The Blackwater Draw Formation (Quaternary): a 1.4-Plus-M.Y

The Blackwater Draw Formation (Quaternary): A 1.4-plus-m.y. record of eolian sedimentation and soil formation on the Southern High Plains VANCE T. HOLLIDAY Department of Geography, Science Hall, University of Wisconsin, Madison, Wisconsin 53706 ABSTRACT by Holliday and Gustavson (1989). The Black- Leonard, as well as Brown (1956), concluded water Draw Formation, a sheet-like body of that the "cover sands" were of eolian origin, The Southern High Plains of northwestern sandy to clayey sediment covering more than probably derived from the Pecos River valley to Texas and eastern New Mexico are mantled 100,000 km2, is the principal surficial deposit of the west and southwest. Frye and Leonard by a vast (>100,000 km2) sheet of Quater- most of the Southern High Plains of northwest- (1964, 1965) also recognized a strongly devel- nary eolian sediment locally as much as 27 m ern Texas and eastern New Mexico (Fig. 1); its oped soil formed at the surface of the "cover thick and termed the "Blackwater Draw fertile soils support the lucrative agricultural in- sands" and considered it to be a "Sangamon Formation." These sediments generally fine dustry of the region. There has been surprisingly Soil." from southwest to northeast, exhibiting a lin- little research concerning the history of the Reeves (1976) proposed the term "Black- ear relationship of decreasing sand content Blackwater Draw Formation, given its extent water Draw Formation" to replace "cover and sand size and increasing silt content with and economic importance, but the available sands." Reeves also considered it an Illinoian increasing distance, indicating the source area data nevertheless demonstrate that it is more deposit but noted that the term "Sangamon soil" to be the Pecos River valley. As many as six complex and spans much more time than pre- should not be applied to the regional surface soil well-developed buried soils (2.5YR to 5YR viously believed. because of evidence of multiple periods of soil hues, Bt horizons 1-2 m thick, Stage II-III The Southern High Plains, or Llano Estacado, formation. Reeves (1976, p. 222) also identified calcic horizons), similar to the regional Pa- are an extensive, semiarid plateau covering a "thin cover of loess" overlying the Blackwater leustalf and Paleustoll surface soils, occur in about 130,000 km2. The region has a virtually Draw Formation in the northeastern part of the the formation. The buried soils indicate epi- featureless, constructional surface, formed by the Llano Estacado (Fig. 1). sodic sedimentation separated by long peri- eolian deposits of the Blackwater Draw Forma- Several other investigations provide data on ods of landscape stability. Eolian sedimentation (Fig. 1). These eolian sediments rest on eo- the physical properties, age, and origin of the tion probably occurred during prolonged lian and alluvial deposits of the Ogallala Blackwater Draw Formation. Well-developed aridity, and stability and pedogenesis likely Formation (Miocene-Pliocene) and locally on buried soils were recognized in the sediments obtained during subhumid to semiarid condi- lacustrine sediments of the Blanco Formation (Brown, 1956; Allen and Goss, 1974; Hawley tions, similar to those of the late Quaternary. (Pliocene). The otherwise flat High Plains sur- and others, 1976; Holliday, 1988a; Holliday and The presence of the 0.62-m.y. Lava Creek B face is locally interrupted by several dune fields, Gustavson, 1989), indicating that the formation Ash and the 1.4-m.y. Guaje Ash in the forma- numerous small basins with ephemeral lakes is composed of a number of individual layers, tion shows that the deposit accumulated (playas) in many cases associated with lunettes, each deposited episodically. Limited numerical throughout most of the Quaternary. Data and several northwest-southeast-trending dry age control, discussed more fully in this paper, from paleomagnetic, thermoluminescence, valleys or draws, which are tributaries of the and radiocarbon studies suggest that each Red, Brazos, and Colorado Rivers (Holliday, cycle of sedimentation-stability lasted for sev- 1985a). The dune fields and lunettes rest on the eral hundred thousand years and that the last TABLE 1. WEIGHTED MEANS OF ARGILLAN Blackwater Draw Formation, and the playas CHARACTERISTICS FROM Bt HORIZONS OF SOILS IN depositional event occurred at least several and draws are inset into it. THE TYPE SECTION OF THE BLACKWATER tens of thousands of years ago. DRAW FORMATION HISTORY OF INVESTIGATIONS Soil % of grains % perimeter coated Thickness INTRODUCTION w/argillans w/argillans (microns) Frye and Leonard (1957, 1965) first de- Surface 55 42 1.7 bl 80 65 2.8 This paper is a first approximation of the age, scribed the Blackwater Draw Formation, using b2 80 65 2.0 origin, and evolution of the Blackwater Draw the informal term "cover sands," and considered b3 85 55 1.9 b4 80 40 2.3 Formation, expanding on a summary presented the deposit to be of "Illinoian" age. Frye and Geological Society of America Bulletin, v. 101, p. 1598-1607,4 figs., 4 tables, December 1989. 1598 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/12/1598/3380541/i0016-7606-101-12-1598.pdf by guest on 24 September 2021 EOLIAN SEDIMENTATION AND SOIL FORMATION, SOUTHERN HIGH PLAINS 1599 section. (2) A radiocarbon-dated lunette, the Dune site, on the north side of Lubbock (Fig. 1), was studied because it buries the surface of and provides minimum ages for the Blackwater Draw Formation (Holliday, 1985a). Sections (3) at Mount Blanco and (4) in the Tule Basin (Fig. 1) were examined because they contain dated volcanic ashes intercalated with buried soils. (5) A site near Bushland, Texas (Fig. 1), originally studied by Allen and Goss (1974), was re-examined because six buried soils are ex- posed and some pedologic data are already available. Sections were described using stan- dard soil nomenclature (Soil Survey Staff, 1951, 1975; Guthrie and Witty, 1982) (Fig. 2; com- plete descriptions are presented by Allen and Goss, 1974; Gustavson and Holliday, 1988; and Holliday, 1988a). Stages of morphological de- velopment of horizons of carbonate accumula- tion follow the terminology of Gile and others (1966) as modified by Machette (1985). Refer- ence to individual buried soils in this paper is based on their stratigraphic position below the surface, using their horizon designation post- scripts (for example, bl is the first buried soil, b2 is the second buried soil, and so on; Fig. 2). Peds were collected from soil horizons at the Blackwater Draw Formation type section, the Blanco section, and the Tule Basin section for thin-section preparation and soil micromorpho- logical characterization (following the terminology of Brewer, 1976). Illuvial clay and carbonate in thin sections from the type locality were further quantified using a technique developed by Holliday (1982, 1988b). Point counts, usu- ally 200, were made for each thin section. Pres- ence or absence of argillans (pedogenic clay films) around each counted quartz sand grain was recorded. When argillans were encoun- Figure 1. Map of the Southern High Plains, showing the distribution of the Black water tered, the percentage of grain perimeter coated Draw Formation and progressive fining of surface soils to the northeast. Also shown are all was also estimated. Thickness of the argillans localities mentioned in the text, the south-north and southwest-northeast transects of particle was determined by scanning the slide and meas- size (PS) by Seitlheko (1975), and selected rivers and cities. The area of clay loam to clay soils uring a random sample of 20 clay films. is the area of "loess" defined by Reeves (1976, Fig. 1). Weighted means were calculated for the percentages of the grains with argillans, the percentages of the grain perimeter coated with clay, and the clay film thicknesses for each soil (Table 1). Par- suggests that deposition took place throughout area are the result of this downwind fining and ticles of pedogenic carbonate were also counted. much of the Pleistocene (Holliday, 1984,1988a; are not a separate "loess cover." Seitlheko's (1975) data on textural variation Machenberg and others, 1985; Patterson and in the Blackwater Draw Formation offer con- others, 1988). METHODS siderable insight into the source and sedimentary Seitlheko (1975) provided quantitative data history of the deposit and therefore warrant on textural variation of the surface layer of the Good exposures of the Blackwater Draw further discussion and manipulation. The fol- Blackwater Draw Formation. He demonstrated Formation are limited, but five were examined lowing discussion of methods is summarized that the upper bed fines from southwest to as part of this study. (1) The type locality for the from Seitlheko (1975, p. 13-18). Samples taken northeast, supporting the hypothesis of eolian Blackwater Draw Formation, established by from two transects across the Southern High origin from the Pecos Valley (Fig. 1; see Sedi- Reeves (1976) near Lubbock, Texas (Fig. 1), Plains (Fig. 1) were analyzed for particle size mentology discussion below). Furthermore, was described because this was not done when distribution. A south-to-north transect 438 km Seitlheko showed that the silty and clayey surfi- the formation name was proposed and because long included 27 sampling localities, and a cial sediments in the northeastern portion of the there appeared to be several buried soils in the southwest-to-northeast transect 392 km long Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/12/1598/3380541/i0016-7606-101-12-1598.pdf by guest on 24 September 2021 1600 V. T. HOLLIDAY Blackwater Draw F m. type section Mt. Blanco Tute Basin Bush land 0™ profile above A A -A A 5YR 3/4 ASB K Is eroded,. 75 YR 4/6 Bt 7.5 YR 3/2 C ab 5YR 8/4 Bt SIC pr K 5YR 3/6 S¡L m \k Btk 7.5YR 9/1 Btk 10YR3/3 C ab Bt i CL m iff 5YR 5/6 SCL pr 7.5YR 5/4 Btkbl SCL ab it Btbl Btbl 5YR5/6 C sb ìli 5YR8/2 SiC pr Bkbl L m 7.5YR 5/8 m iBkbl 7.5YR 6/4 "Bkbl 5YR8/3 CL m 2- 2- 5YR8/3 ¿è Bk Lava Ck Ash 0.62 m.y.

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