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Plate , , Geologic , and the Great Plains: A Primer for Non-

R. F. Diffendal Jr. University of Nebraska-Lincoln, [email protected]

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Diffendal, R. F. Jr., ", Space, Geologic Time, and the Great Plains: A Primer for Non- Geologists" (1991). Great Plains Quarterly. 591. https://digitalcommons.unl.edu/greatplainsquarterly/591

This Article is brought to you for free and open access by the Great Plains Studies, Center for at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Great Plains Quarterly by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. PLATE TECTONICS, SPACE, GEOLOGIC TIME, AND THE GREAT PLAINS A PRIMER FOR NON,GEOLOGISTS


For most Americans, "The Great Plains" evokes of the first technologies to affect the images of grasslands, dust storms, prairie , Plains, and it favored the development of some Indians on horseback, cowboys and wheat , types of vegetation, particularly grasses, over and perhaps flat valleys crossed by braided rivers others, such as woody . Farming and carrying a heavy load of and gravel, ex­ ranching have had vast impacts on the vege­ tremes of , and a typified by an tative cover, on the soils, and on wildlife dis­ alternation of droughts and wetter periods. Ge­ tribution. And dams, built across rivers to ologists picture such general images, too, but control flooding, to produce hydroelectric power, they also see radical changes in the landscape or to divert for irrigation or other uses, over periods expressed in millions rather than have produced major changes in the behavior hundreds of . Geologically speaking, hu­ of the rivers. man activities on the Great Plains are too re­ While historians look at the past on a human cent to have much of a place in the broad time scale, for geologists human time is simply geologic of the region, but they have the most recent frame in an unfinished movie. certainly influenced both the use and the ap­ Each frame is composed of a mosaic of ­ pearance of the region today and hence the wide snapshots. Pieces of the film are missing, terms in which we understand it. was one but what is left is usually in a chronological sequence. What geologists seek to do is to trace their way back through time as it is recorded on this broken film to explain the development R. F. Diffendal, Jr., is professor and research ge­ of a particular place, a region, or even the entire ologist at the University of Nebraska-Lincoln. A . The geologic history of any region is tied Fellow of the Geological Society of America, he has up with the as a whole, because spent the last fifteen years studying the events happening far away can leave an indel­ of Nebraska, on which he has published ible impression on the region under consider­ many articles. ation. Recent volcanic eruptions, for example, have injected into the dust [GPQ 11 (Spring 1991): 83-102] that have altered the weather or that have been

83 84 GREAT PLAINS QUARTERLY, SPRING 1991 deposited as ash for great distances downwind rates of rocks and have from their sources. The 's holistic view changed in response to the actions of and of the earth should include the overall conse­ and also of plants, and thus the elements quences of evolving as well as purely geo­ have laid down various kinds of soils in places logical forces. Beginning in 1972, ]. E. Lovelock and thicknesses that changed over time. Or­ hypothesized that all life makes up "a single ganisms have evolved, emigrated or immi­ living entity, capable of manipulating the Earth's grated, and changed in abundance. Yet for the atmosphere to suit its overall needs and en­ past 66.5 million years, since the uplift of the dowed with faculties and powers far beyond those Rocky and the withdrawal of the of its constituent parts." Lovelock suggested that shallow from the North American mid­ living might influence not only cli­ , this region has maintained the basic mate change but also , for­ identity that we associate with the Great Plains. mation, and the movement of the earth's lithospheric plates. is also at least partly attributable to the solar cycle of variations in the earth's orbital eccentricity, ax­ In order to understand what it is that geol­ ial tilt, and precession of the equinoxes due to ogists know about the Great Plains, it is nec­ variation in the earth's and position essary to understand a little bit about the history relative to other . l ]. E. Oliver linked of the of geology and of how geologists these plate movements, differences in the understand what it is that they are doing. In amount of coming from the , and the 1700s geologists began describing a chro­ other factors to make a "compromise " of nological sequence of layers of , upsetting climatic change. 2 earlier ideas of a relatively stable and recently My purpose in this paper is to explain for the created world. Geologists recognized that the non-geologist what some of these factors are, layers at the bottom of the sequence were the to give a history of how geologists have come oldest and that the layers were usually deposited to understand them, and then to go beyond horizontally. A third idea, that any feature cut­ Oliver's general model to focus on the relation­ ting across or deforming several strata had to ship between climate change and the geological be younger than any of the affected strata, also development of the Great Plains as it has been helped in constructing a time frame. When sim­ influenced by major geological events both out­ ilar types of appeared in rocks great dis­ side and within the region. These events in­ tances apart, they served as indexes to match clude movements of the earth's rigid up or correlate the strata. By the latter part of plates with attendant and the nineteenth century, when the concept of volcanism, cyclic changes in level and gla­ was sweeping the biological , ciation, broad uplifts and downwarps of the con­ evolutionary of organisms allowed tinents, the evolution of new environment­ geologists to establish a fairly precise worldwide changing organisms, and meteorite impacts. chronological sequence of rock strata. In the While most of these affect an area far greater twentieth century, with the discovery of the than just the Great Plains, local events also principles of radioactive dating, geologists ten­ produce regional change. For example, the al­ tatively established time spans for formation of titude of the Plains was once considerably lower rock sequences and increasingly refined these than it is at present. Folding, faulting, and ig­ dates to the point that many correlations are neous activity have thrust up mountains in parts now excellent. 3 of the region. Rivers have cut valleys and then rocks are older than 570 million filled them back in, wholly or partially, with years before present (mybp); the oldest of these sediments. The climate has been wetter and • contain no known fossils, while somewhat dryer, hotter and colder, than it is now. Natural younger Precambrian rocks have been found so PERIOD LITHOSHERE PLATE GENERAL TIMING OF AND CENOZOIC MOUNTAIN BUILDING (m.y.) MOVEMENTS VOLCANISM AND GLACIATION N. America Caribbean S.Amerlca N.Zealand

Holocene v 1------11 0.0 I V G v v v~ v v ~ I~V gG V V G Plei$ofacene VI~IV ~~ v~vG I ~v I II~V V G 1.651-1 I~_ -.::iG_ ~_~_ r y - vg1- - v_ -- ~ ,Y- _~-~--v_ G G- : ccC-1 i~l:~~ I~ I~~v V I v v G? o-s-u-'e-o-,fc:l-sl.,..hm-u-s-o--:f-=p-on-o-m-o----I~ ~ I v V <..) c Pliocene " ! G V VG v (5 co VG G V G "0 N I 5.2 I : Collision of Ausl,olio and ,Indonesia; IV IV Iv Iv vG V G ~ z" ~ II I ~ ~ I 0 1---+------1125 2 I I Africa and Europe Continued; v G Z .~ • I Uplift in Centrol America v V G ~ Iv~ v I ~ W Q) c <..) f- " 1------11 36.Q I Iv v v ~ I "'"0 :- Indio Begins to Collide with Asia " !I 1-----1154.0 I I I v I Collision of Africa and EurOPE! ~ Jt I l It: III I_I I V InIV V __ v ___1 ___ --,- __ - '" Australia Breaks from Antarctica -­ ~ ~ ~ <..) Late ~~ Cretaceous I Indio Breaks from Antarctica I Early ~ V-:~-I~ ~ ~ ~ :~ o 131 I S. America and Africa Breakup ll I N Lote li v :1 If~- I t ~ I Breakup of ; Formation of o Middle (f) I Atlantic w 210 ::2: 250 286 J.. Pennsyl- 'c van ian 0," .0" 320 ~o Missis- late LEGEND FOR SYMBOLS USED ABOVE <..) o~ UQ) sippian N 360 Consolidation of Continental o Lithosphere to Form of ~ General Period of Mountain Building W 408 --l Pangaea



t? ~ ./

OO'+------~----~------_+_r----,?------~~~~~~--~ 0°

" », \) d PRESENT DAY

FIG. 2. Changes in the positions of masses over the last 60 my. (Modified after Smith and Briden, note 7, courtesy of Cambridge University Press.) PRIMER FOR NON-GEOLOGISTS 87 far to contain only primitive invertebrate ani­ the geology and along this fit, and mals, , , and (Fig. 1). The the evidence of past climate changes that seemed remaining younger rocks, including those that explicable only in terms of . 4 contain more advanced fossils, were formed dur­ Other geophysicists, however, insisted that the ing parts of three major time spans or , the could not float about like rafts on (570-250 mybp) , Mesozoic (250-66.5 the seas of a denser mass of solid rock. Geol­ mybp) , and Cenozoic (66.5 mybp to present). ogists studied spinoffs of 's ideas or pro­ The fossils from the rocks formed during these posed other explanations for the similarities, eras are the preserved remains of life that more such as the development and later disappear­ and more resembles the organisms alive today ance of land bridges across the , to avoid as one progresses from the oldest to the youngest the problems involved in the drift concept. 5 rocks in the sequence. Geologists define the By the early 1960s, however, oceanographers systems of rocks formed during periods by major and other earth scientists produced data that changes in the types of fossils and types of rock showed that the entire of the earth is bro­ strata. Subdivisions are made according to def­ ken into larger and smaller pieces that move inite but less dramatic changes in the rock for­ with respect to each other. 6 New crustal ma­ mations and their fossils. In this paper I will terial is generated along oceanic ridges deal primarily with the geologic history of the and consumed under oceanic trenches. Within Great Plains during the part of the Cenozoic a decade of the production of these new data era from 37 mybp, when perhaps the oldest that satisfactorily explained the basic mechan­ Cenozoic rocks in western Nebraska were de­ ics, the idea of sea floor spreading and plate posited, to 1.65 mybp, the time of the depo­ tectonics had produced the major conceptual sition of the youngest sediments. I have revolution in geological sciences in the twen­ avoided discussing closer to the present tieth century. Not only did the concept explain because the is still too complex the jigsaw puzzle, but it allowed geologists to for the type of analysis I am performing here. I develop a coherent theoretical framework that will begin by discussing those events that had linked the fit with the origins of mountains, an impact on most of the earth and then con­ , and , as well as other sider those that were more regional in scale. phenomena. The theory describes a giant supercontinent, Pangaea, that had formed by the end of the LITHOSPHERE PLATE MOVEMENTS Permian Period (250 mybp) and that broke apart Plate tectonics has now been so widely de­ as new plates formed and carried continental scribed in the popular media that the concept fragments to their present positions. Figure 2 is familiar to any school child, but I well re­ shows the many changes in land/ rela­ member that in the late 1950s, when I was an tionships that have taken place over the last undergraduate majoring in geology, some of my sixty million years. 7 These changes in the po­ professors were extremely scornful of the idea, sitions of continents and changes in the shapes already several decades old, that the continents and connections of seas and oceans had pro­ had not stayed fixed in their places throughout found effects on climate and atmospheric/ time but had moved or drifted in respect to one oceanic circulation patterns, and they also trig­ another. German meteorologist and geophysi­ gered other events, such as the formation of cist was the foremost propo­ mountains. Earlier had held that nent of the drift idea, which so elegantly mountain formation had occurred at regular in­ explained such geological phenomena as the tervals worldwide, but plate tectonics theory amazing jigsaw-like fit of the eastern coastline suggests that mountain building has been going of the Americas with the western coastlines of on more or less continuously in one part of the Europe and Africa, the striking similarities of earth or other throughout geologic time. 8 Most 88 GREAT PLAINS QUARTERLY, SPRING 1991

PHOTO 1. Chimney Rock, Morrill County, Nebraska. The white layer near the base of the rock is Lower Whitney volcanic ash. The darker rock above it is Whitney Member of Brule Formation, while the spire is basal Arikaree Group. Photograph courtesy of R. F. Diffendal, Jr.

studies of plate tectonics have focused on the major mountain belts on land today (Figs. 1, areas where plates have broken apart or ground 3). Mountain ranges affect atmospheric circu­ together, but these forces have also had a great lation and precipitation. 9 Clearly the tremen­ impact on regions like the Great Plains, floating dous mountain uplift of the last 66.5 million seemingly untouched in the middle of the plate. years has played a major part in changes in world climate, as recent simulations have MOUNTAIN FORMATION confirmed.

When the leading margins of moving plates VOLCANISM collide, the rocks crumple and (break with displacement). Deeper down, rocks melt and Volcanism has been widespread along and flow upward, forming intrusive masses beneath adjacent to plate boundaries (Fig. 1). Volcanic the surface of the land or erupting in volcanoes activity has been particularly important to the with flowing lava. Mesozoic (250-66.5 mybp) Great Plains during two time spans (Figs. 4, 5). and Cenozoic (66.5 mybp to present) defor­ Eruptions to the south and west from sixty-five mation resulted in the formation of most of the to about seventeen million years ago produced PRIMER FOR NON-GEOLOOISTS 89

huge volumes of volcanic ash (particles less than two mm in diameter) that were carried into the GLACIATION atmosphere and deposited in thick accumula­ tions to the east in intermontane basins and on Glaciation has been part of earth's history parts of the Great Plains. II In the last seventeen on and off since the earliest geological era {Fig. million years, lesser episodes of volcanism have 1).14 Evidence for multiple glaciations during also sent ash onto the Great Plains. the Epoch (c. 1.65 mybp to present) Volcanism plays an important role in climate includes glacial deposits and other on change. The fine of ash an erupting vol­ land and on continental shelves and, on the cano injects into the atmosphere reduces the deep sea floors, both ice rafted deposits and solar radiation reaching the earth's surface. Ma­ variations in marine shells that indicate jor episodes of volcanism during the last 66.5 changing sea temperatures. million years correlate with periods of sharply Similar evidence indicates that glaciations lowered temperature. 12 R. S. White hypothe­ occurred several times back to the beginning of sized from th~s that massive volcanism would the Oligocene Epoch (36 mybp) in the northern cause major worldwide cooling, reduction in hemisphere. 15 Pre-Pleistocene glacial deposits , huge , and, because and landforms also have been found in the erupted gases contain carbon dioxide, sulfates, southern hemisphere {Fig. 1).16 Geologists dif­ and chloride which combine with fer about the placing of the boundary between to form acids, a decrease in the alkalinity of the the Pliocene/Pleistocene epochs (Fig. 1). While surface of the oceans. 13 most would date it at about 1.65 mybp, others


FIG. 3. Positions of Cenozoic Mountain Belts. (Modified after Umbgrove, note 8.) 90 GREAT PLAINS QUARTERLY, SPRING 1991

o 300MILES


FIG. 4. Acidic volcanic sites in the western and northern Mexico, 65 to 17 million years ago. Minimum former extent of Arikaree Group stippled. (Modified after Stewart and Carlson, note 10 [Plate 11-1); Swinehart, Souders, DeGraw, and Diffendal, note 11; and McDowell and Clabaugh, note 38.) PRIMER FOR NON-GEOLOGISTS 91

o 300MILES f-I--,,--JIL....,-, -r-, ''--_-II o 300 KILOMETERS

FIG. 5. Acidic volcanic sites in the western United States and northern Mexico, 17 million years to present. Minimum former extent of Ogallala Group stippled. (Modified after Stewart and Carlson, note 10 [Plate 11-2]; Swinehart, Souders, DeGraw, and Diffendal, note 11; and McDowell and Clabaugh, note 38.) 92 GREAT PLAINS QUARTERLY, SPRING 1991

TIME DEPTH {Km} Below -2.70 At 0 m.y. --4.029 - -4.718 --5.350

-2.70 After 20 m.y. - -4.029 - -4.718 - -5.350

-2.70 After 40 m.y. - -4.029 - -4.718 - -5.350

-2.70 After 60 m.y. - -4.029 - -4.718 --5.350

-2.70 After 70 m.y. --4.029 - -4.718 --5.350

4000 2000 o 2000 4000 KM

FIG. 6. Change in oceanic ridge profile when spreading rate at ridge is reduced from 6 cm/yr to 2 cm/yr at 0 million years. Top of ridge is about 2.70 km below sea level. (Modified after Pittman, note 18 [Figure 2).) PRIMER FOR NON-GEOLOGISTS 93 place it back at 2.8 million years ago. 17 No and on newly emergent continental shelves. glacial deposits older than this disputed time Rises seem to correlate to major periods of depo­ period have been found on or adjacent to the sition in these valleys. Far as they were from Great Plains, but the older Cenozoic glaciations an ocean, the rivers and streams of the Great noted above also affected the Great Plains be­ Plains responded thus to changes in sea levels. cause they were implicated in coincident cli­ mate changes, and their effect on sea level FLORAL CHANGES worldwide would have affected and river entrenchment on the Plains. Plants may also have had an important im­ pact on climate change. Although scientists have not yet been able to date the advent of an­ SEA LEVEL CHANGES giosperms (flowering plants) with certainty, de­ Scientists have known for centuries that sea ciduous angiosperms are among the fossil levels have not remained constant. Using seis­ of the Early Period (131-98 mybp), mic techniques that allow them to pick out and these angiosperms have become more and changes in sediments and to define the presence more abundant up to the present. 19 Vascular of erosion surfaces between major masses of plants (the majority of visible terrestrial plants), strata, geologists have worked out a sea-level which have well-developed conductive systems curve showing the changes, their frequencies, from roots to leaves, and their root-associated and their magnitudes. In the 1970s Peter Vail fungi, increase the breakdown of soil . and his colleagues at Exxon used these tech­ The deciduous angiosperms have higher net nu­ niques to work out a sea level change curve trient losses than do gymnosperm (coniferous) back through geologic time, a curve that con­ , which makes for overall increased tinues to be refined as new data are found. W. weathering rates where angiosperms dominate. C. Pittman related some major rises and falls T. Volk has associated this -caused in­ to changes in the size and number of submarine crease in the decomposition of rocks and soil ridges, the plate-generating areas of the lith­ over the past 131 million years with the in­ osphere (Fig. 6).18 Undersea ridges, underlain creased losses of potassium, calcium, and mag­ by hot rock that erupts into volcanic activity, nesium ions from the soil and the capture of are the places where heat expands the litho­ the ions by marine organisms in which oxides sphere. When a ridge ceases to be active, the of the ions combine with carbon dioxide to form rocks beneath it cool and contract, displacing carbonate . If this relationship be­ less sea water and producing a fall in sea level. tween soil decomposition and marine organisms When such ridges cover more of the sea floor, has been correctly hypothesized, then the growth the opposite sequence takes place. These effects and diversification of the angiosperms, includ­ are geologically very slow and account for long­ ing the grasses of the Great Plains, has been a term rises and falls in sea level on the order of factor in the systematic removal of carbon diox­ tens to hundreds of millions of years. Vail and ide from the atmosphere over the past 66.5 mil­ his colleagues called the various long and slow lion years and may have been an important changes in sea level first and second order cycles. contribution to overall through­ The much more rapid third order cycles of rises out this time period. 20 and falls they discovered are probably the result Studies of the distribution of fossil floras dur­ of glaciation and deglaciation, possibly com­ ing the (66.5-1. 65 mybp) in the north­ bined with tectonics. ern hemisphere support the idea that substantial Sea level rises and falls, whatever their causes cooling occurred at the Eocene/Oligocene and magnitudes, have had major impacts on boundary (36 mybp) and that have other aspects of earth's geologic history. Falls continued to be cool since then. Vastly accel­ have caused major river erosion on land erated erosion of the continents during the last 94 GREAT PLAINS QUARTERLY, SPRING 1991 fifteen million years may also be the result, at ATTEMPTS AT SYNTHESIS least in part, of floral evolution. 21 The idea of constructing a coherent geologic record of all these simultaneous events is daunt­ METEORITE IMPACTS ing, but at least two scientists have attempted A number of researchers have suggested that such syntheses. In 1966 J. F. Simpson tried to cornets or meteorites strike the earth at regular link mountain formation, sea level change, cli­ intervals. One theory is that these extraterres­ mate change, change in the earth's magnetism, trial objects are brought into the solar and and origination of organisms into when "Nemesis," a hypothetical companion star one interrelated whole. More recently A. Low­ to our sun with a very eccentric orbit, ap­ rie has related major geologic events, including proaches our . If such objects are the effects of meteorite impacts, to one another large enough, their collision with the earth over the past twenty-five million years. 2l Lowrie would inject huge dust into the atmo­ is continuing this work and has helped me in sphere and cause worldwide climate changes, attempting to associate climatic cycles with the , and other major geologic events. 22 geologic record of the Great Plains.

FIG. 7. Magnitude of vertical movement in thousands of meters in the conterminous United States over the last 10 million years. (Figure modified after Gable and Hatton, note 27.) PRIMER FOR NON-GEOLOGISTS 95

the sediments produced by their erosion were deposited by rivers in valleys radiating out from REGIONAL EROSION AND FILL the mountains, while when other areas were Thus far I have been discussing geologic later uplifted an overlay of sediments of different events on a worldwide scale. Now it is time to composition was deposited over the older beds. look specifically at the geologic history of the During the last ten million years,28 parts of the Great Plains. The Laramide , the Rocky Mountains were elevated as much as three mountain-building episode that produced the thousand meters or more, while parts of the Rocky Mountains and hence defined the Great adjacent plains were uplifted up to fifteen Plains in contrast, began in the Cretaceous (131- hundred meters during the same time (Fig. 7). 66.5 mybp) and culminated in the Eocene (54- The rain shadow effects produced by such in­ 36 mybp). Throughout the late Eocene, streams creases in altitude changed the climate from eroded the mountains down to a plain of low humid to semiarid. relief. The period of perhaps fifteen million years, As deeply buried blocks of Precambrian rocks from fifty to about thirty-five million years ago, broke and shifted with periodic differential was thus marked by widespread erosion followed movement, the overlying rocks were draped in by soil formation across this broad area. Deep folds or faulted. Subsequent erosion smoothed and extensive weathering of minerals in areas off the sharp edges of these surface inequalities, with warm, humid climates leaves behind bright creating hills and valleys. 29 At least some of this red and yellow oxide and -rich ancient movement has continued into the present. Ge­ soils (paleosols) indicative of such environmen­ ologists have also found faulting, or breaks across tal conditions. Parts of the erosion surface and strata, in rocks of the Tertiary (66.5-1.65 mybp) paleosols are preserved in both the Rocky in parts of eastern Wyoming, northwestern Ne­ Mountains and in the adjacent Great Plains. 24 braska, and southwestern South Dakota. 30 during the Paleocene and Eocene (66.5-36 mybp) took place in basins between GEOLOGIC HISTORY OF NEBRASKA AND mountain uplifts and on parts of the adjacent ADJACENT AREAS, 37-1.65 MYBP Great Plains region, presumably up to the gen­ eral level of the erosion surface. 25 Little evi­ In order to correlate geologic events in Ne­ dence of deposition during this time span has braska to the larger picture described in the first been found in Nebraska, either because the sed­ half of this article, it is necessary to establish a iments were eroded away at most sites after their precise and accurate dating system for the period deposition or because they are buried beneath in question. The classification of the Tertiary younger beds. The oldest part of Nebraska's System of rock formations (three-dimensional Chadron Formation is very Late Eocene, while mappable masses of rock having some distinc­ one site in northeastern Nebraska has yielded tive features) in Nebraska was initially worked Early Eocene fossils from river deposits, both out in some detail by N. H. Darton in 1899 circumstances suggesting that this phase of the and was refined nearly fifty years ago by A. L. depositional record has for the most part been Lugn and others. Scholars have subsequently worn away.26 published many works that add details to the classification or revise it to a greater or lesser degree. 31 T. M. Stout's work is of particular im­ REGIONAL UPLIFTS portance, because in it he addresses questions Uplifts have affected both the Rocky Moun­ of origins of the Tertiary sediments and causes tains and the adjacent Great Plains for the last of their deposition and of the erosional surfaces thirty-six million years, although they do not separating them one from another. According seem to have affected the whole area simulta­ to Stout, deposition and erosion are cyclic, with neously.27 Thus as certain areas were pushed up, erosion occurring during cold, dry periods on 96 GREAT PLAINS QUARTERLY, SPRING 1991



FIG. 8. Distributions of Tertiary geologic groups, formations, and members in western Nebraska compared to coastal onlap curve, eustatic curve, meteorite ages, and known gliu:ial and cold climate episodes. (Scale of geologic units after Swinehart, Souders, DeGraw, and Diffendal, note 11, and Stout, note 31.) Width of box indicates general amount of western Nebraska covered by unit. Wavey lines show erosion surfaces. Diagonal lines denote no deposits recognized. (Cycles, onlap curve and eustatic curve after Haq and others, note 39; Deep sea hiatuses !crosshatched] after Keller and Barron, note 39; Glacial episodes [crosshatched] after Armentrout, note 15, Mercer, note 15, Lowrie, note 23; overlap of episodes indicates differences in reports of authors. Crater [0] ages after Alvarez and Muller, note 39.) Lines of onlap curve noted by circled stars are major sea level declines.

the Great Plains that coincide with major gla­ pIes. New data from studies have allowed ciation and lowered sea levels in other parts of my colleague J. B. Swinehart to revise the clas­ the world. Deposition would then occur when sification of the Tertiary rocks in Nebraska (Fig. the glaciers melted and sea levels rose. 8). More recent work by Swisher and Prothero Refinement of radiometric and other dating has resulted in some slight changes in dates for techniques over the last two decades have en­ older units. 32 abled scientists to determine the geologic ages Swinehart and others have recently discussed of key beds in formations and soils quite pre­ in some detail the basic of the Cenozoic cisely, sometimes with a standard deviation of deposits in western Nebraska (Fig. 8). The White less than half a million years on multiple sam- River and Arikaree groups (a group is two or PRIMER FOR NON-GEOLOGISTS 97

more consecutive formations) are mainly com­ 10.5 million years ago appears to coincide with posed of valley fills of wind-borne volcanic de­ the erosion surface between the Valentine and bris carried into Nebraska from the west and Ash Hollow formations. The Ash Hollow sed­ southwest. Some stream-transported sediments iments are different in texture, geometry, and also occur in these fills, particularly but not prominence of paleosols when compared to older exclusively at the bases of the groups and for­ beds, so this formation break may be made more mations. On the other hand the Ogallala Group prominent in classifications. On the other and Broadwater Formation deposits are princi­ hand, some formations may have been eroded pally stream-transported sediments with minor away during a later and depo­ volcanic ash contribution. Paleosols occur in sition, thus producing gaps in the correlations. all of the groups and formations. Gravels derived from the erosion of the Rocky The boundaries between most of the various Mountains occur in the basal Chadron For­ formations and groups seem to coincide quite mation, in some Whitney Member fills, in the precisely with the worldwide events chronicled Gering Formation, and in the Runningwater earlier in this article. The volcanic debris com­ and younger formations of the Ogallala Group prising much of the White River and Arikaree (Fig. 8). They are particularly abundant in the groups is the right age to have come from the Ash Hollow Formation and the younger Pli­ explosive volcanism in the western United States ocene Broadwater Formation. These pulses of and northern Mexico (Figs. 4, 5). Late Oli­ coarser correlate well with periods of gocene- (29-19 mybp) strata on renewed uplift in the Rocky Mountains and es­ the coastal plain, approximately the same pecially well with times that Vail and others age as the Arikaree Group, also have significant have identified with relative sea level rises, that air fall volcanic debris as well as stream-trans­ is, times which sediment is being deposited ported fragments preserved in ma­ largely in river valleys rather than being trans­ jor valley fills. 33 Stream sediments begin to ported out to sea. predominate in the Ogallala Group at an age The glacial model proposed by Stout and that coincides with the period of quiescence of generally accepted by this author must also be volcanism in the west about seventeen million compared to the Nebraska Tertiary sequence if years ago, which was followed by generally less we are to claim an overall correlation of events explosive volcanism. on the Great Plains with those of the rest of Most of the major erosional breaks repre­ the globe. 34 According to Vail and his co-work­ senting gaps in time between formations and ers, their generalized first and second order sea groups fit points of greatest relative sea level level cycles are related to such plate tectonics decline on the curve developed by Vail and his phenomena as increase or decrease in oceanic co-workers (Fig. 8). These correlations are re­ ridge volume with consequent changes in sizes markable, as are the fits of many of the rock of ocean basins, but some or all of the more formation boundaries with the least generalized rapid third order cycles result from glaciation cycles of the curve of sea level rise and fall. The and deglaciation. Known onsets of glaciation incidences in which these third order cycles do correlate with the beginnings of third order sea not appear to match our geologic formations level cycles and with erosional formation may simply reflect a lack of subdivision of the boundaries in some places, but the currently formation rather than a lack of correlation. For recognized match in timing between these events example Swinehart and his colleagues have not is not nearly so good as that between relative as yet divided the Ash Hollow Formation into sea level changes and erosional formation named subdivisions, but I recognize that at least boundaries (Fig. 8). As scientists acquire more three cut and fill sequences are present in the data the correlation between glacial cycles and formation. These seem to fit the changes on geologic formations may improve, but perhaps, the curve. A major cycle break occurring about as Vail and his colleagues have claimed, short- 98 GREAT PLAINS QUARTERLY, SPRING 1991

PHOTO 2. Banner County, Nebraska. An older river bed (1), including scour holes and potholes (P), has been eroded into Brule Formation. The younger river bed (2) has been carved into Brule and overlying Gering Formation. Photograph courtesy of R. F. Diffendal. Jr.

term mountain building may be related to some flourishing in places where they do not live third order cycles, rather than glaciation/degla­ today? Furthermore, the paleosols of the Ogal­ ciation. lala Group do not give evidence of having been deposited during increasingly arid times. There should be more evidence of aridity than simply SOILS enriched paleosols whose As several researchers have noted, paleosols origins do not always reflect aridity. Wind trans­ seem to reflect a general trend of increasing ported sediments that do indicate aridity have aridity throughout the 37 to 1. 65 million- been recognized in parts of the Ogallala in Texas, span of geologic history I am considering. 35 One but no such deposits have been discovered in anomalous finding that seems to argue against Nebraska and adjacent areas. 36 In fact rivers the increasing aridity is the existence of abun­ deposited almost all of the strata of the Ash dant and widespread fossil hackberry pits and Hollow Formation. A small, older loess-like unit casts of these trees in even the youngest Ash below the Ash Hollow Formation, which may Hollow deposits. If it was really so very arid or may not be part of Ash Hollow, is at the near the end of Ash Hollow Formation depo­ base, the wrong place to indicate increasing sition, why were all of those trees alive and aridity, since aridity increases wind erosion and PRIMER FOR NON-GEOLOGISTS 99

tectonic plates with resultant mountain for­ mation,37 igneous activity, 38 and changes in ocean circulation and ocean basin sizes and shapes interacts with variations in the earth's solar cycle and possibly collisions with meteors throughout geologic time. Climate changes and evolution of flowering plants and of may be driven by these events or, according to Lovelock, may be driving some of them. On the Great Plains, rivers rising in the Rocky Mountains eroded sediments from upstream and deposited them on the Plains along with air­ fall volcanic debris. Volcanic ash from the south and west blanketed much of the Plains between thirty-seven and seventeen million years ago. Renewed uplift of the mountains over the last nineteen million years provided a continuing source of sediment for the rivers to lay down upon the Plains. Throughout the whole period discussed in this article the sea level of the repeatedly rose and fell in response to changes in plate tectonic activity and to the waxing and waning of glaciers. Sea level falls are probably related to the erosion of river valleys, while rises probably are related to sediment deposition in PHOTO 3. Close up of pothole in Photo 2. Basal those valleys. Riding on the center of a lith­ Gering Fonnation gravel fill (left center) of the river ospheric plate, the Great Plains nonetheless re­ pothole eroded into Brule Formation siltstones. Photo­ graph courtesy of R. F. Diffendal, Jr. sponded to the multiple geologic events that defined the geologic transformation of the world. 39 therefore wind deposits indicate a period of rel­ NOTES atively high aridity, higher than that at the time A. Diffendal, V. Souders, and two anonymous streams were depositing sediments. Evidently reviewers offered helpful suggestions that greatly im­ many questions about the geologic history of proved this article. the Great Plains remain unanswered, but we 1. J. E. Lovelock, "Gaia as Seen Through the can draw some conclusions. Atmosphere," Atl1Wspheric Environment 6 (1972): 579, Gaia a New Look at Life on Earth (New York: , 1979), The Ages of Gaia (New York: CONCLUSION W. W. Norton and Company, 1988); J. Imbrie and K. P. Imbrie, Ice Ages-Solving the Mystery (Hillside, A study of the geologic history of Nebraska New Jersey: Enslow Publishers, 1979). and adjacent parts of the Great Plains during 2. J. E. Oliver, Climate and Man's Environment the Cenozoic era reveals that the area developed (New York: John Wiley and Sons, 1973); W. L. according to the geologic rhythms of the region Donn, , 4th ed. (New York: McGraw Hill, 1975); W. F. Ruddiman, W. L. Prell, and M. E. and the rest of the world. The events that shaped Raymo, "Late Cenozoic Uplift in Southern Asia and the Plains were synergistic and reflect the idea the American West: Rationale for General Circu­ of a dynamic living planet, the Gaia hypothesis lation Modeling Experiments," Journal of Geophysical expressed so well by Lovelock. Movement of Research 94 (1989): 18, 379-91; J. E. Kutzbach et 100 GREAT PLAINS QUARTERLY, SPRING 1991 a\., "Sensitivity of Climate to Late Cenozoic Uplift of the ," American Scientist 77 (1989): 564- in Southern Asia and the American West: Numerical 73; and B. McGowran, "Fifty Million Years Ago," Experiments," Journal of Geophysical Research 94 American Scientist 78 (1990): 30-39. (1989): 18,393-407; W. F. Ruddiman and]. E. Kutz­ 9. See note 2 above. bach, "Forcing of Late Cenozoic Northern Hemi­ 10. P. ]. Coney, "Cordilleran Tectonics and North sphere Climate by Uplift in Southern Asia American Plate Motion," American Journal of Science and the American West," Journal of Geophysical Re­ 272 (1972): 603-28; P. ]. Coney, "Mesozoic-Ceno­ search 94 (1989): 18, 409-27; S. Manabe and A. zoic Cordilleran Plate Tectonics," in R. B. Smith Broccoli, "Mountains and Arid Climates of Middle and]. P. Eaton, eds., "Cenozoic Tectonics and Re­ ," Science 247 (1990): 192-95. gional of the Western Cordillera," Geo­ 3. W. B. N. Berry, Growth of a Prehistoric Time logical Society of America Memoir 152 (1978): 33-50; Scale (San Francisco: W. H. Freeman and Co., 1968) W. S. Snyder, W. R. Dickinson, and M. L. Silber­ is an excellent review of the development of the time man, "Tectonic Implications of Space-time Patterns scale, written for non-specialists. See also D. R. of Cenozoic in the Western United Prothero and C. C. Swisher, "Magnetostratigraphic States," Earth and Letters 32 (1976): Correlation and Ar-Ar Dating of Late Eocene-Oli­ 91-106; D. C. Noble, "Some Observations on the gocene Terrestrial Sequences of ," 28th Cenozoic -tectonic Evolution of the Great International Geological Congress Abstracts, 1989, pp. Basin, Western United States," Earth and Planetary 2-642. Science Letters 17 (1972): 142-50;]. H. Stewart and 4. A. Wegener, The Origin of Continents and Oceans ]. E. Carlson, "Generalized Maps Showing Distri­ (1929; trans. and rpt. New York: Dover Publications, bution, , and Age of Cenozoic Igneous Rocks 1966). in the Western United States," in Smith and Eaton, 5. A. Holmes, Principles of Physical Geology (Lon­ eds., "Cenozoic Tectonics" (this note above), pp. don: Ronald Press, 1944), drift mechanism proposed 263-64. on pp. 505-9; E. Mayr, ed., "The Problem of Land 11. G. A. !zett, "Late Cenozoic Connections Across the South Atlantic with Special and Deformation in Northern Colorado and Adjoin­ Reference to the Mesozoic," American Museum of ing Areas," in B. F. Curtis, ed., "Cenozoic History Bulletin 99 (no. 3, 1952): 81-285. of the Southern Rocky Mountains," Geological Society 6. A. Cox, ed., Plate Tectonics and Geomagnetic of America Memoir 144 (1975): 179-209; D. L. Black­ Reversals (San Francisco: W. H. Freeman and Co., stone, ]r., " and Cenozoic History of 1973) is a collection of articles on the topics and the Laramie Basin Region, Southeast Wyoming," in historical sketches of the development of these ideas; Curtis, "Cenozoic History" (this note above), pp. ]. M. , Plate Tectonics, 2nd ed. (Washington, 249-79; ]. B. Swinehart, V. L. Souders, H. M. D.C.: American Geophysical Union, 1980) is a col­ DeGraw, and R. F. Diffendal, ]r., "Cenozoic Paleo­ lection of major articles on the topic. of Western Nebraska," in R. M. Flores 7. A. G. Smith and ]. C. Briden, Mesozoic and and S. S. Kaplan, eds., "Cenozoic Paleogeography Cenozoic Paleocontinental Maps (Cambridge: Cam­ of the West-central United States," Society of Eco­ bridge University Press, 1977) is a fine collection of nomic Paleontologists and Mineralogists, Rocky Moun­ map reconstructions at mostly twenty million year tain Section, Rocky Mountain Paleogeography Symposium intervals; A. G. Smith, A. M. Hurley, and ]. C. 3 (1985): 209-29. Briden, Paleocontinental World Maps 12. D. I. Axelrod, "Role of Volcanism in Climate (Cambridge: Cambridge University Press, 1981), and and Evolution," Geological Society of America Special ]. C. Briden, A. M. Hurley, and A. G. Smith, "Pa­ Paper 185 (1981). leomagnetic Data and Mesozoic-Cenozoic Paleocon­ 13. R. S. White, "Igneous Outbursts and Mass tinental Maps," Journal of Geophysical Research, B 86 Extinctions," EOS 70 (1989): 1480, 1490-91. (no. 12, 1982): 11, 631-56 are good sources for past 14. See note 2 above. positions of continents. 15. ]. M. Armentrout, "Late Climati­ 8. ]. H. F. Umbgrove, The Pulse of the Earth (The cally Induced Depositional Cycles, Robinson Moun­ Hague: Martinus Nijhoff, 1947) and W. H. Bucher, tains, Yakataga District, Southern ," TER­ Deformation of the Earth's Crust (Princeton: Princeton QUA '82 Program with Abstracts, pp. 8-9; ]. M. Ar­ University Press, 1933) give older theories, while mentrout, "Glacial Lithofacies of the Neogene Yak­ more recent reports on this topic include A. M. ataga Formation, Robinson Mountains, Southern Spenser, ed., "Mesozoic-Cenozoic Orogenic Belts," Alaska Range, Alaska," in B. F. Molnia, ed., Geological Society of London Special Publication 4 (1974); Glacial-marine Sedimentation (New York: Plenum Press, P. ]. Coney, "Cordilleran Tectonics and North 1983), pp. 629-65; K. G. Miller, R. G. Fairbanks, American Plate ," American Journal of Sci­ and G. S. Mountain, "Tertiary Isotope Syn­ ence 272 (1972): 603-28;]. F.Shroder, ]r., "Hazards thesis, Sea Level History, and PRIMER FOR NON-GEOLOGISTS 101

Erosion," Paleooceanography 2 (1987): 1-19; L. Mar­ Societies Transactions 36 (1986): 497-509. inovich, "Molluscan Evidence for Early Middle Mio­ 24. D. E. Trimble, "Cenozoic Tectonic History cene Glaciation in Southern Alaska," Geological of the Great Plains Contrasted with that of the Society of America Bulletin 102 (1990): 1591-99. Southern Rocky Mountains-A Synthesis," Moun­ 16. Spenser, "Mesozoic-Cenozoic Orogenic Belts," tain Geologist 17 (No.3, 1980): 59-69; S. M. Cole­ Coney, "Cordilleran Tectonics," Shroder, "Hazards man, "Map Showing Tectonic Features of Late of the Himalayas," and McGowran, "Fifty Million Cenozoic Origin in Colorado," U.S. Geological Sur­ Years Ago" (all note 8 above); J. H. Mercer, "Cen­ vey Map 1-5666 (1985); G. J. Retallack, "Late Eocene ozoic Glaciation in the Southern Hemisphere," An­ and Oligocene Paleosols from Badlands National Park, nual Review of Earth and Planetary Sciences 11 (1983): South Dakota," Geological Society of America Special 99-132. Paper 193 (1983). 17. Geologists who date the boundary at 2.8 mybp 25. W. R. Dickinson et al., "Paleogeographic and include L. A. Smith, " of the Stra­ Paleotectonic Setting of Laramide Sedimentary Ba­ totype Pleistocene Sections at Vrica, ," TER­ sins in the Central Rocky Mountain Region," Geo­ QUA '83 Program with Abstracts, p. 30; J. L. Lamb, logical Society of America Bulletin 100 (1988): 1023- W. W. Wornardt, T-C Huang, and T. E. Dube, 39; S. M. Cather and C. E. Chapin, "Alternative "Practical Application of Pleistocene Eustacy in Off­ Interpretation," Geological Society of America Bulletin shore , " in C. A. Ross and D. Hamen, 102 (1990): 256-58. eds., "Timing and Depositional History of Eustatic 26. C. C. Swisher, III, and D. R. Prothero, "Sin­ Sequences--Constraints on Seismic ," gle 4°AR/39AR Dating of the Eocene-Oligo­ Cushman Foundation for Foraminiferal Research Special cene Transition in North America," Science 249 Publication 24 (1987): 33-39. (1990): 760-62; M. R. Voorhies, "Early Eocene 18. C. E. Payton, ed., "Seismic Stratigraphy-Ap­ (Wasatchian) from Northeastern Ne­ plications to Hydrocarbon Exploration," American braska-the Lewis and Clark Local , Geological Association of Petroleum Geologists Memoir 26, has Society of America Rocky Mountain Section Abstracts many excellent articles on the subject including sev­ (1987), p. 340. eral by P. Vail and colleagues explaining the devel­ 27. Trimble, "Cenozoic Tectonic History" and opment of the sea level curve; W. C. Pittman, III, Coleman, "Map Showing Tectonic Features" (both "Relationship Between Eustacy and Stratigraphic Se­ note 24 above); J. D. Love, "Cenozoic Sedimenta­ quences of Passive Margins," Geological Society of tion and Crustal Movement in Wyoming," American American Bulletin 89 (1978): 1389-1403. Journal of Science 258-A (1960): 204-14; J. McPhee, 19. M. A. Knoll and W. C. James, "Effect of the Rising from the Plains (New York: Farrar, Straus, Gi­ Advent and Diversification of Vascular Land Plants roux, 1986), pp. 46-55; D. J. Gable and T. Hatton, on Weathering Through Geologic Time," "Maps of Vertical Crustal Movements in the Con­ Geology 15 (1987): 1099-1102. terminous United States Over the Last 10 Million 20. T. Volk, "Rise of Angiosperms as a Factor in Years," U. S. Map 1-1315 (1983). Long-term Climatic Cooling," Geology 17 (1989): 28. Gable and Hatton, "Maps of Vertical Crustal 107-11. Movements" (note 27 above); Manabe and Broccoli, 21. J. A. Wolfe, "A Paleobotanical Interpretation "Mountains and Arid Climates," Ruddiman and of Tertiary Climates in the Northern Hemisphere," Kutzbach, "Forcing of Late Cenozoic Northern American Scientist 66 (1978): 694-703; T. W. Don­ Hemisphere Climate," and Kutzbach et al., "Sen­ nelly, "Worldwide Continental Denudation and Cli­ sitivity of Climate" (all note 2 above). matic Deterioration During the Late Tertiary­ 29. S. A. Sonnenburg and R. J. Weimer, "Tec­ Evidence from Deep-sea Sediments," Geology 10 tonics, Sedimentation, and Petroleum Potential, (1982): 451-54. Northern Denver Basin, Colorado, Wyoming, and 22. M. Davis, P. Hut, and R. A. Muller, "Ex­ Nebraska," Colorado School of Mines Quarterly 76 (no. tinction of by Periodic Comet Showers," Na­ 2, 1981); T. S. Ahlbrandt and W. Groen, "The ture308 (1984): 715-17; W. Alvarez and R. A. Muller, Goshen Hole Uplift-a Brief Review of its Geologic "Evidence from Crater Ages for Periodic Impacts on History and Exploration Potential," Mountain Ge­ the Earth," Nature 308 (1984): 718-20; R. A. Muller, ologist 24 (no. 2, 1987): 33-43. Nemesis (New York: Weidenfeld and Nicolson, 1988). 30. Swinehart et al., "Cenozoic Paleogeography" 23. J. F. Simpson, "Evolutionary Pulsations and (note 11 above). Geomagnetic Polarity," Geological Society of Ameri­ 31. A. L. Lugn, "Classification of the Tertiary can Bulletin 77 (1966): 197-204; A. Lowrie, "Model System in Nebraska," Geological Society of America for Fine-scale Movements Associated with Climate Bulletin 50 (1939): 1245-76; C. B. Schultz and C. and Sea Level Changes Along Louisiana Shelfbreak H. Falkenbach, "The Phylogeny of the Oreodonts­ Growth Faults," Gulf Coast Association of Geological Parts 1 and 2," American Museum of Natural History 102 GREAT PLAINS QUARTERLY, SPRING 1991

Bulletin 139 (1968): 410-15; T. M. Stout, "The Com­ 36. Two important articles are D. A. Winkler, parative Method in Stratigraphy-the Beginning and "-bearing Eolian Unit from the Ogallala End of an ," Nebraska Academy of Sciences Group (Miocene) in Northwestern Texas," Geology Transactions 6 (1978): 1-18. (Stout would probably 15 (1987): 705-8; T. C. Gustavson and D. A. Wink­ disagree with the dates, ages, and some formation ler, "Depositional of the Miocene-Pliocene names shown as equivalent to his groups. The rocks, Ogallala Formation, Northwestern Texas and Eastern however, remain the same no what we call New Mexico," Geology 16 (1988): 203-6; L. R. Gard­ them.) See also M. F. Skinner, S. M. Skinner, and ner, D. F. Williams, P. Helland, and R. F. Diffendal, R. J. Gooris, "Stratigraphy and Biostratigraphy of Jr., "Current Studies on Climate in Source Areas Late Cenozoic Deposits in Central Sioux County, and Depositional Sites During Latest Ogallala Group Western Nebraska," American Museum of Natural (Late Neogene) Deposition, Western Nebraska," History Bulletin 158 (1977, art. 5): 263-371; Swine­ TER-QUA '91 Abstracts with Programs (1991): 7. hart et aI., "Cenozoic Paleogeography" (note 11 37. Spenser, "Mesozoic-Cenozoic Orogenic Belts" above). (note 8 above). 32. Swinehart et aI., "Cenozoic Paleogeography" 38. F. W. McDowell and S. E. Clabaugh, "Ig­ (note 11 above); Swisher and Prothero, "Single­ nimbrites of the Sierra Madre Occidental and Their Crystal