decreased precipitation with main- of the Basin and Range province and are formed mainly by fuag- tenance of the present seasonal dishibution to climates with greatly ments of intrusive rocks of mid-Tertiaryage. They are north of increased winter precipitation (Van Devender and Spaulding,7979; the SacramentoMountains, an extensive highland in south-cen- Hawley, 1993). Many of these models were proposed to account tral New Mexico. The rock glaciersin the central and west region for the water budgets of pluvial lakes or were based on the alti- are in the Datil-Mogollon section, a transitional zone between tudinal distribution of cryogenic deposits that are assumed to the Colorado Plateauand the Basinand Rangeprovince that forms indicate lower orographic snow lines and timberlines. Other mod- a large highland in west-centralNew Mexico and east-central els were inferred from fossil plant assemblagesof packrat middens Arizona (Hawley, 1986). They are composed predominantly of and from pollen records of pluvial lakes. debris derived from extrusive rocks of mid-Tertiary age. Galloway (1970) proposed a cold and dry model of late Wis- consin climate in the southwest United States utilizing undated solifluction deposits in the Sacramento Mountains of south-cen- tral New Mexico. He postulated a 1,300-1,400-m lowering of tim- New A4exlco berline resulting from a temperature decrease of 10 to 11'C for all seasons and a 10-207o reduction in precipitation. Brakenridge GEOLOGY (1978) also presented evidence for a cold and dry late Wisconsin (full-glacial) climate in the American southwest. He inferred a . Scienceand Service 1,000-m depression of the orographic snowline, timberline, and tssN 0196-948X cryogenic deposits and proposed a 7 to 8'C cooling in all seasons Volume 16, No. 4, Noyember 1994 and no significant increase in annual precipitation. Van Devender et al. (1984) and Van Devender (1990) made Editor: Carol A. Hjellming several inferences about the climate in the Sacramento Mountains Publishedquarterly by in south-central New Mexico during the late Wisconsin (16,000 New Mexico Bureau of Mines and to Mineral Resources 18,000 years B.P.) utilizing macroplant fossils from packrat a division of New Mexico Institute of middens. They suggested that summers were much colder than Mining & Technology today with temperatures probably resembling present tempera- BOARD OF REGENTS tures for late spring or early fall. Winters were not much colder Ex-Officio than today, and most of the precipitation was in winter and spring Bruce King, Gooernorof Nru Meico Alan Morgan, Superintendentof Publiclnstruction with only modest amounts of summer rainfall. Van Devendel and Appointed Spaulding (1979) and Spaulding et al. (1983), using similar data, Diane D. Denish, Prcs.,1992-197, Albuquerque advocated J. Michael Kelly, Sec.lTreas.,1992-1997, RNweU comparable models for an extensive region of the Lt. Gen. Leo Marquez, 7989-1995, Albuquoque southwest United States. SteveTorres, L91-1997, Albuquerque Recent studies in New Mexico present evidence Charles Zimmerly, 1997-7997, Socorro for variable New Mexico Instihrte of Mining & Technology climatic conditions during the late Wisconsin suggesting that all President.. ....... DanielH.L6pez of the models noted above may be, at least, paitially correct. New Merico Bureauof Mines & Mineral Resourc-es Directorand StateGeologisf .. Charles E. Chapin Phillips et al. (1985)proposed a late Wisconsin climatic model for Sulrciptions: Issued