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INFORMATION ONLY Grokqgczt Soaocdi Amscnr= 0C Calcic soils of the southwestern United States 00 3fi," NI. Mochette U.S. Geoloical Sunve Denver Federal Center Denver. Colorado 022S5 ABSTRACT Calck soils are Commonly developed in Quaternary sediments thoughout the arid and semiarid pars of the soutbhesterm United States. In alluvial chronosequences, these soils have reiWonal varitions in their content of secondary calcium carbonate (CaCO3) because of (i) the combined effects of the age of the soiL f2) the amount, seasonal distribuWon, and concentration of Ca In rainfilL and 13) the CaC03 content and net influx of airborne dust. sik. and sand. This study shows that the morphology and amount of secondary CaCOj IcS) are valuable correlation tools that can also be used to date caick sols. The structures in calic soils are dues to their age and dlssoldtion-precipitation history. Two additional stages of carbonate morphology. which are more advanced than the four stages previously described. arn commonly formed in mide Pleistocene and older soils. Stage V morphology includes thick laminae and incipient pisolites. whereas Stage %I morphology inchxes the products of muliple cycles of brecciation. pisoite formation, and %holesale relamination of breccia fragments. Caici soils that have Stage VI morphology are associated with the latel?) Miocene contructional surface of the Ogallaha Formation of eastern New Mexico and western Texts and the earlyl?) Pliocene Mormon Mesa surfac of the Muddy Creek Formation eas of Lis Vegas. Nevada. Thus. caicic soils an represent MiMfn of years of formation and. in many cases, provide evidence o(clmatic. sedimentologic. and geologic events not otherwise recorded. The whol-prorile secondary Ca&CO3 content IcS) is a powerful developmental index for cak soils: eS is derined as the weight of CaCO. in a I-cm- vertical column through the soil fg'm 2 L This value is calculated from the thickness. CaC3O conceWtrS ton. and bulk density of caocic horizons in the soil. 1Sce Soil Survey Staff. 1975. p. 45-46. for a complete defnition of caklic horizon.) CaCOj precipitates in the soil through leaching of external Ca'- that is deposited oan the surface and in the upper part of the soi, generally in the A and B horios The cS content. maximum stage of CaCO3 morphology. and accumulation rate of CaCO3 in calck soils of equivalent age can vary over large regions of the southwestern United States in response to regional climatic patterns and the influx of Ca'" dissolved in rainwater and solid CaCO3. "Preliminary•ranium-trend ages and cS contents for relict soils of the Las Cruces. New Mexico. chron•sequence show that 100.000- to S0000O-year-old soils have imilar average rates of CaCO3 accumulatior. Conversely. soils formed during the past 50.000 years have accumulated C•O0C about twice as fast. probably because the amount of vegetative cover decreased in the Holocene and. hence, the potential supply of airborne Ca'" and COCO3 to the soil surface increased. The quantitative soil-development Index cS can be used to estimate the age of calcic soils. This index can also be used to correlate soils formed in unconsolidated Quaternary sediments both locally and regionally. to compare rates of secondary taCO, accumula tion. and to study landscape evolution as it applies to problems such as earthquake hazards and siting o(fcritical fciliies. PDR 1 2 I R UTMtaoeul _ _ _ _ __hev INTRODUCTION ,.. , . ,•.I Quantitative ind of soil dcvpne ame powefl and I iI funhamC l tools for esmating Ages of Qualerna•y soils. but such indices have no been widey applied in geology as yet. However. many soils and tbeir paent maeials cam scarcely be dated otherwise and thus, inadcquaw age contra limits the value of many elogi inCveigatom This pa describes a quanita. tive index that a-ssees the develomn of cakic so& that is. those arid and semiarid soils that have sigificaun awcmulatios of secoary calcium carbonate. This index can be ued to (I) estimate the duratia of soil fomnatio. (I) correlate Qua. tcrnuy sediments i3 which they are formed, and (3) analyze spatial and temporal variations in rates of secondary CaCO3 accumulation. Calci soils are hcrei defined as having simnificat accumu hutio o f secondary calcium carboate Not The term carbon. ate is used hereafter for both calcium carbonate and magesum --- carbonate, which may occur as a minor constiuem m calcic soi5& According to pedologic criteria they must have a cakic horizon. In the field, such soils ar characterized by a la.er cau least 15cm thick) of secondary carbonate thai is enriched in comparison to the soirs parent matCrial If Quaternary sedimets or sois contain app•oiate mate Firen I. Ckvpraphi and cWtural feaumr- of the southwestrn United rial. tey can be dated by one o soex l analya mehod States mentoned in thi x:• Organic or inorganic carbon can be dated by Ce of uranium disequilibrium methods. Ages o" vokanic rocks can be dated by They form widely in unconsolidated sediments during periods the K-Ar method. and their associated tephra lash and pumice) ranging from as little as a few thousand years to as much also be dated by the fission-uta technique or correlated as Scan several million years. Most calcic soils in the Southwest have using chemical and mineralogical characteisics ftepbrochrmno formed since middle and late Pleistocene time, but some highly oly,. These techniques. espeially when used with srazigraphic advanced ones have bee= forming since Pliocene or late and geomorphic Miocene evidence. help solve many problems in Quater. time on stable surfaces such as the Mormon Mesa of southern nary geology. Because of advances in dating techniques, age con. Nevada and the High Plains of Colorado. New Mexico. and trol in many Quaternary geologic st has been much Teas. improved. Unformnaely. This report is based on most Quaternary sediments lack reconnaissance and detailed soil abundant datable material and thus their straigraphis are CoM studies in the southwestern United States during 1974 through monly based on less exacting yet wid•y app•ical criteria. 1977 u contract to the U.S. Nucler Regulatory Commis Soils, however, mamnt mmlanind suaces. These soils range sion. Prelimunary results were published by EaahZP and from thin. incipien to weak prolile on young IHolocne) ainu Machene in 1977. Since then. unxd numerous US. Geological vium. to thick. strong pfiks on old (Peistocee and Plioccne) Survte programs, further information has been collected in New alluvium. The vast majority of soth. howve. ae bew these Mexico and in the Basin and Range province near Beaver. Utah extremes. Calcic sods are at the arid end-member of a broad (Fig. 1;Machet. 1982). spectrum of climates under which soils form. This study deal only with rlict soils (as redefined by Ruhe, 1965. and aused by NOMENCLATURE AND DISTRIBUTION OF Birkdand. 1974). Relict sois are those that have formed continu CALCIC SOIL AND PEDOGENIC CALCRETE ously since their parent material was deposite The age ofa relicst soil should closely approximate that Ofits Parent material because the soil is the cumulative product of soilormation since the deposit became geomorphically stable that is isolate from depo The term -calic" (Blake., 1902) has been broadly applied sition or erosion. to deposits of secondary calcium carbonate of various origins. Cakc soils are particulady well suited to quantitative as often irrespective of their geomorphic and stratigraphic relations. sesments o(soil deVlOp•CnL In the United States these soils are phsical, and chemical characteristi and genesis. Although widespread in the Southwest. but they also extend into the north. geologists in the United States generally associate caliche with soil em Basin and Range province and into the Midwest (Fig. I). carbonate, the indiscriminate use of the term has created semantic Cack sods of the so.hwewsrn United S•zer 3 problems. For example. the term has been applied loosely to he horizon to describe prominent layers soluble salts of sodium in nitrate deposits of of carbonate accumulation. northern Che a•ad The K-horion nomenclature is particularly useful Peru and to gyjeum deposits in Death Valley. California for geologic (cited in applications because it allows field diffrentiation Goudic. 1973. p. 8-9). Such usage can lead to misconceptioi of weak calcic M. hiizonM such as Bk horizons. esPecially i(comparisos and inaerprctations are made rom moderate to strong K hori. in Otbe a& zons. According to their definition, sratigapc or a geinetc context. In generaL the Seam Calid the K horizon must have be more than 90 percent K.fabric. a should be abandoned for more specific omenclastue. diagnostic soil fabrlic in which ".. in-e-grained authigenic Calci soils (an informal trm) arc herei dfined carbonate occurs as an essentially as ft se continuous medium, It coats or engulfs, soils that contain a signfian amount of and commonly separates secondary carbonat e. and cements skeletal Pebbes. generally in the form of calcic horizons). The term sand. and silt grains... (Gile and -borizon is other. 1965. p. 74. used to descnbe a thrmedimensional body (a laveri ofsoil man ria- For the index developed here. such horios should reprei it Disuibudon new additions of secondary carbonate to a soil profile. rather tha redistibutions of primary (allogenic) carbonate. Calcic horizor ts Calcic soils may range from layers of carbonate-coated Pebbles or carbona are found in many areas of the western United e States where warm and periodically to predominately filaments and nodules to thick. massive, and indurated carbonat dry di e- mates esult in torric. usatic enriched layers. With increasin age. caicic borzons become s or xcric soil-moisture regimes (se D Soil Su'v Suaff. 1975. well developed that their structural morphology and eneral ap for descriptions ofsoil-moistur regimes .
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